REACTION OF INCOMPATIBLE ISOLATE OF PYRICULARIA-GRISEA ON RICE LEAVESSTRIPPED OF EPICUTICULAR WAX REFLECTS BLAST CONDUCIVENESS OF SOILS

Upland rice cultivars were evaluated in the greenhouse for susceptibil ity to the rice blast disease caused by Pyricularia grisea Sacc., on t wo upland soils from the Philippines previously considered to be ''bla st conducive'' and ''blast non-conducive''. Under monocyclic inoculati on tests plants grown in conducive soil showed significantly greater l esion development than plants of the same cultivar grown in non-conduc ive soil: cultivars considered to be susceptible to the isolates used showed increased number of susceptible-type lesions; resistant cultiva rs showed increased number of hypersensitive resistant-type lesions. A similar effect was observed under polycyclic tests where several gene rations of the pathogen were allowed to develop on the test plants. Di lution of conducive soil with non-conducive soil resulted in a corresp onding reduction of disease severity, although this was most pronounce d on resistant cultivars. Removal of leaf epicuticular waxes (LEW) usi ng organic solvents increased the number of resistant-type lesions on resistant cultivars grown in both soils following inoculation. Suscept ible plants were not suitable for quantifying the relative blast condu civeness of a soil because of the extreme environmental sensitivity of the bioassay and the tendency of lesions to coalesce. Comparing numbe rs of resistant-type lesions on leaves of plants stripped of LEW and i noculated with an incompatible P. grisea isolate among plants grown in different soils proved to be a satisfactory means of distinguishing t he relative blast conduciveness of soils under controlled conditions. This method was field tested in eastern India and results corroborated farmer assessment of which soils were blast conducive. Using incompat ible isolate-cultivar combinations and LEW-free leaves is proposed as a simple bioassay for assessing blast conduciveness of soils and shoul d prove useful in regional characterization of rice blast risk.