Growers Alerted to Vine Malady 'Red Blotch'
UC Davis researchers report to Napa Viticulture Group
A new threat to vines?
Called “red blotch” for its distinctive appearance, Wolpert said the new malady is not caused by known viruses or other plant diseases. He said that plants at the Oakville Experimental Station show the condition, and that it seems to be spreading there. It may be localized, but he has also noticed similar symptoms elsewhere in Napa Valley.
“I don’t mean to be alarmist,” he said, but admitted, “We don’t know what it is yet.” He added that it could be due to unique nutritional or other conditions.
Instead of the deep purple of leafroll virus, red blotch causes red leaves with pink/red veins on the reverse side. The reddening appears primarily at the base of the shoots.
Of great concern—particularly in a cool year like this one—is that affected vines accumulate lower sugar levels: They have at least 2º Brix less than healthy, green vines. This condition is consistent across the affected vines.
The Napa Valley Grapegrowers is working with UC Davis to identify Napa Valley vineyards that may have red blotch. Growers there—or elsewhere—observing these symptoms in their vineyards should contact Wolpert at email@example.com or (530) 754-6245.
Update on phylloxera
The major part of the session was devoted to an update by Andy Walker, who reported recent findings about phylloxera, the bug that devastated European grapevines planted on their own roots until growers learned to use resistant rootstocks with grafted vinifera scions.
Some of the findings were surprising.
First, many new varieties of phylloxera have evolved and been identified; Walker and others have been testing their impact on popular rootstocks and vines. They gathered many on a trip from Boston to Texas, stopping every 50 miles to collect samples from vines. “Grapevines grow everywhere, if you know where to look,” he said.
He found phylloxera to be prevalent everywhere, too, and expects it will also eventually be found in places where it’s not now considered a problem—including some emerging wine regions.
Walker said it’s not clear how the louse is evolving, since it’s generally considered to reproduce asexually. Recent studies, however, suggest that it may have a sexual stage, which could help explain the new strains. It also develops wings, which could explain some dispersal.
Imported rootstocks and vines, some imported illegally, may be another source of new phylloxera strains.
Of late, many rootstock nurseries in Yolo and Solano counties (also at UC Davis and the USDA Clonal Germplasm Repository) have been infected with a strain of bacteria that causes small galls in their leaves. These are most severe where the rootstocks are grown on the ground without trellises: Rootstock suckers act as a conduit to the vinifera leaves, but very few galls form on vinifera varieties.
This foliar phylloxera can reduce nursery production, but doesn’t seem to affect vineyard production. Walker added that phylloxera is rarely seen above ground.
The forms of phylloxera that feed on leaves and those that feed on roots seem distinctive, however.
Interestingly, phylloxera lives on and feeds on the root tips of even some rootstocks considered resistant, but it doesn’t necessarily damage the vine. Some rootstocks seem particularly resistant to strains of phylloxera to which others are susceptible.
The popular 1103 rootstock is susceptible to root tip feeding, but not damaging feeding to established roots. The rootstock has such large root systems that it seems able to compensate. The bugs feed on St. George, but don’t seem to affect it significantly, perhaps for the same reason.
The original strains of phylloxera that in the past devastated California vineyards planted on their own roots are still here, but new ones seem to have evolved or have been imported.
Phylloxera doesn’t kill plants by feeding on them but by causing wounds that allow various fungi into the plant. These fungi cause galls that girdle the roots and cut them off. Ironically, phylloxera can live on the severed roots and may even find them a superior food source. Disking fields can also sever roots with the same impact: Infected roots can live as long as seven years.
Walker and his colleagues are searching for improved rootstocks resistant to new strains of phylloxera, with special interest in V. berlandieri. Unfortunately, resistance to phylloxera isn’t the only consideration: Drought tolerance and resistance to salinity are important, and nematodes and viruses are other increasing threats.
Walker pointed out that growers break cardinal rules of agriculture by not rotating crops or leaving fields fallow for long periods, and they tend to use the same rootstocks over and over, increasingly the likelihood of pests developing resistance.
Walker recommended the text Wine Grape Varieties in California as a current guide to rootstocks.
An audience member put him on the spot by asking for his recommendations. He said that if he were planting his own vineyard in a virgin site, he’d p robably use a clean stock of 110R, but warned that it’s susceptible to viruses and has no nematode resistance.