Chemicals for the Management of Paddy Blast Disease

Abstract

Blast disease of rice caused by Magnaporthe oryzae has remained to be the biggest threat for rice production across the world. Several strategies such as cultural, biological, host plant resistance, chemical and integration of different strategies have been developed to manage this devastating disease in farm situations. Poor bio-efficacy of biocontrol agents, limited availability of resistant cultivars in desired varieties and continuous evolution of pathotypes have forced the rice farmers to depend much on fungicides for minimising the yield losses caused by blast disease. Several molecules with the diverse mode of action have been developed and registered against blast disease since the 1960s, and the work is being carried out continuously in search of better molecules. Compared to the fungicides developed in the 1970s, present-day fungicides are better in terms of higher efficacy and lower environmental impacts. In this chapter, we attempt to summarise all the chemicals developed and recommended for the management of paddy blast. This chapter also includes some of the novel chemicals which interfere with the infection process without causing any toxicity to the pathogen, and also those that trigger resistances in rice plants against blast disease.

5.1 Introduction

Chemicals are vital elements of effective disease management programs. Over the past 200 years, fungicides are being used to protect the plants from fungal diseases. Even though from the beginning there is an increase in the cultivation of crops and the treatment of these crops for the disease management within the limited range of available chemicals, the number of applications and bio-efficacy of chemicals have significantly increased, especially after World War II. In the late 1960s and 1970s, a large number of efficient fungicides with systemic function and new structures, which were not available in the previous products, were introduced. These chemicals consist of benzimidazoles, carboxanilides, dicarboximides, sterol demethylation inhibitors (DMIs), morpholines, 2-amino-pyrimidines, phosphorothiolates and phenylamine. However, in the late 1980s new fungicides were released which were corresponding to the existing fungicides belonging to the DMIs exhibiting enhanced bio-efficacy properties and more environmentally friendly. Over the last decade, a number of new chemical compounds such as aniline pyrimidines, benzamides, carboxylic acid amides, phenyl-pyrroles and quinone outside inhibitors (QoIs, strobilurin analogues) were commercially introduced for plant disease management (Brent and Hollomon 2007).

Rice is an important cereal crop of the world which feeds two-thirds of the world population. Like other cultivated crops, rice production is also limited by the number of biotic problems which include blast, a destructive fungal disease. Rice blast caused by an ascomycetous fungi Magnaporthe oryzae (anamorph, Pyricularia oryzae) is the most destructive disease on rice crop in most of the rice-cultivating ecosystems of the world. The most common ways followed for the rice blast management under field conditions include manipulation of the sowing time or date of transplantation, cultivation of resistant varieties or hybrids, application of chemical fungicides, controlled fertilisers especially nitrogenous, and irrigation schedules (Mbodi et al. 1987; Moletti et al. 1988; Naidu and Reddy 1989; Georgopoulos and Ziogas 1992). Among various strategies employed for the management of blast disease, chemical-based control has been widely adopted in many countries (Mariappan et al. 1995). Seed treatment and foliar application with the systemic fungicides were found to be very effective in reducing the blast disease (Manandhar et al. 1985; Sah and Karki 1988; Manandhar 1984; Chaudhary and Sah 1997; Chaudhary 1999).

In the early days, introduction of organo-mercuric compounds for seed treatment was later extended for the field application for controlling rice blast. This practice greatly contributed to the protection of rice from blast disease which helped in the improved production of rice. But the report of mercury poisoning causing the ‘Minamata disease ’ in Japan in the year 1956 changed the chemical use pattern for the management of paddy blast. Even though the later findings proved that the ‘Minamata disease’ was caused by industrial wastes, the use of organic mercuric chemicals for plant disease control was prohibited, and over time, non-mercuric fungicides with a superior mode of action were developed against the blast disease of rice (Yamaguchi 2004).

Several anti-blast chemicals have been developed over time which can be grouped into the following types based on their origin and mode of action:

Fungicidal chemicals (which directly affect the fungal physiology):

• Fungicides of microbial origin

• Anti-blast chemicals inhibiting sulfhydryl enzyme

• Anti-blast chemicals inhibiting cell division

• Sterol biosynthesis inhibitors (SBIs)

• Anti-blast chemicals inhibiting the membrane phospholipid biosynthesis

• Fungicides targeting fungal membrane permeability

• Fungicides suppressing fungal respiration

• Combination fungicides with multiple modes of action

Non-fungicidal chemicals (which indirectly interfere with disease development):

• Melanin biosynthesis inhibitors (these chemicals inhibit only infection process and are not directly lethal to the fungal pathogen)

• Inducers of the systemic acquired resistance (SAR)

5.2 Fungicidal Chemicals Directly Affecting the Pathogen Physiology

This group includes the molecules which are lethal to fungi by inhibiting one or more physiological processes in the fungal cell. These molecules directly affect the pathogen and thereby control the disease caused by them.

5.2.1 Fungicides of Microbial Origin

• (a) Blasticidin S

  Blasticidin S was the prime agricultural antibiotic which was developed successfully in Japan from the actinomycetes (Streptomyces griseochromogenes) by isolating from the filtrates of this actinomycete culture in the year 1958 by Takeuchi and associates. Later its effect in curing the rice blast disease was found, and the evaluation of this chemical was conducted during 1959–1960 through field trials.

