Introduction

Faba bean (Vicia faba L.) is an important legume food crop, particularly in Mediterranean countries, the Middle East and China (Emeran et al. 2011). Approximately 2.5 million ha of faba bean are planted in the world (Wang et al. 2014). In developing countries (primarily in Asia and Africa), the most common use of faba bean is as human food. China is the largest producer in the world, planting 2 × 106 ha per year (Duc 1997). Fusarium wilt is prone to occur in continuous monocropping systems. The causal agent of Fusarium wilt is the destructive soil-borne disease Fusarium oxysporum f. sp. fabae (FOF), which causes serious economic losses worldwide and limits faba bean production (Stoddard et al. 2010). FOF is difficult to control due to its ability to survive in the soil for long periods, even in the absence of faba bean (Stoddard et al. 2010). Many methods have been used to control Fusarium wilt, such as the use of resistant cultivars, chemical fungicides, and different agronomic measures, but these methods are not practical, effective or economical (Yang et al. 2014). Intercropping, growing two or more crops in close proximity, has long been employed as a simple and non-expensive alternative for disease control, and it appears to be a more promising solution than petrochemical-based pesticide applications (Gao et al. 2014; Li et al. 2018). Among the successful examples of this strategy are the inhibition of watermelon Fusarium wilt caused by Fusarium oxysporum f. sp. niveum in a watermelon-aerobic rice intercropping system (Hao et al. 2010), the suppression of red crown rot (Cylindrocladium parasiticum) in maize with soybean intercropping (Gao et al. 2014), and the alleviation of pepper Phytophthora blight (Phytophthora capsici) by intercropping with maize (Yang et al. 2014). More than 79% of the studies on this topic have shown that intercropping can reduce the severity of diseases caused by fungi, bacteria and viruses compared with monocropping (Boudreau 2013). Therefore, intercropping appears to be a promising strategy for controlling soil-borne diseases. However, there are few reports on how intercropping can help crops resist soil-borne pathogens.

It is well known that root exudates play a vital role in influencing the interactions with neighboring plants and microbes (Xu et al. 2015). Plant species, varieties, developmental stages and environmental factors regulate the quantity and quality of root exudates (Xu et al. 2015). Root exudates include a variety of carbon-containing primary metabolites (such as sugars and amino acids) and more complex secondary compounds (Hao et al. 2010). Amino acids exuded from the roots affect rhizosphere microorganisms and can serve as a source of nutrients or inhibitors of microbial growth (Hao et al. 2010; Xie et al. 2013). Most amino acids significantly promoted the germination of the spores of Fusarium oxysporum f. sp. niveum and sporulation by this fungus (Hao et al. 2010). The components of amino acids exuded from the roots are altered under biotic and abiotic stresses (Xie et al. 2013). The sugar content of the exudates of rice roots was significantly lower than that of the watermelon roots, which might limit the growth and development of Fusarium oxysporum (Hao et al. 2010).

Phenolic acids exuded from the roots have been suggested to act as attractants or signaling molecules (Lanoue et al. 2010; Mandal et al. 2010). Relatively high concentrations of p-coumaric acid and ferulic acid could increase the occurrence of strawberry seedling anthracnose crown rot (Tian et al. 2015). The secretion of p-hydroxybenzoic acid (p-HA), ferulic acid, and cinnamic acid from the roots of watermelon stimulates Fusarium oxysporum f. sp. niveum (FON) spore germination, sporulation, and growth (Lv et al. 2018). The phenolic acids in the root exudates of intercropped plants inhibited spore germination by FON and regulated the rhizosphere microbial community (Gao et al. 2014; Xu et al. 2015). Therefore, intercropping may potentially be used to control Fusarium wilt in faba bean.

