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Effects of Temperature and Alfalfa Entry on Seedling Reaction to Stem Nematode¹

Because of a need for additional information regarding the effects of temperature on the reactions of alfalfa (Medicago sativa L.) seedlings from diverse sources to stem nematodes (Ditylenchus dipsaci (Kühn) Filipjev), we inoculated germinating seeds of 101 alfalfa varieties and experimental lines (entries) with 25 stem nematodes and grew the seedlings for 24 days at 16, 20, and 25 C. After 24 days the seedlings were rated for cotyledonary. node swelling and stained for counts of numbers of hem. atode adults, larvae, and eggs in plant cotyledons, the cotyledonary node, and the hypocotyl.

Greater cotyledonary-node swelling was observed at 16 and 20 C than at 25 C. A few entries differed in cotyledonary-node swelling; however, most of the entries were similar. Greater numbers of larvae and eggs were found at 16 and 20 C than at 25 C. Some entries differed for numbers of adults, larvae, and eggs counted in the seedlings. Greater numbers of larvae and eggs occurred in the hypocotyl than in the cotyledonary node or cotyledons. Number of adults, larvae, and eggs in the seedlings increased, particularly in the cotyledonary node and hypocotyl, as the cotyledonary-node swelling of the seedlings increased.

Due to our inability to easily separate resistant and susceptible entries on the basis of visual symptoms or nematode population counts, we concluded that the usefulness of an early-stage seedling evaluation similar to what we used is of questionable value to the breeder.

Key Words: Medicago sativa • Ditylenchus dipsaci • Disease • Breeding

² Research Agronomists, ARS, Prosser, Washington, and Reno, Nevada, respectively; Nematologist and Associate Agronomist, Washington State University, Irrigated Agriculture Research and Extension Center, Prosser, WA 99350.

Received for publication August 8, 1973.