Moniliophthora perniciosa mycelia quantification in infected tomato and cacao tissues

Abstract

Witches' broom is one of the main diseases that occurs in cacao (Theobroma cacao), corresponding to one of the most limiting factors to cocoa production in South America. Moniliophthora perniciosa, the causal agent of the disease, is a basidiomycete with a hemibiotrophic lifestyle. Generally, hemibiotrophic fungal infection involves a transient and asymptomatic biotrophic phase. However, M. perniciosa presents a peculiar extensive and symptomatic biotrophic phase. The witches' broom characteristic symptoms of the biotrophic phase are loss of apical dominance and swelling and proliferation of lateral shoots in the infected branches. Despite the presence of the fungus at low densities in the apoplast of the host plants, this phase is characterized by expressive biochemical and physiological changes. Moniliophthora perniciosa can infect distinct hosts, being classified into biotypes. The C-biotype infects cacao, whereas the S-biotype colonizes members of the Solanaceae family, presenting similar symptoms of infection. In the tomato "Micro-Tom", a model system to study the pathogenicity of M. perniciosa and host response, the inoculation with M. perniciosa S-biotype at the shoot apex and axillary meristems of the first pair of leaves results in swelling of the stem close to the region of infection, but also led to changes in phenotype of leaves, petioles, fruits, and roots. Therefore, we aim to answer if such distant changes are result of hormonal, metabolic or physiological changes and to verify if the fungus presents systemic behavior. In "Micro-Tom", we verified the presence of M. perniciosa in stems, petioles, leaves, and fruits. The objective of this study is to quantify M. perniciosa C-biotype in the base, middle and top of cacao green brooms using qPCR, contributing with the knowledge related to the correlation of fungal biomass with the development and severity of symptoms and to identify the presence of the pathogen in distant regions from the infection. This method will potentially serve as a risk assessment technique for M. perniciosa infection, which may detect diseased plants in the field with poorly visible or asymptomatic symptoms and assist in the improvement and control/management programs of "witches' broom disease" in cocoa producing properties.