Biological Control of Botrytis 

 Grey Mold in Strawberry 



John C. Sutton 



Department of Environmental Biology, University of Guelph, Ontario 



The fungus Botrytis cinerea, commonly 

 known simply as Botrytis, is probably the most 

 destructive pathogen of strawberry worldwide. 

 Botrytis is the chief cause of fruit rot in strawber- 

 ries, but it also is able to infect leaves and 

 flowers. The disease it causes is referred to 

 widely as gray mold after the sporulation struc- 

 tures the fungus produces on affected tissues. 

 Fruit rot symptoms may appear in berries as 

 they ripen in the field or after they have been 

 picked. For more than a half century, the chief 

 means of controlling fruit rot has been fungicide 

 applications when the crop is flowering and 

 fruiting. Finding fungicides which effectively 

 control gray mold is becoming increasingly diffi- 

 cult, and in the absence of satisfactory alterna- 

 tive measures, the vulnerability of strawberry 

 crops to Botrytis is increasing. 



The heavy dependency of fruit rot manage- 

 ment on fungicides is threatened by public per- 

 ceptions and concerns regarding fungicide resi- 

 dues in the foliage and fruits at picking time, and 

 by declining availability of effective fungicides. 

 Many consumers now reject berries from treated 

 crops regardless of care taken by the grower in 

 using fungicides or the presence of any residues. 

 Botrytis has an extraordinary record of develop- 

 ing resistance to fungicides such that on some 

 farms only one or two registered fungicides 

 remain effective. New fungicides are not being 

 registered. The demise of fungicide programs 

 prompted us to explore alternative approaches 

 to fruit rot management, in particular, biologi- 

 cal control of Botrytis through the use of various 

 microfungi and bacteria. 



The general concept of biological control of 

 plant diseases is straightforward, but few suc- 

 cessful systems of biocontrol have been devel- 

 oped since pioneering efforts in the 1930's. In 



essence, biological control involves the applica- 

 tion to the crop of microorganisms that do not 

 harm the crop but suppress disease-causing 

 organisms and thereby control disease. Reports 

 that organisms successfully suppressed disease 

 under controlled conditions but failed to do so in 

 the field seem endless. We can be certain from 

 many studies, however, that biocontrol of micro- 

 organisms, including pathogenic fungi and bac- 

 teria, happens all the time in the microbial "dog- 

 eat-dog" worlds of crops, plant residues, and soil. 

 At certain times, biological and other natural 

 li mits on populations of pathogens simply are 

 insufficient to prevent disease outbreaks. In the 

 case of strawberry fruit rot, we considered that 

 it may be possible to augment natural forces 

 that suppress disease, provided that we could 

 find the right kinds of control organisms and 

 learn how and when to use them. We were under 

 no illusions that shotgun techniques or a super- 

 ficial approach would provide the desired re- 

 sults. Our research program was, as a conse- 

 quence, comprehensive and required detailed 

 information on gray mold, the strawberry micro- 

 flora, and complex interactions of pathogen, 

 control organisms, strawberry plant, and micro- 

 climate. 



A search for organisms that suppress Botrytis 

 effectively under a wide range of weather and 

 crop conditions was started in 1986. We chose to 

 look for possible biocontrol organisms in straw- 

 berry plant materials collected in growers' fields 

 and in research field plots. The rationale was 

 that organisms with strong adaptation to straw- 

 berry as well as suppressive activity toward 

 Botrytis would be more likely to function well as 

 biocontrol agents in the field than would organ- 

 isms from other sources such as soil or culture 

 collections. No fields with suspected high levels 



Fruit Notes, Winter, 1992 



