Introduction

Fusarium dry rot is one of the most important storage diseases in potato tubers (Solanum tuberosum L.), and affects almost all commonly grown potato cultivars (Leach and Webb 1981). Potentially, Fusarium dry rot can cause great yield losses with up to 60% of the tubers affected (Secor and Salas 2001). Fusarium dry rot pathogens infect through wounds on tubers caused mainly by handling during planting, harvesting, and grading.

The disease is caused by several species of Fusarium (Boyd 1972; Secor and Salas 2001) that in addition to potato have a wide host range including, e.g., cereals, legumes, and beetroot (Peters et al. 2008b). Thirteen Fusarium species are considered as causal agents of Fusarium dry rot in potatoes worldwide (Cullen et al. 2005). In Great Britain and in the Nordic countries, the most common species isolated from potato has been Fusarium coeruleum (Libert) Sacc. (Bjor 1978; Olofsson 1976; Peters et al. 2008a; Seppänen 1983). In other parts of Europe, in northern and western China and in North America Fusarium sambucinum (Fückel) sensu stricto is considered to be the most significant causal agent of Fusarium dry rot (Du et al. 2012; Secor and Salas 2001). In North Dakota, Fusarium graminearum (Schwabe) in addition to F. sambucinum was reported to be the most prevalent species causing Fusarium dry rot (Estrada et al. 2010). In a recent study on Fusarium dry rot in Michigan, Fusarium oxysporum Schlechtendal emend. Snyder & Hansen was the most prevalent species, but F. sambucinum was the most aggressive (Gachango et al. 2012).

Control strategies for Fusarium dry rot include use of resistant cultivars and cultural practices such as crop rotation, use of disease free seed, and wound healing prior to storage. Biological control agents and ultraviolet radiation are also used, as well as chemical control (Al-Mughrabi et al. 2013; Bojanowski et al. 2013; Bång 1992; Gachango et al. 2012; Peters et al. 2008a; Ranganna et al. 1997). However, chemical control is not common in use against Fusarium dry rot in Norway. Diagnostic tools providing fast identification and quantification of Fusarium dry rot pathogens could in principle validate the recommendations on the control strategy. Moreover, such tools can be used to detect latent infections in tubers pre-storage, to validate their storability and/or suitability as seed potatoes. Several molecular assays for detection of different Fusarium spp. found in potato and other crops have been developed (Cullen et al. 2005; Halstensen et al. 2006; Nicholson et al. 1998).

In Norway, the problems with Fusarium dry rot in potatoes seem to have increased the last decade. The objective of the current study was to identify which Fusarium species currently are causing Fusarium dry rot in commercial potato production in Norway, including the extent of regional variation and the effect of agronomic and storage factors. We also aimed to test the suitability of available real-time PCR assays for detection of Fusarium spp. common in Norway and, if needed, develop new assays.

Materials and Methods

Sample Collection

In total 238 potato tuber samples from fields in all major potato growing districts in Norway, covering a distance of more than 2000 km of the country from north to south, were collected in October 2010, 2011, and 2012. The main potato production areas in Norway are situated in the eastern and southeastern parts of the country, and hence, most of the samples came from these areas (Table 1). Different cultural practices are used in different regions; hence, some cultivars are only grown in specific regions of Norway. Cultivars sampled from different regions are listed in Table 1.

Table 1 Number of potato samples from different regions and years and cultivars sampled in each region
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The samples were collected by agronomists, farmers, and store managers. Information about geographical origin and potato cultivar was also collected. The samples originated from 26 different cultivars. Only cultivars of which we received seven or more samples were included when analyzing the effect of cultivar (Table 4). In the other analyses, all 26 cultivars were included. The remaining cultivars, with less than seven representatives, included Arielle, Bruse, Gulløye, Juno, Kuras, Odin, Ostara, Pimpernel, Polaris, Sava, Solist, Tivoli, and Van Gogh. The majority of the samples were from potatoes intended for crisps and fries; hence, the cultivars Asterix, Lady Claire, and Saturna comprised more than 40% of the samples in this study. Each sample consisted of 100 tubers collected from box storages: 10 tubers just under the top layer from 10 different boxes from different heights in the storage. The samples were stored at 4 °C and 95% RH in experimental storage facilities for 6 months prior to wounding. A pre-study showed that tubers could be left in storage under these conditions without changes in Fusarium population and contaminations.

