Rice Blast Epidemics Initiated by Infested Rice Grain on the Soil Surface

Abstract

Field experiments were conducted in 1996 and 1997 with a marked strain of Pyricularia grisea to determine if inoculum from infested rice grain could cause primary infections and sustain a rice blast epidemic during the growing season by giving rise to leaf, collar, and neck symptoms. The marked strain, a sulfate nonutilizing (sul) mutant of P. grisea, was grown on autoclaved rice seed for 7 days at 25°C. Infested rice grains were applied to the soil surface at the time of plant emergence (approximately 10 days after planting) at densities of 0, 0.5, 5, 25, and 50 grains per 0.1 m2 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 emergence 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 emergence. Leaf blast increased more rapidly in plots with 25 and 50 infested grains per 0.1 m2 than in plots with less inoculum pressure (0.5 and 25 infested grains per 0.1 m2), although in 1996, leaf blast incidence recorded at midseason in plots containing 0.5 and 5 infested grains per 0.1 m2 was 41 and 55%, respectively. At the end of both seasons, the sul mutant was recovered from over 90% of the leaf, collar, and neck blast lesions except 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 1996. Comparisons of disease progress on leaves between the marked strain and the parental wild-type strain under field conditions indicated that development of disease caused by the sul mutant was similar to disease caused by the wild-type strain.

Tissue-Adapted Invasion Strategies of the Rice Blast Fungus Magnaporthe oryzae

“…New seedling infestations after winter or crop rotation periods are unlikely to occur from conidia as a source of inoculum (Valent, 2004). Instead fungal inoculum conserved in the soil in the form of mycelium on plant remnants (e.g., straw or seed; Ou, 1985;Long et al, 2001) or, alternatively, resting structures (microsclerotia) might set the infection cycle in motion starting from roots as a primary port of entry (Figure 6;Sesma and Osbourn, 2004;Valent, 2004). The omnipresence of rice blast wherever rice is cultivated (http://www.knowledgebank.irri.org) lends further support to this model.…”

Origins of Host-Specific Populations of the Blast Pathogen Magnaporthe oryzae in Crop Domestication With Subsequent Expansion of Pandemic Clones on Rice and Weeds of Rice

“…Although sexual reproduction 600 BP), and, more recently, as a result of European colonization, to Africa and North and South America occurs in vitro (isolates from Eleusine tend to show high fertility), the morphological structures, perithecia, are ‫005-003ف(‬ BP) (Oka 1988). Introduction of O. sativa to Africa also brought it in contact with the more recently not observed in nature (Hebert 1971;Silué and Nottéghem 1990;Zeigler 1998). There is indirect evidomesticated ‫0053ف(‬ BP), closely related, West African domesticated species, O. glaberrima (Chang 1976).…”

“…Dispersal of rice can also disperse demic clones specialized for pathogenicity against rice cultivars. rice blast (Long et al 2001). Even if rare, seed transmission has probably introduced, and reintroduced, rice…”

Varietal Differences in Rice (Oryza sativa L.) Resistance to the Shield Bug, Aspavia armigera (Fabricius) (Hemiptera: Pentatomidae)

“…Production practices such as varietal selection impact insect populations and damage because some rice varieties are more tolerant to biotic stresses than others (Long et al, 2001). In fact, one of the ideal methods of minimizing insect damage on rice is to cultivate resistant varieties (Akinsola, 1985).…”