Environmental factors affecting expression of resistance to pink snow mold caused by Microdochium nivale in winter wheat

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Abstract
The effects of prehardening growth, cold-hardening temperatures, duration of cold hardening, light intensity, and light period during cold hardening on the development of resistance to Microdochium nivale in winter wheat were studied under controlled environment conditions. Resistance was expressed as the median lethal incubation days (LI50) measured by the optimum temperature inoculation method of T. Nakajima and J. Abe. Plant growth at 20:15 °C (light:dark) had the largest effect on augmenting resistance to M. nivale in winter wheat, but conditioning at low temperatures was essential for expression of resistance. Low temperature conditioning at 6–4 °C under low light intensities initiated a rapid development of M. nivale resistance; this process was slower at 4–12 °C. ‘PI 173438’, resistant to snow molds but not to low temperatures, required lower temperatures during cold hardening for full expression of resistance to M. nivale than ‘Nanbukomugi’, which was moderately resistant to snow molds and low temperatures. When conditioned at 2 °C, the plants subjected to the dark remained susceptible but developed resistance rapidly when exposed to low light intensities of 150 μmol ∙ m−2 ∙ s−1. Extending the light period from 8 to 16 h did not affect the expression of resistance to M. nivale. These results suggest that the pattern of development of snow mold resistance is substantively different from that involved in freezing tolerance, although both appear to be conditioned by low temperatures. Keywords: Monographella nivalis, Fusarium nivale, Triticum aestivum L., cold hardiness, snow mold, winter wheat.