Isolation, Cultivation, and Identification of Fusarium

Fusarium spp. were isolated and identified as described by Peters et al. (2008a). Briefly, all tubers from each sample were wounded on both sides to a depth of 4 mm using a sterile wounding device consisting of four spikes (each with a diameter of 1 mm) in a quadratic square (20 mm on each side). The tubers were placed in paper bags and incubated in experimental storage rooms holding a temperature of 10 °C and 95% relative humidity for 6 weeks.

After the incubation period, a cut exactly through each wounding point toward the center of the tuber was made with a flame-sterilized knife to detect potential Fusarium dry rot development. Tubers with typical Fusarium dry rot symptoms (shallow or sunken and wrinkled necrotic areas) in the wounding site or in natural infection sites on the tuber were registered. Rots were assumed to be conical for the purpose of analysis. Therefore, the volume of the rot was recorded and calculated using the formula: volume = 1/3πhr 2, where r is half the width of the rot and h is the depth of the rot. Four pieces of tissue from the edge of the rotten area were transferred to potato dextrose agar (PDA) with 200 mg l−1 streptomycin. The PDA plates were incubated at room temperature for 7–10 days in alternating 12 h of light and 12 h of darkness. Cultures resembling Fusarium were then transferred to fresh PDA and subsequently to synthetic nutrient agar (SNA) with a small piece of autoclaved filter paper. The pure cultures were identified to species based on conidial morphology, production of chlamydospores, growth characteristics, and colony pigmentation as described by Leslie and Summerell (2006) and Gerlach and Nirenberg (1982).

DNA Extraction and Molecular Identification of Fusarium spp.

Fusarium isolates were grown on PDA plates for 2–3 weeks at room temperature prior to DNA extraction. Mycelium was scraped off the agar plate using a scalpel. The mycelium was homogenized using a pestle and mortar and liquid nitrogen. DNA was extracted from the homogenized mycelium (100 mg) using DNeasy Plant Mini Kit (QIAGEN) according to the instructions provided by the manufacturer. DNA concentrations were estimated by comparing the intensity of genomic DNA bands in agarose gel to a DNA marker with known concentrations, or by using a NanoDrop spectrophotometer. To confirm species identity, DNA extracts of approximately 15% of the isolates were tested using PCR-based assays for F. avenaceum (Fries) Sacc. (Halstensen et al. 2006), Fusarium culmorum (W. G. Smith) Sacc., and F. sambucinum (Cullen et al. 2005). F. coeruleum was tested with the assay developed and described in this study (see below).

Statistics

All statistical analyses were performed using SAS 9.4. Logistic regression was used to analyze the data. The individual Fusarium species were modeled and analyzed one by one. Potato cultivar, geographical region, and year were used as fixed factors. All data was converted from number of isolates found per sample of 100 tubers to 0 or 1 representing absence or presence of the Fusarium species, respectively. Prevalence of the individual Fusarium species in geographical region and potato cultivar is given in incidence and probability. Incidence is given as percentage of infected tubers per sample and probability, which indicate the likelihood of finding the given Fusarium species in a certain region or cultivar, is calculated from the formula: P(Fusarium spp. = 1) = 1 − (e^estimate / (1 + e^estimate)) where the estimate is given in the outcome of the logistic regression. Differences between potato cultivars and geographical regions were tested by Tukey’s multiple comparison method.

Primer Design and Real-Time PCR for F. coeruleum

Five isolates of F. coeruleum originating from different parts of Norway were used for development of the assay. The internal transcribed spacer (ITS) regions (ITS1 and ITS2) of the rDNA genes of the isolates of F. coeruleum were amplified with the universal primers ITS1 and ITS4 (White et al. 1990). The PCR products were purified and sequenced at GATC Biotech AG. Sequence data were assembled and analyzed using the DNAStar Seqman II software. Five identical sequences were compared to other Fusarium sequences using ClustalW2, and primers and a TaqMan MGB probe were designed using Primer Express 2.0 and manual evaluation of the sequence alignment. A real-time PCR assay was selected because it is less labor intensive and less prone to contamination than standard PCR. Moreover, it is fast, sensitive, and amenable to quantification. Real-time PCR reactions were performed using TaqMan® Universal PCR Master Mix, 200 mM MGB probe (Fcoer1 P) (5′-FAM-CAGCGAGACCGCCAC-3′) and 300 mM of primer F (Fcoer1 F) (5′-TGTTAGCTACTACGCAATGGAAGCT-3′) and primer R (Fcoer1 R) (5′-GCCGGCCCCGAAATC-3′), with 2 μl template DNA in a total volume of 25 μl. PCR reactions were performed in duplicate in a 7900 HT Fast Real-Time PCR System (Applied Biosystem), and real-time data were analyzed using the Sequence Detection System version 2.2.1 (Applied Biosystems). The cycling protocol was as follows: 2 min at 50 °C, 10 min at 95 °C, 45 cycles of 95 °C for 15 s, and 60 °C for 1 min.

