Agent Of Powdery Mildew Research Articles

First Report of Powdery Mildew Caused by Erysiphe (Golovinomyces) orontii on Veronica spicata in Italy.

Veronica spicata (spike speedwell) is a perennial garden species belonging to the family Scrophulariaceae. During the summer through fall of 2004 and 2005, severe outbreaks of a previously unknown powdery mildew were observed in several gardens near Biella (northern Italy). Upper surfaces of leaves were covered with a white mycelium and conidia, and as the disease progressed, infected leaves turned yellow and died. Very rarely was the mycelium observed on the lower surface of leaves or on petioles and flowers. Foot cell was cylindric and measured 19.2 to 25.7 × 10.8 to 14.3 μm (average 21.9 × 12.0 μm). Conidia were hyaline, ellipsoid, brought in short chains (three conidia per chain), and measured 22.2 to 40.8 × 13.6 to 21.6 μm (average 30.1 × 17.0 μm). Conidiophores measured 45.5 to 74.0 × 10.4 to 11.0 μm (average 59.4 × 10.6 μm). Fibrosin bodies were absent. Cleistothecia were never observed on the samples collected. The ITS region (internal transcribed spacer) of rDNA was amplified using the primers ITS4/ITS6 (3) and sequenced. BLASTn analysis (1) of the 504 bp obtained showed an E-value of 0.0 with Erysiphe (Golovinomyces) orontii (2). The nucleotide sequence has been assigned GenBank Accession No. DQ386696. Pathogenicity was confirmed by gently pressing diseased leaves onto leaves of five healthy Veronica spicata plants. Five noninoculated plants served as controls. Inoculated and noninoculated plants were maintained in a greenhouse where temperatures ranged between 15 and 28°C. After 15 days, typical powdery mildew symptoms developed on inoculated plants. Noninoculated plants did not show symptoms. The pathogenicity test was carried out twice. To our knowledge, this is the first report of the presence of powdery mildew on V. spicata in Italy. Sphaerotheca fuliginea has been reported as the causal agent of powdery mildew on V. spicata (4). Specimens of this disease are available at DIVAPRA Collection, University of Torino.

Development of Erysiphe pulchra, the causal agent of powdery mildew, on leaf disks of susceptible and resistant flowering dogwood (Cornus florida)

Understanding the relative contribution of the different resistance components is necessary to develop selection schemes and accelerate resistant-cultivar development. This study was conducted to investigate spore germination, infection-structure formation, and fungal development of Erysiphe pulchra, the causal agent of powdery mildew, on leaf disks of six cultivars or lines of flowering dogwood (Cornus florida) with different levels of resistance. The cultivars and lines tested were grouped into the following three resistance categories: highly susceptible (‘Cherokee Daybreak’ and MW 94-60), moderately susceptible (‘Cherokee Princess’ and MW 95-25), and resistant (‘Cherokee Brave’ and ‘Karen’s Appalachian Blush’). Percentages of spore germination and secondary-appressoria formation were not significantly different among the cultivars and lines. Significantly less percent germinated conidia with branched hyphae were observed on resistant cultivars than on the moderately susceptible cultivar or line, which was less than on the highly susceptible cultivar or line. Infection efficiencies were significantly different among cultivars and lines in the three resistance categories, except that there were no differences between ‘Cherokee Princess’ and the resistant cultivars. Resistant cultivars supported shorter latent periods than moderately and highly susceptible cultivars or lines, but no differences in latent period were detected between the later two resistance categories. The recently released ‘Karen’s Appalachian Blush’ expressed higher levels of resistance to powdery mildew than did ‘Cherokee Brave’, as indicated by the longer latent period and reduced relative sporulation of the pathogen.

Genetic Variation and Population Structure of the Grape Powdery Mildew Fungus, Erysiphe necator, in Southern France

Erysiphe necator, the causative agent of powdery mildew in grapevine, was introduced into Europe from North America during the middle of the 19th century. Our objective was to analyze the genetic variation and the population structure of the fungus in southern France. The sample comprised 101 isolates and was mainly of flag shoot origin, i.e., infection of sprouting shoots after overwintering of mycelium in buds. RAPD analysis identified different haplotypes that clustered in two genetic groups (A and B). The most frequent haplotypes of each group were found in several different locations in two areas separated by 100 km and throughout the 3 year period. Several haplotypes of both groups originated from flag shoots and were recovered over successive years indicating that there is no correlation between genetic group and overwintering mode. All isolates of group A were of mating type +, but those in group B could be either + or −. Lower genotypic diversity was detected within group A than within group B. These results were consistent with the hypothesis that group A reproduces only asexually.

