Fig 1.
The Moniliophthora perniciosa life cycle in Theobroma cacao.
Infection begins when fungal basidiospores penetrate the plant through stomata or wounds. In the first stage of the disease, M. perniciosa develops as a swollen monokaryotic mycelium that grows exclusively in the extracellular space of the living plant tissue. Infection of shoots induces drastic morphological alterations resulting in the characteristic “green broom” structure, though infection can also occur in other tissues (fruits and flowers). After one to three months of biotrophic infection, necrosis of the plant tissue occurs, giving rise to the “dry broom” structure. Necrotic tissue is colonized intracellularly by thin dikaryotic mycelium, which is characterized by the presence of clamp connections—a cross structure formed by hyphal cells that ensures the presence of two nuclei in each fungal cell. After alternating rainy and dry periods, basidiomata are formed from necrotrophic hyphae, completing the pathogen life cycle. Illustrations by Diana Carneiro.
Fig 2.
The pathogenic lifestyle of M. perniciosa is an exception within the Marasmiaceae family of basidiomycetes, which is mostly composed of saprotrophic litter and wood-decomposing fungi.
The genus Moniliophthora includes the hemibiotrophic sister species M. perniciosa and M. roreri, the two major pathogens of Theobroma cacao. Notably, it also encompasses a still poorly characterized grass endophyte, suggesting that the pathogenic lifestyle of M. perniciosa may have evolved from an endophyte ancestral. The tree was constructed based on Bayesian inference using regions of the genes 25S, 18S, ITS/5.8S and Rbp1 (large fragment of the RNA polymerase II). Sequences were retrieved from Aime & Phillips-Mora (2005) [5] and Matheny et al. (2006) [34]. Numbers next to the branches represent the posterior probabilities. The species Schizophyllum radiatum was used as outgroup.