Some organic acids can stimulate plants to produce a more effective environment for pathogen antagonists, such as Paenibacillus polymyxa SQR-21 (Ling et al. 2011). For example, colonization by the biocontrol agent Bacillus amyloliquefaciens T-5 in tomato roots was influenced by organic acids exuded from the roots, and the swarming motility and chemotactic responses were significantly induced by citric acid, malic acid, fumaric acid and succinic acid, which induced maximal chemotactic response and swarming motility (Tan et al. 2013). However, Wu et al. (2015) found that under gnotobiotic conditions, the chemotaxis ability in vitro and the recruitment of Ralstonia solanacearum to tobacco roots were significantly increased by citric acid and malic acid. Oxalic acid could not only significantly induce the chemotactic response but also increase the biofilm biomass of Ralstonia solanacearum. The interaction between plant root exudates and soil-borne pathogens suggests that different types and concentrations of organic acids released from root systems have varying effects on pathogens (Ling et al. 2013). A study by Ren et al. (2015) indicated that the levels of phenolic acids, organic acids and amino acids exuded from watermelon roots increased significantly when the roots were infected with FON. However, data on the potential of wheat intercropping to control Fusarium wilt in faba bean by regulating the root exudates are limited. It is notable that the disease altered the composition of the root exudates, which could be mitigated by wheat intercropping with faba bean. Thus, we hypothesized that wheat intercropping controlled the Fusarium wilt by modifying the phenolic acid, organic acid, sugar and amino acid contents exuded by the roots of faba bean. The objectives of this study were to (1) Assess the effect of wheat intercropping on the Fusarium wilt of faba bean and (2) characterize the differences in the composition of root exudates produced by plants infected with FOF in monocropping and intercropping systems.

Materials and methods

Materials

Local varieties of wheat (Tricum aestivum L. var. Yunmai 53) and faba bean (Vicia faba L. var. 87–147) were used as plant materials. A total of 150 faba bean seeds and 300 wheat seeds were used for germination, and seeds that germinated well were selected and transplanted into pots. A total of 72 faba bean seedlings and 96 wheat seedlings were involved. FOF was isolated from an infected faba bean plot. Spore suspensions of the pathogen were obtained by adding 10 mL of sterile water to a 14-day-old Petri dish and rubbing the surface with a sterile L-shaped spreader. The suspension was then filtered through four layers of cheesecloth. The spore concentration was determined using a hemocytometer (Ren et al. 2015).

The pot experiment was conducted from October 2015 to January 2016 in a greenhouse at the Yunnan Agricultural University (YNAU), Kunming, Yunnan Province. Sandy loam soil was used. The physicochemical properties of the soil were as follows: total organic carbon = 14.5 g kg−1; available N = 59.8 mg kg−1; available P = 29.9 mg kg−1; exchangeable K = 52.1 mg kg−1 and pH 6.5 (1:2, soil:water ratio). Small stones and visible plant debris were removed from the soil using a 5 cm sieve. After sterilization, the soil was mixed with sterile sand in a ratio of 2:1 (sand:soil, w/w), which resulted in a growth medium that was loose and easily used to sample roots. The growth medium was amended with basal fertilizers as follows (mg kg−1 soil): N 100 (Ca (NO3)2·4H2O), P2O5 100 (KH2PO4), and K2O 100 (KNO3).

Experimental design

The experiment consisted of four treatments: (1) faba bean monocropping with no FOF inoculation (M) in which six faba bean seedlings were grown alone in the pot; (2) faba bean monocropping with FOF inoculation (M + F); (3) faba bean/wheat intercropping with no FOF inoculation (I) in which three faba bean seedlings were grown and the other side with 12 wheat seedlings (the distance between the faba bean and wheat seedlings was 10 cm); and (4) faba bean/wheat intercropping with FOF inoculation (I + F). Four replicates per treatment were used, resulting a total of 16 pots in a randomized block design.

The faba bean seeds were treated in 10% (v/v) H2O2 for 30 min and germinated in a saturated CaSO4 solution in the dark for 12 h and in a porcelain tray in the dark for 48 h. After germination, the seeds were sown in plastic pots (34 cm in diameter, 25 cm in height) filled with 10 kg growth medium. During the growth period, the plants were irrigated with tap water each day, and the water content was maintained at 50–80% of the maximum water holding capacity. Natural light was utilized in the experiment. The temperature was maintained between 20 °C and 25 °C. No pesticides, fungicides or herbicides were used throughout the growth period, and the position of the pot was changed randomly every five days. The faba bean plants of the M + F and I + F groups were inoculated with FOF spores (1.0 × 106 mL−1; 200 mL per pot) near the roots 20 days after the faba beans were sown. The FOF conidia were prepared by growing plate cultures on PDA at 28 °C for 14 days in the dark to induce sporulation as previously described (Wu et al. 2015). Faba bean seedlings were sampled and harvested when the seedlings of the M + F and I + F group exhibited wilt symptoms at 40 days after the wheat and faba bean had been sown.

The faba bean seedlings were pulled out of the soil. The rhizosphere soil was obtained by shaking the soil. The rhizosphere soil was stored in a sterile bag at 4 °C, and colony forming units (CFUs) were determined within 3 days. The concentration of Fusarium in the rhizosphere soil was determined by the number of CFUs. The number of Fusarium CFUs in the rhizosphere soil was determined by a standard dilution plating procedure, and Fusarium colonies were cultured on media selective for Fusarium (Komada 1975). Four replicates were used per treatment (Ren et al. 2008, 2015).

