Black spot disease or Alternaria black spot (ABS) of pecan (Carya illinoinensis) in South Africa is caused by Alternaria alternata. This fungal pathogen impedes the development of pecan trees and leads to low yield in pecan nut production. The present study investigated the in vitro effect of six fungicides against the mycelial growth of A. alternata isolates from ABS symptoms. Fungicides tested include Tilt (propiconazole), Ortiva (azoxystrobin), AgTin (fentin hydroxide), and Bellis (boscalid + pyraclostrobin). All fungicides were applied in 3 concentrations (0.2, 1, and 5 μg mL-1). Tilt and Bumper 250 EC containing propiconazole active ingredient (demethylation Inhibitors) were the most effective and inhibited all mycelial growth from up to 6 days post-incubation. The other active ingredients (succinate dehydrogenase inhibitors, organotin compounds, and quinone outside inhibitors) showed 75-85% mycelial growth inhibition. The effective concentration to inhibit mycelial growth by 50% (EC50) was estimated for each isolate and fungicide. The overall mean EC50 values for each fungicide on the six isolates were 1.90 μg mL-1 (Tilt), 1.86 μg mL-1 (Ortiva), 1.53 μg mL-1 (AgTin), and 1.57 μg mL-1 for (Bellis). This initial screening suggested that propiconazole fungicide was the most effective for future field trials test and how these fungicides could be used in controlling ABS disease.
Keywords: Alternaria alternata; Carya illinoinensis; azoxystrobin; boscalid; disease management; fentin hydroxide; fungicide sensitivity; propiconazole; pyraclostrobin.