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Alternaria leaf blight disease caused by Alternaria brassicae in mustard (Brassica juncea (L) Czern and Coss) was studied in two crop seasons, 1992 and 1993 in Nepal at Nawalpur, Sarlahi (Tarai) and Khumaltar, Lalitpur (mid hill). At Nawalpur, epidemics of the disease was recorded for both seasons. Weather conditions like humidity, temperature and frequent rainfall played key role for the epidemics. In experimental fields, the disease appeared first in mid-December when the relative humidity was more than 80% with maximum temperature ranging between 18-25 0 C and minimum between 10-14 0 C. At Khumaltar, incidence of the disease was low in both seasons due to low average temperature from December to February. Among cultivars, Krishna and Pusabold were less susceptible than Varuna. Both mancozeb and iprodione had effectively reduced disease in the sprayed plots and increased seed yield by 48% and 130% respectively. The correlation between disease severity and yield, and yield components was negative and highly significant. Average yield loss was estimated to be in the range of 32 to 57%. Seed infection was also significantly higher in non sprayed treatment than sprayed one. The disease showed a negative effect on oil content causing losses on oil between 4.2 to 4.5%.

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.

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