Infection pathways of Botrytis cinerea on selected wine grape cultivars

[摘要] ENGLISH ABSTRACT:An understanding of the infection pathways of Botrytis cinerea in grape bunches will helpto combat this devastating pathogen of grape. Many studies have been done to determine thepossible infection pathways of B. cinerea. Most of these studies made use of artificialinoculations that deposit groups of conidia on the plant surface. The deposition of clusters ofconidia is not a common phenomenon in nature. The aim of this study was to investigate theinfection pathways of (i) naturally- as well as (ii) artificially inoculated B. cinerea conidiaduring all the phenological stages of three wine grape cultivars, and to compare the (iii)pathogenicity and virulence, on grape and nectarine fruit, of isolates obtained from differenthost plants.In the natural infection study the occurrence of Botrytis cinerea and subsequent diseaseexpression at different positions in bunches of wine grapes (cultivars Chenin Blanc, Shirazand Chardonnay) was determined from 1999 to 2001. Different techniques were used todetect viable inoculum at different positions (rachises, laterals, pedicels, and the peicel end,cheek and style end of berries) in bunches. Isolations were made on Kerssies' B. cinereaselective medium, or bunches were used untreated, or treated with paraquat. Paraquat wasused to terminate host resistance and to promote the development of the pathogen from thetissues. The material was used untreated to detect the pathogen on the surface, or weresurface-sterilized to detect mycelia (latent infection) in the tissue. In the artificial inoculationstudy, bunches of wine grapes (cultivars Chenin Blanc, Chardonnay and Shiraz) at pea size,bunch closure, and harvest were dusted with dry conidia of Botrytis cinerea in a settlingtower and incubated for 24 h at high relative humidity (±93%). Following incubation, thebunches were divided in two groups. The one group was surface-sterilised in 70% ethanolfor 5 s, the other group was left untreated. Bunches of the sterile group, and from theuntreated group were used for isolation. From each bunch rachis segments, laterals, pedicelsand berry skin segments (from the pedicel-end and cheek) were removed. The sections wereplaced in Petri dishes on Kerssies' B. cinerea selective medium and on a water agar mediumsupplemented with paraquat, and incubated at 22°C under diurnal light. Occupation by thepathogen was positively identified by the formation of sporulating colonies of B. cinerea onthe different tissues. Lastly, in the virulence and pathogenicity experiment on grape andnectarine fruit Botrytis cinerea isolates, which were obtained from different host plants, werecompared by simulating natural infection. Cold-stored fruit, considered highly susceptible toB. cinerea were therefore inoculated with single, airborne conidia of the pathogen. Differenttests were conducted to assess surface penetration and lesion formation. Isolations weremade from fruit skins on Kerssies' B. cinerea selective medium. Nectarine fruit were treatedwith paraquat, and grape berries were frozen for 1 h at -12°C. Paraquat and freezing wereused to terminate host resistance and to promote the development of the pathogen from thetissues.In the natural infection studies B. cinerea occurred in a consistent pattern in bunches of thethree cultivars. B. cinerea consistently developed from the tissue of the rachis, laterals,pedicel and pedicel-end, but not from the berry cheek. The rachis, lateral and pedicelcontained much higher levels of B. cinerea than any position on the berry. Furthermore, thepathogen consistenly occurred at relatively high levels on the rachises throughout the season.Collectively, the data showed that in the Western Cape province, B. cinerea occured moreregularly in wine grape bunches during the early part of the season, than later in the season.The data of the artificial studies confirmed the findings made with the natural infectionstudies. In these experiments the pathogen resided more often on the structural bunch partsthan on the berries. Overall, the isolation studies revealed that conidia occurredpredominantly on the rachis. The incidence of B. cinerea was furthermore constantly high inthe inner bunch after each inoculation, and in bunches of different maturities. The datatherefore indicated that, when available, conidia penetrated loose and tight clustered bunchesin a similar way. Finally, in the virulence and pathogenicity experiments the results showedclearly that no host specialisation exists in the B. cinerea isolates used in this study.From these studies it is clear that in the Western Cape province B. cinerea occurs morereadily in the inner structural parts of the bunches and more so during the earlier parts of theseason. These findings should be considered when planning and implementing diseasecontrol programmes.

[发布机构] Stellenbosch University