Ten grams of rhizosphere soil samples were weighed and placed in a conical bottle containing 90 mL sterile water. The conical flask was placed on a shaker and shaken in the greenhouse for 20 min. The soil sample was fully mixed with water, and then incubated for 20–30 s, resulting in the 10−1 dilution. One milliliter of the diluent above was added to 9 mL sterile water and mixed to produce a 10−2 diluent. A total of 0.1 mL of a 10−2 diluted solution was applied to the culture dish of the Fusarium oxysporum selective medium, and the number of colonies of Fusarium oxysporum was counted after 4 d.

Collection of root exudates

The faba bean roots were cleaned with deionized water to remove the soil particles and other minor impurities. Three seedlings from each pot were immersed in a plastic cup containing 100 mL of deionized water. During the collection process, the cups were covered with a black plastic cover to avoid contamination and light. Microbial activity was inhibited by adding a drop of concentrated phosphoric acid. The root exudates were collected for 4 h, filtered with a 0.45 μm Millipore membrane (Burlington, MA, USA), lyophilized, and stored at −20 °C. The lyophilized powder was dissolved in deionized water and then used for analysis.

Plant dry biomass analysis

Three plants per replicate of faba beans were harvested 40 days after sowing. The plants were divided into shoots and roots, dried in an oven at 105 °C for 15 min, placed at 70 °C for three days and weighed.

Assessment of the disease incidence

The classification standard for faba bean Fusarium wilt is as follows: 0 indicates no visible symptoms; 1 indicates mild wilt symptoms on the leaf and stem; 2 indicates moderate wilt symptoms; 3 indicates severe wilt symptoms; and 4 indicates plant death. The incidence of disease was defined as the number of wilted plants divided by the total number of plants per pot, multiplied by 100%. The disease index (DI) for each pot was defined using the following formula:

$$ \mathrm{Disease}\ \mathrm{index}=\frac{\Sigma \left(\mathrm{Number}\ \mathrm{of}\ \mathrm{diseased}\ \mathrm{plants}\ \mathrm{at}\ \mathrm{each}\ \mathrm{level}\times \mathrm{level}\right)}{\mathrm{The}\ \mathrm{highest}\ \mathrm{level}\times \mathrm{total}\ \mathrm{number}\ \mathrm{of}\ \mathrm{plants}\ \mathrm{investigated}}\times 100 $$

Determination of soluble sugar, amino acids, phenolic acids and organic acids in the faba bean root exudates.

The exudates from the faba bean were filtered through a 0.22 μm filter prior to determining the contents of the phenolic compounds, organic acids, amino acids and sugars. Phenolic and organic acids from the faba bean root exudates were isolated and identified using high performance liquid chromatography (HPLC) (Agilent 1200, Waldbronn, Germany). The phenolic acids were determined using a Welchrom-C18 column (4.0 mm × 250 mm, 5-Micron, Agilent, Santa Clara, CA, United States) as described by Hao et al. (2010). The mobile phase consisted of methyl alcohol (A) and 1% acetic acid solution (pH 2.59) (B) with a gradient elution. The wavelength of UV detector was adjusted to 280 nm. The column temperature was maintained at 30 °C (Ren et al. 2016). Phenolic compounds in the root exudates were identified using an HPLC system (SPD-20A, Shimadzu, Tokyo, Japan) with p-hydroxybenzoic acid (p-HA), vanillic acid (VA), syringic acid (SA), ferulic acid (FA), benzoic acid (BA), salicylic acid (SA), and cinnamic acid (CA) included as standard phenolic compounds. All the chemicals purchased were analytical grade, and the solvents used were HPLC spectral grade. The major peaks were identified by comparing the retention time with that of the matching standard. The organic acids were determined with a chromatographic column, Zorbax SB-Aq (4.6 mm × 250 mm, 5 mm) as described by Lv et al. (2018). The detector wavelength was 215 nm, and the column temperature was 35 °C. Standard organic acids included citric acid, oxalic acid, succinic acid, malic acid, trans-aconitic acid and fumaric acid for HPLC analysis.

Anthrone colorimetry was used to determine the content of the soluble sugars, and an automatic analyzer (Biochrom 30) was used to analyze the types and contents of amino acids.

