Laboratory tests for leaf resistance to Botrytis squamosa in onions

Two laboratory methods of testing for resistance to Botrytis squamosa in Allium cepa, A. fistulosum and hybrid material derived from them were developed. In one method, seedlings were sprayed with conidiospores, incubated for 3 days at 15°C (2 days at 100% r.h.) and the numbers of lesions per leaf assessed. This test was destructive. The second method was non-destructive, the plants being retained for breeding purposes. Leaf segments (25 × 15 mm) or leaf discs (14 mm diameter) were inoculated individually with a drop (0·014 ml) of spore suspension, and maintained at 15°C and 100% r.h. with a 12 h photoperiod. The length of the latent period, denned as the number of days for 50% of the leaf portions to produce lesions bearing visible mycelium, was determined for each onion line in a test. The reciprocal (1/t) of the latent period was used to test for significant differences between lines by analysis of variance. With both test methods, spores were applied at concentrations that discriminated relative resistance most readily. In both seedling and leaf segment tests A. fistulosum was more resistant than A. cepa to B. squamosa. The leaf disc test indicated that cultivars of A. fistulosum differed in their resistance. In hybrid material resistance was intermediate or more like that of A. fistulosum.