| Two problems which deal with basic concepts of disease control are addressed: stability of horizontal resistance and efficacy of pathogen-induced resistance under field conditions.;The first problem was addressed because an Arkansas little-leaf cucumber breeding line with horizontal resistance to anthracnose had been found to develop a severe anthracnose in a cultural study planting at the Arkansas Agricultural Experiment Station Vegetable Substation at Kibler. Isolates of Colletotrichum lagenarium Pass. (Ell. & Halst.) made from this planting and from plantings in subsequent years at the same location were tested on seedlings of differential cucurbit hosts. The varieties used permitted race determination and assessment of whether the Kibler isolates were pathogenic specifically on little-leaf types. The overwintering ability of the pathogen population was also monitored witha trap plots. Isolates were found to be Race 3 and were not specific to little-leaf cucumbers. Instead, the two little-leaf types tested were the most resistant. Overwintering occurred during each of the 3 winters of the study. These results indicate that, contrary to theoretical predictions, management of horizontally resistant varieties, especially by crop rotation, is necessary for disease control when highly aggressive pathogens have the ability to overwinter.;The second problem dealt with modification of techniques for inducing resistance with pathogenic C. lagenarium to evaluate efficacy in relation to a commercial production situation and to determine whether such treatment is capable of producing unacceptable levels of disease in the plants it supposedly protects. Weather factors at the time of inoculation were important in establishment of infection and symptom development, and two of five studies failed because of unfavorable conditions. Although protective inoculation showed some effect in short-term evaluation of field studies, no long-term protection occurred. No long-term effects of protective inoculation were evident, and short-term effects were not consistently found. In the two studies carried through fruiting, both foliar and fruit infection occurred in protected plots. These results indicate that, in a commercial production situation, the level of resistance that might be contributed by induced resistance is low, and that the risk of causing epidemics is too great to consider use of this method. |