Sampling and DNA Extraction of Potato Peel and Soil

To evaluate the ability of the developed real-time PCR assay to detect F. coeruleum in tubers, we tested 40 potato peel samples from 2010 and 39 from 2012. These samples, consisting of 20 tubers each, came from the same fields as the tuber samples used for the survey. One peel strip (1–2 mm thick) including both rose and heel end was taken from each of 20 tubers using a hand held potato peeler. The peels were mixed with 50 ml SPCB buffer (120 mM sodium phosphate, 2% CTAB, 1.5 M NaCl, pH 8.0) (Lees et al. 2002) and homogenized using a kitchen blender for 2 min. A single 1.5 ml aliquot was taken from each sample for DNA extraction. DNA extraction was performed using the Molestrips DNA Plant kit (Mole Genetics, Lysaker, Norway) according to the manufacturer’s protocol.

The plant internal control primers (COX F and COX RW) and probe (COX) based on a previously described assay designed for the cytochrome oxidase (COX) gene (Weller et al. 2000) were used to detect host DNA, providing confirmation that DNA extraction was successful and thereby avoiding false-negative results for F. coeruleum.

The F. coeruleum assay was also tested with soil samples. In 2010, soil samples were taken after harvest from the 40 potato fields where the tuber samples were grown. Within these fields, soil was sampled from 50 to 60 points (0–10 cm deep) 10 m apart in a W-shape pattern using a soil corer. DNA was extracted from soil suspensions (60 g soil added 120 mL SPCB buffer) by physical disruption in a minibead beater using the method described by Cullen et al. (2001).

Results

Fusarium Species in Norway

In the years 2010–2012, a total number of 238 samples comprising 23,800 tubers were collected from commercial potato stores in Norway. Fusarium was isolated from 3% of the tubers divided over 47% of the samples; in total, 718 isolates were recovered and these were identified to seven species (Table 2). Most (98.4%) of these isolates were attributed to one of four Fusarium species: F. coeruleum, F. avenaceum, F. sambucinum, and F. culmorum. Other fungi isolated from rotted tissue of the sampled tubers included Boeremia foveata (Foister) Aveskamp, Gruyter & Verkley, Colletotrichum coccodes (Wallr.) Hughes, Cylindrocarpon spp. Wollenweber, Helminthosporium solani (Durieu & Montagne), Polyscytalum pustulans (M.N.Owen & Wakefield) M.B.Ellis, Rhizoctonia solani (A.B.Frank) Donk, and Penicillium sp., all potential plant pathogens.

Table 2 Relative frequencies (%) of different Fusarium species isolated from commercial potato tubers sampled from Norwegian stores during 2010–2012
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Isolates were identified as F. coeruleum by their appearance of white-bluish mycelium after 2 weeks on PDA and production of weakly curved 4 septate macroconidia (25–50 × 5 μm) from slimy cream colored sporodochia around the filter paper on SNA. A previously developed F. coeruleum specific real-time PCR assay (Cullen et al. 2005) gave unexpectedly high Ct values (28.0–32.0), which lead us to develop a new test for this species (see below). The new test confirmed the identification based on morphology for 41 isolates (Ct values ranging from 17.7 to 22.9).

F. avenaceum produced white to rose mycelium after 2 weeks on PDA and long and slender 3–5 septate macroconidia from pale orange sporodochia around the filter paper on SNA. F. avenaceum specific real-time PCR assay (Halstensen et al. 2006) confirmed the identification based on morphology for 45 isolates (Ct values ranging from 23.1 to 29.1).

F. sambucinum was identified by production of white, yellow, or red to salmon pink mycelium on PDA and falcate, slender 3–5 septate macroconidia with a foot-shaped basal cell and pointed apical cell from orange sporodochia produced on SNA. F. sambucinum specific real-time PCR assay (Cullen et al. 2005) confirmed the identification for eight isolates (Ct values ranging from 19.2 to 25.0). Three isolates identified morphologically as F. sambucinum gave only weak signals in the real-time PCR test (Ct values ranging from 33.9 to 36.3). There was some cross-reactivity with other species giving Ct values from 32.6 to 44.1.

