The effects of the fungicides azoxystrobin (a strobilurin) and epoxiconazole (a sterol biosynthesis inhibitor) on phyllosphere fungi, senescence and yield were studied in winter wheat in field trials free of visible disease and under controlled environmental conditions. In two field trials, treatments with each of the two fungicides prolonged green leaf area retention and increased yield compared with untreated control plots. Azoxystrobin maintained green leaf area for longer than epoxiconazole and, in one trial, treatments with azoxystrobin gave a greater yield response than epoxiconazole. Mycelial growth on leaf surfaces, mainly originating from saprophytic fungi, was reduced by each of the fungicides. Papilla formation and hypersensitive reactions, almost exclusively against infection attempts by Mycosphaerella spp. (most probably M. graminicola), occurred with high frequency in the leaves. These defence reactions presumably incurred a significant energy cost, accelerating plant senescence. Fewer defence reactions were recorded in azoxystrobin-treated leaves than in epoxiconazole-treated and untreated leaves. Inoculation in a glasshouse experiment with the saprophytic fungi Alternaria alternata and Cladosporium macrocarpum accelerated wheat senescence. Control of the saprophytes by azoxystrobin or epoxiconazole treatments caused a delay in the accelerated senescence, but without significant increase in above-ground biomass and yield. Neither fungicide influenced senescence, above-ground biomass or yield in noninoculated wheat plants. In growth chamber experiments azoxystrobin inhibited spore germination and mycelial growth of A. alternata and C. macrocarpum. Epoxiconazole had little inhibitory effect on spore germination, but strongly inhibited mycelial growth of both saprophytes. Both fungicides reduced A. alternata-induced papilla formation in wheat leaves, with epoxiconazole being more effective. Inoculation with either of the two saprophytes did not significantly increase wheat leaf respiration, in contrast to inoculation with the nonhost pathogen Erysiphe graminis f.sp. hordei. Treatment with azoxystrobin did not affect this latter increase in respiration whereas it was reduced by epoxiconazole treatment. It is proposed that the greater inhibition of infection attempts from Mycosphaerella spp. by azoxystrobin, compared with epoxiconazole, may account for the greater yield given by azoxystrobin in field plots.

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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.

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.