Search

Almost one-third of the world supply of cocoa is produced in the Americas but in many countries maximum production is severely curtailed by witches’ broom disease. The pathogen (Crinipellis perniciosa) causes overdevelopment of cocoa meristematic tissues leading to hypertrophied and proliferated growth, to give the characteristic witches’ broom (escoba de bruja/vassoura de bruxa/balai de sorciere). The fungus also infects many pods, which fail to ripen and the beans either become liquified or adhere to the pod husk and are commercially valueless. The disease is described in Chapter 2.

Methods are described to collect and store basidiospores of Crinipellis perniciosa and to inoculate seedlings of cacao (Theobroma cacao) for evaluation of resistance to witches'-broom disease. Basidiospores of C. perniciosa collected overnight in a 16% glycerol solution (containing buffer and Tween 20) remained viable but did not germinate until the collecting solution was diluted to 3% glycerol. Basidiospores freshly collected or stored in the collecting solution for 1 week at 4 degrees C, or stored in liquid nitrogen for up to 18 months remained viable and, after application to cacao seedlings, caused witches'-broom. Witches'-broom developed on inoculated susceptible seedlings placed in a dew chamber for 4 to 6 h at 25 degrees C, but did not develop on seedlings after 2 to 4 h of wetness. Maximal percentages of seedlings became diseased with 15 h of wetness. Differences in percentage of witches'-broom were evident among susceptible and putatively resistant cacao families inoculated with basidiospore concentrations of 10,000 and 12,500 spores per ml; however, higher concentrations (25,000 to 100,000 per ml) resulted in nearly 100% disease. Likewise, inoculation of flushes with large succulent leaves resulted in high percentages of disease in all families. Discrimination among families by percentages of diseased seedlings was achieved when small (0.3 to 1.5 cm) leaves were inoculated. Procedures for basidiospore collection and storage, inoculation of cacao seedlings, and control of pre- and postinoculation conditions support the effective and efficient screening of cacao for resistance to witches'-broom disease.

Infection biology of Crinipellis perniciosa on vegetative flushes of cacao (Theobroma cacao) was studied by fluorescent microscopy. Basidiospores germinated only if they landed on wet tissue or were suspended in water and sprayed onto the plant parts; those landed on dry surfaces were no longer viable. Interruption of the period of wetness during the prepenetration phase resulted in irreversible termination of the infection process. Growth of germ tubes on the surface of young flushes was strongly oriented toward natural infection sites (stomata) and wounds. On hardened flushes, growth of germ tubes was random (...)

Basidiocarps of C.p. developed in 4-5 wk on mycelial mats supported by sterilized pieces of witches' brooms from cacao, and mats continued to produce basidiocarps for an additional 8 wk. Multibasidiospore and single-basidiospore isolates of the fungus from T.c., Solanum spp. and lianas produced basidiocarps on mycelial mats

Basidiocarps of Polyporus hydnoides and Fomes sclerodermeus were recovered from a 750-yr-old level of a water-saturated archaeological site on Hontoon Island, Volusia County, Florida. The excellently preserved basidiocarps could be observed as if they were present-day specimens. We suggest that archaeological wet sites are an untapped reservoir of extremely well-preserved fungi and other paleobotanical specimens.

Bacterial pathogens have not been reported in association with Theobroma cacao. However, three strains of Agrobacterium tumefaciens isolated from chrysanthemum induced galls in seedling cacao plants grown in a greenhouse. Since T. cacao is thus a proven host for the crown gall bacterium, the Ti plasmid of A. tumefaciens may possibly be used as a vector for the introduction of selected DNA into cacao