F. culmorum was identified by rapid production of white to orange-red mycelium and short, robust, and thick-walled 3–4 septate macroconidia from orange sporodochia produced on SNA. The species identification of five isolates was confirmed with F. culmorum specific real-time PCR assay (Cullen et al. 2005) (Ct values ranging from 18.6 to 20.2).

The most commonly isolated species was F. coeruleum comprising 59.6% of the total Fusarium isolates (Table 2). F. coeruleum was the most prevalent species in 2010 and 2012, whereas F. avenaceum was found slightly more than F. coeruleum in 2011. F. coeruleum was found in 17.2% of the samples and 1–88 isolates were recovered from each sample. Two samples from 2012, cultivars Berber and Rutt (each 100 tubers), were heavily infected with F. coeruleum and resulted in 81 and 88 isolates, respectively. These samples affected the total number of isolates greatly and resulted in a high total isolate number (415) this year. F. avenaceum was the second most prevalent species comprising 27.2% of the isolates. It was found in 27.7% of the samples (1–18 isolates per sample); hence, F. avenaceum was found in more samples than the other Fusarium species. None of these samples, however, was highly infected with F. avenaceum, indicating that F. avenaceum is a less aggressive species. F. sambucinum was the third most prevalent species, comprising 6.4% of the isolates, and was identified in 8.8% of the samples (1–10 isolates per sample). The relative frequency of F. sambucinum was considerably lower in 2010 than the two following years. F. culmorum was the fourth most prevalent species, comprising 5.2% of the isolates and was identified in 6.3% of the samples (1–9 isolates per sample). The species F. cerealis (Cooke) Sacc., F. graminearum and F. equiseti (Corda) Sacc. were less prevalent (0.2 to 0.6% of the isolates in 0.4 to 1.3% of the samples).

Volume of the dry rot was measured. The mean size of dry rot lesion for the four most prevalent Fusarium species varied from 3.6 to 10.1 cm3. However, there was large variation in lesion development within the Fusarium species and there was no significant correlation between Fusarium species and lesion size.

Real-Time PCR Assay for F. coeruleum

Ten different isolates, identified morphologically as F. coeruleum, were analyzed with the real-time PCR assay developed by Cullen et al. (2005). The isolates gave Ct values ranging from 28.0 to 32.0, which is relatively high considering the fact that DNA templates were extracted from pure isolates. After several failed attempts to optimize the assay by Cullen et al. (2005), we developed a new TaqMan assay targeting the ITS1 region at a different site than the previously developed assay for F. coeruleum. The new real-time PCR assay was tested with various concentrations of primers (ranging from 50 to 900 mM) and probe (ranging from 50 to 250 mM). The optimal primer concentrations were 300 mM, and the optimal probe concentration was 200 mM. The TaqMan assay could reliably detect 10−15 g of F. coeruleum DNA. The standard curves for the real-time PCR test showed high correlation coefficient (R 2 = 0.99), indicating a reproducible linear response in detection of increasing concentrations of F. coeruleum DNA (Fig. 1).

Fig. 1
figure 1

Standard curve for F. coeruleum real-time PCR assay

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The assay was tested on 97 Fusarium isolates from Norway selected at random from the survey. DNA from isolates of F. coeruleum gave Ct values ranging from 17.7 to 22.9 (Fig. 2). The other Fusarium species, including F. avenaceum, F. cerealis, F. culmorum, F. equiseti, and F. sambucinum gave Ct values ranging from 34.6 to 44.4 or did not give any Ct value (shown as Ct value = 0).

Fig. 2
figure 2

Ct values for 97 isolates belonging to six different species of Fusarium (F. avenaceum, F. cerealis, F. coeruleum, F. culmorum, F. equiseti, and F. sambucinum) when tested with F. coeruleum real-time PCR assay

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The F. coeruleum real-time PCR assay was tested for the ability to detect individual Fusarium species in potato peel samples and soil samples. In total, 79 potato peel samples were collected in 2010 and 2012 and 40 soil samples were collected in 2010. No Fusarium was detected with certainty in the samples from potato peel or from soil. However, there were weak signals of F. coeruleum in five potato peel samples giving Ct values ranging from 34.6 to 39.1 and in one soil sample giving a Ct value of 36.7. The COX internal control targeting the host plant DNA of potato yielded stable Ct values of around 25 for all the plant samples, indicating an overall high quality of the DNA extracts.

