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Three-year cycle combines with high moisture: Botrytis bunch rot may again challenge grape producers

This has been one of the wettest California winters in at least five years.

That is the good news. It will ensure California grape growers good vineyard moisture to start the season, and it should mean adequate irrigation water for the growing season from an above average snow pack.

The bad news is that this could be a year when Botrytis bunch rot once again becomes a more challenging menace, according to Doug Gubler, University of California Extension plant pathologist.

“Botrytis hits California vineyards hard every three years and this could be one of those years with all the moisture we have had,” Gubler told a group of growers and pest control advisers recently at a grape pest control strategies workshop sponsored by Bayer CropScience.

Botrytis requires free water and 95 percent humidity to germinate spores, said Gubler. Spores germinate at temperatures ranging from 50 to 85 degrees, with 65 being the optimum temperature. It takes only about two hours for an infection to start at 65 degrees with high humidity and free water, said Gubler.

It is difficult for grape growers to avoid Botrytis since it can be found just about anywhere on the vine and even in dead blossoms and leaves on the vineyard floor. It overwinters in grape mummies left on the vine or left undestroyed in the vineyard floor.

Growers and PCAs can evaluate Botrytis in the fall before pruning. Gubler said tiny black spots in bleached areas on canes are overwintering Botrytis fungus.

Most widespread

“Botrytis is one of the most widespread fungi in the world. It can be found on table tops, on doorways, in parking lots — just about everywhere,” said the widely recognized grape disease expert.

“It can also be introduced into a vineyard from nursery stock,” he added.

It will invade early, causing early-season shoot blight following spring rains. Botrytis likes succulent tissue versus mature green material. Flowers can be infected during bloom, but the bunch-rot causing Botrytis generally becomes dormant until sugar concentrations increase in berries.

Leaking infected or damaged berries serve to spread the disease. Free water collecting where berries touch also can be the source of a beginning infection. Berries damaged by birds, insects, machinery or any other mechanical means may become infected at any time under the right environmental conditions since all it requires is free water. Wind can also spread spores.

“A leaking berry is a source of free water,” said Gubler.


Dry spells should not lull growers into believing they will not have a problem. The fungus stops growing when it is dry. All it takes is rain or irrigation water to trigger the latent bunch rot spores in multiple vineyard locations.

Several years ago Gubler attempted to rid a vineyard of all Botrytis, but what was left was actually a 30 percent infection rate, enough to initiate an outbreak under the right environmental conditions.

Leaf removal and or fungicides can control the diseases.

Removal of basal buds early in the season to expose small bunches to more heat, reduce humidity and increase ventilation. Gubler said allowing wind in the canopy has been identified as the key to preventing Botrytis with leaf removal. On cordon-trained vines, UC recommends removing only leaves from the side of the vine that receives afternoon shade.

Changing the “architecture” of bunches to minimize grape contact can also reduce Botrytis. Some growers, said Gubler, use plant growth regulators to lengthen bunches.

Decreasing fertilizer to reduce the amount of succulent growth also can help prevent Botrytis. “Tissue more hardened off is less susceptible.”

Gubler's early work found that strategic leaf removal can give the same control as three fungicide applications.

However, fungicides are generally necessary to control bunch rot, particularly if a grower does not remove leaves. Leaf removal can be expensive if done by field crews. The number of sprays is dependent on the level of leaf removal. In the coastal areas, UC recommends beginning leaf removal at late bloom and continue to berry set. In the warmer Central Valley, UC suggests removing leaves from bloom to berry set or hedge mid-season to open the canopy.

Fungicides are important and can be made between bloom and pea-size berries early and before rainfall, especially at bloom and at verasion. “The most important fungicide application is the pre-close application in almost all cases,” said Gubler.

Coverage is very critical with Botrytis control materials. Leaf removal can obviously help with that coverage.


He cited a wide range of fungicides that continue to be effective. However, there is a new one coming into the market later this year from Bayer CropScience, Scala, that has looked “very good” in Gubler's trials over the past two years.

“I first heard about Scala at a conference in New Zealand in 1993. Everybody in the world was impressed with it. It has only taken 12 years to get here,” he said.

Scala will add another fungicide to the resistance management program.

“We are excited about it coming into the grape and strawberry market. It is a new product and historically when a new fungicide is introduced it really works well. That was the case when Elevate came into the market,” said Gubler, who expects Scala to move to the head of the class when it is introduced.


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