Blast is a devastating disease of rice caused by the fungus Pyricularia oryzae. The disease is present wherever rice is grown, and was ranked as the third cause of global yield losses among rice pests and diseases. Under conditions favorable to the disease, attacks on the panicle result in very severe yield losses of 80 to 100%. The role of infected straw and seed as sources of primary inoculum in blast disease epidemics is well known. The role of alternative hosts is yet to be confirmed.
The current study sought to assess if wild rice is a major source of inoculum for cultivated rice by comparing the genetic structure of P. oryzae populations from both hosts. Two hundred and eighty-two strains were genotyped with 12 Simple Sequence Repeats (SSR) markers. Cross infectivity of 20 P. oryzae isolates was also assessed using pathogenicity tests. Samples were collected from cultivated and wild rice organs with blast symptoms in irrigated and lowland areas of Mali in the Koulikoro, Sikasso, and Ségou regions. Under controlled conditions, P. oryzae isolates from Oryza longistaminata wild rice were pathogenic to cultivated rice and had a similar range of cultivar compatibility to isolates from cultivated rice.
Results of pathogenicity tests suggest that P. oryzae isolates from wild rice (O.longistaminata and O. barthii) have the potential to attack cultivated rice in the field. However, populations of P. oryzaeon cultivated and wild rice (O. longistaminata and O. barthii) were genetically differentiated. A very large fraction (94.4%) of the multilocus genotypes identified were specific of one host species or the other. Thus, although physically close, each rice species hosts a different population of the pathogen, and our results support the hypothesis that wild rice is not a major source of blast inoculum for cultivated rice.
Diariatou Diagne, Henri Adreit, Joëlle Milazzo, Ousmane Koita, and Didier Tharreau published this work in Plant Pathology Journal:
TITLE IMAGE: Pyricularia oryzae isolates from wild rice species are pathogenic to cultivated rice, but each rice species hosts a different pathogen population, showing that wild rice is a minor source of blast inoculum for cultivated rice.
