Assessment of Seed Treatments for Management of Pathogens Causing Seed Rot, Seedling Damping-off, and Seedling Root Rot in Alfalfa ( <i>Medicago sativa</i> )

Seed rot and damping-off of alfalfa (<i>Medicago sativa</i> L.) is caused by a soilborne disease complex resulting in thin initial stands of plants with continuing damage during wet soil conditions decreasing forage yields and winter survival. Apron (metalaxyl) and Apron XL (mefenoxam) have been used on alfalfa seeds for decades to reduce damping-off and early season root rot. Although several other anti-oomycetes (oomicides) and fungicides are labeled for use on alfalfa seeds, there is little information on their efficacy against specific pathogens or the disease complex. Our objective was to test chemical seed treatments to identify those with broad antimicrobial activity to aid in establishing alfalfa stands where pathogen complexes are present. First, 10 products were tested for mycelial growth inhibition of nine alfalfa pathogens. The two most promising, EverGol Energy and Intego Solo, were active against <i>Globisporangium ultimum, Pythium irregulare, P. sylvaticum, P. paroecandrum</i>, <i>Aphanomyces euteiches</i>, and <i>Phytophthora medicaginis</i>. Protection of seedlings in agar plate bioassays and infested soil assays against <i>G. ultimum</i> and <i>Pythium</i> species by EverGol Energy was like that of Apron XL, whereas Intego Solo was not effective. The EverGol Energy seed treatment provided modest protection in bioassays with <i>A. euteiches</i> and <i>P. medicaginis</i> for susceptible cultivars, although it did not improve protection for resistant cultivars. Combined seed treatments did not improve seedling health in single pathogen bioassays or experiments with naturally infested field soil with multiple pathogens. Seed treatments with Apron and Apron XL were the most effective in providing partial protection against the disease complex in field soil. These results suggest that greater resistance is needed for managing the disease complex.