Screening methodology for post-harvest fungal resistance in alfalfa

Forage deterioration due to field and storage fungi represents a major economic loss for hay producers. A series of experiments was conducted to develop a methodology for screening alfalfa plants for resistance to post-harvest fungal colonization. Pure cultures of Aspergillus glaucus, Aspergillus repens, Aspergillus flavus, Aspergillus versicolor and Aspergillus fumigatus, isolated from alfalfa forage sampled during field wilting and storage, were established and maintained to pr duce a 10⁵ spore mL^-1 suspension containing an equal proportion of spores from each Aspergillus spp. Alfalfa leaves were dipped in concentrations of propionic acid solution to simulate levels of resistance to fungal activity. Fully-developed trifoliate leaves were placed adaxial side down onto petri plates containing a dichloran (2 mg mL^-1) and glycerol (18% vol/vol) agar. The Aspergillus spp. spore suspension was sprayed onto each petri plate and the plates were incubated under dark conditions at 25°C and 70% relative humidity. Plates were monitored daily for percent of leaf area colonized. The random screening of a small population of cloned alfalfa genotypes using this screening procedure indicated that variation in post-harvest resistance to fungal attack does exist for greenhouse and field-grown plants harvested at an early vegetative stage. Leaf dry matter was not related to genotype variation in post-harvest susceptibility to fungal colonization; however, leaf soluble carbohydrate level was negatively correlated (P < 0.05) with leaf area colonized. The screening procedure did not detect consistent differences among genotypes when dried leaves were used.