Response of Brassica oleracea var. capitata to Wound and Spray Inoculations with Xanthomonas campestris pv. campestris

Eighteen cabbage breeding lines and cultivars were evaluated for resistance to black rot caused by Xanthomonas campestris pv. campestris following wound and spray inoculations at the juvenile and mature stages. Plants were evaluated using four inoculation procedures (juvenile wound, juvenile spray, mature wound, and mature spray) in replicated greenhouse and field experiments. The breeding lines Badger #16, Cornell 101, Cornell 102, NY 4002 and accession PI 426606 exhibited high levels of resistance following all inoculation procedures. `Silver Dynasty' was the most resistant commercial cultivar based on the four tests, yet ranked 12th following the juvenile wound inoculation. The juvenile spray inoculation had a high correlation with both wound and spray inoculations in field experiments (0.89 and 0.86, respectively); however, the juvenile wound inoculation did not correlate well with mature wound and spray inoculations (0.58 and 0.51, respectively). The results indicate that the juvenile wound inoculation is not the most appropriate approach for determining field resistance in Brassica oleracea, and that resistant material could be selected against using this approach. A high correlation between juvenile spray inoculation disease severity ratings and mature plant resistance indicates that plants can be evaluated effectively at the juvenile stage for mature plant resistance to black rot.

033 Characterization of Downy Mildew Resistance in USDA Broccoli Inbreds

Downy mildew, caused by Peronospora parasitica (Pers. ex Fr.), is one of the most economically important diseases in broccoli (Brassica oleracea L. Italica group). Previous studies reported that resistance to downy mildew in broccoli depends on plant age and that seedling resistance appears to be independent of mature-plant resistance. The objectives of our studies were to evaluate resistance and susceptibility of USDA broccoli inbreds to downy mildew and to investigate the interaction between the host and pathogen at two plant stages with single or double inoculation. Multiple screening tests at both cotyledon and three-expanded leaf stages using 38 entries, including USDA inbreds and commercial hybrids, were conducted in randomized complete-block designs. In these tests, every leaf of each plant was thoroughly sprayed with P. parasitica isolate PP1 at a concentration of 10,000 sporangia per ml at both stages. Ratings for downy mildew reaction phenotype were made at 9 days postinoculation on a 0-9 scale of increasing disease severity. We found significant phenotypic variation to infection among broccoli entries. We observed three general phenotypes: 1) resistance at both stages; 2) susceptible at cotyledon stage combined with resistance at three-expanded leaf stage; and 3) susceptibility at both stages. Additionally, inoculation at the cotyledon stage had no effect on inoculation at the three-expanded leaf stage.

Natural resistance to bean yellow mosaic potyvirus in subterranean clover

Seed of 31 named cultivars and 11 other genotypes of Trifolium subterraneum (subterranean clover) was sown in 3 major and 3 smaller scale field experiments. In these, bean yellow mosaic virus (BYMV) was spread from infector plants to test rows by naturally occurring aphids. The numbers of clover plants developing BYMV symptoms in each row were counted at different times. Based on the results from at least 2 field experiments, different genotypes were assigned to 1 of 5 BYMV infection categories (A-E), ranging from greatest (A) to least (E) infection. Category A was found to contain genotypes that were latest maturing while, except for cv. Rosedale (ssp. brachycalycinum) which was in E despite being of intermediate maturity, category E contained earliest maturing genotypes. Except for cv. Rosedale, when the percentages of plants with BYMV symptoms in the 3 main experiments were plotted against days to flowering previously established for each genotype, the two were closely related. However, when young plants of cultivars in categories D or E were aphid-inoculated with 2 BYMV isolates, they became infected to a similar extent to young plants of 2 late-maturing cultivars from category A. This suggests that, except for cv. Rosedale, the extent of BYMV infection obtained in the field was determined by plant physiological age, plants of late-maturing genotypes being easier for viruliferous aphids to infect with BYMV than plants of early-maturing genotypes of the same age but physiologically older, a phenomenon known as 'mature plant resistance'. When plants of cv. Rosedale were aphid-inoculated with 7 different isolates of BYMV, a systemic hypersensitive reaction developed with 5 isolates but not with two others. This suggests that cv. Rosedale has 'strain specific hypersensitive resistance' to BYMV. No systemic hypersensitive reaction developed when the same 7 BYMV isolates were aphid-inoculated to cv. Dwalganup and named selections Pink Flowered and Northam Early, genotypes previously reported to react hypersensitively to potyvirus infection. The field screening procedure employed is suitable for use by T. subterraneum breeding programs seeking to develop new cultivars with resistance to BYMV. Cultivar Rosedale may prove a suitable source of resistance to use as a parent in crossing programs aimed at producing cultivars with natural BYMV resistance.

Plant Age and Isolate Source Affect Expression of Downy Mildew Resistance in Broccoli

Resistance to downy mildew [Peronospora parasitica (pers.) ex. Fr.] in broccoli (Brassica oleracea L. Italica Group) depends on plant age. Seedling resistance seems to be independent of mature-plant resistance, where a mature plant is defined as having eight or more leaves. Our results suggest that, by using mature-plant resistance, an almost continuous variation instable levels of mature-plant resistance can be developed. Similarities in the response of mature plants of various lines to isolates from California, Washington, New York, and South Carolina indicated that the predominant race was the same at all locations. Correlations between resistant and susceptible responses to isolates from California, Washington, New York, and South Carolina varied from r = 0.48 to 0.74 depending on isolate source. The results indicated that selecting for high levels of resistance in mature plants at one location should result in good resistance elsewhere in the United States. Selecting immature plants (three to six leaves) may provide less reliable results due to the transitional status of the plant; i.e., between seedling and mature plant.