Aetiology and toxigenicity of Fusarium graminearum and F. pseudograminearum causing crown rot and head blight in Australia under natural and artificial infection

Globally fusarium head blight (FHB) of wheat is predominantly caused by Fusarium graminearum (FG) and crown rot (FCR) by F. pseudograminearum (FP). While both FG and FP can cause FHB in Australia, the reasons why the morphologically and culturally similar FG is not a major FCR pathogen has remained elusive. Using aetiology and toxigenicity, this study clarifies the contrasting roles of FG and FP in FCR and FHB in Australia. Naturally infected wheat from 42 sites during 2010 FHB epidemics, and wheat inoculated with either pathogen to induce FCR or FHB at three field plantings in 2011, were used to determine pathogen prevalence and deoxynivalenol (DON) content of the crown, stem base, stem top, rachis and grain. As the primary aetiological agent, FP prevalence in the crown correlated with FCR severity while FG in grain and/or the rachis correlated with FHB severity. FG was an effective colonizer of the crown and stem base but colonization was symptomless. DON content was linked to FG biomass in all tissues except the crown, where FP biomass was the main contributor. Of the 30 measures derived to analyse pathogen fitness in 2011, 10 described the superior fitness of FG for FHB; six defined FP fitness for FHB including inoculum dispersal; and eight defined FCR fitness of both FP and FG. FG had superior FHB fitness but weak saprophytic survival may have undermined its FCR fitness.     

Development of three fusarium crown rot causal agents and systemic translocation of deoxynivalenol following stem base infection of soft wheat

Fusarium pseudograminearum, F. culmorum and F. graminearum are the most important fusarium crown rot (FCR) causal agents. They have the common ability to biosynthesize deoxynivalenol (DON). To elucidate the behaviour of each of the three species, a comparative study was carried out to investigate symptom progression, fungal systemic growth and translocation of DON following stem base inoculation of soft wheat. FCR symptoms were mainly localized in the inoculated area, which extended up to the second node for all inoculated species. Only the most aggressive strains caused symptoms up to the third node. Real‐time quantitative PCR showed that fungal colonization reached the third node for all the tested species, but a low percentage of plants showed colonization above the third node following inoculation with the most aggressive strains. Fungal growth was detected in symptomless tissues but none of the three species was able to colonize as far as the head tissues. However, even if the pathogens were not detected in the heads, DON was detected in head tissues of the plants inoculated with the most aggressive strains. These results demonstrate that F. pseudograminearum, F. culmorum and F. graminearum, under the same experimental conditions, follow a similar pattern of symptom progression, fungal colonization and DON translocation after stem base infection. Differences in the extent of symptoms, fungal colonization and mycotoxin distribution were mainly attributable to strain aggressiveness. These findings provide comparative information on the events following infection of the stem base of wheat by three of the most important FCR casual agents.     

Trichothecene production is detrimental to early root colonization by Fusarium culmorum and F. graminearum in fusarium crown and root rot of wheat

Fusarium culmorum (Fc) and F. graminearum (Fg) belong to the predominant causal agents of fusarium crown and root rot (FCR) in wheat. While many studies have been done to investigate crown rot, including stem base infection, root colonization and mycotoxin production associated with root rot is not well understood. In this study the impact of mycotoxins on the colonization of wheat roots and stem bases was analysed by using Fc and Fg isolates that varied in both quantity and types of trichothecenes they produce. Seedling inoculations in growth chambers with a high deoxynivalenol (DON)- and 3-acetyldeoxynivalenol (3ADON)-producing isolate led to more severe symptoms and 20-times greater colonization of the stem base, as measured by Fc DNA accumulation, than isolates that produced less DON/3ADON. In contrast to stem base colonization, in vitro inoculations of roots with a Tri5 deletion mutant deficient in Fg trichothecene production led to three-times higher colonization than the wildtype. Furthermore, an Fc isolate that produced low levels of zearalenone resulted in twice the level of colonization of a high DON/3ADON-producing isolate included in the study. When root inoculation with a low DON/3ADON-producing Fc isolate was supplemented with exogenous DON, DON production decreased by more than half per unit weight of Fc DNA, and root colonization doubled compared to the untreated control. Therefore, in contrast to its potential role as an aggressiveness factor in stem base infection, trichothecene production by Fc and Fg is detrimental to the early stages of wheat root colonization in FCR.     

