Physiology and biochemistry of compatible and incompatible responses of Theobroma cacao to Verticillium dahliae

摘要

Verticillium wilt of cocoa caused by V. dahliae Kleb. has become an increasingly serious problem in Brazil, the second largest cocoa producer in the world. The disease has long been known in Uganda, and may explain why cocoa has not become a significant crop there; recently Verticillium has also affected cocoa in Colombia (7). Infected cocoa plants usually exhibit sudden wilting and subsequent necrosis of leaves and fine branches, but occasionally defoliation occurs without necrosis (2). Similar defoliating and non-defoliating forms of symptom development can be found in other Verticillium wilts such as on cotton. Tzeng and DeVay (12) showed enhanced ethylene production in cotton plants infected with a defoliating isolate compared to a non-defoliating isolate. The physiological bases for development of wilting and leaf abscission in Verticillium of cocoa are unknown and are investigated in this paper. Likewise the nature of resistance of cocao to any pathogen is poorly understood and there is noinformation on resistance to V. dahliae (9). Resistant genotypes offer the only practical solution to control of this disease, largely because the host is perennial and the pathogen soil-borne. The enormous variation within a population from the Amazon basin of over 20,000 assumed accessions of Theobroma spp. maintained by the Ministry of Agriculture in Brazil has not yet been adequately exploited (7). A few lines have been found with resistance to undefined isolates of V. dahliae. We recently developeda stem-puncture method of inoculation which has much improved the previously inefficient screen for resistant lines. This technique distinguishes between susceptibility and resistance to V. dahliae in 15-day seedlings within 3 weeks (7). It also facilitates a study of host-parasite interactions which can be notoriously difficult with vascular pathogens. Inoculation of mature plants via roots leads to non-synchronous, chronic infection which in cocoa can take ca. 90 days to manifest symptoms. Critical biochemical studies are not practical or subject to clear interpretation in such systems. Here, we describe the chemical nature of defence of certain cocoa genotypes following stem-puncture with V. dahliae.