  This antibiotic gives excellent control of rice blast disease upon spraying at low concentration (10–20 ppm). It inhibits the protein synthesis in rice blast fungi (Misato et al. 1959) by interfering the ribosomes during the peptidyl transfer (Yukioka et al. 1975). At a higher dose, this antibiotic causes mild-to-moderate phytotoxic effect on rice and other crop plants. Benzyl-amino-benzene-sulfonate, a derivative of blasticidin S, was established with low phytotoxicity to plants without compromising the disease control efficacy, and this has been produced commercially for the practical use.

  The inhibitory action of blasticidin S is mainly on mycelial growth of the fungus and was found to be 10–100 times more powerful than that of organomercuric fungicides. Therefore, it gives excellent disease control, especially against neck blast. The residues of blasticidin S in the environment are less persistent and easily broken down by sunlight and also by microorganisms in the soil.

  Blasticidin S exhibited the prominent blast control and was found to be effective as a curative fungicide. But it is an inferior protectant compared to modern fungicides with extreme toxicity to cultivated plants (Ou 1985).

• (b) Kasugamycin

  It is also an actinomycetous antibiotic that belongs to the aminoglycoside group of antibiotics isolated from S. kasugaensis in 1965 from the soil in Japan by Umezawa and associates . Though there is a nonsignificant in vitro antifungal activity of kasugamycin, it has exhibited the curative action for rice blast under field condition. This is mainly because the hydrogen ion concentration in the tissue of rice plants is around pH 4.5–5.0 which favours the inhibition of the fungal growth by kasugamycin that requires low pH for its activity. Generally, the antibiotics of the group aminoglycosides act by inhibiting protein synthesis by misreading of codons, while on the contrary kasugamycin was found to specifically interfere with the initiation complex formation by preventing the aminoacyl-tRNA binding to RNA-30S ribosomal subunit complex.

  Kasugamycin is effective on rice blast and has been widely used in agriculture since 1965 (Ishiyama 1965; Singh et al. 2014). This was the major fungicide used for the control of paddy blast with 4–5 sprays per cultivation season during the early 1970s, and this antibiotic accounted for 90% of the chemicals that were applied for the paddy blast control (Miura et al. 1975). Use of kasugamycin on rice also has added advantage of low or no phytotoxicity on rice at higher doses. After the repeated application of this antibiotic intensively for several years, its efficacy was reduced due to development of resistant field strains. Afterwards, kasugamycin was used in combination with other chemical fungicides with the varied mode of action. Fortunately , it has been observed that the proportion of resistant strains declined rapidly after discontinuing the sole use of kasugamycin for the management of blast.

5.2.2 Anti-blast Chemicals Inhibiting Sulfhydryl Component of Cellular Enzyme

The group includes the complex fungicides, ethylene bis-dithiocarbamates (EBDCs)/dithiocarbamates, that were used for managing various diseases from late 1940s, exclusively in the complex form with zinc (zineb) or manganese (maneb) or a combination of manganese and zinc (mancozeb) (Morton and Staub 2008). Among them, zineb and mancozeb have been used for managing blast disease of rice.

• (a) Zineb : It is a polymeric complex of zinc with a dithiocarbamate. It is a protective fungicide that controls a broad range of diseases. The chemical is fungitoxic only when it is exposed to air, where on exposure it is converted into an isothiocyanate, and thereby inactivates enzymes of the fungi bearing sulfhydryl (SH) groups. This also functions by disturbing the enzyme activity within fungi whenever the metal exchange between zineb and enzymes of fungi occurs. Effectiveness of zineb in controlling blast disease under moderate-to-severe disease severity is well reported (Singh et al. 2014).

• (b) Mancozeb : It is a protective contact fungicide with multiple sites of action. It is developed by combining two dithiocarbamates, viz. zineb and maneb. The mancozeb inactivates enzymes of the fungi bearing sulfhydryl (SH) groups of amino acids within fungal cells, thereby interrupting the respiration, lipid metabolism, and synthesis of adenosine triphosphates (ATPs) .

  This fungicide is widely used for the management of rice blast throughout the world for its superior efficacy against blast disease. Presently, mancozeb is being used as a component with single-site target curative fungicides in many combination products to enhance bio-efficacy and also to prevent the development of fungicide resistance.

5.2.3 Anti-blast Chemicals Inhibiting Benzimidazole Cell Division

This class of fungicides includes the most extensively used fungicide in agriculture such as carbendazim. It is a broad-spectrum systemic fungicide and effectively controls many fungal pathogens of the phylum Ascomycota and the Basidiomycota. Within plants, the benzimidazoles are systemic or mobile and the fungicide is degraded by the microbes in the soil and water, thereby limiting its soil toxicity. Through the phenomenon of photolysis and hydrolysis, this fungicide is metabolised in the plant system. The fungicide kills the cells of the fungi during the mitosis stage of the cell division by specifically targeting the mitotic spindle microtubule synthesis by binding to the β-tubulin and thereby inhibiting the polymerisation of β-tubulin by interacting with it directly.

• (a) Carbendazim: It is a systemic and broad-spectrum benzimidazole fungicide. Although its exact mode of action is unknown, carbendazim is found to suppress the microtubule assembly by binding to tubulin at an unspecified site, thereby resulting in the halt of cell cycle at the G2/M phase and initiation of apoptosis. Carbendazim is the member of the class of benzimidazoles (2-amino-benzimidazole) and the primary amino group of benzimidazoles is substituted by a methoxy-carbonyl. For the management of paddy blast, it has been used in different ways, viz. seed treatment, seedling dip and foliar sprays of standing crop.

