Tuesday, April 19, 2011

What may happen on a vineyard floor this time of the season

During this winter, I received a few questions about a sanitation (= removal of infected tissues) as a part of disease managementOur understanding is that most of the grape pathogens can survive on infected tissues over the winter to cause diseases on next springSome survive on infected berry tissues (e.g., black rot) or leaf tissues (downy mildew), and others survive on cane or wood tissues (e.g., Phomopsis, Botryosphaeria).  Thus, my recommendation is a removal of any infected tissues (berries, leaves, and canes) from the vineyard

Many major grape diseases we deal with such as powdery mildew (which survives in the bark as a fruiting body over winter) are polycyclic diseases, meaning they have a multiple generation of disease cycle within a seasonWhen we compared the risk of having an outbreak of the disease throughout a season on these polycyclic diseases, the sanitation may or may not provide a significant difference  because each generation will produce millions of sporesTherefore, even if we remove the inoculum at the beginning, soon or later, it can cause a big outbreak of the disease.

However, what I think the sanitation is useful for is a suppression of the risk of disease development at early in the seasonRight now, buds are open and shoots are starting to form, and everything is moving quite rapidlyDuring this period, tissues are susceptible to many diseases, and since it is grown rapidly, even if you apply a fungicide, there will be new unprotected tissues produced within a few daysUsually, the environmental conditions during this period are relatively unfavorable for many common grape diseases (except Phomopsis, which can infect at lower temperature ranges), thus, the risk of having an infection is lowHowever, if you have abundant spores in your vineyard, the situation may be differentEither by a probability or by a genetic difference among the population, some individual may be able to infect tissues even at lower than optimal temperature range.

Once they establish a foothold this early in the season, you may encounter a higher risk of a disease outbreak later in the season because they are already in a preparation of next generation of spores.  (It will also depend on the weather conditions in the future, but I'm simply talking about a potential risk.)  Thus, it makes more sense to me to clean up your vineyard floor for both disease management and aesthetic purpose.

Some may argue that infected tissues from the last season will be decomposed by the spring.  I thought of it too, but based on what I found last week on my vineyard, things are not ready for decomposition yet.
From this picture (you can click to enlarge), you can see twigs and berries which seems to be intact and covered with fungal mycelium or fruiting bodies.  But please keep in your mind that it may or may not be pathogenic.

However, some of them look like Botrytis spores, which is known to be active in a lower temperature range.  (You probably have seen a gray mold on strawberry in your refrigerator.  It's the same pathogen.)
That's about it for today.  I will post results of disease risk assessments once this rain is over.  By the way, both our Chardonnay and Merlot are at 100% bud break.  Our Cabernet Sauvignon is at bud swell stage.

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