Spray Deposition and Control of Botrytis cinerea on Grape Leaves and Bunches: Part 1 (Table Grapes)

Insufficient quantity but also quality of spray deposition on susceptible grapevine tissue (i.e. target sites) andfavourable conditions for pathogens could lead to control failure during high disease pressure situations.To determine deposition quantity and quality benchmarks for biologically effective spray deposits,bunches and leaves of table grapes (Waltham Cross) were sprayed at various growth stages, using differentapplication volumes of a mixture of fenhexamid and a fluorescent tracer pigment and subsequently dustedwith dry conidia of Botrytis cinerea where after infection levels on pedicels, receptacles and leaves weredetermined. Pigment deposition quantity and quality were determined from photos of sprayed parts takenwith a digital camera under a stereo microscope and black light illumination at ×30 or ×10 magnificationsand assessed with digital image and Hoerl regression analyses. The deposition quantity resulting in 75%control of B. cinerea infection (FPC75 values) was calculated from biological efficacy curves (sigmoidalregression analyses) for leaves and for each growth stage, for pedicels and receptacles. Deposition quantityand quality measurements correlated favourably with Botrytis infection. An optimal deposition value forthe control of B. cinerea was determined by increasing spray volume, however by increasing spray volumeand deposition quantity or quality levels past this optimum will not significantly improve disease controlfurther. It was indicated that efficacy of agricultural chemicals could be influenced by improving bothdeposition quantity and quality, quantifiable by digital image analyses of fluorescent pigment deposition.FPC75 values obtained in this study can be used as benchmarks to evaluate future spray application invineyards