Fusarium species from nepalese rice and production of mycotoxins and gibberellic acid by selected species.

Infection of cereal grains with Fusarium species can cause contamination with mycotoxins that affect human and animal health. To determine the potential for mycotoxin contamination, we isolated Fusarium species from samples of rice seeds that were collected in 1997 on farms in the foothills of the Nepal Himalaya. The predominant Fusarium species in surface-disinfested seeds with husks were species of the Gibberella fujikuroi complex, including G. fujikuroi mating population A (anamorph, Fusarium verticillioides), G. fujikuroi mating population C (anamorph, Fusarium fujikuroi), and G. fujikuroi mating population D (anamorph, Fusarium proliferatum). The widespread occurrence of mating population D suggests that its role in the complex symptoms of bakanae disease of rice may be significant. Other common species were Gibberella zeae (anamorph, Fusarium graminearum) and Fusarium semitectum, with Fusarium acuminatum, Fusarium anguioides, Fusarium avenaceum, Fusarium chlamydosporum, Fusarium equiseti, and Fusarium oxysporum occasionally present. Strains of mating population C produced beauvericin, moniliformin, and gibberellic acid, but little or no fumonisin, whereas strains of mating population D produced beauvericin, fumonisin, and, usually, moniliformin, but no gibberellic acid. Some strains of G. zeae produced the 8-ketotrichothecene nivalenol, whereas others produced deoxynivalenol. Despite the occurrence of fumonisin-producing strains of mating population D, and of 8-ketotrichothecene-producing strains of G. zeae, Nepalese rice showed no detectable contamination with these mycotoxins. Effective traditional practices for grain drying and storage may prevent contamination of Nepalese rice with Fusarium mycotoxins.

Seedborne Infection of Rice by Pyricularia oryzae and Its Transmission to Seedlings.

Seedborne infection of rice by Pyricularia oryzae and its transmission to seedlings were studied quantitatively with naturally infected seeds of three rice cultivars collected from three locations in Nepal. A linear relationship on a logistic scale was found between panicle symptoms and seed infection, i.e., the more symptoms the higher seed infection. However, healthy-looking panicles and branches of panicles could also yield infected seeds. Postharvest measures such as winnowing and sun-drying significantly reduced seed infection by P. oryzae and filled grains had a lower degree of infection than unfilled grains. Sporulation of P. oryzae was most often confined to the embryonal end of germinating seeds. In contrast, most of the nongerminating seeds had sporulation all over the seed surface. Transmission of P. oryzae from seeds to seedlings, studied under various seeding conditions, showed that the transmission rate was always low. Thus, a seed sample with 21% seed infection resulted in less than 4% seedlings with blast lesions. Seed transmission was found for light covering of the seeds with soil or for moist seeding without covering. Transmission was rarely found when seeds were completely covered, and never in seedlings raised under water seeding conditions. Lower infection frequency was observed in seedlings raised in unsterilized soil than in seedlings raised in sterilized soil. Also, percent recovery of P. oryzae from infected seeds was higher in sterilized soil than in unsterilized soil and declined with time. Seedlings grown under low temperature (15 to 20°C) conditions did not develop blast lesions but when the same plants were transferred to high temperature (25 to 30°C) conditions, blast lesions were detected. This confirmed the latent infection in seedlings by P. oryzae grown under low temperature conditions.

Suppression of Rice Blast by Preinoculation with Avirulent Pyricularia oryzae and the Nonrice Pathogen Bipolaris sorokiniana.

ABSTRACT Avirulent isolates of Pyricularia oryzae and isolates of Bipolaris sorokiniana, a nonrice pathogen, were used to suppress rice blast caused by P. oryzae. In greenhouse experiments, both fungi substantially reduced leaf blast when applied 24 h or more before the pathogen. B. sorokiniana, but not avirulent isolates of P. oryzae, systemically reduced disease in leaf 5 when applied to whole plants at the four-leaf stage. In field experiments, both fungi were able to reduce neck blast significantly. No increase in grain yield was obtained by using avirulent isolates of P. oryzae, whereas five sprays with B. sorokiniana from seedling to heading stages increased the grain yield in two of three experiments conducted at two locations in Nepal. The significant increase in yield was observed under high inoculum pressure of P. oryzae. Induced resistance is suggested to be involved in the suppression of disease.