不同小麦品种(系)对赤霉病的抗性和麦穗组织中DON毒素积累分析

 为明确不同小麦品种(系)对赤霉病的抗性和麦穗组织中DON毒素积累水平,培育和利用抗赤霉病和DON毒素积累的品种提供资源和依据,本研究采用单小花滴注接种法对河南省的106个小麦品种(系)抗赤霉病性进行鉴定分析,并用ELISA测定了病穗组织中DON毒素水平。结果表明不同小麦品种(系)对赤霉病的抗性有显著差异,106个小麦品种(系)中未发现抗病和中抗材料,中感品种(系)有华育198、郑麦103和春丰0021等14个,占13.2%;感病的有曌式2010-06、百农898和中麦63等92个,占86.8%。不同小麦品种(系)籽粒、颖壳和穗轴中DON毒素积累水平有显著差异,籽粒中DON毒素水平在(0.70~287.63)mg/kg之间,其中郑03876、豫保1号和中麦63 的DON毒素水平在2 mg/kg 以下,为抗毒素材料;其他的103个品种DON毒素水平大于2 mg/kg;颖壳和穗轴中的DON毒素水平在(51.03~392.87)mg/kg之间,普遍比籽粒中DON毒素含量高。籽粒中DON毒素水平与小麦品种(系)的平均病害严重度间呈极显著正相关。     

Within-field variability of Fusarium head blight pathogens and their associated mycotoxins

Within-field variability in the Fusarium head blight (FHB) and its associated mycotoxins was studied in four European countries. At each of 14 sites, each FHB pathogen and associated mycotoxins were quantified in 16 quadrat samples at harvest. Overall, the incidence of quadrat samples with detectable and quantifiable pathogen DNA was significantly lower in the grain than in the corresponding chaff. Deoxynivalenol (DON) was the most frequently detected toxin in the samples and its accumulation was most strongly associated with the presence of Fusarium graminearum. Nivalenol (NIV) accumulation was significantly associated only with the presence of F. culmorum. Zearalenone (ZON) accumulation was strongly associated with the presence of all three pathogens (F. graminearum, F. culmorum and F. poae). The levels of both DON and ZON concentrations were positively related to the amount of F. graminearum DNA in the grain or in the chaff. The presence/absence of FHB pathogens within a single quadrat appeared to be independent of each other. The presence of a particular FHB pathogen and the amount of its DNA, as well as the associated mycotoxin(s), varied greatly among samples at each site. This study demonstrated the large extent of within-field variability of FHB and its associated mycotoxins, and the importance of representative sampling in FHB studies.     

Virulence and toxin synthesis of an azole insensitive Fusarium culmorum strain in wheat cultivars with different levels of resistance to fusarium head blight

Fusarium culmorum causes head blight, produces toxins and reduces yield and quality of cereals. To prevent damage caused by fusarium head blight (FHB), azole fungicides are mainly applied. The occurrence of insensitivity to azoles is a major problem in agriculture. The present study shows that a tebuconazole insensitive strain of F. culmorum can be readily produced in the laboratory, but that the resulting strain of the fungus is of lower fitness in vitro. Insensitivity was confirmed microscopically and by cell viability and metabolic activity. The tebuconazole insensitive strain shows cross insensitivity to nine important azoles. In addition, plants inoculated with the insensitive F. culmorum strain showed no reduction of FHB symptoms and deoxynivalenol (DON) content after tebuconazole treatment, compared to an inoculation with the sensitive strain. Use of wheat cultivars carrying a high resistance level (i.e. cv. Toras) was the most effective method for reducing symptoms and decreasing DON content, independent from the level of fungicide insensitivity of the F. culmorum strain. In conclusion, resistant cultivars and a fungicide mixture which combines different mechanisms of action in fungal metabolism should be applied to avoid fungicide insensitivity of Fusarium spp. in future.     

