Quantification of spray coverage on grape bunch parts and the incidence of Botrytis cinerea

[摘要] ENGLISH ABSTRACT: Various studies revealed that Botrytis cinerea, the causal pathogen of Botrytis bunchrot, is mostly associated with pedicels, rachises, laterals and berry bases, and not with berryskins as previously understood. Provided that sufficient coverage of inner bunch parts wasachieved, laboratory studies have shown that fungicides can effectively reduce the amount ofB. cinerea at the various positions in bunches, and prevent infection and symptom expressionat all growth stages. The same efficacy was, however, not achieved with the same fungicideswhen using conventional spraying methods in vineyards. Poor disease control on fruit andleaves in vineyards is attributed to inappropriate timing of fungicide applications and/orinsufficient coverage of susceptible tissue. Previously, spray coverage evaluations in SouthAfrica were based on the use of water-sensitive cards. A variety of other methods have beenused to assess spray coverage in vineyards, but none of these methods could assess spraydeposits on a very small, three-dimensional area of interest such as the susceptible grapebunch parts. The methods were furthermore dependent on human objectivity, which lacksquantitative measuring and speed of measurement. Suitable technology to determine spraycoverage on susceptible bunch parts is, therefore, not available.The aim of this study was to develop a protocol to visualise and quantify spraydeposits in grape bunches, specifically on the inner bunch parts and to use the protocol todetermine the effect of different levels of spray cover on artificially inoculated B. cinereagrape bunches, in order to facilitate future determination of minimum effective coveragelevels for effective B. cinerea control.A spray coverage assessment protocol using fluorometry, photomicrography anddigital image analyses was developed to measure spray coverage on susceptible grape bunchparts. Among several fluorescent pigments tested, a yellow fluorescent pigment (SARDIFluorescent Pigment) from Australia was selected on the basis of its small particle size (2.45 -4.90 μm). Bunches were sprayed at pea size and bunch closure with different volumes of amixture of fenhexamid and the yellow fluorescent pigment. Sprayed parts from bunches wereilluminated under black light (UV-A light in the 365 nm region) and visualised under a stereomicroscope at 20 x magnification. Photos of the berry skin, pedicel and rachis were takenwith a digital camera (Nikon DMX 1200). Image analysis of photos was done with Image-Pro Discovery version 4.5 for Windows (Media Cybernetics) software. The total area ofdeposited pigment in selected areas of interest (AOI) was calculated. The percentage areacovered was subsequently calculated for each AOI. Good correlation was evident betweenthe parameters, sum of objects and percentage area covered. Bunch parts at pea size generallyhad higher coverage values than at bunch closure. Spray applications earlier in the seasonwould therefore result in higher and more effective spray coverage of the susceptible bunchparts. Similar deposition trends were observed on the inner bunch parts (pedicel and rachis).These were, however, significantly different from berry skins, which had significantly higherlevels of spray deposits than the inner bunch parts. The variance component analysisindicated that the highest variance was observed for berries and bunches, and substantiallyless for image readings. For the same accuracy, means for percentage coverage values of atleast 10 bunches per treatment (1 part per bunch and 3 readings per part) will be sufficient.In order to determine the biological efficacy of different levels of spray coverage on B.cinerea incidence on grape bunches, bunches were sprayed at pea size and bunch closure withdifferent volumes of a mixture of fenhexamid and a yellow fluorescent pigment and thepercentage fluorescent pigment coverage on pedicels was determine. Bunches weresubsequently dusted with dry airborne conidia of B. cinerea in a settling tower and incubatedfor 24 h at high relative humidity (98%). Infection was determined by estimating the amountof B. cinerea infections occurring on sprayed bunch parts with isolations on to paraquat andKerssies mediums. Linear regressions for the part x stage combinations of percentage B.cinerea incidence on different bunch parts were fitted on mean coverage levels. An increasein spray cover caused linear reductions in levels of B. cinerea on susceptible bunch parts.Higher B. cinerea incidences were recorded at pea size. Furthermore, higher B. cinereaincidences were found on paraquat medium for both stages, than on Kerrsies medium. Theinformation gathered from this study will be used to facilitate future determination ofminimum effective coverage levels for effective B. cinerea control in grape bunches.In these validation experiments, the results clearly showed that the protocol can beused to determine the effect of different levels of spray coverage on B. cinerea incidence andthat an increase in spray coverage will decrease B. cinerea incidence. The informationgathered from this study will be used to facilitate future determination of minimum effectivecoverage levels for effective B. cinerea control in grape bunches and subsequently be used asbenchmarks to evaluate spray application in vineyards.

[发布机构] Stellenbosch University