Statistical analysis

Before the statistical analyses, all the data were tested for normality using the Kolmogorov-Smirnov test and were investigated for their homogeneity of variance using Levenes’s test and inspection of the residual plots. We used a one-way ANOVA to test the differences between treatments of the same substance. All the data were analyzed using SPSS version 13.0 (SPSS 13.0, Inc., Chicago, IL, USA) software with significance accepted at P < 0.05 and were expressed as the means ± standard errors.

Results

Effects of intercropping on Fusarium wilt in faba bean

Following inoculation with FOF, wheat intercropping significantly reduced the incidence and disease index of the faba bean compared with monocropping (Fig. 1 a, b). There was no significant difference in shoot dry weight between monocropping and intercropping after inoculation with FOF (Fig. 1c). And following inoculation with FOF, the root weight was significantly higher in the intercropping treatments than those in the monocropping treatments (Fig. 1d). In addition, following inoculation with FOF, the number of FOF per gram soil was significantly lower in the intercropping treatment than that of the monocropping (Fig. 1e).

Fig. 1
figure 1

a, Incidence of Fusarium wilt; b, Disease index of Fusarium wilt; c, Faba bean shoot dry weight; d, Faba bean root dry weight; E, Number of FOF. M-F, faba bean monocropping without FOF inoculation; M + F, faba bean monocropping with FOF inoculation; I-F, faba bean/wheat intercropping without FOF inoculation; I + F, faba bean /wheat intercropping with FOF inoculation. All the values are presented as the mean ± SE. The different letters on the mean values indicate significant differences among the treatments (P ≤ 0.05)

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Intercropping and inoculation with FOF-modulated faba bean root exudates

Phenolic acid secretion from the faba bean roots was induced by FOF infection in both the monocropping and intercropping systems. Fewer phenolic acids were secreted from the faba bean roots in the intercropping system compared with the monocropping system after inoculation with FOF. Most of the phenolic acids present in the root exudates of faba bean in the intercropping system were significantly decreased after FOF infection (Fig. 2). In contrast, the amounts of vanillic acid, syringic acid, benzoic acid, SA and p-HA secreted by the roots were increased by inoculation with FOF. However, the quantity of syringic acid, benzoic acid and SA was dramatically reduced in the root exudates of faba bean after FOF inoculation in the wheat intercropping treatments (Fig. 2a, b).

Fig. 2
figure 2

Quantity of phenolic acids in the root exudates of faba bean and the total phenolic acids after FOF inoculation. VA, vanillic acid; SyA, syringic acid; FA, ferulic acid; BA, benzoic acid; SA, salicylic acid; p-HA, p-hydroxybenzoic acid; CA, cinnamic acid. All the values are presented as the mean ± SE. The different letters on the mean values of the same phenolic acid indicate significant differences among the treatments (P ≤ 0.05)

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The secretion of organic acids from the roots of FOF-inoculated faba bean plants was higher compared with that of the non-inoculated faba bean plants (Fig. 3). The quantities of malic acid, tartaric acid, citric acid, succinic acid and fumaric acid secreted from the roots of faba bean were 40.2–85.0% lower in the intercropping system compared with the monocropping system after inoculation with FOF (Fig. 3a, b). Intercropping did not affect the secretion of trans-aconitic acid after inoculation with FON.

Fig. 3
figure 3

Quantity of organic acids in the root exudates of faba bean and total organic acids after inoculation with FOF. ND. none detected. All the values are presented as the mean ± SE. The different letters on the mean values of the same organic acid indicate significant differences among the treatments (P ≤ 0.05)

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Seventeen types of free amino acids were detected in the faba bean root exudates, including as cystine acid (Cys), phenylalanine (Phe), tyrosine (Tyr), threonine (Thr), lysine (Lys), valine (Val), methionine (Met), leucine (Leu), glutamic acid (Glu), serine (Ser), alanine (Ala), isoleucine (Ile), glycine (Gly), aspartic acid (Asp), histidine (His), arginine (Arg) and proline (Pro) (Table 1). FOF inoculation significantly increased the contents of free amino acids in the faba bean root exudates. The amount of each amino acid present in root exudates was increased by FOF inoculation with the exception of proline. The concentration of the 16 free amino acids that was detected was 1.9–98.75% lower in the root exudates of the intercropped plants inoculated with FOF than in the monocropped plants inoculated with FOF. Furthermore, the cysteine, phenylalanine, tyrosine and threonine contents in the exudates of the intercropped plants inoculated with FOF decreased by 94.8%, 92.5%, 90.2% and 89.5%, respectively, compared to the monocropped plants inoculated with FOF (Table 1).

