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 leaves, without defoliation. The underlying mechanisms that precede the occurrence of these two distinct responses were studied in root inoculated seedlings under glasshouse conditions. Rapid decreases in total transpiration, stomatal conductance and midday leaf water potential were closely associated with the onset of foliar symptoms, indicating that water stress is a major cause of symptom development. Water stress was most pronounced when plants were inoculated with a 'non-defoliating' isolate. In contrast, a 'defoliating' isolate induced accumulation of ethylene 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 application of the ethylene inhibitor silver thiosulphate. Increased symptoms, water stress and ethylene production occurred in upper leaves and coincided with more intensive colonization of this part of the plant by the pathogen. This unusual pattern reflects the vascular anatomy of the host.