Regional Variation in Fusarium Occurrence

The incidence of and probability of finding the four most important Fusarium spp. varied between regions (Table 3). There were no significant differences between years. The probability of finding F. coeruleum was highest in northern Norway and significantly different from eastern Norway (Table 3). F. coeruleum was present in all sampled regions in 2010–2012 except in central Norway in 2012, where there was no incidence of Fusarium spp. at all (Fig. 3). There was a high incidence of F. coeruleum in northern Norway all three years compared to the other regions, and in 2012, no other species were detected. In southeastern and southwestern Norway, F. coeruleum was the most common species in 2010 and 2012; especially in 2012, the number of isolates collected in west Norway increased due to the, before mentioned, two heavily F. coeruleum infected samples (cultivars Berber and Rutt; each 100 tubers, 81 and 88 isolates per sample) (Fig. 3). F. avenaceum was found in all regions all years except in northern and central Norway in 2012. It was the most common Fusarium species found in eastern Norway all years (Fig. 3). F. sambucinum was isolated from the potato tuber samples in eastern and southwestern Norway in all years, as well as from central Norway in 2011 and southeastern Norway in 2012 (Fig. 3). The probability of finding F. sambucinum was significantly higher in southwestern than in eastern and southeastern Norway (Table 3). F. sambucinum was not found in northern Norway (Table 3). F. culmorum had the total lowest incidence of the four Fusarium spp., but it was found in all Norwegian regions (Fig. 3). The probability of finding F. culmorum was significantly higher in southwestern than in eastern Norway (Table 3).

Table 3 Incidence (% infected tubers per sample) of different Fusarium spp. and probability (P(Fusarium spp. = 1)) of finding these Fusarium spp. on potato tubers sampled from different regions of Norway 2010–2012
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Fig. 3
figure 3

Incidence of Fusarium spp. on potato tubers in five regions in Norway in 2010–2012 (n = number of Fusarium isolates)

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Effect of Cultivar on Fusarium Occurrence

The incidence of and probability of finding one of the four Fusarium species in 13 different potato cultivars are given in Table 4. The incidence of F. coeruleum was high (≥11.0) in Asterix, Berber, Laila, Mandel and Rutt. However, the probability of finding F. coeruleum was highest in Berber and Rutt. The incidence of F. avenaceum was high in Berber and so was the probability. The incidence of F. sambucinum was high in Berber and Rutt. However, the probability of finding F. sambucinum was low (≤0.002) in all cultivars. F. culmorum was found more in Rutt than the other species. There were no significant differences between cultivars for any of the Fusarium species: F. coeruleum (P = 0.743), F. avenaceum (P = 0.927), F. sambucinum (P = 1.000), and F. culmorum (P = 0.939).

Table 4 Incidence (% infected tubers per sample) of Fusarium spp. and probability (P(Fusarium spp. = 1)) of finding Fusarium spp. on tubers of different potato cultivars sampled in 2010–2012 (number of samples in parentheses)
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Discussion

During the period 2010–2012, Fusarium spp. were only found in 3% of the tubers, but Fusarium species—in varying number—were present in almost half of the samples (each 100 tubers), showing the potential risk of Fusarium dry rot development in Norwegian potatoes if the right conditions are present.

F. coeruleum was the Fusarium species most frequently isolated from potato tubers in the samples, in total 428 isolates, which amounted to 59.6% of all the Fusarium isolates (Table 2). The high prevalence of F. coeruleum was consistent with previous findings in Norway (Bjor 1978). F. coeruleum was also found to be the most common species isolated from potato in Great Britain (Peters et al. 2008a), Sweden (Olofsson 1976), and Finland (Seppänen 1983). F. coeruleum was especially prevalent in northern Norway, which might be influenced by a crop rotation dominated by potato, the only described host for F. coeruleum. In 11 out of 12 samples, potato was also grown the year before on the same field and the most common cultivars grown were Mandel and Gulløye. These cultivars are known as susceptible to Fusarium dry rot, with score 1 of 9 where 9 is most resistant (Møllerhagen 2014). Two samples (from cultivars Berber and Rutt) heavily infected with F. coeruleum collected in southwestern Norway resulted in a high total number of isolates in 2012. These two samples show the potential risk of Fusarium infections in storage. The probability of finding F. coeruleum was high in Berber and Rutt in all three years of the survey. Møllerhagen (2014) confirms the susceptibility of Rutt (score 1), and we found Berber and Rutt to be very susceptible to Fusarium spp. in another study (Heltoft et al. 2015). We noticed that Berber and Rutt share one of their parents, Alcmaria, but no information regarding resistance to Fusarium spp. is registered for this cultivar (www.europotato.org).

