Some pathogen factors affecting inoculation of cacao seedlings with Verticillium dahliae

Some factors that may influence the incidence of experimental infection of cacao (Theobroma cacao L.) seedlings by Verticillium dahliae Kleb. have been studied. Incidence of infection increased with increased conidial concentration of the inoculum from a numerical threshold of approximately 5 x 10 exponent 4/ml, below which infection did not occur. Infectivity of conidial suspension decreased with increasing age of culture from which they were harvested; a 60 per cent incidence of infection was obtained from conidia harvested from cultures 1-2 weeks old, and only 10 per cent from 12-week old cultures. Incidence of infection also increased with increase in amount of nitrogen, but not carbohydrates, in the culture medium. Conidial viability was not affected by either soluble derivatives of the culture medium, or by age of culture. Isolates of V. dahliae from cacao, cotton and okra were equally pathogenic to cacao. They all induced typical wilt symptoms on okra, cacao and cotton, with severity of wilt symptoms on the three hosts decreasing in that order