Fusarium graminearum and Fusarium pseudograminearum caused the 2010 head blight epidemics in Australia

Wheat crops in southeast Queensland (Qld) and northern New South Wales (NSW) were infected with fusarium head blight (FHB)‐like symptoms during the 2010–11 wheat growing season. Wheat crops in this region were surveyed at soft dough or early maturity stage to determine the distribution, severity, aetiology and toxigenicity of FHB. FHB was widespread on bread wheat and durum, and Fusarium graminearum and/or F. pseudograminearum were diagnosed from 42 of the 44 sites using species‐specific PCR primers directly on spikelets or from monoconidial cultures obtained from spikelets. Stem base browning due to crown rot (CR) was also evident in some samples from both states. The overall FHB and CR severity was higher for NSW than Qld. Deoxynivalenol (DON) concentration of immature grains was more than 1 mg kg^?1 in samples from 11 Qld and 14 NSW sites, but only 13 of 498 mature grain samples sourced from the affected areas had more than 1 mg kg^?1 DON. DON concentration in straw also exceeded 1 mg kg^?1 in eight Qld and all but one NSW sites but this was not linked to DON concentration of immature grains. The proportion of spikelets with positive diagnosis for F. graminearum and/or F. pseudograminearum and weather‐related factors influenced DON levels in immature grains. The average monthly rainfall for August–November during crop anthesis and maturation exceeded the long‐term monthly average by 10–150%. Weather played a critical role in FHB epidemics for Qld sites but this was not apparent for the NSW sites, as weather was generally favourable at all sites.     

Comparison of pathogenicity of the Fusarium crown rot (FCR) complex (<Emphasis Type="Italic">F. culmorum</Emphasis>, <Emphasis Type="Italic">F. pseudograminearum</Emphasis> and <Emphasis Type="Italic">F. graminearum)</Emphasis> on hard red spring and durum wheat

Fusarium species involved in the Fusarium crown rot (FCR) complex affect wheat in every stage of development from seedling to grain fill. This study was designed to compare the aggressiveness of the FCR complex members including F. culmorum, F. pseudograminearum and F. graminearum in causing seedling blight, decreased plant vigour and crown rot. To assess their relative pathogenicity, two hard red spring wheat cultivars and two durum wheat cultivars were inoculated in the field with five isolates from each of the three species for two years. Significant differences in patterns of pathogenicity were identified. In particular, F. culmorum caused greater seedling blight while F. pseudograminearum and F. graminearum caused greater crown rot. Greatest yield reductions were caused by F. pseudograminearum. Cultivar differences were identified with respect to seedling disease and late season crown rot. No interactions were identified between cultivar performance and isolates or species with which they were challenged.     

Fungi on roots and stem bases of asparagus in the Netherlands: species and pathogenicity

