Pathological factors affecting persistence in alfalfa with emphasis on diseases incited by Fusarium and Colletotrichum species

There is a range of biotic and abiotic factors that contribute to persistence of alfalfa. Enhanced disease resistance to lethal fungal pathogens may contribute to increased individual plant survival and hence, long-term persistence of production alfalfa stands. In order to investigate the effect of disease on alfalfa persistence, experiments were undertaken to examine the role of lethal and chronic fungal pathogens on alfalfa.; Disease incidence and severity data were recorded over a six-year period on sixteen cultivars representing six decades of cultivar improvement. The major diseases present that were considered to be limiting to plant survival were Fusarium wilt and anthracnose. Total disease incidence over the six years for both diseases was highest in cultivars released prior to 1980. Percent stand after five years showed a negative correlation with total disease incidence (reflecting Fusarium wilt, anthracnose, Rhizoctonia stem blight and Sclerotinia stem and crown rot). In the fourth production year, a random sample of surviving plants from six cultivars were selected from the field study, established in the greenhouse and randomly intermated, creating six new populations. The progeny of the six new populations that had undergone one cycle of field selection were tested in the greenhouse against the progeny of the same unselected plant material as was initially planted at the field site. Percent of resistant individuals for Fusarium wilt and anthracnose was higher in the populations that underwent field selection than the corresponding parent material in all cultivars. The results of this study provide evidence that the survivors, or those individuals exhibiting persistence in the field study, possessed resistance to the lethal diseases present. Reduction in yield becomes apparent as the stand ages and plant loss ensues.; During the course of the previous study, an isolate of Colletotrichum trifolii was identified at high frequency in modern cultivars with known levels of high resistance to anthracnose. This isolate, designated OH-WA-520, was retained and subjected to further studies to determine the race identity of the isolate. Results of these studies show a race reaction inconsistent with either C. trifolii race 1 or race 2. Isolate OH-WA-520 proved virulent on race differential alfalfa cultivars Saranac AR and Saranac but avirulent on Arc. Previous studies in Australia have shown similar results and have termed these isolates C. trifolii race 4. This is the first reported isolate reaction consistent with C. trifolii race 4 in the US. (Abstract shortened by UMI.)...