Studies on Resistance to Downy Mildew in Cucumber (Cucumis sativus L.) Caused by Pseudoperonospora cubensis (2010)

CALL, ADAM DEAN. Studies on Resistance to Downy Mildew in Cucumber (Cucumis sativus L.) Caused by Pseudoperonospora cubensis. (Under the direction of Todd C. Wehner, M.S.)\nDowny mildew, caused by the oomycete pathogen Pseudoperonospora cubensis (Berk. And Curt) Rostov, is a major foliar disease of cucumber (Cucumis sativus L.) Currently, high yield and quality in the presence of downy mildew is achieved with multiple fungicide applications. Most of the currently grown cultivars have some resistance to downy mildew. Prior to a 2004 outbreak in the United States, host resistance was sufficient to control the disease, and downy mildew was only a minor problem on cucumber. There are currently no cultivars that show resistance at a level equal to that seen prior to 2004. However, differences in resistance among cultivars exist, ranging from moderately resistant to highly susceptible. Both host resistance and fungicides contribute to control of downy mildew for growers. Experiments were conducted to identify resistant and high yielding cultigens and and to determine the contribution of resistance and fungicides to overall control of downy mildew.\nThere were three major experiments. All experiments rated disease using a 0 to 9 scale (0=none, 1-2=trace, 3-4=slight, 5-6=moderate, 7-8=severe, 9=dead). The objective of experiment 1 was to identify new sources of resistance to downy mildew among plant introduction accessions from the U.S. National Plant Germplasm System, elite cultivars, and breeding lines of cucumber. The 1300 cultigens were tested at Clinton NC, USA, and Skierniewice, Poland during 2005-2007 under natural field epidemics of the disease in unreplicated trials. Mean ratings for downy mildew leaf damage in the germplasm screening\nranged from 1.0 to 7.3 in North Carolina and from 0.3 to 9.0 in Poland. The 40 most resistant and 10 most susceptible cultigens, along with 22 check cultivars were further evaluated in replicated field and greenhouse experiments in North Carolina and India in 2007 to 2009 for a total of eight environments (year by location). A fungicide component was added in 2008 and 2009 in that one environment each year was a field treated weekly with applications of Previcur Flex and Mancozeb alternating with Bravo and Tanos. Results from the retest study in NC confirmed the results of the initial screening study. The most resistant and most susceptible cultigens in the screening study were also the most resistant and most susceptible cultigens in the field retest. Cultigens were found that significantly outperform checks. High yielding and tolerant cultigens were also been identified, which could be used in developing improved cultivars.\nThe objective of experiment 2 was to identify cultivars having high yield and resistance to the new downy mildew. The experiment had 86 cultigens, three locations (Clinton and Castle Hayne, NC, and Bath, MI), three years (2007 to 2009) and 4 replications per location. None of the cultigens tested in this study showed a high level of resistance, although differences in resistance and yield among cultigens do exist.\nThe objective of experiment 3 was to evaluate different fungicide treatments, chosen to represent different levels of efficacy, combined with different levels of resistance (resistant - M 21, moderate - 'Sumter', susceptible - 'Wisconsin SMR-18') among cultigens for the effect on disease severity and yield. There were six and twelve replications in 2008 and 2009, respectively. Cultigen had a large effect in both years. As expected, using a resistant cultivar significantly improved resistance and most yield traits compared to a susceptible cultivar. Fungicide has a smaller effect on resistance traits and larger effect on total yield and percent marketable yield. To achieve maximum yield both a resistant cultivar and fungicide spray program should be used.