Rice blast epidemics initiated by infested rice grain on the soil surface

Field experiments were conducted in 1996 and 1997 with a marked strain of P yricularia grisea to determine if inoculum from infested rice grain could c ause primary infections and sustain a rice blast epidemic during the growin g season by giving rise to leaf, collar, and neck symptoms. The marked stra in, a sulfate nonutilizing (sul) mutant of P. grisea, was grown on autoclav ed rice seed for 7 days at 25 degreesC. Infested rice grains were applied t o the soil surface at the time of plant emergence (approximately 10 days af ter planting) at densities of 0, 0.5, 5, 25, and 50 grains per 0.1 m(2) in plots planted to the blast susceptible cv. M-201. Leaf blast symptoms were first detected in the plots containing infested grain 35 days after plant e mergence in both 1996 and 1997. The sul mutant was isolated from more than 90% of the lesions sampled from rice seedlings 35 to 45 days after plant em ergence. Leaf blast increased more rapidly in plots with 25 and 50 infested grains per 0.1 m(2) than in plots with less inoculum pressure (0.5 and 25 infested grains per 0.1 m(2)), although in 1996, leaf blast incidence recor ded at midseason in plots containing 0.5 and 5 infested grains per 0.1 m(2) was 41 and 55%, respectively. At the end of both seasons, the sul mutant w as recovered from over 90% of the leaf, collar, and neck blast lesions exce pt for one sample date in 1996. Rice blast was not detected in the control plots (no infested grain) in 1997 and not until 65 days after planting in 1 996. Comparisons of disease progress on leaves between the marked strain an d the parental wild-type strain under field conditions indicated that devel opment of disease caused by the sul mutant was similar to disease caused by the wild-type strain.