The role of the mediterranean fruit fly, Ceratitis capitata, in Botrytis bunch rot of grape

[摘要] ENGLISH ABSTRACT:Botrytis bunch rot of grape is caused by Botrytis cinerea Pers. :Fr. Conidia of thepathogen, which is dispersed by wind, water droplets and by insects, can penetrate the intactgrape berry cuticle, but disease expression occurs only under predisposing conditions. Sincerelatively high infection rates often occur in vineyards, predisposing factors must play afundamental role in primary infection and subsequent disease occurrence. Insects can play avery important role in this regard by depositing inocula at wound sites during feeding and byproviding fresh wounds during their oviposition and feeding activities. The aim of this studywas (i) to determine the potential of the Mediterranean fruit fly to transfer B. cinerea andother bunch and fruit rot fungi in natura, (ii) to investigate the transport, deposition andsubsequent disease expression on grape berries in vitro, and (iii) to investigate fruit flyactivities and the nature of deposited conidia and mycelia of B. cinerea by aid of digitalphotography and epifluorescence microscopy, respectively.Two Sensus fruit fly traps containing the para-pheromone, Capilure, were installed inorchards and five neighboring vineyards on four farms in the Stellenbosch region. Ceratitisfruit flies were collected weekly, identified and counted to determine the fluctuations in fruitfly population. Following field collection, the fruit flies were plated on Kerssies' B. cinereaselective medium and the number of flies yielding the pathogen was recorded. Two fruit flyspecies, C. capitata and C. rosa, were captured during the study period. Ceratitis rosanumbers comprised only 1% of the total number of fruit flies captured. Ceratitis capitatanumbers, and the percentage B. cinerea contaminated flies generally increased after harvest inthe different orchards and vineyards. Following harvest, the percentage flies yielding B.cinerea was higher in vineyards compared to orchards. Furthermore, in each vineyard anincrease in percentage B. cinerea contaminated fruit flies was preceded by a correspondingincrease in its neighboring orchard. The levels of both Penicillium and Alternariacontaminated fruit flies stayed high throughout the investigation period, especially afterharvest of the orchard cultivars. Low incidence of Aspergillus, Mucor and Rhizopus spp.were recorded on C. capitata. These findings suggest that the Mediterranean fruit fly may play an important role in the dispersal of inocula of fungi associated with postharvest decayfrom early-maturing stone fruit orchards to mid- and late-maturing wine grape vineyards, andin disease induction under conditions unfavourable for natural infection.Three experiments were conducted to determine the potential of fruit flies in provoking B.cinerea decay. In the first experiment, transport of conidia and disease expression wereinvestigated on rachis segments bearing unwounded berries only. In the second experiment,the effect of wounding on disease expression was investigated. In the third experiment, theeffect of inoculum type (mycelia and conidia) on transportation and disease expression wasinvestigated on rachis segments bearing unwounded berries, and on segments with woundedberries. The table grape cultivar, Dauphine, and the wine grape cultivar, Shiraz, were used atvéraison, two weeks before harvest and harvest, and the transport studies were conducted inethanol-disinfected perspex cages. Disease expression was studied in dry (~56% RH),ethanol-disinfected perspex chambers incubated at 22°C. The isolations from berries revealedthat the flies deposited, without preference, high amounts of B. cinerea at various positions onthe grape berry's surface. The freezing studies showed that the deposited conidia germinatedand penetrated the berry skin at various positions. However, B. cinerea developed more oftenat the pedicel end than on the cheek or style end, which indicated a peculiar interactionbetween B. cinerea, the fruit fly and host tissue at this part of the berry. This phenomenonwas substantiated by the finding that B. cinerea also developed more often at the pedicel endof berries that were not frozen. Further evidence for this interaction was found on intactberries exposed to flies that carried mycelia after feeding on berries without sporulatingcolonies of the pathogen, but showing symptoms of slippery skin. Significantly more decaydeveloped on wounded berries compared to the unwounded berries and more so at the woundsite. In addition, female fruit flies were responsible for significantly more decay developmentthan male fruit flies. The study thus proved that the Mediterranean fruit fly can promote B.cinerea disease development under conditions unfavorable to natural infection.The activities of the Mediterranean fruit fly, Ceratitis capitata, on grape berries weremonitored by aid of digital photography. In addition, the deposition of conidia and mycelia ofBotrytis cinerea at three sites (pedicel end, cheek and style end) on the grape berry,germination of the fungal structures after dry (±56% RH) and moist (±93% RH) incubationand wounds inflicted during ovipositioning were examined with an epifluorescencemicroscope. The observations revealed that the fruit fly's activities were generally restricted to the grape berry. They visited the grape berry cheek more often, but visitations to thepedicel end of berries increased substantially from véraison to harvest, indicating thepossibility of nutrient leakages at this site. Microscopy revealed that the flies depositedconidia singular, in feeding packages and in faecal excrements on the berry surface. Theconidia in feeding packages were ensheathed by salivical fluids and occurred in clusters of 10to 50 conidia. An average of 60% of the conidia in feeding packages germinated under dryconditions (±56% RH). Conidia that passed through the intestinal tract of the fruit fly and thatwere deposited in faecal excrements were deformed and low in viability. These conidia didnot occur in cluster format, but were proportionally spread with the faeces on the surface ofthe grape berry. Conidia that were deposited singular and in faecal excrements did notgerminate unless incubated under moist conditions (± 93% RH). Wounds inflicted by femalefruit flies during ovipositioning were most frequently observed on the cheek. Thispredisposition to B. cinerea infection of grape berries by the activities of fruit flies, suggestedan important role for the flies in the initiation of Botrytis bunch rot epidemics in vineyards.

[发布机构] Stellenbosch University