It is suggested to use other contact fungicides with the carbendazim in an alternate manner or using as tank mix application of carbendazim with contact fungicides to manage the development of fungicidal resistance. The carbendazim in combination with other fungicides gave effective results in managing paddy blast compared to treatment with carbendazim alone (Pramesh et al. 2016a).

5.2.4 Sterol Biosynthesis Inhibitors (SBIs)

Sterols are the important lipid molecules found in plants, animals and fungi naturally. The sterol that is found in the fungal cell membrane is commonly called as ergosterol (ergosta-5, 7, 22-trien-3β-ol). The name ergosterol is derived from disease ergot, from which it was first isolated from the members of genus Claviceps. Ergosterol is an important constituent of the cell membrane of yeast and other fungi and serves various similar functions as that of cholesterol in animal cells. Its unique presence in the higher fungi may be because of the exposure of these organisms to harsh climatic situations, viz. extremely fluctuating humidity and moisture in their particular ecological niches like the surfaces or within plants or in soil. Since the ergosterol is found in the fungal cell membrane, its absence in plants and animals makes it a useful target for the development of antifungal chemicals.

Sterol biosynthesis inhibitors (SBIs) are assorted into different FRAC (Fungicide Resistance Action Committee) groups based on their mode of action as G₁, G₂, G₃ and G₄, where each group has compounds exhibiting different target sites in the sterol biosynthesis pathway. The chemicals under the groups G₁, G₂ (amines including spiroxamine) and G₃ (hydroxyanilides) are only used as agrochemicals. The members of the group G₄ are of pharmaceutical importance. They are called as demethylation inhibitors (DMIs) because of the target site of fungicides, which in G₁ being sterol C₁₄ demethylase. Most of DMIs belong to the triazole group of fungicides and account for nearly 90% of SBIs.

SBIs are found to exhibit long-lasting broad-spectrum systemic action with protective and curative activity and also has a relatively slow development of field resistance (Kuck et al. 2012).

• (a) Triazole fungicides: The triazole fungicides are the members of the group demethylation inhibitors (DMIs), which were introduced during the mid of 1970s. Triazoles comprise legion members, among which hexaconazole, tebuconazole and tricyclazole are used for the control of paddy blast. Triazole fungicides act by inhibiting a specific enzyme, C₁₄-demethylase, which is responsible for the production of sterol in the fungal cell membrane. Ergosterol is very much necessary for the structure and function of cell membrane which makes it an essential factor for the functional cell wall development. Application of these compounds leads to the abnormal growth of the fungi, thereby causing the eventual death.

The triazole compounds act on the biochemical pathway of sterol synthesis in a bit different way. Even though it exhibits similar results as with other fungicides of this group leading to abnormal fungal growth and eventual death, the main difference in these fungicides lies in their activity spectra. The triazoles were found ineffective against germination of spores because of the presence of enough sterols in the spores which is sufficient to form germ tubes. Some of the fungal spores were even found to comprise sterol that is adequate to produce infection structures; hence, in some of the fungal diseases, triazoles are ineffective in preventing the host from infection (Mueller 2006).

The application of triazoles can be carried out as early-infection treatments or as a preventive spray. But the applications of the fungicide must be done early during the infection process of the fungi. Few of the fungicides of this group have anti-sporulation characteristics, where the application of such fungicide will inhibit sporulation and thereby slow down the development of the disease. However, the triazole fungicides are ineffective if the fungus starts sporulation on infected plant tissue.

Use of tricyclazole for managing different stages of blast disease of rice has been reported by many research groups (Froyd et al. 1976). Presently it is the most widely used curative fungicide for blast disease management in India.

5.2.5 Anti-blast Chemicals Inhibiting the Membrane Phospholipid Biosynthesis

The organophosphorus compounds were initially introduced as an insecticide, but some of these compounds were found to be effective against the rice blast. The phosphorothiolate compounds having anti-blast activity were introduced in Japan during the year 1963 and they are still one among the major fungicide groups for managing paddy blast. The discovery of phosphorothiolate (PTL) compounds with antifungal activity has led to the development of fungicides, viz. iprobenfos (IBP), edifenphos (EDDP) and isoprothiolane, which were used at a greater extent for the management of rice blast.

• (a) IBP: It is an organophosphorus compound with high water solubility and systemic action within the plant, which favours the use of this fungicide as soil and water surface application in the paddy fields.

• (b) EDDP: It is a non-systemic, organophosphorus compound with impressive antifungal activity against the rice blast pathogen. It acts as an efficient blast control fungicide when used as a foliar spray because of its less solubility in water and less stability in plants (Uesugi 2001).

  IBP and EDDP specifically act by inhibiting the conversion of phosphatidylethanolamine into phosphatidylcholine (Kodama et al. 1979). Phospholipid is an important constituent of a fungal cell membrane; inhibition of the synthesis of phosphatidylcholine causes impairment of membrane permeability and affects the associated enzyme activities. Among various systemic chemicals analysed, the best blast control was observed with IBP on application to the water surface of paddy (Yamaguchi 1974).

• (c) Isoprothiolane: It is a systemic organophosphorus compound that primarily acts by preventing the fungus invasion into the plant system (Araki and Miyagi 1976; Taninaka et al. 1976). Even though isoprothiolane (di-isopropyl 1, 3-dithiolan-2-yliden-malonate) is chemically different from the other PTL fungicide compounds, the cross-resistance between isoprothiolane and other PTL compounds intimates resemblances in their mode of action (Katagiri and Uesugi 1977).

