Masters Degrees (Plant Pathology)

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Browsing Masters Degrees (Plant Pathology) by Subject "Adjuvants"

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    The use of adjuvants to improve fungicide spray deposition on grapevine foliage
    (Stellenbosch : Stellenbosch University, 2009-03) Van Zyl, Sybrand Abraham; Fourie, P. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Sufficient fungicide deposition on the target site is an essential requirement for effective chemical management of fruit- and foliar diseases such as grey mould of grapevines. Control failure is often attributed to insufficient quantitative deposition on susceptible grapevine tissue. However, in high disease pressure situations control failure might also be attributed to poor qualitative deposition. The primary objective of spray technology is to optimise deposition, of which the plant surface is a critical component in the spray application process, specifically in the retention of spray droplets. Adjuvant technology is reported to improve the wettability and spread of droplets by surface-acting-agents on the target surface and thereby improve deposition and retention of the fungicide active ingredient. However, this relatively new spray technology on viticulture and horticultural crops, and possible effects of adjuvants on epicuticular wax affecting plant disease development, needs to be investigated. Moreover, the development of useful prescriptions for adjuvants by determining water volumes and adjuvant dosages is required for different pesticide tank mixes. The aims of this study were, firstly to determine the effect of selected adjuvants on quantitative and qualitative spray deposition on grapevine leaves and subsequent biological efficacy of a fungicide, and secondly to evaluate selected adjuvants under field conditions and determine the effects of adjuvant dosage and spray volume on deposition. Leaves were sprayed under similar laboratory conditions to pre-run-off with 1 mL of a mixture of fenhexamid (Teldor® 500 SC, Bayer) at recommended dose, a fluorescent pigment (SARDI Fluorescent Pigment, 400 g/L EC; South Australian Research and Development Institute) at 0.2 L/100 L, as well as 15 selected commercial adjuvants to manipulate the deposition quality of a given quantity of deposited spray. Spray deposition on leaves was illuminated under black light (UV-A light in the 365 nm region) and visualised under a stereo microscope (Nikon SMZ800) at 10× magnification. Photos of sprayed leaf surfaces were taken with a digital camera (Nikon DMX 1200). Digital images were quantitatively and qualitatively analysed with Image-Pro Discovery version 6.2 for Windows (Media Cybernetics) software, to determine spray deposition. The sprayed leaves were inoculated with 5 mg dry airborne conidia of Botrytis cinerea in a spore settling tower and incubated for 24 h at high relative humidity (≥ 93%). Leaf discs were isolated onto Petri dishes with paraquat-amended water agar and rated 11 days later for development of B. cinerea from isolated leaf discs. B. cinerea incidence on the upper and lower surfaces of water sprayed leaves averaged 90.4% and 95.8%, respectively. Despite full spray cover of leaves, applications with fenhexamid alone did not completely prevent infection and resulted in 34.6% and 40.8% B. cinerea incidence on the upper and lower surfaces of leaves, respectively. Through the addition of certain adjuvants, B. cinerea incidences were significantly lower (2.9-17.1% and 10.0-30.8%, respectively), while some adjuvants did not differ from the fungicide-only treatment, even though they might have improved spray deposition. The effects of Hydrosilicote and Solitaire alone and in combination with fenhexamid on germinating Botrytis conidia on leaf surfaces were studied in a histopathology study using epifluorescence microscopy. Distinct differences were observed in conidium mortality, germination and germ tube lengths between adjuvants alone and in combination with the fungicide, which might be attributed to indirect effects of the adjuvant mode of action on B. cinerea. The laboratory study clearly demonstrated the potential of adjuvants to improve the bio-efficacy of a fungicide directly through improved deposition on grapevine leaf surfaces. For the vineyard evaluations, the same fluorometry, photomicrography and digital image analysis protocol were used to assess quantitative and qualitative spray deposits under varying adjuvant dosage and volume applications. The Furness visual droplet-rating technique was initially included to determine optimum spray volume with a STIHL SR400 motorised backpack mistblower by assessment of pigment deposition on Chardonnay leaves under illuminated black light. Both assessment protocols showed that quantitative spray deposition increased with increasing spray volume applications of 40 L/ha to 750 L/ha, but decreased at 900 L/ha, possibly due to run-off. The addition of selected adjuvants at recommended dosage and at 600 L/ha demonstrated the potential of adjuvants to increase quantitative and qualitative deposition significantly on upper and lower leaf surfaces. Agral 90, BB5, Nu-film-P, and Solitaire significantly improved deposition on upper and lower leaf surfaces compared with the fenhexamid only and water sprayed control. Break-thru S 240 and Villa 51 did not improve quantitative deposition, although remarkably better qualitative deposition was obtained. An adjuvant dosage effect (within the registered dosage range) was evident, especially those retained on the upper leaf surfaces. Agral 90 and Nu-film-P affected significant improvement of spray deposition at the higher, but not at the lower dosage tested. Solitaire improved deposition at the lower dosage tested, whereas reduced deposition at the higher dosage was attributed to excessive spray run-off. No significant improvement of spray deposition was observed for both dosages tested with Villa 51. Spray mixtures with adjuvants Agral 90 and Solitaire yielded similar deposition values at 600 L/ha compared with the fenhexamid only control at 900 L/ha, but reduced deposition at the higher spray volume, possibly due to spray run-off. This study clearly demonstrated the potential of adjuvants to improve quantitative and qualitative deposition, but highlights the necessity to match adjuvant dosages and application volumes on the spray target to achieve maximum spray deposition.