Vineyard Management


Get rid of the mummies!


Different strategies are required for control of ‘Noble Rot’ and other fungus infections. By Gary Strachan


et’s start in Section 5 of the 2010 Best Practices Guide for Grapes. We generally associate the various rots with fungal infections: Powdery mildew, bunch rot, and sour rot. Each of these has different effects on the crop, different disease suppression strategies, and different optimal conditions for development.


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There is no “Universal Soldier” that will control all of them. This was an interesting year. As we approached harvest, the most troublesome infection was Botrytis. In some cases it showed up just before harvest. Infected fruit could be dropped, and even if a small amount of infected fruit slipped into the picking bin, it probably would not have caused a problem. On the other hand, we had a lot of rain this year, so there was the potential for big problems if the dry, wrinkled berries (sometimes called Noble Rot) Botrytis developed into bunch rot.


With bunch rot, the berries become soft and sticky. Secondary infections with other fungal species such as Aspergillus, Penicillium, and Rhizopus produce off flavours, increase volatile acidity (vinegar smell) and produce a breeding ground for an explosive increase in fruit fly population. Fruit flies then quickly spread the infection throughout the vineyard and there is no registered insecticide for fruit flies.


Many growers rarely encounter Botrytis. If powdery mildew is controlled by a regular spray program, then leaf and berry surfaces are not disrupted by the mildew infection and


may become dry and shriveled, as in harvestable “noble rot”. If you pick and sample these berries, they taste very sweet, often with greater than 30 percent sugar. If the berries are sticky, perhaps with some broken berries, this is cause for concern. My advice has been to drop clusters if they were sticky, but if they were shriveled and not sticky, and you had a salvage strategy for winemaking, be sure they were harvested before the next rainfall.


The second type of Botrytis infection that I noticed this fall was on the upper part of the grape clusters and shoulders. This can originate from a late-season infection from windborne spores. In one case I observed, the grower confirmed he had encountered an unusually heavy amount of dust this summer. Fungal spores can be carried in dust. Berry skins are reasonably resistant to Botrytis infection unless the waxy bloom layer has been interrupted by a sticking agent, mildew, or mechanical abrasion.


There is no ‘Universal Soldier’ to beat themall.


remain more or less resistant to Botrytis, dependent on grape variety.


We had a cool, wet, spring followed by a summer with none of the 40-degree weather that is lethal to fungi. The optimal growth temperature for Botrytis is 15 to 20 degrees and high (90 percent) humidity. That sounds like rainy day conditions to me.


I noticed two styles of Botrytis infection. The first was a more or less classic appearance of bunch rot. The damaged berries were mid way down the cluster, which indicated that the initial infection probably occurred sometime between bloom and bunch closure.


This type of infection is almost impossible to halt by the time you observe it. In some cases, the fungus may infect the berry stems shortly after berry set. This can cause the stems to turn brown and harden and the attached berries to wilt and stop growing. If fungal growth occurs later in the season, the berries


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The flavour changes that occur with a Noble Rot infection are worth a brief discussion. When Botrytis infects a berry the mycelium grows deeply into the berry, as opposed to the near-surface growth that occurs with mildew. This level of intimate metabolism with the whole berry produces a flavour spectrum in the juice that is unique to a Noble Rot infection. It is much more attractive than the musty, earthy flavours that we associate with other fungal infections.


I liken the flavour to pineapple or perhaps mandarin orange. If the growing conditions are right, a delicious nectar can be produced, such as the sauternes of France or the tokays of Hungary. If things go badly, the result may simply be a wine that tastes moldy, or worse yet, like solvent or vinegar. With few exceptions, the sprays that affect Botrytis have little effect on mildew and vice versa. It is for this reason that any vineyard with a history of Botrytis should be especially monitored, not only to keep mildew under control, but also Botrytis. It can give you a false sense of security to assume that mildew control will also control Botrytis. Check the list of control agents in the 2010 Best Practices Guide. While you’re at it, don’t forget that fungal diseases often over-winter in those mummified berries that were dropped on the ground. If you can, cultivate them into the soil. Try not to leave the mummies where you drop them. Take them away to a compost heap or somewhere they won’t re-infect your vineyard. Give the mummies a good burial. — Gary Strachan can be reached at gestrachan@alum.0.edu


British Columbia FRUIT GROWER • Winter 2010-11


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