Effects of Timing of Powdery Mildew Infection on Carbon Assimilation and Subsequent Seasonal Growth of Potted Chardonnay Grapevines

Potted Chardonnay (<i>Vitis vinifera</i> L.) grapevines were inoculated with conidial suspensions of <i>Uncinula necator</i> (Schw.) Burr., the causal agent of powdery mildew of grape, just prior to bloom (early), just after the estimated 5-mm berry stage (late), or they were not inoculated but were treated with myclobutanil fungicide to maintain foliage free of the disease (control). The effects of the timing of initial infection on net carbon assimilation (<i>A</i>), carbon partitioning, stomatal conductance (g_s), internal CO₂ concentration (C_<i>i</i>), and transpiration (<i>E</i>) were studied over two seasons. Early vines had reduced <i>A</i> relative to late and control vines until the estimated preveraison period, when late vines also showed reduced <i>A</i>. Early vines had reduced fresh and dry weights compared to other treatments. Leaves on both early and late vines senesced earlier than those on control vines. During the following growing season, shoot lengths and fresh and dry weights were negatively correlated with the length of the disease development period in the previous growing season.

UPDATE ON POWDERY MILDEW RESISTANCE SCREENING IN WATERMELON

Powdery mildew has been reported on Citrullus lanatus in Africa and Europe for the past 9 years, and in the United States for the past 6 years. During this time, it has occurred in the main watermelon production areas in the U.S. and has been documented in nine states (South Carolina, Georgia, Florida, Oklahoma, Texas, Maryland, New York, Arizona, and California). This is of great concern to the watermelon industry since powdery mildew is difficult to control and can have a severe impact on yield and fruit quality due to loss of photosynthetic area and sunscald. Finding resistant C. lanatus germplasm is needed for the development of commercial varieties containing this resistance. This report summarized the status of an ongoing project to screen the entire USDA–ARS C. lanatus germplasm collection. Currently, the collection is being screened for race 1 and race 2 Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.), the causal agent of powdery mildew in C. lanatus in the United States. Resistance genes appear to exist for both races and the genes conferring resistance to race 1 appear to be different than race 2 resistance genes. Allelism tests are currently in process to determine the number of resistance genes present.

UPDATE ON POWDERY MILDEW RESISTANCE SCREENING IN WATERMELON

Powdery mildew has been reported on Citrullus lanatus in Africa and Europe for the past nine years, and in the United States for the past 6 years. During this time, it has occurred in the main watermelon production areas in the U.S. and has been documented in nine states (South Carolina, Georgia, Florida, Oklahoma, Texas, Maryland, New York, Arizona, and California). This is of great concern to the watermelon industry since powdery mildew is difficult to control and can have a severe impact on yield and fruit quality due to loss of photosynthetic area and sunscald. Finding resistant C. lanatus germplasm is needed for the development of commercial varieties containing this resistance. This report summarized the status of an ongoing project to screen the entire USDA–ARS C. lanatus germplasm collection. Currently, the collection is being screened for race 1 and race 2 Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.), the causal agent of powdery mildew in C. lanatus in the United States. Resistance genes appear to exist for both races and the genes conferring resistance to race 1 appear to be different than race 2 resistance genes. Allelism tests are currently in process to determine the number of resistance genes present.

First Report of Powdery Mildew Caused by an Oidium sp. on Spiraea japonica in Italy.

Spiraea japonica is an old-fashioned shrub widely grown in Italy in parks and gardens. In the summer of 2003, severe outbreaks of a previously unknown powdery mildew were observed in some parks and gardens in the city of Torino (northern Italy). Infected leaves became covered on both sides with a white mycelium. As the disease progressed, infected leaves turned reddish and eventually dropped prematurely. The presence of powdery mildew infections on leaves sometimes caused their distortion and growth reduction. Frequently, mycelium was observed also on the stem. Conidia were hyaline, ellipsoid, cylindrical, or dol iform, born in chains, measured 21.0 to 38.4 × 10.8 to 14.4 mm, and showed fibrosin bodies. Cleistothecia were not observed. The pathogen was identified as Oidium subgenus Fibroidium (1,2). Pathogenicity was confirmed by spraying leaves of healthy potted S. japonica plants with a conidial suspension (105 conidia per ml) prepared in sterile water from diseased leaves. Three plants were inoculated and three noninoculated plants served as control. The artificial inoculation was carried out twice. After artificial inoculation, plants were maintained in a growth chamber at 25°C. After 20 days, powdery mildew symptoms developed. Microsphaera alni and Podosphaera oxyacanthae were described as causal agents of powdery mildew on S. japonica in the United States (3), while Sphaerotheca spiraeae was considered the causal agent of a powdery mildew observed in Japan (4) and more recently in Poland.

