DEVELOPMENT OF RESISTANCE TO MICRODOCHIUM-NIVALE IN WINTER-WHEAT DURING AUTUMN AND DECLINE OF THE RESISTANCE UNDER SNOW

The effect of autumn climate on the development of resistance to pink snow mold (Microdochium nivale) in winter wheat was estimated in mid-December. Changes in resistance over time under snow cover were also determined. Resistance in December was closely correlated with the accumulated degree-days above 0 degrees C from sowing. The number of days of incubation at which 50% of the plants are killed (LI(50)) was lowest in the cool autumn in 1988 and highest in the warm autumn in 1989. Temperatures below 5 degrees C were also required for expression of resistance. Differences in LI(50) between resistant and susceptible cultivars were most apparent in late autumn. Continuous snow cover was found to reduce resistance to pink snow mold. The rate of decline of the resistance in cv. Nanbukomugi during the winters under snow was lower than in cv. PI 173438 and cv. Kitakamikomugi. The resistance to pink snow mold was correlated with the amount of etiolated growth at 25 degrees C. This indicates that exhaustion of food reserves during prolonged snow cover predisposed wheat plants to snow mold diseases.