Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/1997
Publication Date: N/A
Interpretive Summary: Alfalfa is a major crop of much of the U.S., but in the south it has not been widely used because of some disease problems. One of the serious problems is the disease called Sclerotinia crown and stem rot. This is a disease of the cool season which can lead to loss of all of the alfalfa on a newly planted field. Recently, breeders have shown that selection can improve the level of resistance to this disease. The evaluation procedure used to select more resistant alfalfa plants was modified so disease development could be monitored on individual stems from the fourth day after inoculation with disease to the fourteenth day. Earlier evaluations were made exclusively on the fourteenth day. Using this new evaluation procedure, it became apparent that plants selected for resistance were of two types. One type stopped disease development after eight days, but the second type had slow but continuous development of disease. With this information, plant breeders can now focus on selecting those plants which can stop disease development instead of just slowing it. It might be possible to include both types of disease resistance in the same plants, but future research will show that effect.
Technical Abstract: A stem-tip inoculation procedure has been successfuly used on alfalfa (Medicago sativa L.) with divergent selection for resistance to Sclerotinia trifoliorum (Eriks). One unreported observation made in early studies was that disease development was not continuous for some alfalfa plants over the fourteen days of incubation. A more in-depth study of disease development was needed to quantify the variability, and the stem-tip inoculation procedure was modified for this purpose. Inoculated stems were placed in plastic sleeves and daily marks were made on the plastic to show the development of necrosis over time. Eight plants previously selected for their higher levels of resistance and eight plants selected for minimal resistance (susceptibility) were evaluated. The susceptible plants developed necrosis described by a linear equation with 0.9 to 1.3 cm necrosis d-1. Of the more resistant plants, six had a cessation of disease development about eight days after inoculation and two had a linear development of disease at rates much less than that measured on the susceptible selections. Statistical analysis suggested disease resistance might be effected by either of two qualitatively different systems. There was a visual difference in the responses as well. When disease development was stopped, a dark border usually developed between the healthy and necrotic tissues which was not seen when disease continued to develop. If there are two systems causing resistance, their combination might further increase resistance to Sclerotinia.