Levantamento preliminar de raças de Sphaerotheca fuliginea no Brasil

Sphaerotheca fuliginea é o agente causal do oídio, uma das principais doenças do meloeiro (Cucumis melo) e de outras cucurbitáceas no Brasil. O controle é feito principalmente pelo uso de fungicidas, mas o uso de cultivares resistentes pode ser uma opção dentro de um programa de manejo da doença. Este fungo apresenta especificidade fisiológica em melão, sendo a raça 1aparentemente a predominante no Brasil e no mundo. O conhecimento das raças prevalecentes em determinada região é de vital importância para os melhoristas. Neste trabalho, 31 isolados foram obtidos de diversas espécies de cucurbitáceas, provenientes de diferentes estados do Brasil. Todos os isolados foram identificados como S. fuliginea. Os isolados foram multiplicados em casa-de-vegetação e avaliados quanto à virulência em uma série de cultivares diferenciadoras de melão, uma cultivar de melancia, uma de pepino e uma linhagem de abóbora. Vinte e um dos 31 isolados foram identificados como raça 1 e oito como raça 2. Os dois isolados obtidos de melancia mostraram-se específicos à mesma pois só infectaram a cultivar desta espécie. Os resultados confirmam que a raça 1 é a predominante no Brasil. A raça 2 também está presente no DF, apesar de parecer estar adaptada apenas às condições de cultivo protegido, pois os oito isolados desta raça foram obtidos de melão, pepino e abóbora cultivados em estufa ou casa-de-vegetação. Este é o primeiro relato da raça 2 de S. fuliginea no Distrito Federal.

First Report of Powdery Mildew Caused by Oidium Subgenus Pseudoidium on Salvia scabra in Italy.

Salvia scabra Thunb. is grown and used on the Italian Riviera as a potted plant and used in gardens. During the spring of 2003, severe outbreaks of a previously unknown powdery mildew were observed in a collection of Salvia spp. maintained at an experimental station at Albenga (northern Italy). Powdery mildew was observed only on S. scabra. Leaves were covered with white exophytic mycelia on both surfaces. As the disease progressed, infected leaves turned yellow and died. Conidia were single, hyaline, cylindric, and measured 21.3 to 35.5 × 12.5 to 22.5 μm (average 28.2 × 18.4 μm). Observations made with a light microscope revealed that foot cells were cylindric and appressoria lobed. Cleistothecia were not observed. The pathogen was identified as Oidium subgenus Pseudoidium (1,2), and pathogenicity was confirmed by gently pressing diseased leaves onto mature leaves of healthy, 40-day-old S. scabra plants. Five plants of S. scabra were used as replicates. Noninoculated plants served as controls. Inoculated and noninoculated plants were maintained in a growth chamber at 20°C. After 5 days, typical symptoms of powdery mildew developed on inoculated plants. Noninoculated plants did not show symptoms. To our knowledge, this is the first report of the presence of powdery mildew on S. scabra in Italy as well as in the world. Erysiphe polygoni DC. (Oidium subgenus Pseudoidium) and E. cichoracearum DC. (Oidium subgen us Reticuloidium) were previously reported as causal agents of powdery mildew on other species of Salvia (S. officinalis and S. sclarea) (3,4). Specimens of this disease are available at the DIVAPRA Collection at the University of Torino.

First Report of Powdery Mildew Caused by an Oidium sp. on Papaver nudicaule in Italy.

