Screening, selection, and genetics of resistance to Alternaria diseases in Brassica oleracea

Mist chamber and field inoculation procedures were developed to identify fifteen accessions of Brassica oleracea that possess partial resistance to Alternaria brassicicola. The optimum mist chamber conditions for identifying host plant resistance were 100% RH and 20{dollar}spcirc{dollar}C, with a 5{dollar}spcirc{dollar}C drop in night temperature possibly beneficial. Lower temperatures require a longer incubation period, and higher temperatures reduced the level of resistance expressed. The optimum inoculum concentration was a suspension of 50,000 conidia ml{dollar}sp{lcub}-1{rcub}{dollar}.; Resistance to A. brassicicola was correlated with resistance to A. brassicae, but the correlation was not high enough to ensure that selections for resistance to one organism would also be resistant to the other.; The genetics of resistance to A. brassicicola indicates that both dominance and additive gene action are involved. The dominance effects appear to be due to a single major gene, which alone contributes a moderate level of resistance, but in the presence of sufficient modifier genes the resistance is fairly high. Most sources appear to have the same type of resistance, and may contain the same major gene, with differences in the level of resistance due to the presence or absence of modifier genes.; Other types of resistance were identified, but most of these sources did not combine well with the other sources. One exception may be a curd-resistant cauliflower, which has shown virtual immunity to curd symptoms, although the leaves can become severely infected.; That the best sources of resistance contain the same alleles for resistance is supported by the results after two cycles of recurrent selection, which did not result in an increase in resistance more than that found in the best sources that created the population. Breeding for resistance using these sources would be best accomplished using the most resistant accession in a pedigree selection or backcross breeding program. The broad-sense heritability was shown to be consistently above 70%, and the narrow-sense heritability ranged from 19 to 30%. The high broad-sense heritability indicates that this resistance would probably be acceptable in the creation of hybrid cultivars with resistance using only one of the hybrid parents carrying this resistance.