Association of Alfalfa Radicle Lenght with Rhizoctonia Damping‐Off

Seedling damping-off is one of several important diseases of alfalfa (Medicago sativa L.) caused by Rhizoctonia solani Kühn. While evaluating alfalfa seedlings for resistance to damping-off, we observed that those populations with faster germination and radicle growth rate generally appeared to be more resistant to damping-off. Therefore, this study was done to determine if alfalfa populations vary in radicle growth rate or length, and if radicle length is associated with resistance to seedling damping-off caused by R. solani. We determined radicle growth rate by measuring radicle length at 48 h after placing seeds in moist conditions. Resistance to damping-off was visually scored according to amount of radicle symptoms. In one experiment, groups of seedlings with long radicles averaged 21.4% resistance, compared with 13.9% for seedlings with short radicles. In another experiment, resistance was about 35% for the seedlings with long radicles versus 11% for short radicles across two plant populations. The correlation coefficient between radicle length at inoculation and resistance score 5 d later (lower score indicates higher resistance) was −0.29 (P < 0.01, n = 200); this moderate correlation coefficient and the variability in resistance within radicle length groups suggest that factors in addition to age and radicle length condition resistance. We recommend selecting seedlings for resistance within groups of common radicle length to reduce non-genetic influences on selection progress.