  The isoprothiolane acts by inhibiting the penetration and elongation of infection hyphae by affecting the formation of infection peg by cellulase secretion. A study conducted by Raji and Louis (2007) revealed that the effectiveness of isoprothiolane against leaf and neck blast of paddy was excellent.

5.2.6 Fungicides Targeting Fungal Membrane Permeability

This group includes chemical ferimzone. It is a systemic fungistatic compound that was developed for the management of paddy blast. The fungistatic ferimzone is not detrimental to the blast pathogen in vitro, but it leads to the leakage of the acidic electrolytes specifically from the M. oryzae mycelia. It acts by impairing the membrane permeability and thereby affecting the influx and efflux of salt ions or specific molecules. Because of its fungistatic nature, ferimzone is used as a mixture with other fungicides as a synergistic component (Okuno et al. 1989). Ferimzone-containing two products, Blacin and Nonblas, were registered in 1991 in Japan and released into the market. Blacin contains fthalide in addition to ferimzone and Nonblas contains tricyclazole with ferimzone to escape resistant strain development and to provide more effective blast control (Matsuura et al. 1994).

Leaf blast field trials were conducted in Japan by using the ferimzone and it provided good control of blast in each application. It was observed that the ferimzone has high curative activity against the disease (Matsuura et al. 1994).

5.2.7 Fungicides Suppress Fungal Respiration

This class includes the important fungicides which target mitochondrial respiration of fungi. Sauter et al. (1995) proposed the usage of the term “strobilurins” for the members either occurring naturally or produced synthetically for this class of fungicides due to the natural molecule strobilurin A, from which the fungicides are derived with structural variations. Strobilurins A and B were the first extracted natural compounds from the fungus Strobilurus tenacellus . These compounds were originally known as β-methoxyacrylates, or β-MOAs, because of its toxophore structure, which is an active part of the molecule that is responsible for its toxic effect. Eventually, different molecules with a similar mode of action were synthesised with a toxophore structure analogous to strobilurin A, but not that of β-MOA.

A variety of strobilurin compounds are currently used in agriculture for managing many fungal diseases. The bc1 complex of the mitochondrial respiration is involved in transferring electrons from ubiquinol to the cytochrome c oxidase. These chemicals block the electron transfer chain at the ubiquinol-oxidising site (Qo) in the inner mitochondrial membrane and thereby inhibiting the ATP synthesis and cellular respiration (Shen et al. 2014).

The naturally occurring strobilurins were less active and photo-insensitive; hence the strobilurins were optimised for the fungicidal activity and photostability, and developed commercially. They were first released in 1996 and the current market has more than ten strobilurin fungicides with 23–25% fungicide sales globally. Several strobilurins, viz. azoxystrobin, kresoxim-methyl, metominostrobin, picoxystrobin, pyraclostrobin and trifloxystrobin, were developed commercially and used for the paddy blast management.

• (a) Azoxystrobin : It is a broad-spectrum methoxyacrylate class of fungicide with a systemic activity which is a derivative of the naturally occurring strobilurin. It is found very effective in managing paddy blast on its application during heading stage of the crop. However, the common trend of a single application of this fungicide in the late crop season to manage both sheath blight and blast has led to less efficient control of rice blast (Groth 2006). Nowadays, this molecule is also being used as a component of a combination product with the triazole group of fungicides.

• (b) Kresoxim-methyl : It is a systemic and broad-spectrum fungicide exhibiting both protective and curative activity. It has good residual activity apart from the broad-spectrum disease control and hence imparts extended duration of disease control. Kresoxim-methyl is effective even at very low concentrations and inhibits the spore germination of the fungus on the host tissue, thereby preventing the infection and spread of the disease. It protects from a broad range of fungal diseases and has a primary role in plant disease management.

  It can cure the already advanced infections and halts the sporulation and symptom expression, thereby impeding further advancement of the disease. These characteristics make kresoxim-methyl an excellent compound in integrated disease management (IDM) programs. For the management of paddy blast, kresoxim-methyl showed higher protective and curative performance against M. oryzae isolates exhibiting excellent efficacy (Chen et al. 2015).

• (c) Metominostrobin: It is a broad-spectrum methoxyacrylate fungicide with systemic activity . It acts by inhibiting the respiration of fungi by blocking the electron transport in the cytochrome-bc1 segment of the respiratory chain in the inner membrane of mitochondria. However, the cells of M. oryzae mycelia induce cyanide-resistant respiration to resume the respiratory cycle on the blockage of the cytochrome-mediated pathway by metominostrobin. The superoxide anion is thought to be responsible for the induction of the cyanide-resistant respiration mechanism involving the metominostrobin-dependent induction. Flavonoids are water-soluble polyphenols found in plants and they are capable of scavenging the superoxide anions that were produced due to the obstruction of the electron flux via the section of cytochrome-bc 1 complex and thereby inhibiting the cyanide-resistant respiration induced by the metominostrobin activity. In this way, the metominostrobin successfully manages the rice blast pathogen with the help of host plant factors (Mizutani et al. 1996).

  Metominostrobin exhibits best disease-controlling activity immediately after application against rice blast. Furthermore, it shows long-lasting activity when applied to paddy water. It can control not only leaf blast but also ear blast even at 60 days after submergence (Mashiko et al. 2001; Gaikwad and Balgude 2016).

