2.10.8
BIOCONTROL OF GREY MOULD OF APPLE: WHY ARE SOME YEASTS EFFICACIOUS AGENTS , WHEREAS OTHERS ARE NOT?

AB FILONOW

Department of Entomology and Plant Pathology, Oklahoma State University, Stillwater, OK 74078, USA

Background and objectives
Grey mould caused by Botrytis cinerea limits the economic value of apples in post-harvest storage. Wounds of apple must be protected from B. ;cinerea infection. As an alternative to fungicides, yeasts have been shown to biologically control grey mould [1]. It is important to understand why some yeasts are effective biocontrol agents, whereas others are ineffective. Competition for sucrose by yeasts was suggested to have played a role in grey mould control [1]. The present study investigated the role that competition for sugars by three yeasts played in protecting wounds from grey mould and assessed the contribution of other factors to biocontrol efficacy of the yeasts.

Results and conclusions
Cryptococcus laurentii BSR-Y22 or Sporobolomyces roseus FS-43-238, but not Saccharomyces cerevisiae BY-1 reduced grey mould when applied to wounds of Golden Delicious apples which were stored at 22C for 7 ;days or 3C for up to 2 ;months. Cell-free filtrates of nutrient broth cultured with yeasts did not inhibit conidial germination of B. ;cinerea. Competition for sugars by these yeasts as a mechanism of antagonism was investigated at 22C. Populations of C. laurentii and S. roseus in wounds were six to nine times greater than those of S. cerevisiae from 1-7 ;days following inoculation. Yeasts mixed with conidia in sterile, dilute solutions of fructose, glucose or sucrose, or in dilute apple juice inhibited conidial germination compared with no-yeast controls. S. cerevisiae was less effective in this regard than C. laurentii or S. roseus. HPLC analysis showed that yeasts rapidly depleted sugars from juice or sugar solutions, whereas conidia did not. Yeasts did not alter the pH or oxygen content of dilute juice to the detriment of conidial germination. Yeasts in wounds utilized more 14C-labelled fructose, glucose or sucrose than conidia of B. cinerea over 48 ;h, but yeasts did not differ in their utilization of sugars. Utilization of sup>14C-sugars by yeasts in vitro was greater at most sampling times for conidia; uptake after 48 ;h was always greater for yeasts and addition of nitrogen did not after this result. Utilization of sup>14C-sugars by S. ;roseus in vitro was greater than the other yeasts. Uptake and utilization of sup>14C-fructose by C. ;laurentii or S. ;roseus was greater than S. ;cerevisiae , but utilization of glucose or sucrose by C. ;laurentii and S. ;cerevisiae was similar, and uptake of these sugars by these yeasts did not differ. Results strongly suggested that competition for sugars by yeasts played a role in the biocontrol of grey mould, but some other factor(s) most likely contributed to differences in efficacy between the yeasts. Conidia of B. ;cinerea on moist membrane fifters stuck on the inside of a sealed 500-ml glass jars showed increasing germination compared with the control when 1-20 ;l of ethyl acetate were injected into the jars. Injected volumes greater than 20-30 ;l were toxic to conidia. When yeasts were mixed with conidia inside the jars and 2-10 ;l of ethyl acetate injected, C. ;laurentii and S. ;roseus reduced conidial germination more than S. ;cerevisiae, which typically did not differ from the controls. Ethyl acetate injections of greater than 10 ;l were typically toxic to yeasts. These findings suggest that fungal interactions with ethyl acetate may contribute to biocontrol.

Reference
1. Filonow AB, Vishniac HS, Anderson JA, Janisiewicz WJ, 1996. Biological Control 7, 212-220.