A survey was made to identify the most important soilborne fungal pathogens of asparagus crops in the Netherlands. Ten plants were selected from each of five fields with a young (1–4 y) first planting, five fields with an old (6–13 y) first planting and five fields with a young replanting. The analysis included fungi present in the stem base and the roots of plants with symptoms of foot and root rot or showing growth decline without specific disease symptoms. Isolates of each species were tested for pathogenicity to asparagus on aseptically grown plantlets on Knop's agar. Symptoms were caused byFusarium oxysporum, F. culmorum, Botrytis cinerea, Penicillium verrucosum var.cyclopium, Cylindrocarpon didymum, Phialophora malorum, Phoma terrestris andAcremonium strictum. F. oxysporum was by far the most common species and was isolated from 80% of the plants. Not all of its isolates were pathogenic to asparagus. Symptoms were caused by 67%, 78% and 93% of the isolates obtained from young first plantings, old first plantings and replantings, respectively.F. culmorum was isolated from 31% of the plants. Two other notorious pathogens of asparagus,F. moniliforme andF. proliferatum, did not occur in our samples.Species causing symptoms in the vitro test that were found on more than 5% of the plants were additionally tested for their pathogenicity in pot experiments.F. oxysporum f.sp.asparagi caused severe foot and root rot, significantly reduced root weights and killed most of the plants.F. culmorum caused lesions on the stem base often resulting in death of the plant.P. terrestris, a fungus only once reported as a pathogen of asparagus, caused an extensive root rot, mainly of secondary roots that became reddish. The fungus was isolated in only a few samples and is not to be regarded as an important pathogen in Dutch asparagus crops.P. malorum caused many small brown lesions on the stem base and incidentally also on the upper part of small main roots. This is the first report of its pathogenicity to asparagus. The fungus is one of the organisms inciting spear rust and it reduced crop quality rather than crop yield.P. verrucosum var.cyclopium andC. didymum did not cause symptoms in pot experiments.Because of its predominance on plants with foot and root rot and its high virulence,F. oxysporum f.sp.asparagi was considered to be the main soilborne pathogen of asparagus in the Netherlands.     

Effect of Dose Rate of Azoxystrobin and Metconazole on the Development of Fusarium Head Blight and the Accumulation of Deoxynivalenol (DON) in Wheat Grain

Glasshouse studies were undertaken to determine if fungicides used for the control of Fusarium head blight (FHB) result in elevated concentrations of the trichothecene mycotoxin, deoxynivalenol (DON) in harvested wheat grain. Metconazole and azoxystrobin, at double, full, half or quarter the manufacturer's recommended dose rate, were applied to ears of wheat (cv. Cadenza), artificially inoculated with conidia of either Fusarium culmorum or F. graminearum. Metconazole demonstrated high activity against both pathogens, reducing significantly the severity of FHB and the DON concentrations at each of the four dose rates tested when compared to untreated controls. Applications of azoxystrobin significantly reduced FHB and DON compared to unsprayed controls. However, their effectiveness was significantly less than that of metconazole and no dose rate response was observed. Quantification of the amount of trichothecene-producing Fusarium present in harvested grain was determined using a competitive PCR assay based on primers derived from the trichodiene synthase gene (Tri5). Simple linear regression analyses revealed strong relationships between the amount of trichothecene-producing Fusarium present in grain and the DON concentrations (r ²=0.72–0.97). It is concluded that fungicides, applied for the control of FHB, affect DON concentrations indirectly by influencing the amount of trichothecene-producing Fusarium species present in wheat grain. There was no evidence that fungicide applications directly increase the concentration of DON in grain.     

Differential Control of Head Blight Pathogens of Wheat by Fungicides and Consequences for Mycotoxin Contamination of Grain

Fusarium head blight of wheat is caused by a disease complex comprised of toxigenic pathogens, predominantly Fusarium spp., and a non-toxigenic pathogen Microdochium nivale, which causes symptoms visually indistinguishable from Fusarium and is often included as a causal agent of Fusarium head blight. Four field trials are reported here, including both naturally and artificially inoculated trials in which the effect of fungicide treatments were noted on colonisation by Fusarium and Microdochium, and on the production of deoxynivalenol (DON) mycotoxin. The pathogen populations were analysed with quantitative PCR and samples were tested for the presence of the mycotoxin DON. Application of fungicides to reduce Fusarium head blight gave a differential control of these fungi. Tebuconazole selectively controlled F. culmorum and F. avenaceum and reduced levels of DON, but showed little control of M. nivale. Application of azoxystrobin, however, selectively controlled M. nivale and allowed greater colonisation by toxigenic Fusarium species. This treatment also lead to increased levels of DON detected. nobreak Azoxystrobin application two days post-inoculation increased the production of DON mycotoxin per unit of pathogen in an artificially inoculated field trial. This result indicates the potential risk of increased DON contamination of grain following treatment with azoxystrobin to control head blight in susceptible wheat cultivars. This is the first study to show differential fungicidal control of mixed natural pathogen populations and artificial inoculations in field trials.     