• (d) Picoxystrobin: It is a broad-spectrum strobilurin compound with both preventive and curative nature and acts by inhibiting fungal respiration. This compound exhibits better curative property in comparison to the azoxystrobin in many crop diseases.

• (e) Pyraclostrobin: It is a systemic and broad-spectrum fungicide exhibiting excellent fungicidal properties with translaminar and loco-systemic activity. It is curative, protective and eradicative which makes it better in comparison to other fungicides. It is absorbed rapidly by plants and retained largely in the leaf cuticle by the waxes. Because of its translaminar property, it gives the best control of the disease on both the leaf surfaces. It has a very confined vapour phase and acropetal and basipetal leaf movement activity. The preventive spray with the concentration as low as 0.1 ppm successfully suppresses the spore germination of a majority of pathogens.

  Pyraclostrobin was assessed for its efficacy and found to be most effective by recording least percent disease index (PDI) for leaf blast disease (Pramesh et al. 2016b).

• (f) Trifloxystrobin: This has been successfully used as a component of combination product along with other groups of fungicides such as triazoles. From the 1990s, this fungicide was extensively used for the effective management of fungal diseases of various crops (Liu et al. 2013). It is grouped as highly toxic to non-target aquatic species but as non-toxic to bees, agriculturally important insects , earthworms and mammals (Junges et al. 2012; Shen et al. 2014).

5.2.8 Anti-blast Chemicals with Combined Mode of Action

Many fungicide combinations with different modes of action have been developed for the management of paddy blast. The important combination products that were found effective against paddy blast include difenoconazole 11.4% + azoxystrobin 18.2% SC, mancozeb 63% + carbendazim 12% WP, copper oxychloride 45% + kasugamycin 5% WP, picoxystrobin 6.78% + tricyclazole 20.33% SC, tricyclazole 18% + mancozeb 62% WP and tebuconazole 50% + trifloxystrobin 25% WG.

• (a) Difenoconazole 11.4% + azoxystrobin 18.2% SC: It is a combination fungicide containing difenoconazole and azoxystrobin. It exhibits a dual-systemic nature with broad-spectrum fungicidal activity with protective and curative action. It acts by inhibiting the spore germination at an early stage of pathogen development. Thus, it protects the crop against invasion by fungal pathogens. Once it is absorbed by plants systemically, it inhibits the fungal penetration and haustoria formation by obstructing the sterol biosynthesis in the fungal cell membrane and thereby preventing the disease development. Singh et al. (2019) reported the use of azoxystrobin 18.2% + difenoconazole 11.4% SC for the paddy blast with the best control of the disease.

• (b) Copper oxychloride 45% + kasugamycin 5% WP: It is a new combination product containing copper oxychloride (COC) and kasugamycin, which has the power of fungicide and bactericide together to prevent the bacterial-fungal complex in multiple crops. Its dual mode of action makes it an effective and powerful tool to protect crops from fungal and bacterial diseases. It is a contact and systemic fungicide which interferes with the enzyme system of spores and mycelium and inhibits protein biosynthesis.

  A spray with a combination of COC and antibiotic (copper oxychloride 45% + kasugamycin 5% WP) was found effective against leaf blast among different combinations (Kumar and Veerabhadraswamy 2014), and suitable for the management of paddy blast under field conditions.

• (c) Carbendazim 12% + mancozeb 63% WP: It is a combination fungicide having a mixture of carbendazim and mancozeb. One of the components, mancozeb, remains on the surface of the plant and prevents the development of the disease by acting as the contact fungicide. Whenever mancozeb is exposed to air, it becomes fungitoxic due to the conversion into an active isothiocyanate that acts by inactivating the fungal SH (sulfhydryl) groups of enzymes and thereby disturbing the functioning of enzymes in the fungal cell. Another component of the mixture, carbendazim, is absorbed by the plant due to its systemic activity and protects the plant from invading pathogen and also acts as a curative agent. It inhibits the fungal germ tube development and appressoria formation, thereby preventing the growth of mycelia.

  The use of the fungicide combination involving carbendazim 12% and mancozeb 63% WP against the paddy blast has resulted in less disease incidence (Pramesh et al. 2016a).

• (d) Picoxystrobin 6.78% + tricyclazole 20.33% SC: It is a combination of fungicides containing picoxystrobin 6.78% + tricyclazole 20.33% SC with dual mode of action, both preventive and curative for effective management of blast disease of paddy. It is an effective systemic fungicide that gets rapidly absorbed and translocated all over the plants. After spraying, the chemical combination is absorbed rapidly by the plant and moves towards leaf tips. The fungicide acts by effectively blocking the penetration of fungus on germination, and thereby preventing the establishment of the pathogen at the infection court. Tricyclazole-treated spores are unable to penetrate as it cannot synthesise melanin and by doing so cannot generate enough turgor pressure to rupture host cuticle. Tricyclazole also inhibits spore formation and release from sporophores and whenever the spores are formed, they are less virulent.

