WATER RELATIONS AND ETHYLENE PRODUCTION AS RELATED TO SYMPTOM EXPRESSION IN COCOA SEEDLINGS INFECTED WITH DEFOLIATING AND NON-DEFOLIATING ISOLATES OF VERTICILLIUM-DAHLIAE

Some isolates of Verticillium dahliae can induce severe defoliation on cocoa plants and others lead to wilting then desiccation of the leave s, without defoliation. The underlying mechanisms that precede the occ urrence of these two distinct responses were studied in root inoculate d seedlings under glasshouse conditions. Rapid decreases in total tran spiration, stomatal conductance and midday leaf water potential were c losely associated with the onset of foliar symptoms, indicating that w ater stress is a major cause of symptom development. Water stress was most pronounced when plants were inoculated with a 'non-defoliating' i solate. In contrast, a 'defoliating' isolate induced accumulation of e thylene in newly developed leaves, where the first symptoms generally appeared. This hormone appeared to be responsible for the accelerated senescence and defoliation, as demonstrated by reversal with the appli cation of the ethylene inhibitor silver thiosulphate. Increased sympto ms, water stress and ethylene production occurred in upper leaves and coincided with more intensive colonization of this part of the plant b y the pathogen. This unusual pattern reflects the vascular anatomy of the host.