Iceland poppy (Papaver nudicaule L.) is grown on the Italian Riviera for export as a cut flower and its importance in the industry is increasing. During the spring of 2003, severe outbreaks of powdery mildew that had not been reported previously, occurred in several commercial plantings grown outdoors in containers near Imperia in northern Italy. Mycelium was observed on all green organs of the plant (leaves, stems, and petioles). Both surfaces on the leaves were affected, and heavily colonized leaves were distorted. As the disease progressed, diseased leaves turned yellow and died. The disease did not affect the flowers. Conidia were hyaline, cylindric, and measured 10.8 to 29.8 × 16.8 to 48.0 μm (average 13.7 × 37.3 μm). Foot cells were cylindric and appressoria unlobed. Fibrosin bodies were not present, and cleistothecia were not observed. The pathogen was identified as Oidium sp. subgenus Pseudoidium (1,2). Pathogenicity was confirmed by gently pressing leaves with visible sporulation onto the leaves of five 90-day-old P. nudicaule plants that were healthy and free of symptoms. Five noninoculated healthy plants served as controls. Inoculated and noninoculated plants were maintained in a growth chamber at 15°C with a 12-h photoperiod. After 7 to 10 days, typical symptoms of powdery mildew developed on inoculated plants but not on noninoculated plants. Previously, Erysiphe cruciferarum Opiz ex L. Junell has been reported on P. nudicaule in all continents, and E. cichoracearum DC. and E. polygoni DC. have been reported in several countries, including Italy, as a causal agent of powdery mildew on other species of Papaver including P. rhoeas and P. strigosum. To our knowledge, this is the first report of powdery mildew caused by an Oidium sp. on P. nudicaule in Italy. The voucher specimen is kept in the department collection at the University of Torino.

Monitoring physiological races ofPodosphaera xanthii (syn.Sphaerotheca fuliginea), the causal agent of powdery mildew in cucurbits: Factors affecting race identification and the importance for research and commerce

Identification of the physiological races ofPodosphaera xanthii (syn.Sphaerotheca fuliginea), the causal agent of powdery mildew in cucurbits, is based upon the differing responses of various melon cultigens to the pathogen. Eight races of the pathogen have been identified to date in the USA, Africa, Europe and around the Mediterranean Sea, and four new races were reported from greenhouse melons in the major growing area of Japan. Plant responses to powdery mildew may be affected by environmental factors such as light intensity, temperature and humidity, as well as by age and nutritional status of the plants. The same factors affect the accuracy and reliability of race identification. In an attempt to overcome those obstacles, the genetic diversity ofP. xanthii was studied using molecular markers. Unfortunately, no correlation was found between DNA polymorphism and the race of the pathogen as identified by biological tests. The usefulness of race identification as a guide for the grower in selecting appropriate cultivars is limited because changes or shifts in the pathogen population are common. Such changes may be found among growing seasons, geographic regions and hosts, and also within a single greenhouse during a single season. On the other hand, race identification is important for basic research and is especially important for the commercial seed industry, which requires accuracy in declaring the type and level of resistance to powdery mildew in its products.

ALTERNATIVE CONTROL TO POWDERY MILDEW (ERYSIPHE POLIGONI D. C. SIN) ON MANGO IN SUBMEDIO SÃO FRANCISCO RIVER IN BRAZILIAN SEMI-ARID REGION

The experimental area was installed in a commercial orchard in Nilo Coelho, Petrolina, Brazil to evaluate the efficiency of chemical and biological agents in the control of Erysiphe polygoni D. C. Sin., causal agent of powdery mildew in mango 'Tommy Atkins' trees. The study was conducted in the second semester of 2001, under 22°C to 30°C, and 45 to 82 RH. Nine treatments were evaluated: seven chemical compounds, a biological agent formulation, and an absolute control. The formulation DE 975 + Fenarimol was applied at the beginning of flower development. Its efficiency was compared to a single application and 4 ten day-interval applications of an standard product (Folicur 200 CE - 100ml/100L of water. The results showed that the treatment - DE 975 + Fenarimol 50ml/150L of water, was the most efficient for the control of powdery mildew reaching 0% of infection in whole cycle of the culture.