• (e) Tebuconazole 50% + trifloxystrobin 25% WG: It is a new combination fungicide containing tebuconazole 50% + trifloxystrobin 25% WG. This combination is a broad-spectrum and systemic fungicide that is having both protective and curative properties and along with the disease control, it also improves the quality and crop yield. In rice, this combination also protects against the dirty panicle disease and the sheath rot in later stages of the crop. Tebuconazole is basically a demethylase inhibitor (DMI) and interferes during the process of the formation of fungal cell wall structures, thereby stopping the fungal growth and reproduction. Trifloxystrobin is a strobilurin fungicide and acts by impairing the fungal respiration by interfering in the mitochondrial electron transport chain.

  The evaluation of different fungicide combination was carried out and it was found that fungicide trifloxystrobin 25% + tebuconazole 50% performed better with least disease incidence over other chemicals (Pramesh et al. 2016b).

• (f) Tricyclazole 18% + mancozeb 62% WP: It is a combination fungicide with both systemic and contact activity and contains the tricyclazole and mancozeb. The systemic part of tricyclazole on spraying is absorbed rapidly by the plant and moves towards leaf tips. The fungicide acts by effectively blocking the penetration of the fungus on germination and thereby preventing the establishment of the pathogen at the infection court. In the rice blast disease, melanin is required for the hardening of appressorium, and whenever the pigment formation is affected, the formed appressoria fail to successfully penetrate the host surface. Another compound in the combination, mancozeb, is a broad-spectrum contact and protective fungicide. Whenever the mancozeb is exposed to air it becomes fungitoxic due to the conversion into an active isothiocyanate that acts by inactivating the fungal SH (sulfhydryl) groups of enzymes, and thereby disturbing the functioning of enzymes in the fungal cell.

  Chethana (2018) evaluated the efficacy of different fungicides and their combination for the rice neck blast and found that the fungicide combination containing tricyclazole and mancozeb performed better by effectively reducing the disease incidence.

5.3 Non-fungicidal Chemicals to Control Rice Blast Disease

Majority of the chemicals that were used for the management of paddy blast are non-fungicidal and prevent the disease development by acting on fungal metabolism, viz. melanin biosynthesis, or by inducing the defence mechanism in plants, such as systemic acquired resistance (SAR). They are non-lethal to fungal cell but interfere with the secondary metabolite synthesis during plant infection and disease development, and thus help to control the disease.

5.3.1 Melanin Biosynthesis Inhibitors (MBIs)

A class of chemical compounds that are known to block specifically the melanin biosynthesis pathway in the pathogen secondary metabolism are called melanin biosynthesis inhibitors (MBIs). These chemicals have been used for a long time for the management of paddy blast (Hamada et al. 2014).

Melanin is high-molecular-weight, black- or brown-coloured pigment which is negatively charged, hydrophobic and formed from the indolic and phenolic compounds by oxidative polymerisation (Wheeler and Bell 1998). Melanin protects the microorganism from different harsh and toxic environments, and hence acts as the fungal armour. It protects the fungi from the ultraviolet rays (UV), extreme environmental conditions, enzymatic lysis, desiccation, antimicrobial drugs, phagocytosis, oxidants and heavy metal ions (Pal et al. 2013). The fungus is known to synthesise melanin either via L 3–4 dihydroxyphenylalanine (l-DOPA) pathway which is found in Basidiomycota or 1,8-dihydroxynaphthalene (DHN) pathway that is usually seen in Ascomycota (Bell and Wheeler 1986).

Melanin is required for the appressorium to penetrate host surface. Accumulation of melanin in the walls of appressorium improves the rigidity and cell capacity to withstand high turgor pressure that is developed during the penetration of host surface (Woloshuk et al. 1980, 1983). The accumulation of DHN melanin is at the inner layer of the appressorial cell wall which on accumulation changes the porosity of the cell wall, thereby preventing the efflux of larger molecules from the cell with increased osmotic gradient due to the enhanced intracellular glycerol concentration. The increased osmotic gradient will result in an accelerated influx of water molecules into the cell leading to the generation of high turgor pressure (>8.0 MPa), which enables the physical penetration of hard barrier like plant surface (Howard et al. 1991).

Magnaporthe produces a dark brown melanin pigment via penta-ketide pathway from 1,8-dihydroxynaphthalene (1,8-DHN) in order to overcome the host self-defence mechanism; the pathogen has developed an infection process that successfully causes disease in plants. Whenever the germinating fungal hyphae identify the host surface characteristics, viz. lipophilicity and hardness, the hyphae differentiate into appressoria through which it infects the harder host surface. The appressoria formation involves the expression of many genes and signal transduction within the pathogen.

For successful penetration of the host, the melanised appressoria are very much essential. Conidia of the fungi germinate and form germ tube, where the tips of the germ tube differentiate into appressoria. The formation of melanin layer in between the cell wall and the plasma membrane leads to maturation of the appressoria that is capable to penetrate the host cuticle by producing a high osmotic pressure within the cell. Hence, the inhibitors of the melanin biosynthesis can effectively control rice blast disease development (Yamaguchi and Kubo 1992).

Melanin synthesis inhibitors, viz. carpropamid, fthalide, isoprothiolane, pyroquilon and tricyclazole, exhibit excellent control against blast disease even though they are non-toxic to the growth of M. oryzae mycelia. The main reactions in DHN melanin biosynthesis pathway involve reduction and dehydration by enzymes. Therefore, the enzymes, viz. reductase and dehydratase, act as targets for producing inhibitors of melanin biosynthesis (Table 5.1).

