Gray mold, caused by the fungus Botrytis
cinerea, damages developing and harvested fruit. Less
noticed, but just as important to the life cycle of Botrytis,
are the leaf infections, generally visible on the dead or dying
leaves on the ground inside a row of strawb
erries. The symptom growers and customers
notice is rotting berries, which will develop a gray and powdery
appearance. The fruit becomes covered with the gray mycelium and
spores of the fungus, which can lead to secondary infections of
adjacent fruit.
The pathogen spends most of its time
on leaves, overwintering on dead and living leaves primarily as
resting mycelium. In the spring, the fungus sporulates on these
leaves given appropriate weather conditions. Moist conditions
of 14 hours or more with temperatures ranging from 64 to 73o
F are ideal for germination of Botrytis spores. The fungus
produces thousands of spores on each dead or aging leaf. These
spores are spread by wind, rain or any action which moves the
plants. After leaves have been wet 14 hours, the spores will ha
ve
infected previously healthy tissue. Spores from leaves are generally
present during bloom. Petals and other parts of older flowers
are prime infection sites. Lush foliage and excessive tissue growth
are also ideal for Botrytis, since the interior of the
plant canopy will stay wet longer and the "softer" tissue
is more easily infected. These infections may destroy developing
fruit immediately, or become dormant until the fruit begins to
ripen. Secondary infections may occur when spores that cling to
ripening fruit germinate in moist packaging conditions after the
fruit is harvested, causing uncontrollable storage rots. See the
figure: Botrytis Fruit Rot Disease Cycle.
Recently, Dr. Jay Pscheidt at Oregon State reviewed the fungicide test results for gray mold treatments. The following is an edited version of his comments.
"There has been an enormous amount of information generated about fungicide control of gray mold on strawberries. Data are easily found as far back as 1955 and include some 60 trials from around North America. After reviewing this material, I am able to provide a ranking of the various fungicides used over the years.
Group A - Ronilan > or = Rovral > Benlate >> Group B or C
Group B - Thiram > or = Captan > or = Ziram > Group C
Group C - Syllit (Cyprex or dodine)
Captan can be used as a reference point since it was used most often in about 30 trials. Captan's average effectiveness was about 60% control but ranged from 0% to 100% control. Captan did better under light disease pressure and poorly under heavy disease pressure."
Dr. Pscheidt's review leads to some interesting conclusions. Obviously, Group A is the best choice for control, except that resistance to these materials occurs easily. While he didn't explicitly list it, Topsin M would perform much like Benlate. However, Benlate can suppress predator mite populations and is not recommended in IPM programs. Syllit is poor enough in terms of control that we don't recommend its use on Botrytis. The best choice on strawberry remains some combination Ronilan or Rovral with a fungicide from Group B. By limiting the number of applications of Ronilan and Rovral to no more than three applications per season and combining them with a protectant fungicide from Group B, control will be highly effective and resistance problems will be minimized.
While sulfur and copper are both registered
on strawberry, their efficacy relative to the fungicides listed
above has not been evaluated. Based on the other crops, I would
expect that the control would be less than that of Group A and
Group B fungicides. 
Daniel Cooley, University of Massachusetts Extension
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
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