Determining the rain fastness of mancozeb on apple leaves and the development of a benchmark spray deposition model for apple scab.

The contact fungicide mancozeb forms an integral part of apple scab (Venturia inaequalis) management in South Africa. Effective fungicide spray deposition is required for optimal disease management. It is important to link spray deposition to biological efficacy by developing a spray deposition benchmark model. Rainfall is an important factor that influences disease control by foliar applied pesticides. The projects objectives are to (i) determine if a yellow fluorescent pigment is a suitable tracer for mancozeb formulations, (ii) develop a mancozeb spray deposition benchmark model for apple scab and (ii) determine the rain fastness of different mancozeb treatments (Dithane M-45 800 WP NT, Ventum 800 WP, and Ventum 800 WP combined with a sticker-spreader adjuvant Nu-Film P).

The rain fastness of the three mancozeb treatments was studied by quantifying the percentage fluorescent pigment loss using macorphotography, fluorometry and image analyses and the percentage loss in mancozeb residues (manganese ions). Benchmark development involved apple seedling leaves being treated with a concentration range of mancozeb and fluorescent pigment (0, 0.15 ×, 0.3 ×, 0.45 ×, 0.6 × and 1.0 ×), followed by inoculation with conidia. Venturia inaequalis control was assessed using a basic fuschin based staining technique and visual assessment. The in vitro production of V. inaequalis conidia was investigated using a cellophane technique.

A yellow fluorescent pigment was shown to be a suitable tracer for five different mancozeb formulations. Simulated rain applied to apple seedlings at a constant rainfall intensity of 5 mm/h at five different rainfall volumes (0, 1, 5, 10 and 15 mm) resulted in no significant differences between the three mancozeb treatments. Although a good correlation (r = 0.726 to 0.783) existed between FPC% and Mn-ions, the response of FPC% and Mn-ion differed somewhat as was evident from the slopes of exponential regression models. A significant loss (11.95%) in mancozeb residue occurred after applying 1mm of rain, but no significant differences in losses (26.32 to 31.67%) occurred after applying 5 to 15mm of rain. The benchmark model development work showed that the staining technique was useful for quantifying V. inaequalis infection within 6 days, but it underestimated percentage control relative to the visual assessment of lesions after 3-4 weeks. Complete control was observed for all mancozeb concentrations based on visual lesion assessment. No function could thus be fitted to deposition quantity data versus disease control. The cellophane agar plate technique was optimized for the in vitro production of V. inaequalis conidium inoculum.

The study provided valuable information for the assessment of mancozeb deposition, mancozeb rainfastness, and benchmark model development. The yellow fluorescent pigment can be used as an excellent cost effective tracer for mancozeb deposition on apple seedling leaves, and will also be helpful for identifying trends on the effect of rain on the persistence of mancozeb. Mancozeb is relative rain fast following the application of 5 to 15mm rainfall at moderate intensity. Although a benchmark model could not be developed, a rapid staining technique for quantification of V. inaequalis disease severity was identified that can be optimized further.