| Target leaf spot of cucumber, incited by Corynespora cassiicola, is a serious problem in some areas of the United States. Tests were conducted in the greenhouse to evaluate varietal reaction, the inheritance of resistance, the relationship between resistance and powdery mildew susceptibility, true leaf versus cotyledon inoculation methods, the host specificity of two isolates, and the incorporation of resistance into 'Poinsett 83'.;Crosses between resistant varieties and the susceptible 'Poinsett' indicated that resistance found in 'Cornell SR 551' and 'Wisconsin SMR 18' is governed by the same major gene found in 'Wisconsin 2757'. Another major gene governs resistance in 'Marketmore 70'. Other genes may be responsible for slight and probably unimportant differences in resistance.;The backcross-derived, mildew-resistant counterparts of 'SR 551', 'SMR 18' and 'Marketmore 70' were all susceptible to Corynespora. Double inoculation of the F(,2) populations of 'PMR 551' x 'SR 551' and 'PSMR 18' x 'SMR 18' showed a tight linkage between mildew resistance and Corynespora susceptibility, but a few segregates with resistance to both diseases occurred with recombination of about 10%.;Inoculations of the first true leaf proved to be more reliable than those of cotyledons. The latter resulted in very erratic lesion production in some cases.;Of sixty entries inoculated, most were susceptible, although a greater percentage of slicing than pickling varieties was susceptible.;A backcross program to incorporate target leaf spot resistance from 'Wisconsin 2757' into 'Poinsett 83' is currently underway. Resistance was maintained at its original level through three backcrosses, selecting for a single dominant gene after each.;A leaf spot pathogen which has regularly defoliated 'Poinsett' and progenies derived from it at Ithaca, NY was identified as Ulocladium cucurbitae; this is the first reported occurrence in the U.S. Numerous 'Poinsett 83' families, 'PMR 551' x 'Poinsett 83' backcross progenies, commercial varieties and plant introductions were evaluated for field resistance. The F(,1) of 'PMR 551' x 'Poinsett 83', as well as 'PMR 551' itself exhibited the highest level of resistance.;Attempts to find a successful greenhouse screening technique for Ulocladium by manipulating humidity, temperature, plant age and inoculum concentration were unsuccessful. |