OBSERVATIONS ON THE POPULATION BIOLOGY OF THE GRAPE POWDERY MILDEW FUNGUS UNCINULA NECATOR

Random Amplified Polymorphic DNA (RAPD) analysis and mating type distribution were used to investigate the population biology of the phytopathogenic fungus Uncinula necator (Schw.) Burr., the causal agent of powdery mildew of grapevine. It is known that the fungus has two overwintering strategies, mycelium and conidia in dormant buds or cleistothecia, but their relative importance in disease epidemiology is still undetermined. Recently, the existence has been hypothesized of two genetically separated biotypes in U. necator that would be related with its overwintering modes: a biotype overwintering as conidia and mycelium in buds would infect shoots and leaves early in the season; the other biotype would overwinter as cleistothecia and infect bunches. RAPD analysis was carried out on 374 isolates of U. necator collected in Southern Italy. Statistical analysis of variation clustered the isolates into two major groups according to the time in the season when they were sampled in vineyards, confirming the existence of two different biotypes into the fungal species. Similar proportions of the two mating types were found in the pathogen’s populations, as well as into each biotype, even in single vineyards. Therefore, the two mating types of the fungus are not separated either in space or time. Sexual crosses between isolates belonging to the two biotypes were fertile and yielded viable ascospore progeny. These results suggest that meiotic recombination can be an important source of genetic variation in U. necator and cleistothecia can play an important role in its overwintering.

Arthrocladiella - a genus of powdery mildew agents new for Serbia and Montenegro

Arthrocladiella Vassilkov, a fungal genus new for Serbia and Montenegro, is described and illustrated as the agent of powdery mildew on plants of the species Lycium halimifolium Mill. (common matrimony vine). In view of the fact that the genus Arthrocladiella is monotypic, taxonomic characteristics of the species Arthrocladiella mougeotii (Lev) Vassilkov in the anamorphic and teleomorphic stages of development are given in the present paper together with symptoms of the disease.

First Report of Powdery Mildew on Asclepias curassavica in Italy.

Asclepias curassavica is grown in Italy for landscape use in parks and gardens. In 2001, severe outbreaks of a previously unknown powdery mildew were observed in commercial farms located in Albenga (northern Italy). Powdery mildew developed on the lower surface of leaves of container grown plants. The upper surface of infected leaves turned reddish and sometimes yellow. Infected tissues wilted, and leaves dropped prematurely. Conidiophores emerged through the stomata. Conidia were hyaline and had variable shape, mitrate when located at the apex of the conidiophore, and subcylindric when borne in the middle of a chain. Conidia measured 46 to 74 × 12 to 24 μm. Cleistothecia were not observed. The causal agent was identified as Oidiopsis sp. Pathogenicity was confirmed by gently pressing diseased leaves on leaves of healthy A. curassavica plants. Plants were maintained in a growth chamber at 20 to 24°C after inoculation. After 12 to 14 days, powdery mildew signs developed. Leveillula taurica has been reported as causal agent of powdery mildew on A. carassavica in India and on Asclepias sp. in Morocco (1). Also in our case, L. taurica should be the causal agent, since the genus Leveillula has as anamorph the genus Oidiopsis (2). In Switzerland, Phyllactinia suffulta has been described as incitant of a similar powdery mildew on A. syriaca.

Identification of a microsatellite marker associated with Pm3 resistance alleles to powdery mildew in wheat

AbstractThe Pm3 resistance locus, located on chromosome 1A in wheat, confers race‐specific resistance to the obligate biotrophic fungus Blumeria graminis (DC) E.O. Speer f. sp. tritici, the causal agent of powdery mildew. Several Pm3 alleles are still effective in controlling the disease in Europe. A genetic map was constructed to map the Pm3g allele in the recombinant inbred line progeny from the cross ‘RE9001’ (susceptible) בCourtot’ (resistant). Two microsatellite markers were closely mapped to Pm3g. The PSP2999 marker, which cosegregates with this allele, was shown to detect the presence of the Pm3g resistance allele in other cultivars. A collection of 56 wheat cultivars or advanced lines carrying one Pm3 allele was used to assess the allele‐specific amplification of the PSP2999 marker. The same amplification pattern was obtained for lines with Pm3a, Pm3b, Pm3e, Pm3f and Pm3g alleles. Twenty genotypes carrying Pm3d showed a specific amplification pattern. This marker allowed the detection of the Pm3d allele in highly resistant lines whose resistance gene combinations were unknown. It was concluded that PSP2999 is a useful marker to detect Pm3 alleles in parents and to manage them in breeding programmes.

