Inoculum sources of Fusarium oxysporum f.sp. cepae on onion in the Western Cape Province of South Africa

Fusarium basal rot of onion, caused by Fusarium oxysporum f.sp. cepae, is one of the leading causes of post-harvest and storage losses within the Western Cape region in South Africa. Several vegetative compatibility groups (VCGs) of F. oxysporum f.sp. cepae have been associated with bulb rot in mature onions in South Africa, of which VCG 0425 predominates. Our study investigated seed and seedling transplants as potential sources of inoculum of F. oxysporum f.sp. cepae, and whether VCG 0425 is associated with these materials. Fusarium isolation studies from 13 seed lots showed that seven of the seed lots were infected with either moderately or highly virulent F. oxysporum f.sp. cepae isolates. The infection frequency of seed lots was between 0.17 and 0.50%, and only two of the seed lots were infected with VCG 0425. The seedborne nature of F. oxysporum f.sp. cepae was confirmed by showing that a green fluorescent protein (GFP)-labelled F. oxysporum f.sp. cepae VCG 0425 transformant could be transmitted from infected soil to bulbs, and from there to the seed stalks and seeds. Onion seedling transplants from nurseries were also implicated as a source of inoculum. The incidence of F. oxysporum f.sp. cepae in nurseries increased as the season progressed from 2.7% at the 6-week-old growth stage to 5.7% at the 14-week-old stage. The F. oxysporum f.sp. cepae isolates from transplants were highly to moderately virulent. However, none of the isolates proved to be VCG 0425. Most (>86%) of the F. oxysporum isolates from seed and seedlings were not F. oxysporum f.sp. cepae and were not pathogenic to onion. Altogether, the results indicate that onion seed and seedlings are inoculum sources of F. oxysporum f.sp. cepae in the Western Cape Province, but that VCG 0425 is rarely associated with these sources.