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GRAY MOLD ON STRAWBERRIES 



Daniel R. Cooley and William J. Manning 

 Department of Plant Pathology 



One of the most aggravating and economically damaging strawberry diseases is 

 gray mold. It strikes after primary investments in plants have been made, and presents 

 an ever-present, difficult to manage, disease problem. A conservative estimate gauges 

 yield lost from fruit rots in general at about 10%. In the Northeast, the majority 

 of this loss is caused by Botrytis cinerea, the gray mold fungus. Gray mold is a 

 particular problem because, unlike most other berry rots, it often attacks living plants 

 in the field as well as the harvested fruit. 



The pathogen is well-adapted for survival. Botrytis occurs on a wide-range of 

 hosts, and while an isolate from a different host may not be as virulent as a strawberry 

 isolate, in most cases B. cinerea isolates can cause some degree of gray mold. Botrytis 

 also has the ability to colonize dead or living tissue. The fungus will often establish 

 itself on dead or aging plant tissue and move from that tissue to healthy areas. Petals 

 and other parts of older flowers are likely to be colonized first. These infections 

 may destroy the developing fruit immediately, or may remain latent until the fruit 

 is well-ripened or harvested. As the fruit ripens, it becomes easier for the fungus 

 to attack it, because endogenous enzymes in the fruit break down mechanical barriers 

 to fungal infection. Substrates for fungal growth become more available, and 

 biochemical defenses decrease as the fruit ripens, making a ripe strawberry an excellent 

 habitat for Botrytis and other rotting fungi. 



Botrytis cinerea inoculum is very common. In strawberry plantings in the Northeast, 

 Botrytis usually overwinters in dead leaves and other decaying plant tissue, using 

 survival structures called sclerotia. In spring, as the temperature warms, the sclerotia 

 germinate and produce spores. These spores are carried about strawberry plantings 

 by air currents, splashing moisture or insects. In the presence of free water on the 

 plant, the spores germinate. Though the fungus is active over a wide range of 

 temperatures, optimum conditions for a Botrytis epidemic occur when temperatures 

 are approximately 60 to 70° F, and relative humidity exceeds 90%. If temperatures 

 and humidity are optimal, an epidemic can occur in as little as 28 hours. One of the 

 driving forces behind this rapid disease development is the prolific reproductive 

 capacity of the fungus. Gray mold is an example of what is called a 'compound interest 

 disease'. That is, each reproductive unit, in this case a condium or spore, can produce 

 hundreds of new spores, which can each produce hundreds more spores, and so on 

 as long as environmental conditions are favorable. Each generation progresses rapidly 

 in warm, humid weather. With conidia multiplying at a high 'interest' rate, disease 

 pressure becomes severe. Add to this the fact that initial inoculum is virtually always 

 present, and gray mold becomes extremely difficult to manage. Figure 1 shows a 

 diagram of the disease cycle for gray mold. 



Most management efforts aimed at gray mold rely heavily on fungicides. To be 

 effective, fungicides must be applied at proper times. Most effective programs attempt 

 to prevent initial infections in the spring, and concentrate on protecting the plant, 

 particularly the flowers and fruit, from infections when environmental conditions 

 favor the disease. To be effective, a layer of fungicide must be kept on the plant 