Chlorophyll-Deficient Mutant in Oak (Quercus petraea L.) Displays an Accelerated Hypersensitive-Like Cell Death and an Enhanced Resistance to Powdery Mildew Disease

Plants of the discovered chlorophyll-deficient mutant of oak (ML) display enhanced disease resistance to the fungus Erysiphe cichoracearum, causal agent of powdery mildew. Quantitative imaging of chlorophyll (Chl) fluorescence revealed that the net photosynthetic rate (PN) declined progressively in both untreated and invaded ML leaves as well as in inoculated wild-type (WT) leaves. Images of non-photochemical fluorescence quenching (NPQ) in both untreated and infected mutant leaves suggested that the capacity of Calvin cycle had been reduced and that there was a complex metabolic heterogeneity within the ML leaf. The ML mutant accumulates reactive oxygen species, ROS (H2O2) from the oxidative burst followed by spontaneous cell death that mimic the hypersensitive response. Reduction in pathogen sporulation on ML leaves correlated with the accumulation of soluble saccharides and a more rapid induction of defence responses including expression of some defence proteins (β-1,3-glucanase and chitinase). Unlike to WT plants, ML- conferred phenotype activates and/or de-represses multiple defence responses, making them more easily induced by pathogens.

Fungi of the genus Uncinula in Serbia

Investigations of fungi of the family Erysiphaceae were carried out between 1981 and 1999. Five species of the genus Uncinula Lév. were collected and studied: U. adunca (Wallr.: Fr.) Lév., U. clandestina (Biv.-Bern.) Schroet., U. necator (Schwein.) Burr., U. prunastri (DC.) Sacc., and U. kusanoi Syd. They were registered as agents of powdery mildew on 14 host plant species. U. adunca, U. clandestina and U. prunastri, as well as 12 host plants, are recorded from this region for the first time. Furthermore, the first record of U. kusanoi on Celtis australis from Europe is included. Taxonomic characteristics of the fungi concerned are presented, localities are recorded, and the frequency of occurrence is noted.

The Distribution of Early Season Grapevine Shoots Infected byUncinula necatorfrom Year to Year: A Case Study in Two California Vineyards

<i>Uncinula necator</i> (Schw.) Burr., the causal agent of powdery mildew on grapevines (<i>Vitis vinifera</i> L.), survives the winter in the San Joaquin Valley in California predominantly by perennation in buds. Infected shoots arising from these buds (flag shoots) are important inoculum sources early in the growing season. Studies were conducted in two San Joaquin Valley vineyard sites over a three-year period to determine whether infected shoots tended to occur on the same vines from year to year. The studies also had the goal to determine whether the occurrence of infected shoots was influenced by the location of spurs retained from shoots infected the previous year. The number of vines with infected shoots in two consecutive years was greater than expected for a random infection process. The incidence of infected shoots arising from spurs retained from shoots infected the previous year was significantly greater than expected for a random shoot infection process. However, most infected shoots arose on spurs that were not infected the previous year, and they were not clustered around spurs retained from shoots infected the previous year. Removal of spurs retained from shoots infected the previous season during dormancy, or removal of flag shoots early in the season as a control measure by itself will probably not sufficiently reduce flag shoots incidence the following years

Genetic differentiation in the French population of Erysiphe cichoracearum, a causal agent of powdery mildew of cucurbits

The relative incidence of Erysiphe cichoracearum and Sphaerotheca fuliginea, both agents of powdery mildew of cucurbits, was determined from 275 samples of mildewed leaves of cucurbits collected in 1994 from five regions of France. E. cichoracearum was identified in 9 to 39% of the mildewed leaf samples from four of the regions but was not detected in samples from the Mediterranean island of Corsica. The genetic structure of the French population of E. cichoracearum was examined using RFLPs of the ribosomal internal transcribed spacers amplified by PCR, random amplified polymorphic DNA (RAPD) markers, pathogenicity and mating‐type tests. Forty‐one isolates, including one from England, were analysed. Cluster analysis from 147 RAPD fragments using 16 primers revealed the existence of three distinct genetic lineages corresponding to three rDNA haplotypes (designated groups A, B and C). Bootstrap, genetic diversity, gametic disequilibrium and private allele analyses supported this differentiation. The genetic differentiation observed in the French population was not related to the geographical origin of the isolates. Group A isolates may be more specialized on melon as, with one exception, they were of race 1 (growth on four of the five melon cultivars tested) in comparison with group B and C isolates, which were of race 0 (growth on IranH only). Thus, the genetic differentiation observed may indicate a host‐specialized subdivision within the French population of E. cichoracearum from cucurbits. Gametic disequilibrium analysis among RAPD loci and biological observations suggest that the sexual stage is of minor importance for epidemics of E. cichoracearum on cucurbits.