Sources of Alternaria carthami inoculum in safflower

Abstract

Spread of the disease Alternaria leaf blight (Alternaria carthami) from infected safflower seed and stubble was studied at Biloela in central Queensland to determine the importance ofthese inoculum sources in the initiation of epidemics. Seed infection levels of 20-55% resulted in 1.4-2.0% emerged diseased seedlings in the field. Levels of 1.0% seed infection have previously caused severe disease outbreaks in commercial crops. Visual appraisal of seed health correlated highly with laboratory, glasshouse and field assessments of diseased seedlings. Glasshouse assessment of emerged diseased seedlings gave the best indication of expected disease incidence in the field. Seed germination in the laboratory correlated poorly with emergence in the glasshouse and the field. Incidence of A. carthami on seedlings following soil incorporation of diseased stubble in November 1977 diminished from 28% in May 1978 to 0% in September 1980. Burning of diseased stubble in November 1977 failed to eliminate the disease, but reduced the number of emerged disease seedlings by 66% in May 1978. Dusting of healthy seed with a fungicide increased total emergence, but it did not control the spread of the fungus from the infected stubble as emergence of diseased seedlings was also increased.