Polymyxa graminis, a promiscuous vector of viruses on crops in tropical areas.
Polymyxa graminis, an obligate root endoparasite of monocotyledonous plants does not cause direct damage but is the vector of up to 15 plant pathogenic viruses of which 3 occur in the tropics (Indian peanut clump virus in the Indian subcontinent, peanut clump virus in Africa and rice stripe necrosis virus in Africa and South and Central America). The development of PCR methods for detecting and quantifying P. graminis confirmed the role of this parasite in the transmission of viruses to groundnut and allowed a determination of the plant species playing a key role in the carry-over and spread of the disease. These studies have revealed an unsuspected promiscuous behaviour for P. graminis, i.e. by multiplying extensively in cereals and infecting (with or without developing further) a wide range of species including fortuitous hosts such as groundnut and other dicotyledonous crops. This protist acquires, carries, protects in its resting spores over many years and transmits viruses depending on the vegetation pattern. The outcome is that several crop or weed species not normally affected by the viral infection actually influence the epidemiology of the disease by participating in the build-up or depletion of viruliferous inoculum. Strategies such as crop rotation, varietal selection or trap cropping were therefore developed to manage P. graminis. Nevertheless, these strategies need to be adjusted to deal with the prevailing form species of P. graminis that are characterized by specific ecological and molecular features and by differences in their host ranges.