Table 5.1 Melanin biosynthesis inhibitor groups

Melanin biosynthesis inhibitors (MBIs) targeting scytalone dehydratase (MBI-D) (called as dehydratase inhibitors) were invented in the year 1998 as specific and efficient fungicides to manage rice blast. These fungicides were released as granule fungicides for the treatment of a nursery box. The usage of MBI-D fungicide compounds unfolded rapidly due to the numerous advantages of box treatment nursery, viz. reduced quantity and frequency of application, effectiveness over a long time and reduced working hours (Suzuki et al. 2010).

However, during the year 2001, it was reported that MBI-D failed to manage rice blast and the resistant isolates to MBI-D were confirmed and their use was discontinued as a countermeasure to overcome developing resistance. After 2003, the use of other types of MBI targeting polyhydroxy-naphthalene reductase (MBI-R) fungicide (called as reductase inhibitors) was increased and it restored the level of use of MBIs which subsisted before the release of MBI-D fungicides in 1998 (Suzuki et al. 2010).

5.3.1.1 Reductase Inhibitors

These MBI act by inhibiting the two reduction steps between 1,3,6,8-trihydroxy-naphthalene (1,3,6,8-THN) and scytalone and between 1,3,8-THN and vermilion (Chida and Sisler 1987b). Fthalide (FTL), tricyclazole (TCZ) and pyroquilon (PRQ) are the members of this group. These were introduced earlier in granular form and applied to the irrigated paddy by submerged application or by nursery box treatment.

• (a) Fthalide: It is an excellent protectant fungicide with extended residual activity. Its primary metabolites are found to have negligible phytotoxicity and mammalian non-toxicity on detailed examination for its behaviour and metabolic fate (Tokuda et al. 1976). Fthalide application inhibits the melanin biosynthesis in the fungal appressoria, thereby interfering the host penetration by the pathogen (Chida and Sisler 1987a).

• (b) Pyroquilon: It has a similar mode of action as that of other melanin biosynthesis inhibitors but with an accumulation of scytalone and 2-hydroxyjuglone (2-HJ) predominantly (Yamaguchi et al. 1982). The application of pyroquilon and tricyclazole induces the accumulation of flaviolin at high concentration, thereby indicating inhibition at another step, which is catalysed by the similar enzyme. This fungicide reduces sporulation of blast fungus and thereby controls the secondary infections under field conditions. Results of the field studies of tricyclazole and pyroquilon have shown the long-term disease control property when applied as either a soil drench, foliar spray or submerged application.

  A study revealed that seed treatment with pyroquilon in susceptible cultivars keeps leaf blast severity very low. Seed treatment at moderate rates with pyroquilon has been found to reduce the severity of leaf blast under low disease pressure. When a susceptible cultivar was used under high disease pressure, the maximum disease control was observed with the application of this fungicide at higher rates. Seed treatment of more resistant cultivars with pyroquilon had no or negligible effect. Thus, the study has shown that the use of pyroquilon for seed treatment gives satisfactory control during the vegetative phase of rice against leaf blast in susceptible cultivars (Prabhu and Filippi 1993).

• (c) Tricyclazole: It is a systemic fungicide with protective action and controls rice blast by inhibiting the melanin biosynthesis pathway in the appressorial wall. The fungicide acts by effectively blocking the penetration of the fungus on germination and thereby preventing the establishment of the pathogen at the infection court. Tricyclazole affects neither germination of the spore and appressorial formation nor the mycelial growth but particularly affects fungal penetration by suppressing the melanin biosynthesis. The fungicide inhibits production of the late intermediate in melanin pathway, a vermelone, even at the extremely low concentration (Tokousbalides and Sisler 1978; Woloshuk et al. 1980).

5.3.1.2 Dehydratase Inhibitors

These MBIs act by inhibiting the dehydration at two steps, between scytalone and 1,3,8-trihydroxy-naphthalene (1,3,8-THN) and between vermelone and 1,8-dihydroxy-naphthalene (1,8-DHN). The use of reductase inhibitors for a long time led to the delayed discovery of these compounds for the control of rice blast. A few dehydratase inhibitors were introduced in the late 1990s, viz. carpropamid, diclocymet and fenoxanil.

• (a) Carpropamid: It is a protectant fungicide with systemic activity and was found to be effective against leaf and panicle blast. The fungicide acts by inhibiting the melanin biosynthesis by particularly targeting the dehydratase enzymes and thereby preventing the dehydratase reaction between scytalone and trihydroxy-naphthalene and also vermelone and dihydroxy-naphthalene. In comparison to the older melanin biosynthesis inhibitors, viz. pyroquilon, phthalide and tricyclazole (reductase inhibitors), the carpropamid fungicide has a different site of action which acts by inhibiting the 1,3,8-trihydroxy-naphthalene reductase (Yamaguchi and Kubo 1992).

Kurahashi et al. (1996, 1997) examined the basic activity and biological properties of carpropamid and it was found to be an excellent penetrant for controlling blast. It showed weak activity against oomycetes fungi and it does not inhibit blast fungus spore germination and appressorium formation but even at very low concentration, it strongly inhibits melanisation of appressorium.

5.3.1.3 Polyketide Synthase Inhibitors

• (a) Tolprocarb: It is a novel systemic chemical used for the management of rice blast. The site of action of this fungicide is polyketide synthase (PKS) that regulates the synthesis of polyketide and cyclisation of penta-ketide in melanin biosynthesis pathway (Hamada et al. 2014). In addition to inhibiting melanin biosynthesis, tolprocarb also induces systemic acquired resistance in rice.

