CAUSAL AGENTS OF NECROTIC SPOTS ON FABA BEANS: LYGUS, BOTRYTIS SPP. OR BOTH?

Faba beans (Vicia fabae L.) can have agronomic advantages for pulse producers, such as fixing 30% more N than other pulse crops, tolerance to water-logging, and resistance to lodging. However, faba bean seed quality is often downgraded due to the presence of necrotic spots on the seeds which affects grain marketability. Chocolate spot disease, caused by Botrytis spp., and Lygus spp. are often found concomitantly in faba bean fields. The objective of this study was to determine the role of Botrytis spp. and lygus bug on reducing seed quality under field conditions. In 2015, field surveys were conducted in faba bean growing regions of Alberta. Thirty fields were surveyed at the pod stage to quantify insects and chocolate spot, and samples of seeds collected to rate for damage levels and isolation of fungi. Correlation analysis was used to test for possible associations among insect abundance, chocolate spot disease levels in the field, percent seed infection and seed damage. Chocolate spot incidence and severity was generally low across Alberta, but Botrytis, Fusarium and Alternaria spp. were frequently isolated from seeds. Seed damage was significantly higher for seeds collected from the top, where lygus bugs were abundant, than the middle and bottom canopy, where disease was more abundant. However, there was no significant correlation between insect or disease intensity and seed damage levels. Greenhouse studies to evaluate interactions between lygus and chocolate spot under controlled environmental conditions are underway.