Performance of Systemic Acquired Resistance Products for Control of Glomerella Leaf Spot of Apples in Georgia, 2025

Caused by a Colletotrichum spp. complex, Glomerella leaf spot produces brown spots on leaves and fruits of genetically susceptible apple cultivars, leading to premature leaf drop and a decrease in marketable fruit. Systemic acquired resistance (SAR) chemicals were tested for controlling Glomerella in a 24-year-old block of Malus domestica ‘Golden Smoothee’ apple trees in Blairsville, GA. The standard treatment regimen incorporated chemical fungicides with proven efficacy against Colletotrichum spp., while incorporating resistance management practices such as alternation and tank-mixing. Additional treatments included Actigard plus Kudos (tank-mixed) and Vacciplant as standalone applications in early and late cover sprays, separated by three applications of captan plus a potassium phosphite fungicide. The standard regimen significantly reduced disease, whereas the control was substantially less effective when either Actigard plus Kudos or Vacciplant replaced standard chemical fungicides. This suggests that adding SAR chemicals did not offer comparable or sufficient control when integrated into a standard spray program.