Phytophthora megakarya, a causal agent of black pod rot in Africa

Abstract

In most parts of the world where Theobroma cacao is grown, Phytophthora palmivora is the major concern for causing black pod rot (BPR). Phytophthora megakarya, on the other hand, occurs only in Africa, but represents a major threat to cacao production, the countries of West Africa being the largest producers of cacao in the world. Since cacao did not originate in Africa, P. megakarya obviously only recently (likely prior to 1960) added cacao as a host and this new encounter has resulted in the most severe form of BPR worldwide. Although P. megakarya and P. palmivora are related, both being grouped in clade 4 in current Phytophthora diversity studies, P. megakarya has a distinct chromosomal composition and adaptations that make it particularly aggressive on cacao. P. megakarya has environmental requirements (temperature and rainfall) similar to cacao, and its ability to survive in soil and reinfect cacao pods through inoculum originating in the soil makes it particularly difficult to manage. Not only does P. megakarya survive in the soil for long periods, it also survives on the roots of cacao and other plant/tree species, many of which are cocultivated with cacao. Scientists have continued to make progress in understanding P. megakarya as a pathogen and have developed management tools for the disease it causes. Unfortunately, management tools such as fungicides and labor-intensive sanitation efforts, although effective, can be costly and, in some cases, difficult to obtain/maintain. As a result, farmers have difficulty justifying their use. Efforts toward breeding for tolerance in the crop to P. megakarya-induced BPR are making progress and in the future new planting materials should be able to greatly reduce disease losses if deployed with sound disease management practices. An improved understanding of the pathogen and its interaction with cacao at all levels, especially the molecular level, the deployment of tolerant cacao planting materials, the standardization of screening tools for developing tolerance to disease and determining pathogen diversity, and the refinement and more consistent employment by farmers of management tools already available will be critical for limiting losses due to P. megakarya-induced black pod rot in the future. The containment of P. megakarya to areas where it already exists is critical to cacao production around the world.