Cucurbit downy mildew caused by the oomycete Pseudoperonospora cubensis is economically the most important disease of cucumber (Cucumis sativus L.) (Palti and Cohen, 1980). Studies on the host range of P. cubensis indicate that ≈20 genera are hosts, including 50 species in the Cucurbitaceae and 19 host species genus Cucumis alone (Lebeda, 1992; Lebeda and Widrlechner, 2003; Palti and Cohen, 1980). In 2010, ≈35,840 ha of cucumbers for processing and fresh market were grown in the United States with a value of $378 million [U.S. Department of Agriculture (USDA), 2011]. Other economically important hosts of P. cubensis are melon (Cucumis melo L.), watermelon (Citrullus lanatus), and squash (Cucurbita spp.) (Whitaker and Davis, 1962). The pathogen infects when windblown sporangia are introduced onto cucurbit hosts under favorable environmental conditions.
P. cubensis is a biotroph and, with the exception of oospore production, survives only on living host tissue (Bains and Jhooty, 1976). Previously, oospore production was thought to be rare, but Cohen et al. (2011) reported recently on oospore formation in the laboratory from crosses between different pathotypes, resulting in the production of viable F1 recombinants. In warm production regions such as southern Florida, overwintering occurs on wild and cultivated cucurbits (Bains and Jhooty, 1976). The pathogen can also overwinter on cucumbers grown in greenhouses. Hausbeck (2007) reported the possibility of cucumbers in greenhouses in Ontario as a local source of P. cubensis inoculum for Canada and neighboring states in the Great Lakes region.
Environmental conditions affect overwintering capacity as well as disease development and intensity. Leaf moisture is required for germination of sporangia. Rain, dew, or irrigation can easily supply enough moisture for sporangia to germinate. Under optimum temperature, infection can occur with only 2 h of leaf wetness (Cohen, 1977). The level of occurrence for compatible reactions is a result of the combination of time, moisture, temperature, and inoculum concentration.
Symptoms of cucumber downy mildew occur almost exclusively on the foliage. Infection first appears as small, water-soaked lesions on the underside of leaves. Symptoms vary by cucurbit species but in cucumber lesions are angular, bounded by leaf veins, and turn chlorotic to varying degrees. Sporulation occurs on the undersides of the leaves. Chlorotic lesions eventually turn necrotic and the entire leaf may be affected by the pathogen as the leaf tissue dies. Symptoms vary depending on relative susceptibility of host plants. The most resistant will exhibit a hypersensitive response (HR) with small necrotic or chlorotic flecks and sparse sporulation, whereas leaves of the most susceptible will become completely necrotic within 2 to 3 weeks.
The HR type resistance was first described by Barnes and Epps (1954) in the accession PI 197087. Resistance from PI 197087 was used to develop resistant cultivars, and most current cultivars are thought to have some resistance derived from PI 197087. This resistance proved highly effective for many years until a resurgence of the disease in 2004. Since then, cultivars having resistance tracing to PI 197087 are only moderately resistant in the United States (Call et al., 2012a). However, differences among cultivars do exist, ranging from moderately resistant to highly susceptible. New sources of disease resistance have been reported (Call et al., 2012b) but this resistance has not yet been incorporated into cultivars.
Chemical control of downy mildew is necessary to achieve high yields in the absence of high host plant resistance. The discovery of systemic fungicides was a major advance over protectant fungicides in control of downy mildew. Systemic fungicides, in the absence of resistant biotypes, can provide effective control. Cohen (1979) reported on the effectiveness of two systemic fungicides, prothiocarb and propamocarb (derivative of prothiocarb), against downy mildew. Both prothiocarb and propamacarb were reported to have very good activity against downy mildew. Briggs et al. (2006) reported fluopicolide as having a novel mode of action controlling a wide range of oomycete pathogens. Typically, a protectant fungicide and a systemic fungicide are tank-mixed and alternated weekly with a different tank mix consisting of fungicides with different modes of action. An example of such a program is propamocarb (Previcur Flex) and chlorothalonil (Bravo) alternating with famoxadone + cymoxanil (Tanos) + mancozeb (Manzate).
In this study, we evaluated fungicide programs with different levels of efficacy against downy mildew in combination with cultivars or breeding lines (hereafter collectively referred to as cultigens) having different levels of resistance for their effect on disease severity and yield.