F. avenaceum was the second most commonly isolated species, and it was even slightly more frequent than F. coeruleum in 2011 (Table 2). Furthermore, F. avenaceum was found in more samples than the other Fusarium species, but with a smaller number of isolates obtained per sample, indicating that F. avenaceum is a less aggressive species. F. avenaceum was the second most prevalent species in previous surveys in Great Britain and China (Du et al. 2012; Peters et al. 2008a), and this species was also considered as a relatively weak pathogen by Peters et al. (2008a). The prevalence of F. avenaceum in Norwegian potatoes may be affected by crop rotation, because potato is often grown in rotation with cereals. In Norwegian cereals, F. avenaceum is the most commonly detected species of Fusarium and furthermore, Fusarium has been an increasing problem in cereals in Norway in the last 10 years (Bernhoft et al. 2013). In the present study, F. avenaceum was the most common species found in potatoes in eastern Norway, where a narrow crop rotation with cereals is normal practice. Gachango et al. (2012) also discussed that crop rotation with cereals may have an implication on the prevalence of F. avenaceum in potatoes. However, when statistical analyses were applied in our study, crop rotation did not have a significant effect on the prevalence of the different Fusarium species. This could be a consequence of very few repetitions of the same crop rotation in the data or simply just the fact that F. avenaceum is commonly found in almost all crops grown in rotation with potatoes. Further studies are needed to investigate crop rotation as a factor of increased Fusarium dry rot development in potato.

F. sambucinum was the third most prevalent Fusarium species in the present study. This species, however, was the most commonly isolated species in potato in other parts of Europe, in northern and western China, and in North America (Du et al. 2012; Estrada Jr et al. 2010; Secor and Salas 2001). In a number of studies, F. sambucinum was the most aggressive species in potato (Esfahani 2005; Gachango et al. 2012; Peters et al. 2008a; Wastie et al. 1989) and therefore, it cannot be readily explained why F. sambucinum is not more prevalent in the present study. However, relatively low prevalence of F. sambucinum was also observed in surveys in Michigan and in Great Britain, where it was the third and fourth most prevalent species, respectively (Gachango et al. 2012; Peters et al. 2008a). In particular, F. sambucinum was not found in northern Norway, where the climatic conditions normally are harsher than in the other regions. However, unfavorable climatic conditions cannot be used as an explanation for the absence of this species in northern Norway, because it is widely reported to be common in temperate and cool parts of the world (Leslie and Summerell 2006). Furthermore, F. sambucinum has previously been found in northern Norway (Abbas et al. 1987). A low number of samples per year in this region could also be the reason for the absence of F. sambucinum.

Even though the inoculum levels in the survey were unknown, the results indicate that they in general were very low. This is based on the results from the real-time PCR tests of soil and potato peel samples, where our assay was only able to detect weak signals of F. coeruleum. It can be discussed whether the potato peel should have been incubated for a period for potential enrichment of Fusarium before the molecular test. The high infection rate with F. coeruleum in 2012 and lack of F. coeruleum detected in potato peel that year indicates that the inoculum might have been present in the soil. However, this cannot be verified as no soil samples were analyzed in 2012.

The species identification based on morphology was confirmed by real-time PCR assays specific to each Fusarium species, and all isolates were tested with all assays. However, some problems occurred with the assays. It was found that the F. culmorum specific primers (Cullen et al. 2005) could not distinguish between F. culmorum and F. cerealis. The same cross-reaction was found by Nicolaisen et al. (2009), who developed another F. culmorum specific assay used for detection of the species in cereals. The F. sambucinum specific assay also had cross-reactivity with other species, even after testing different primer and probe concentrations to optimize the assay. However, this species was not considered an important species in Norway and therefore no attempts was made to set up a new assay. In contrast, a new real-time PCR assay was developed for the most prevalent species in the survey, F. coeruleum, because of unexpected high Ct values when using the assay previously developed (Cullen et al. 2005).

Seven species of Fusarium are currently causing Fusarium dry rot in commercial potato production in Norway, with F. coeruleum and F. avenaceum as the most important species. Fusarium spp. were present in almost half of the samples, and heavy infections may occur if the right conditions are present. The results of this study indicated differences between cultivars in resistance to Fusarium rot and can support future control strategies, for example by providing methods and targets for breeding programs However, further studies are needed looking at differences in cultivar resistance.