Inoculation method, temperature, and relative humidity affect leaf and neck anthracnose, a new onion disease in Michigan

Abstract

Leaf and neck anthracnose caused by Colletotrichum coccodes is a newdisease of onion inMichigan. To test the effect of inoculation method, Prince onion seedlings were grown in the greenhouse and inoculated with either a conidial suspension of C. coccodes (alone or with an abrasive agent) or infested millet seed (dry or wet, 2 or 5 g). Foliar disease severity was greater when a conidial suspension (> 39%) was used compared with infested millet seed (≤24.3%). Growth chamber studies were conducted using Infinity onion seedlings that were inoculated with a conidial suspension spray to determine the effects of temperature (15, 20, 25, or 30°C) and duration (0, 12, 24, 48, or 72 h) of high (95 ± 5%) relative humidity (RH) on disease severity (percentage of leaf area with C. coccodes lesions). Significant differences and interactions among temperature and RH were observed. The combination of high temperature (≥25°C) and extended (≥24 h) high RH resulted in > 20% disease severity 28 days postinoculation. Results suggest that onion leaf and neck anthracnose symptoms are likely to bemore severewhen the environmental conditions are ≥25°C with ≥24 h of high RH.