THE INVOLVEMENT OF FLAVOUR VOLATILES IN THE RESISTANCE TO DOWNY MILDEW OF WILD AND CULTIVATED FORMS OF BRASSICA OLERACEA

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Abstract
SUMMARY: The possible involvement of flavour volatiles in the resistance of Brassica oleracea L. seedlings to cabbage downy mildew (Peronospora parasitica (Pers. ex Fr.) Fr.) was investigated using seedlings of a number of commercial Brassica oleracea cultivars. Flavour volatiles are primarily released as a result of tissue damage in brassicas. Only one cultivar tested was resistant to the disease and its tissue macerates yielded the greatest concentration of volatiles. One of these compounds, allyl isothiocyanate, proved to be highly toxic to the pathogen and was produced in much greater concentrations in macerates of cotyledon than first leaf tissue. The ability of seedlings to produce allyl isothiocyanate in large amounts following tissue damage may inhibit the growth of Peronospora parasitica in resistant seedlings. Wild populations of Brassva oleracea L. subsp. oleracea (wild cabbage) were also tested for disease resistance, and the highest proportions of resistant seedlings were found in those populations containing high levels of flavour volatiles. The longest established wild populations possessed the highest average concentrations of flavour volatiles, and it is suggested that selective breeding has resulted in a reduction in the amounts of these compounds in cultivated forms. This change may be a contributory factor towards the lack of resistance shown by modern cultivars.

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