Adams, M.L. & Ojiambo, P.S. 2010 Evaluation of fungicides for control of downy mildew of cucumber and winter squash, Clayton 2009. Plant Dis. Mgt Rpts. 4:V091. Amer. Phytopathol. Soc., St. Paul, MN
Adams, M.L., Ojiambo, P.S. & Thornton, A.C. 2010 Evaluation of fungicides for control of downy mildew and phytotoxicity on cucumber, Faison 2009. Plant Dis. Mgt Rpts. 4:V094. Amer. Phytopathol. Soc., St. Paul, MN
Bains, S.S. & Jhooty, J.S. 1976 Over wintering of Pseudoperonospora cubensis causing downy mildew of muskmelon Indian Phytopathol. 29 213 214
Briggs, G., Mansfield, D., Moloney, B., Gary, S. & Wegmann, T. 2006 The discovery and chemistry of fluopicolide: A new standard for oomycetes disease control Pflanzenschutz-Nachrichten Bayer. 59 141 152
Call, A.D., Criswell, A.D., Wehner, T.C., Ando, K. & Grumet, R. 2012a Resistance of cucumber cultivars to a new strain of cucurbit downy mildew HortScience 47 171 178
Call, A.D., Criswell, A.D., Wehner, T.C., Klosinska, U. & Kozik, E.U. 2012b Screening cucumber for resistance to downy mildew caused by Pseudoperonospora cubensis (Berk. and Curt.) Rostov Crop Sci. 52 577 592
Cohen, Y. 1977 The combined effects of temperature, leaf wetness and inoculum concentration on infection of cucumbers with Pseudoperonospora cubensis Can. J. Bot. 55 1478 1487
Cohen, Y., Rubin, A.E. & Galperin, M. 2011 Formation and infectivity of oospores of Pseudoperonospora cubensis, the causal agent of downy mildew in cucurbits Plant Dis. 95 874
Colucci, S., Thornton, A.C., Adams, M.L. & Holmes, G.J. 2008a Delayed fungicide application to cucumber and the effects of downy mildew severity and yield I, 2007. Plant Dis. Mgt Rpts. 2:V044. Amer. Phytopathol. Soc., St. Paul, MN
Colucci, S., Thornton, A.C., Adams, M.L. & Holmes, G.J. 2008b Evaluation of fungicides for control of downy mildew of cucumber I, 2007. Plant Dis. Mgt Rpts. 2:V043. Amer. Phytopathol. Soc., St. Paul, MN
Colucci, S., Thornton, A.C., Adams, M.L. & Holmes, G.J. 2008c Evaluation of fungicides for control of downy mildew of cucumber II, 2007. Plant Dis. Mgt Rpts. 2:V045. Amer. Phytopathol. Soc., St. Paul, MN
Hausbeck, M. 2007 Downy mildew reported on cucumbers growing in Canadian greenhouses. 17 Feb. 2010. <http://ipmnews.msu.edu/vegetable/vegetable/tabid/151/articleType/ArticleView/articleId/1273/categoryId/110/Downy-mildew-reported-on-cucumbers-growing-in-Canadian-greenhouses.aspx>
Kanetis, L., Adams, M.L. & Holmes, G.J. 2009a Evaluation of fungicides for control of downy mildew of cucumber and winter squash, Clayton 2008. Plant Dis. Mgt Rpts. 3:V073. Amer. Phytopathol. Soc., St. Paul, MN
Kanetis, L., Adams, M.L. & Holmes, G.J. 2009b Evaluation of fungicides for control of downy mildew of cucumber, winter squash, and cantaloupe, Clayton 2008. Plant Dis. Mgt Rpts. 3:V072. Amer. Phytopathol. Soc., St. Paul, MN
Lebeda, A. 1992 Screening of wild Cucumis species against downy mildew (Pseudoperonospora cubensis) isolates from cucumbers Phytoparasitica 20 203 210
Lebeda, A. & Widrlechner, M.P. 2003 A set of Cucurbitaceae taxa for differentiation of Pseudoperonospora cubensis pathotypes J. Plant Dis. Prot. 110 337 349
Palti, J. & Cohen, Y. 1980 Downy mildew of cucurbits (Pseudoperonospora cubensis). The fungus and its hosts, distribution, epidemiology and control Phytoparasitica 8 109 147
SAS Institute 2008 SAS/STAT User’s guide, Release 9.1 edition. SAS Institute Inc., Cary, NC
Schultheis, J.R. 1990 Pickling cucumbers. N.C. State Ag. Ext. Hort. Info. Lflt. No. 14-A
Shetty, N.V., Wehner, T.C., Thomas, C.E., Doruchowski, R.W. & Shetty, V.K.P. 2002 Evidence for downy mildew races in cucumber tested in Asia, Europe and North America Sci. Hort. 94 231 239
U.S. Department of Agriculture 1958 United States standards for grades of cucumbers. 8 May 2012. <http://www.ams.usda.gov/AMSv1.0/getfile?dDocName=STELPRDC5050262>
U.S. Department of Agriculture 2011 Vegetables annual summary 1.27.2011. National Agricultural Statistics Service. 8 Jan. 2012. < http://usda01.library.cornell.edu/usda/current/VegeSumm/VegeSumm-01-27-2011.pdf>
Wehner, T.C. 1984 Cucurbit Genet. Coop. Rprt. 7. p. 31–32
Wehner, T.C. & Shetty, N.V. 1997 Downy mildew resistance of the cucumber germplasm collection in North Carolina field tests Crop Sci. 37 1331 1340
Whitaker, T.W. & Davis, G.N. 1962 Cucurbits. Leonard Hill, London, UK