Resistance in winter wheat lines to deoxynivalenol applied into florets at flowering stage and tolerance to phytotoxic effects on yield

In Europe and North America, deoxynivalenol (DON) is the most prevalent mycotoxin associated with wheat head blight caused by Fusarium graminearum and Fusarium culmorum. Because DON is toxic to plants and enhances the ability of the pathogen to spread within a spike, wheat lines with resistance to DON should be more resistant to head blight. Resistance to DON has been associated with resistance gene Fhb1 that confers resistance to spread within a spike. The objectives of this study were to determine if wheat lines resistant to head blight were also resistant to DON, if genes other than Fhb1 confer resistance to DON, and to identify lines able to fill grain in the presence of DON. Susceptible controls and diverse North American and European winter wheat lines with resistance to head blight were screened for molecular markers linked to known head blight resistance genes, and evaluated in a greenhouse for resistance to DON and relative yield after application of DON to spikes at flowering. Fhb1 appeared to have the unique ability to confer resistance to DON, as measured by the number of DON‐bleached primary florets. However, this resistance did not protect plants from the phytotoxic effects of DON on kernel formation as measured by the relative yield of treated spikes. Furthermore, measuring the relative yield loss following DON application may be useful for identifying lines with tolerance to head blight.     

Epidemiology of root rot caused by <Emphasis Type="Italic">Leptosphaeria maculans</Emphasis> in <Emphasis Type="Italic">Brassica napus</Emphasis> crops

The infection of above-ground tissues of Brassica napus by Leptosphaeria maculans is well understood. However, root infection (root rot) under field conditions, the development of root rot over time and its relationship to other disease symptoms caused by L. maculans has not been described. A survey of B. napus crops was conducted in Australia to investigate the incidence and severity of root rot. Additionally, the pathway of root infection was examined in field experiments. Root rot was present in 95% of the 127 crops surveyed. The severity and incidence of root rot was significantly correlated with that of crown canker; however, the strength of this relationship was dependent on the season. Root rot symptoms appeared before flowering and increased in severity during flowering and at maturity, a pattern similar to crown canker suggesting that the infection of the root is an extension of the crown canker phase of the L. maculans lifecycle. All isolates of L. maculans tested in glasshouse experiments caused root rot and crown canker in B. napus and Brassica juncea. In the field, the main pathway of root infection is via invasion of cotyledons or leaves by airborne ascospores, rather than from inoculum in the soil. Root rot was present in crops in fields that had never been sown to B. napus previously, in plants grown in fumigated fields, and in glasshouse-grown plants inoculated in the hypocotyl with L. maculans.     

Head-blighting populations of Fusarium culmorum from Germany, Russia, and Syria analyzed by microsatellite markers show a recombining structure

