GREENHOUSE, DETACHED-LEAF, AND FIELD TESTING METHODS TO DETERMINE CUCUMBER RESISTANCE TO GUMMY STEM BLIGHT

The effects of leaf age, guttation, stomata and hydathode characterist ics, and wounding on the symptom development of gummy stem blight [Did ymella bryoniae (Auersw.) Rehm] of cucumber (Cucumis sativus L.) were studied to develop a useful germplasm screening method, Older cucumber leaves were more susceptible than younger leaves in field, greenhouse , and detached-leaf tests. Compared to seedlings with true leaves, see dlings at the cotyledon stage were less susceptible, had a smaller var iance for ratings, and were more likely to escape infection. Stomata d ensity and hydathode counts were not correlated with field ratings; bu t, stomata length on older leaves was highly correlated with susceptib ility. In greenhouse and field tests, susceptibility increased as gutt ation increased and actively guttating plants were more susceptible th an nonguttating plants. Phylloplane moisture and/or nutrition were mor e important in the infection process than was stomatal opening. Althou gh important, guttation was not necessary for infection. Dawn inoculat ion of field or greenhouse tests increased leaf symptoms compared with dusk inoculation. The increase was likely due to the free water and n utrients provided by guttation. Genotype ranks and ratings for detache d-leaf tests were not correlated with field results. A useful method, highly correlated (r = 0.82 to 0.96) with field ratings, for screening germplasm in the greenhouse was developed.