Table 1 Effect of intercropping on the free amino acid secretion by the faba bean root after Fusarium oxysporum f. sp. fabae inoculation (μg·g−1 root FW)
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Inoculation with FOF significantly increased the soluble sugar content in the faba bean root exudates. The content of the soluble sugar was 13-fold higher in the root exudates of the monocropped faba bean that had been inoculated with FOF compared with the monocropped faba bean that had not been inoculated. Furthermore, the soluble sugar content in the exudates of the intercropped faba bean inoculated with FOF decreased by 78.8% compared to that in the monocropped faba bean that was inoculated with FOF (Fig. 4).

Fig. 4
figure 4

Content of the soluble sugars in the root exudates. M-F, faba bean monocropping without FOF inoculation; M + F, faba bean monocropping with FOF inoculation; I-F, faba bean/wheat intercropping without FOF inoculation; I + F, faba bean /wheat intercropping with FOF inoculation. All the values are presented as the mean ± SE. The different letters on the mean values indicate significant differences among the treatments (P ≤ 0.05)

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Discussion

Response of the root exudates to FOF infection and intercropping

In this study, inoculation with FOF significantly increased the amount of phenolic acids, organic acids, amino acids and sugars exuded by the faba beans. However, after intercropping with wheat, the amount of phenolic acids, organic acids, amino acids and sugars exuded from the root system of faba bean was alleviated. The response of the root exudates to the FOF infection and intercropping may be the response of the faba bean to environmental stress, which is related to the disease resistance of the faba bean. Alternatively, it may be related to disease control by intercropping.

The relation between the root exudates of faba bean, the occurrence of Fusarium wilt and the mechanism of mitigation by intercropping

Phenolic acids are widely found in plant root exudates. In this study, FOF inoculation promoted the exudation of benzoic acid from the roots. Benzoic acid can increase the activity of cellulase and pectinase (Wu et al. 2009), and greatly promote the ability of FON to produce a mycotoxin, which increased by 610–2630% (Wu et al. 2009). The changes of ferulic acid and cinnamic acid were also notable. Compared with monocropping, intercropping significantly reduced the secretion of these two phenolic acids, and this reduction was not correlated with the inoculation of FOF. The accumulation of ferulic acid and cinnamic acid may contribute to the destruction of the cell structure and the inhibition of the growth of faba bean, rendering the cell more fragile. Therefore, pathogens invade the cell more easily and promote the occurrence of disease. The amount of benzoic acid, ferulic acid and cinnamic acid was significantly reduced in the intercropping treatment of faba bean and wheat, which reduced the damage to the faba bean and alleviated the disease. Therefore, reducing the amount of phenolic acids may be the key to disease control. In addition, salicylic acid is an active component. Following inoculation with FOF, the content of salicylic acid increased sharply in the monocropping treatment, which was obviously the result of FOF stimulation. The intercropping treatment significantly reduced the amount of salicylic acid, but the content of salicylic acid remained high relative to the treatment that lacked inoculation by FOF. Because of this sensitive response of salicylic acid to FOF, salicylic acid may act as a signal substance to activate the resistance of plant and play an active role in the inhibition of FOF growth (Wu et al. 2008; Hao et al. 2010; Lv et al. 2018), rather than as an allelopathic autotoxic substance to inhibit the growth of the plants.

The organic acids exuded from the plant roots inhibit the growth of pathogens in soil, including more than 10 fungi such as Fusarium oxysporum (Momma et al. 2006), and organic acids can aid in the chemotaxis of beneficial bacteria, the plant growth and improve plant disease resistance (Ling et al. 2011). In contrast, organic acids can also provide nutrients for pathogens and promote their growth and reproduction (Lv et al. 2018). Malic acid can promote the production of H2O2 (Peng and Kuc 1992; Wingler et al. 2000). It is well known that the accumulation of H2O2 in plant somatic cells can lead to the damage of protein and DNA, aggravate the degree of membrane lipid peroxidation and affect the normal metabolism of cells (Foyer et al. 1994; Mohanpuria et al. 2007). In this study, inoculation with FOF stimulated the exudation of organic acids from faba bean roots, which may represent the stress response of faba bean to pathogen stress to activate the disease resistance of faba bean and inhibit the growth of the pathogen to alleviate the disease. With the development of disease, the root cells of faba bean were damaged and could not maintain their integrity, resulting in the outflow of cell contents, providing nutrients for the pathogens, causing the pathogens to multiply in large quantities in the rhizosphere and aggravating the disease. In intercropping, the faba bean is protected from cell damage in many ways. This is the reason why the organic acid content of the intercropping inoculation FOF treatment is lower than that of the monocropping FOF inoculation treatment and higher than that of the intercropping inoculation without FOF treatment.

