Submitted to: American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/19/1999
Publication Date: N/A
Interpretive Summary: Downy mildew is one of the most destructive diseases of broccoli and is caused by a fungus. Most broccoli producers spray fungicides to control this disease. Due to concerns about the negative effects of pesticides on the environment and as residues on vegetables, it is desirable to develop varieties of broccoli that have natural resistance to downy mildew and do not require fungicide sprays to prevent leaf, head, and stem infection. The goal of this research was to identify USDA varieties of broccoli that have high levels of natural resistance to downy mildew and to determine if potential resistances are expressed at different growth stages. Our results showed that some varieties were resistant at all growth stages, while others were not resistant as early seedlings but were resistant as they grew older. These results indicate there is more than one gene for resistance among the tested varieties. Certain resistant USDA varieties identified in this work are of interest to commercial and public breeders attempting to develop downy mildew resistant hybrids. Information obtained in these studies will prove useful to those breeders as they characterize resistant lines in their programs. Ultimately, this work will lead to the development and use of new and more downy mildew resistant hybrids that producers can grow without using fungicides.
Technical Abstract: Downy mildew caused by Peronospora parasitica is one of the most damaging diseases of broccoli. Experiments have shown that downy mildew resistance is dependent on plant age and seedling resistance is independent of mature-plant resistance. Objectives herein were to determine: 1)if valid evaluations for downy mildew resistance could be conducted at both the cotyledon and the 3-4 true leaf stages on the same broccoli plants of a given entry; 2) if doubled-haploid (DH) lines exhibit resistance, and if so, to characterize their resistance phenotype(s); and 3) if resistant lines show resistance to isolates of P. parasitica from different regions of the U. S. DH lines (23) and two commercial hybrids were evaluated for reaction to infection by P. parasitica at different stages. Results showed plants can be evaluated for mildew resistance in a two stage process. Inoculation at the cotyledon stage did not offer any cross protection or influence the expression of reaction phenotype (RP) when the same plants were subsequently inoculated at the true leaf stage. Three different RP were identified in DH lines, including: a) susceptibility at both cotyledon and true leaf stages; b) resistance at both cotyledon and true leaf stages; and c) susceptibility at the cotyledon stage with resistance at the true leaf stage. There was no difference between RP against two pathogenic isolates from different geographic regions. Selection for resistance at the cotyledon stage will identify plants with resistance at subsequent stages.