Infection of Rice Seed Grown in Arkansas by Pyricularia grisea and Transmission to Seedlings in the Field

Abstract

Rice blast, caused by Pyricularia grisea, is an important and serious disease of rice (Oryza sativa) in the southeastern United States. The disease sporadically reaches epidemic proportions on susceptible cultivars within fields and over large areas within Arkansas. The main overwintering sources of inoculum reportedly include infected rice stubble, related host species, and infected seed. The objectives of the research were to (i) determine whether rice seed grown in Arkansas were infected with P. grisea, (ii) investigate the relationship between seed infection and seedling disease, and (iii) determine if planting naturally infected seed could lead to the subsequent development of rice blast on seedlings in the field. The results of seed assays showed that P. grisea was detected in samples of foundation, certified, and production seed. Estimated levels of infection by P. grisea of rice seed from 66 samples of rice seed grown in Arkansas ranged from 0 to 10.5%. Planting infected seed in the greenhouse and the field resulted in seedling infection. Planting naturally infected seed may result in disease development (i) from seedlings grown from infected seed planted beneath the soil surface, (ii) from seedlings grown from germinating seed left on the soil surface, (iii) from seed coats, or (iv) from nongerminated seed left on the soil surface after planting. Additional research is necessary to establish the mechanisms of infection of seedlings and to establish disease thresholds for this important fungal pathogen of rice.

Transmission of rice blast from seeds to adult plants in a non‐systemic way

“…Thus, two non‐exclusive scenarios of rice blast epidemics in the field can be proposed. The first scenario is the systemic colonization of the plant from the seed via the stem vascular system, without symptom expression (as proposed by Guerber & TeBeest () and more recently by Marcel et al . ()).…”

“…Few studies have addressed the importance of rice resistance in the early stages of epidemics in younger plants. In previous reports, the fungus was not detected on seeds of highly resistant rice accessions (Guerber & TeBeest, ). Using fluorescent M. oryzae strains enabled the rice blast progression on infested seeds and seedlings to be followed easily and in detail.…”

“…Tests employing different ways of covering seeds with soil (light or complete) and under water seeding (no covering) pointed out that complete covering or seeding under water induce a lower infection frequency (Manandhar et al ., ). Guerber & TeBeest () reported similar experiments in the USA. They showed that 23% of the seed lots examined, including commercial lots, were contaminated and that the corresponding rate of diseased seedlings varied from 0·3 to 11% in greenhouse conditions.…”

“…These results clearly show that the fungus can survive on the grains used for seeding and that these contaminated grains can serve as primary inoculum. However, Guerber & TeBeest () did not detect the fungus in seeds of highly resistant rice varieties. Bernaux & Berti () reported that overwintering of mycelium and conidia could occur between the glumella and caryopsis because of the susceptibility of palea and lemma before anthesis.…”

Sensitivity of Nonexposed and Exposed Populations of Magnaporthe oryzae from Rice to Tricyclazole and Azoxystrobin

“…In Italy, it was described for the first time by Astolfi in 1828 and, since then, blast epidemics are recurring with increasing frequency and severity (2,12). The fungus can overwinter in straw or other rice residues or, in areas characterized by cold winters, in infected rice seed, which serve as primary inoculum for disease epidemics (11,19,28). M. oryzae can infect rice at all developmental stages, causing leaf or collar blast, and, more economically important, neck or panicle blast.…”