After the faba bean was infected with FOF, a large amount of amino acid was exuded from the root system. On the one hand, this response may be the stress response of faba bean, which is closely related to its disease resistance. Alternatively, it may also be that faba beans can not maintain the integrity of cells when they are attacked by pathogens, resulting in the outflow of intracellular substances, so the content of the amino acids that we measured was higher. The exact reasons for this response merit additional study. Among these 17 amino acids, proline is the most interesting. It is well known that the increase of proline in plants is closely related to plant disease resistance (Fabro et al. 2004; Haudecoeur et al. 2009; Yang et al. 2009). In addition, proline, as a signal substance, controls the growth and development of cells and affects the programmed cell death to a certain extent (Székely et al. 2008; Mattioli et al. 2009; Fabro et al. 2004; Deuschle et al. 2004). In this study, both infection with FOF and the intercropping treatment significantly promoted the exudation of proline from the faba bean roots. This may imply that proline has a special function in response to disease. In the early stages of disease stress, proline accumulated in the root system of faba bean in an attempt to resist the invasion of pathogens, and some progress was obtained. As the pathogen proliferates, the disease becomes more serious, and for some seriously damaged cells, proline may induce programmed cell death, avoid the outflow of intracellular content to provide nutrition for pathogens, and prevent the spread of disease, which also protects the faba bean to some extent. The intercropping of wheat and faba bean could significantly reduce the amount of amino acids exuded from the root system of faba bean and increase the amount of proline, which may be an important reason for disease control in the intercropping system.

Soluble sugar provides nutrients for pathogen growth (Hao et al. 2010). After inoculation with FOF, the root exudes a large amount of soluble sugars, which can be reduced by wheat intercropping. The decrease in soluble sugar exudation reduces the nutrients needed for the growth and reproduction of the pathogen, and limits the increase of pathogen density to reduce diseases. Currently, due to the limitations of separation and purification technology, the research on soluble sugars progresses slowly and merits additional study.

Intercropping of wheat and faba bean to control faba bean wilt

Biodiversity is the natural barrier of plant disease epidemics; the sustainable control of crop diseases by using biodiversity is a hot research topic in China and other countries in recent years (Wolfe 2000). Intercropping is the most popular cultivation model to improve crop genetic diversity, and intercropping systems can reduce disease damage, which has been of wide concern. This reduction in disease damage may be due to fewer attacks by pathogens compared with monocropping systems (Lv et al. 2018). In this study, faba bean and wheat intercropping significantly reduced the number of pathogens, which may be one of the reasons for the control of the faba bean Fusarium wilt. Under field conditions, intercropping has proven to be a mode of disease control and yield increase in many systems, and the advantages of disease control and yield increase in faba bean/wheat intercropping are also frequently reported (Chen et al. 2007). In actual production, different soil types, fertility conditions and the environment may also affect disease control in intercropping. For example, research by Liu et al. (2007) found that increasing the K supply had an important effect on the occurrence of maize stem rot, which was closely related to changes in the root exudates. Yang et al. (2014) showed that the effect of intercropping between maize and pepper on pepper blight control was affected by the intercropping distance between the two crops. The closer the intercropping distance, the better the disease control. Our previous studies also found that proper N application in the wheat/faba bean intercropping system could increase the effective control of the faba bean Fusarium wilt (Dong et al. 2010). Therefore, under different soil types, fertility conditions and environmental factors, we need to further verify the effect of the wheat/faba bean intercropping to control Fusarium wilt and study its mechanism.

In conclusion, the phenolic acids, organic acids, amino acids and sugars exuded from the roots of faba bean infected with FOF increased significantly, which may be closely related to the change in the disease resistance of the faba bean, and these compounds can also be used as nutrients to promote the proliferation of the pathogens. Intercropping with wheat can reduce this increase, which may imply that faba bean plants recover from stress, reduce the nutrients needed for pathogen growth, limit pathogen proliferation, and contribute to the alleviation in disease. The results showed that wheat/faba bean intercropping could alleviate Fusarium wilt on faba bean. Therefore, intercropping can be an effective strategy to control Fusarium wilt in faba bean.