Biological control of botrytis leaf blight of onion byGliocladium roseum applied as sprays and with fabric applicators

Gliocladium roseum (5×10⁶ conidia ml^–1) and chlorothalonil (Bravo 500) were compared in two field tests for effectiveness in suppressing leaf blight caused byBotrytis squamosa in cooking onions. The biological control agent and fungicide were applied with sprayers and with specially designed fabric applicators that were mounted interchangeably on an aluminium carriage with bicycle wheels. In the applicators, inoculum and fungicide gravitated from a reservoir down curtains of denim strips and onto portions of onion leaves contacted by the strips. Initial applications were timed using a leaf blight forecasting system (BOTCAST) and three or four subsequent applications were made at weekly intervals. When compared to water checks,G. roseum applied as sprays or by applicators reduced density of leaf spots by about 50–58% during middle and late stages of epidemics. The antagonist was about half as effective as chlorothalonil in suppressing density of spots, regardless of application method. The applicators delivered inoculum in similar densities to proximal and distal halves of leaves, and used at least 35% less volume of inoculum compared to the sprayers. Density of inoculum on leaves treated by applicators was about the same as in spray-treated leaves when canopies were sparse, but less when canopies were dense. It is concluded thatG. roseum has good potential for controlling leaf blight sufficiently to avoid economic yield losses.