The structure and temporal dynamics of the virulence of Pseudoperonospora cubensis (causal agent of cucurbit downy mildew) were studied in pathogen populations in the Czech Republic from 2001 to 2010. A total of 398 P. cubensis isolates collected from Cucumis (Cm.) sativus, Cm. melo, Cucurbita (Cr.) maxima, Cr. pepo, Cr. moschata and Citrullus lanatus were analysed for variation in virulence (pathotypes). Virulence was evaluated on a differential set of 12 genotypes of cucurbitaceous plants. All isolates of P. cubensis were characterized by their level of virulence (classified according the number of virulence factors, VF; low VF = 1-4, medium VF = 5-8, high VF = 9-12): high (75%), medium (24%) and low (1%). The structure and dynamics of virulence in the pathogen populations were expressed by pathotypes using tetrad numerical codes and a total of 67 different pathotypes of P. cubensis were determined. The most susceptible group of differentials was Cucumis spp., while the lowest frequency of virulence was recorded on Cr. pepo ssp. pepo, Ci. lanatus and Luffa cylindrica. A high proportion (c. 90%) of isolates was able to infect cucurbit species Benincasa hispida and Lagenaria siceraria, which are not commonly cultivated in the Czech Republic or elsewhere in central Europe. In the recent pathogen populations (2008)(2009)(2010) there was prevailing frequency (70-100%) of isolates with high numbers (9-12) of virulence factors. 'Super pathotype' 15.15.15 was often observed in the study within the pathogen populations and was one of the four most frequently recorded pathotypes. Pseudoperonospora cubensis populations shifted to a higher virulence over time. From 2009 the pathogen population changed dramatically and new pathotypes appeared able to establish natural and serious infection of Cucurbita spp. and Ci. lanatus, which was not observed in [2001][2002][2003][2004][2005][2006][2007][2008]. Generally, virulence structure and dynamics of P. cubensis populations are extremely variable in the Czech Republic.

Ales leš Lebeda ebeda, Jana ana Pavelkova avelková , Jirı iř í Urban rban and Bozena ožena Sedlakova edlá ková Abstract Cucurbit downy mildew, caused by Pseudoperonospora cubensis, is a major cucumber disease in the Czech Republic. Disease prevalence, host range and disease severity were evaluated from 2001 to 2009. The geographical distribution of P. cubensis was assessed on ca 80-100 locations per year in two main regions of the Czech Republic (central and southern Moravia, and eastern, northern and central Bohemia). Infection by P. cubensis was observed primarily on cucumber (Cucumis sativus) but only on the leaves. During the study, disease prevalence ranged from 66 to 100%. The majority of C. sativus crops were heavily infected at the end of the growing season (second half of August). Generally, P. cubensis was present at high or very high disease severity. The loss of foliage results in the reduction in the quality and quantity of marketable yield of fruit. Pseudoperonospora cubensis was widespread across the whole area of the Czech Republic studied. Very rarely, infection was recorded in muskmelon (Cucumis melo) and Cucurbita moschata. Of other pathogens, the most frequently recorded was the cucurbit powdery mildew (Golovinomyces cichoracearum and Podosphaera xanthii).

Pseudoperonospora cubensis (Berk. & M.A. Curtis) Rostovzev, the causal agent of cucurbit downy mildew, was observed for the first time on Cucurbita moschata Duchesne in the Czech Republic (CR) in August 2009 and repeatedly in September 2010. Recently, C. moschata has not been an economically important crop in the CR; however, related crops C. pepo and C. maxima have increased in importance. Infected plants with P. cubensis were found in two locations: in a hobby garden in north Moravia (Nový Jičín – Kojetín [49°33′48.088″N, 17°59′16.632″E], 2009 and 2010) and in a commercial field in central Moravia (Olomouc-Holice [49°34′31.95″N, 17°17′35.462″E], 2010). The pathogen caused small, angular, yellowish or pale green lesions on the upper leaf surfaces and produced sporangiophores and sporangia on the lower leaf surfaces. The lesions were delimited by leaf veins and later turned necrotic. Sporangiophores were hyaline, branched, and emerged in groups from stomata. Olive brown-to-dark brown sporangia were ellipsoidal to oblong. Our morphological observations confirmed that the pathogen was P. cubensis (2). No previous reports are available of P. cubensis on C. moschata in CR or anywhere in Central Europe. However, P. cubensis is common on C. moschata in some parts of Asia and the United States (1,2). P. cubensis exhibiting clear host specialization has been reported in different countries and geographic areas (2). A C. moschata isolate (PC 88/2009) originating from the naturally infected plants was inoculated (1 × 105 spores per ml and incubation temperature of 18/15°C during light/dark cycles) according to the methodology described by Lebeda and Urban (3) onto the abaxial surface of leaf discs of all genotypes of a differential set of cucurbits for P. cubensis pathotype determination (4). C. moschata (line Novo5, Nohel-Garden, CR) was added to this set. The isolate PC 88/2009 was highly pathogenic to all screened Cucurbita spp. genotypes (C. pepo, C. maxima, and C. moschata). However, no infection was detected on most of the Cucumis accessions; only Cucumis melo subsp. agrestis var. conomon was susceptible. Also, no infection was observed on other differentials (Citrullus, Benincasa, Luffa, and Lagenaria). The pathotype was classified as Pc 4/15/0. This pathotype had not been previously detected in CR. References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.arsgrin. gov/fungaldatabases/ . December 16, 2010. (2) A. Lebeda and Y. Cohen. Eur. J. Plant Pathol. 129:157, 2011. (3) A. Lebeda and J. Urban. Page 285 in: Mass Screening Techniques for Selecting Crops Resistant to Disease. M. M. Spencer and A. Lebeda, eds. International Atomic Energy Agency (IAEA), Vienna, Austria, 2010 (4) A. Lebeda and M. P. Widrlechner. J. Plant Dis. Protect. 110:337, 2003.

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