Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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Some grape varieties are most susceptible to bud PMBM than others, as seen in our bud dissections.
Most wine grapes have very low percentage necrosis in good years, in addition to being highly fruitful. Total fruitfulness is typically above 90%, and the majority of buds usually have two or more cluster primordia.
In 2005 in California,whenmanywine blocks had low fruitfulness, the causewas lack of inflorescence, not PMBN.
At least some varieties tend to have less PMBN in the first several buds, and more in the distal buds. Shiraz, Riesling, and a few other varieties reportedly develop more PMBN than most wine varieties.5
On the other hand, Thompson Seedless vines are highly susceptible to PMBN. Even healthy blocks typically have only 25% to 75% total fruitfulness, and double flower clusters are rare. In addition, the first four Thompson buds normally have less inflorescence and more PMBN than #5 and beyond, which is why Thompsons are cane-pruned.
Flame Seedless also seems to be more susceptible to PMBN than most table grape varieties. Like Thompsons, Flames generally have the most PMBN in the first few buds. Other table grape varieties typically have better fruitfulness and less PMBN than Thompsons and Flames.
The plant-mediated necrosis described above is not the only process that can lead to the death of a bud. While examining buds in November and December, our inspectors seemany buds sick or dead from processes clearly not involving an abscission layer under the shoot primordia and inflorescence inside the bud. These processes are describable but not well-documented. I have not seen any studies examining these other causes of necrosis.
Our presumption is that these dead or sick buds are the result of injurious processes; such as pathogens, heat- or water-stress, toxic mineral imbalances, or frost. I will refer to them collectively as injury-related bud necrosis (IRBN).
Mediated Necrosis
Bud mite feeding causes bubbly or dark scars on the leaf primordia and inflorescence. Even at this magnification (30x), the mites can barely be seen.
In one common scenario, the necrosis appears as a tiny spot on the surface of an inflorescence, or sometimes inside that has a brownish, wet appearance. The necrosis may occur at the base of the inflorescence where it joins the bud or start on the vegetative tip.
In other dissected buds, we see the necrosis spreading tomore of the inflorescence, and eventually it spreads to the whole bud, which eventually dies. This pattern of spreading is reminiscent of a rot pathogen. Our inspectors have never seen any of the filaments or fruiting bodies that are typical of fungi, so if a pathogen is the cause, I suspect it is a bacterium.
Sometimes the entire compound bud will be found badly misshaped, gathered into jagged, reddish, leathery protrusions. The majority of buds on a cane may be in this condition. When we see this in late fall, the cause cannot be determined. My guess is that the cause is some extreme stress, such as high temperature without adequate water or salt, or heavy mite or insect populations.
In 2007, I examined buds froma few ranches after a January freeze. In some vineyards, the freeze did not seem to have a significant
effect, but in at least one vineyard, I saw damage that appeared to be recent and possibly attributable to frost. Grapevines are a temperate climate plant and fairly tolerant of cold winter temperatures.
We cannot always identify the cause of necrosis when we examine the buds in late fall, but every year we learn more and gleanmore information from the appearance of the buds. We can, at least, provide a grower with some clues about whether low fruitfulness resulted from a failure of inflorescence, plant-mediated bud necrosis, or some injurious process.
Bud mite is a strain of the grape Erineum mite, Colomerus vitis, an extremely tiny pest in the rust mite family, Eriophyidae. Materials registered to treat for rust mites should work on bud mites. The Erineum mite strain, which causes white swellings on grape leaves, is rarely seen in commercial vineyards because it is controlled by sulfur. The bud mite strain hides in buds, so it is only exposed to sulfur and other pesticides for a brief period of one or two weeks following bud break.6
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