Browsing by Author "Rossouw, Cornelis Johannes"

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    Mancozeb rainfastness and residue thresholds for control of Venturia inaequalis
    (Stellenbosch : Stellenbosch University, 2016-03) Rossouw, Cornelis Johannes; McLeod, Adele; Fourie, P. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH SUMMARY: An important apple disease world-wide is Venturia inaequalis that causes scab-like lesions on fruit and defoliation of trees. In South Africa, the disease is mainly managed through fungicide sprays that mostly consist of the contact fungicide mancozeb. The volume and intensity of rainfall can influence the efficacy and persistence of mancozeb depositions. Mancozeb spray deposition can be assessed through expensive quantification of Manganese-ion residues, or a cost effective fluorescent pigment of which the coverage is accessed using photomacrography image analyses. The biological efficacy of quantitative mancozeb depositions can be determined through the development of benchmark models. For V. inaequalis, this is challenging since in vitro production of inoculum is difficult, and leaf lesions are slow to develop and not amendable to quantification through image analyses. The study showed that a yellow fluorescent pigment was a suitable tracer for five different mancozeb formulations (Dithane M-45 800 WP NT, Mancozeb 800 WP, Pennfluid, Ventum 800 WP, Vondozeb) since a good Pearson’s correlation (r = 0.779) existed between fluorescent particle coverage (FPC%) and mancozeb residues (Mn-ion (mg/kgDW)).The particle size ranges of Dithane M-45 800 WP NT and Ventum 800 WP were significantly smaller than those off the other formulations, but this did not result in differences in Mn-ion residues realized on apple leaves for WP formulations. The Pennfluid SC formulation deposited markedly less mancozeb than the WP formulations, due to a lower active ingredient label rate used. Rainfastness was evaluated for the Dithane M-45 800 WP NT and Ventum 800 WP formulations, and Ventum 800 WP combined with a sticker-spreader adjuvant Nu-Film P. 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 treatments, based on FPC% and Mn-ion concentrations. 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, showing slower initial loss in FPC% than for Mn-ions and a markedly larger predicted loss by the model’s asymptotic value (61.4% and 32.2%, respectively). 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. Benchmark development for mancozeb deposition for the control of V. inaequalis 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 harvested from naturally infected orchard leaves. Venturia inaequalis control was assessed using a basic fuschin based staining technique and visual assessment. The staining technique was useful for quantifying infection within 6 days, but 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 (0.29 to 8.28 FPC% values) versus disease control (staining or visual assessment). The cellophane agar plate technique was optimized for the in vitro production of V. inaequalis conidium inoculum that can be used in future infection studies. After 1 week, optimum spore production (1.59× 106 conidia/ml) and viability (± 85%) were observed that were significantly higher than at weeks 2 to 4. The study provided valuable information for the assessment of mancozeb deposition, mancozeb rainfastness, and for benchmark model development with regards to the rapid quantification of lesions on leaves and the in vitro production of V. inaequalis inoculum. The yellow fluorescent pigment can now be used as an excellent cost effective tracer for mancozeb deposition of various formulations on apple seedling leaves, and will also be helpful in identifying trends on the effect of rain on the persistence of mancozeb formulations. This has increased research capacity towards evaluating the rainfastness of fungicides using simulated rain, and orchard trials for accessing the effect of spray volumes and machines through fluorescent pigment deposition. Although a benchmark model could not be developed, the development of a model in future will be more feasible since a rapid staining technique for quantification of V. inaequalis disease severity was identified that will just have to be optimized further. Secondly, a cellophane agar plate technique, and isolates with high spore production capacity for axenic conidial production will further facilitate model development.