A recessive resistance gene of alfalfa to alfalfa mosaic virus and impact of high ambient temperature on virus resistance

Twenty plants of alfalfa cv. Beaver were screened for resistance to alfalfa mosaic virus (AIMV) severe strain A-515. ELISA screening on both inoculated and apical leaves at fixed temperature (20°C day, 16°C night) suggested the following three types of clonal response to AIMV infection: extremely resistant, AIMV not normally detected from either inoculated or apical leaves; resistant, AIMV detected from inoculated leaves only and did not spread systemically; susceptible, AIMV detected from both inoculated and apical leaves. When plants in the second category were maintained at high temperature (30°C), AIMV was detected from inoculated and apical leaves 6 and 12 days after inoculation, but was not detectable in the apical leaves thereafter. Plants in the first category remained extremely resistant at all temperatures tested. The results of comparative tests using progeny plants of extremely resistant, resistant and susceptible plants, and of their hybrids, suggested that the resistance to AIMV A-515 was controlled by a temperature-dependent recessive gene.