Downy mildew, caused by the oomycete pathogen Pseudoperonospora cubensis (Berk. & cucumber (Cucumis sativus L.) (Palti and Cohen, 1980). Other economically important hosts of P. cubensis are melon (Cucumis melo L.), watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai], and squash (Cucurbita spp.) (Whitaker and Davis, 1962). Studies on the host range of P. cubensis indicated ≈20 genera, including 50 species in the Cucurbitaceae, to be hosts, of which 19 species are in Cucumis (Lebeda, 1992; Lebeda and Widrlechner, 2003; Palti and Cohen, 1980). The downy mildew present in the United States has the ability to cause disease on formerly resistant (pre-2004) cucumber cultivars. Several races of P. cubensis have been reported in using differential cultigens (Angelov et al., 2000; Bains and Jhooty, 1976a; Inaba et al., 1986; Palti, 1974; Shetty et al., 2002). Studies of past (Barnes and Epps, 1954) and recent (Holmes et al., 2006) epidemics suggest that the pathogen has the potential to evolve. It is likely that there is a new race of P. cubensis present in the United States, but it has not yet been identified as a new race. Therefore, we refer to the currently present P. cubensis as a new strain of cucurbit downy mildew.
Symptoms of cucumber downy mildew appear mostly on the foliage. Infection first appears as small, water-soaked lesions on the underside of leaves. Initially, lesions may be round in shape, becoming angular, because they are bound by leaf veins, and turning chlorotic to varying degrees. Chlorotic lesions may eventually turn necrotic. The most resistant cultigens exhibit a hypersensitive response (HR) type with small necrotic flecks and sparse sporulation. P. cubensis, the causal organism of cucurbit downy mildew, is an obligate biotroph that infects by windblown sporangia that land on the leaf surface (Bains and Jhooty, 1976b). Environmental conditions affect overwintering capacity as well as disease development and intensity (Cohen, 1977). In production regions having a mild winter such as southern Florida, overwintering occurs on wild and cultivated cucurbits (Bains and Jhooty, 1976b). Overwintering also may occur in greenhouses. For instance, Hausbeck (2007) reported P. cubensis on greenhouse cucumber in Ontario, Canada, in 2006 and 2007. Rain, dew, and irrigation supply adequate leaf moisture required for sporangia to germinate. Under optimum temperature, infection can occur with only 2 h of leaf wetting (Cohen, 1977). The level of infection for compatible reactions is a result of the combination of time, moisture, temperature, and inoculum concentration. Inoculum concentration is affected by many factors such as weather, location, proximity to source, cultigen resistance, fungicide effectiveness, and area affected.
Most of the currently grown cultivars have some resistance to downy mildew. In many cases, this resistance can be traced back to PI 197087, originally identified as resistant by Barnes and Epps (1954). PI 197087 was used by Carroll Barnes, at Clemson University, to develop resistant cultivars such as ‘Polaris’, ‘Poinsett’, ‘Pixie’, and ‘Chipper’, which were released from 1961 to 1973 (Wehner and Shetty, 1997). The cultivar Polaris was the first of these to be released, available to growers in 1961. From 1961 to 2003, the dm-1 resistance gene from PI 197087 was sufficient to control downy mildew, and the disease was only a minor problem on cucumber. St. Amand and Wehner (1991) reported an average 2.9% yield loss per year from 1982 to 1988. The pathogen reappeared as a major problem in 2004, causing a 40% loss for cucumber growers (Colucci et al., 2006). Since then, downy mildew has continued to be a major disease of cucumber in the eastern United States, where conditions are favorable for disease. We believe the same strain of downy mildew is causing disease in both North Carolina and Michigan. The change in the pathogen in 2004 affected cucumbers in both locations that were previously resistant. There are currently no cultivars known to have resistance at the level seen from 1961 to 2003. Growers can obtain high fruit yield and quality in the presence of downy mildew by using multiple fungicide applications with moderately resistant cultivars (Call, 2010). The objective of this study was to determine the resistance of previously and currently grown cultivars to the new strain of downy mildew, present since 2004. We also evaluated cultivars for ability to produce high yield in the presence of disease, which we defined as tolerance. Finally, we evaluated the cultivars for components of resistance of their vines: chlorosis and necrosis, stunting, lesion size, and sporulation.
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