Moniliophthora roreri and Moniliophthora perniciosa: Atypical Hemibiotrophs Causing Disease On Cacao

Moniliophthora roreri (frosty pod rot) and Moniliophthora perniciosa (witches’ broom) are hemibiotrophs causing disease in Theobroma cacao (cacao).  Both pathogens have a prolonged biotrophic phase (typically greater than 45 days).  During the biotrophic phase there is no evidence the pathogens form specialized cell wall penetration structures, instead surviving between cells.  The mycelial morphology changes with the onset of necrosis along with a change in nuclear state from haploid to dikaryotic.  M. roreri causes disease on pods and does not form basidiocarps while M. perniciosa forms basidiocarps and infects pods, flower cushions, and stems.  We sequenced the M. roreri genome and transcriptome.  The initial susceptible interaction between M. roreri and cacao is characterized by limited changes in cacao gene expression and slow progressive pod colonization.  Late in the biotrophic phase (30 days after infection), a significant cacao stress response occurs along with limited changes in the pod’s metabolite profile.  Necrosis ensues (60 DAI) along with the large scale reduction of pod metabolites.  Large scale shifts in fungal gene expression occur as the pathogen shifts from biotroph to necrotroph.  M. roreri tolerant cacao clones are available.  Pods from tolerant clones occasionally develop disease and we are characterizing the responses of M. roreri in these rare infections identifying fungal genes important in overcoming resistance.  Results from these and other studies concerning the interactions between Moniliophthora species and cacao will be discussed.  Of particular interest is the possibility that these pathogens began as endophytes, an aspect that may be critical to their pathogenic abilities.