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Effects of Temperature and Incubation Period on the Expression of Resistance to Stem Nematode in Alfalfa¹

Three experimental Mfalfa (Medicago sativa L.) cultivars, resistant to stem nematode (Ditylenchus dipsaci Kuhn) populations found in the Piedmont and Appalachian Mountains of the Eastern USA, have been developed in North Carolina. Preliminary to a genetic study of resistance in these cultivars, the effects of different temperatures and incubation periods on the expression of resistance were determined. Eight cultivars representing a wide range of resistance were studied: three North Carolina experimental cultivars, one Swedish experimental cultivar, 'Lahontan,' 'Talent,' 'DuPuits,' and 'Cherokee.'

At 10, 17, and 24 days after inoculation, resistant cultivars contained fewer eggs and fewer nematodes per plant, and they showed less swelling at the cotyledonary node than susceptible cultivars. Nematodes penetrated resistant and susceptible plants in equal numbers; resistance resulted from the failure of nematodes to reproduce once they entered the plant. The best temperature for nematode reproduction was 19 C. Swelling at the cotyledonary node could be rated 10 days after inoculation, but 24 days at 19 C were required to obtain the best measure of nematode reproduction. Genotype x temperature interaction was not important over a temperature range of 13 to 25 C. Genotype x time (incubation period) interaction was significant (.05 level) over a time range of 10 to 24 days after inoculation. This interaction resulted from the failure of reproduction to occur in the resistant cultivars while reproduction proceeded rapidly with time in the susceptible cultivars. Lahontan contains a low frequency of extremely susceptible plants.

Key Words: Medicago sativa L. • Ditylenchus dipsaci Kuhn

² Graduate Associate, Crop Science Department, North Carolina State University, and Research Agronomist, Crops Research Division, at Raleigh, N. C. 27607.

Received for publication August 17, 1967.