Fusarium culmorum is a haploid, worldwide occurring phytopathogenic fungus causing seedling blight, foot rot, and head blight of cereals and producing the mycotoxins deoxynivalenol (DON) and nivalenol (NIV) associated with health hazards in human and animals. The fungus reproduces asexually by conidiospores, a teleomorph is not known. We analyzed for the first time naturally occurring F. culmorum populations collected randomly in the field from infected wheat heads. A total of 186 isolates, from three populations from Germany (GER), Russia (RUS), Syria (SYR), as well as an international collection (INT) for comparison, were genotyped by 10 microsatellite (SSR, single sequence repeat) markers. A high genetic diversity within the three natural populations and the INT population as well was detected. About 90 % of multi-locus haplotypes (MLH) were unique across populations. The largest part of variance (81 %) was found within populations. Accordingly, population subdivision was low, fixation indices were significant only in one out of six comparisons, while estimates of gene flow (N _m ) ranged from 0.8–4.8. Linkage equilibrium was revealed by the index of multi-locus association and the quotient of observed and expected variance when two linked markers were deleted. DON and NIV chemotypes grouped closely together in a principle coordinate analysis. SYR isolates were partly separated from GER and RUS populations. All population-genetic parameters were in a similar range compared to those for the sexually propagating species F. graminearum. In conclusion, results support the hypothesis of a recombining structure in F. culmorum as revealed by the high genetic variation within populations, a low fixation index and low gametic phase disequilibrium within populations.     

Untersuchungen zur systemischen Infektion von Fusarium culmorum an Winterweizen in Schleswig-Holstein

In north-west Germany Fusarium head blight of wheat (FHB) is frequently caused by Fusarium culmorum and F. graminearum. This damage has, however, mainly been caused by infections during flowering; the incidence of FHB has increased steadily for many years in crop rotations without maize. The authors postulate that F. culmorum can cause systemic infections in wheat. In 2003 they used real-time PCR to analyse more than 1?100 tissue samples from wheat fields in order to determine whether F. culmorum was present in the internodes and spikes of the wheat plants. More than 30% of all analysed samples contained F. culmorum-DNA, and many wheat stems were completely or nearly completely systemically colonized by F. culmorum. These findings are consistent with those of other researchers who have detected a systemic growth of F. culmorum during experiments under controlled conditions. It cannot be automatically assumed that the fungus spreads from the wheat head during flowering; the plant could have become infected through chlamydospores or mycelium on residual plant debris in the soil. Therefore this pathogen must be regarded as a crop rotation disease. F. culmorum is able to severely infect wheat through the roots and stem bases, especially when the soil is dry and warm. Thus, spraying fungicides during flowering is ineffective.     

First report on the flower-rot of Crossandra infundibuliformis, a commercial flower crop in India

Crossandra infundibuliformis (L.) Nees is an important commercial plant in many cities in India, which is useful for bouquets, garlands, and decorations. The crop is affected by a new flower rot caused by Fusarium pallidoroseum (Cooke) Sacc. Severely infected plants had purplish leaves, decayed flowers and shortened internodes and were colonized by the cottony colonies of the fungus. The fungus was isolated and caused the same symptoms on plants after inoculation.     

Bacterial inflorescence rot of grapevine caused by Pseudomonas syringae pv. syringae

Molecular sequencing (rpoB) and standard pathological and microbiological methods identified Pseudomonas syringae pv. syringae (Pss) as the causal agent of bacterial inflorescence rot of grapevines (Vitis vinifera) in three vineyards in Tumbarumba, NSW, Australia in 2006 and 2007. Pss strains from shrivelled berries and necrotic inflorescences of diseased grapevines were used to inoculate leaves and inflorescences of potted cv. Semillon grapevines. Pss caused disease symptoms similar to those experienced in the field, including angular leaf lesions, longitudinal lesions in shoot tissues and rotting of inflorescences from before flowering until shortly after fruit set. High humidity promoted symptom severity. The necrotic bunch stem and leaf lesions were susceptible to the development of Botrytis cinerea infections. Cryo‐scanning electron microscopy (cryoSEM) indicated that Pss entered leaves and inflorescence tissues via distorted, open, raised stomata surrounded by folds of tissue that appeared as ‘star‐shaped’ callose‐rich complexes when viewed by UV light microscopy. In necrotic tissues, cryoSEM revealed Pss within petiole parenchyma cells and air‐filled rachis xylem vessels. This is the first report of inflorescence and hence fruit loss caused by Pss in grapevines. The disease is described as ‘bacterial inflorescence rot’ and regarded as one that expands the previously reported pathology of grapevines caused by P. syringae. This study also indicated that infection by Pss might promote destructive B. cinerea infections when the fungus is already present but latent, although further experimentation is needed to prove such an interaction.     

