Submitted to: HortScience
Publication Type: Abstract Only
Publication Acceptance Date: May 20, 1999
Publication Date: N/A
Technical Abstract: Downy mildew, caused by Peronospora parasitica (Pers. ex Fr.), is one of the most economically important diseases in broccoli (Brassica oleracea L. Italica group). Studies by others reported that resistance to downy mildew in broccoli can depend on plant age and that seedling resistance appeared 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 inoculations. Multiple screening tests at both the cotyledon and the 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 nine 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 interaction phenotypes: 1) resistance at both stages; 2) susceptibility at cotyledon stage combined with resistance at three-expanded leaf stage; and 3) susceptibility at both stages. Inoculation at the cotyledon stage had no apparent inhibitory effect on infection following inoculation at the three-expanded leaf stage.