The application of tolprocarb accelerated genes involved in signalling pathway mediated by the salicylic acid, viz. chitinase-1, β-1,3-glucanase and PBZ1, without accelerating genes related to the signalling pathway mediated by the jasmonic acid. Hence, it was found that tolprocarb also induces the systemic acquired resistance (SAR) in rice plants by accelerating the salicylic acid-mediated signalling pathway (Hagiwara et al. 2019).

The tolprocarb fungicide can be used effectively for the control of rice blast which in turn provides resistance against other pathogens by activating the SRA. Because of the dual mode of action of tolprocarb, the risk associated with the development of resistant isolates is very low.

5.3.2 Inducers of Systemic Acquired Resistance (SAR)

Plant activators or priming effectors are the groups of chemical compounds which are non-fungicidal in action and effectively control rice blast by inducing systemic acquired resistance (SAR) in plants. Induced resistance enables the plants to improve their level of basal resistance against the invading pathogen. Various biotic and abiotic stimuli assist in the activation of induced resistance (Pieterse et al. 2012). Two types of such induced resistance can be seen in plants, viz. systemic acquired resistance (SAR) and induced systemic resistance (ISR). Both these resistance types vary based on the signalling pathway involved in the activation and nature of the elicitor molecules involved (Knoester et al. 1999). The activation of systemic acquired resistance results in signal transduction via salicylic acid (SA)-mediated pathway, thereby producing the PR (pathogenesis-related) proteins (Knoester et al. 1999). Contrastingly, the non-pathogenic microorganisms activate the induced systemic resistance (ISR) that acts through the pathway mediated by jasmonic acid (JA) and ethylene (ET) where there is no accumulation of PR proteins (Knoester et al. 1999).

Many of such chemicals acting as plant activators against rice blast were developed and used for the practical control of the disease. The most commonly used plant activators or defence inducers include acibenzolar S-methyl, probenazole, isotianil and tiadinil. Even though plant activators have no fungicidal activity, they protect plants by activating the resistance mechanism mediated by SA pathway with the accumulation of pathogenesis-related protein (PR protein) (Arie and Nakashita 2007).

• (a) Probenazole (PBZ): It is an effective systemic compound that effectively controls rice blast on application to the root system (Watanabe et al. 1977). The augmentation of the enzyme activity related to the plant resistance was found at the site of pathogen invasion on the application of PBZ to rice plants. The host-mediated defence activity of this chemical compound improves its efficacy for the extended period.

  Minami and Ando (1994) studied the activity of this chemical for the management of paddy blast and found that probenazole successfully induced resistance in the host plant against the invading M. oryzae pathogen. Formation of hypersensitive response (HR) lesions was observed on pretreatment of probenazole to plants due to high activity of SA and accumulation of PR proteins.

• (b) Acibenzolar-S-methyl (BTH): It is a plant defence activator and acts by inducing SAR in plants (Yamaguchi 1998). Even though the structures of BTH and PBZ look chemically similar they are found to have a different site of action. BTH acts by inducing the SAR downstream of the salicylic acid, whereas the active metabolite (1,2-benzisothiazole-l, 1-dioxide; BIT) of PBZ acts at a prior step to the salicylic acid (Yoshioka et al. 2001; Nakashita et al. 2002).

  It works as a preventive spray and needs to be applied prior to commencement of the disease. The best results were obtained with the spray covering the entire plant surface uniformly despite its systemic nature. To ensure the uniform spray coverage, it can be best applied under sufficient water situations by the aerial spay or through ground application. The BTH gives maximum control of rice blast after 4 days of the application and it was found to mimic SAR in plants.

• (c) Tiadinil: It is another priming effector and a novel systemic chemical compound that effectively controls rice blast (Umetani et al. 2003). Within the leaf sheath in inner epidermal tissue of rice, tiadinil acts by inhibiting the growth of pathogen hyphae at the first invaded cell by enhancing the deposition of callose in the invaded cells and thereby hindering the development of hyphae. All these functions and the invaded cell’s cytoplasmic reaction were found to be similar as observed in resistance reactions during the incompatible host-pathogen interaction.

  In the tiadinil-treated plants, the improved expression of genes related to the host resistance, viz. PAL-ZB8, RPR-1 and PBZ1, has been reported, thereby suggesting that tiadinil acts as a plant activator and gives excellent control of rice blast by activating the genes responsible for host resistance against the invading pathogen. It can be effectively used for the management of blast by water application and nursery box treatment (Tsubata et al. 2006).

• (d) Isotianil: It is a resistance-inducing chemical belonging to the isothiazole class which stimulates the natural defence mechanisms of rice plants. This provides resistance with low application rates. Isotianil can effectively control the fungicide-resistant strains of blast fungus as it is an inducer of resistance in host plants to invading pathogens. Isotianil exhibits some excellent characteristics and this chemical can be applied to plants by employing different methods, and even with lower dosage, the effect of this chemical is long-lasting in comparison to the existing other plant activators.

  In a preventive spray test on rice plants against blast, the inoculation of the pathogen 5 days after the chemical spray exhibited better control in comparison to the inoculation of the pathogen on the next day of chemical spray. This indicated the requirement of spray of isotianil at least a few days before the landing of the pathogen spores on host surface (Sakuma et al. 2008). The detailed list of fungicides used in the management of blast disease is summarised in Table 5.2.

Table 5.2 List of chemicals used for the management of blast disease