Relationship Between Growth and Mycotoxin Production by Fusarium species, Biocides and Environment

Fusarium head blight of cereals has, in recent years, become one of the most important pre-harvest diseases worldwide. This paper examines the in vitro efficacy of fungicides to control Fusarium species in cereals and the efficacy in the field on both Fusarium infection of ripening ears as well as their impact on mycotoxin production. Field studies suggest that fungicides such as tebuconazole and metconazole give good control of both Fusarium infection of ears and control of deoxynivalenol (DON) production. However, azoxystrobin and related fungicides are less effective, and grain from treated crops has sometimes been found to have increased concentrations of DON and nivalenol. Studies of isolates of Fusarium culmorum from different parts of Europe showed that complex interactions occur between environmental factors, fungicide type and isolate in relation to growth inhibition and DON production. These studies confirmed the ineffectiveness of azoxystrobin and suggest that environmental stress factors, particularly water availability and temperature, and low fungicide doses may stimulate mycotoxin production by Fusaria in vitro and in wheat grain.     

Infection and colonization of strawberry by <Emphasis Type="Italic">Gnomonia fragariae</Emphasis> strain expressing green fluorescent protein

Gnomonia fragariae is a poorly studied ascomycete, which was recently demonstrated to be a cause of severe root rot and petiole blight of strawberry. The pathogen was genetically transformed with the GFP as a vital marker and hygromycin resistance gene. Several stable transformants were obtained, which did not differ in their phenotype from the wild type isolate. Using one of the GFP-tagged isolates the infection process and colonization of roots and petioles of host plant by the pathogen were studied. Fluorescence microscopy examinations of the inoculated plants at different time points showed that plant infection occurs 24 h after inoculation and intensively continues during first 3 days. The specific penetration sites on epidermal cells and preferences in colonization for certain root and petiole tissues were observed. The pathogen intensively colonized and destroyed cortex of roots and petioles and spread rapidly longitudinally within intercellular spaces. The petioles were colonized by the hyphae, which grew mostly in the intracellular spaces of the cortical cells while in the roots the intracellular growth of hyphae occurred only in the later stages of infection. The fungus was also capable to infect the vascular tissues of petioles although these were not the primary tissues colonized by the pathogen. The mature ascomata were formed on the infected petiole bases several weeks after the inoculation. This study presents a genetic transformation method for Gnomonia fragariae and it demonstrates details on infection process and colonization of root, crown and petiole tissues of strawberry by the pathogen.     

Gnomonia fragariae, a Cause of Strawberry Root Rot and Petiole Blight

Gnomonia fragariae has been occasionally listed among the fungi associated with diseased strawberry plants. However its pathogenicity has not been established. During the investigation on strawberry decline in Latvia and Sweden, a fungus was repeatedly recovered from discoloured root and crown tissues of severely stunted plants. Attempts to induce sporulation of the isolates grown on several agar media resulted in the formation of mature ascomata only on potato carrot agar and oatmeal agar. On morphological grounds and comparisons with reference herbarium specimens these isolates were identified as Gnomonia fragariae. The pathogenicity of the fungus was evaluated initially in the detached leaf assay and subsequently in three bioassays on strawberry plants. All the bioassays showed that G. fragariae was pathogenic on strawberry and capable of causing severe root rot and petiole blight. The symptoms that developed in the greenhouse experiments closely resembled those observed in the fields. The fungus did not cause rapid plant death but growth and development of inoculated strawberry plants was severely affected. To our knowledge this is the first time when pathogenicity of G. fragariae as a root rot pathogen has been clearly established. Our study shows that G. fragariae is one of the serious pathogens involved in the root rot complex of strawberry in Latvia and Sweden.