Witches’ broom disease (moniliophthora perniciosa): History and biology

Abstract

The history of witches’ broom disease is cataloged with special reference to its socioeconomic and ecological impact on the countries and regions affected. In particular, focus is placed on the Brazilian States of Rondônia and Bahia and the disastrous political events that have shaped their histories. The changing taxonomic status of the fungal causal agent-especially, its current placement in the genus Moniliophthora and its relationship to M. roreri-is documented and discussed. Like the frosty pod rot pathogen, M. perniciosa is hemibiotrophic with well-defined parasitic and saprophytic phases: separated morphologically, physiologically, and genetically. The symptomatology is described and illustrated in cacao and its relatives in the Malvales, as well as in the plant orders in which other hosts of the pathogen have been confirmed (Malpighiales and Solanales). The occurrence of a nonpathogenic biotype in the Bignoniaceae (Lamiales) is discussed in relation to the evolution of the pathotypes from benign to malign endophytes of woody plants. The etiology of the disease remains a matter of conjecture and there is circumstantial evidence indicating that the fungus may have developed a symbiotic association with another microorganism to facilitate invasion and disrupt host physiology. The only infective spore is the sexual basidiospore produced in the basidioma (mushroom) and spores are forcibly released and air dispersed over considerable distances as the temperature falls and the humidity rises during the night. The full life cycle of the cacao pathogen is described and illustrated schematically, and compared to that of the nonpathogenic L-biotype on Bignoniaceae. Finally, the areas of research that need to be addressed are prioritized.