New information on powdery mildew of grapevines developed by UC Davis scientists provides a more precise temperature threshold for more economical control of the disease through a computerized predictive model.
One of the researchers, plant pathologist Douglas Gubler, described the revisions during the 2009 Grape Day at the Kearney Agricultural Center, Parlier.
The powdery mildew index, also known as the risk assessment index (RAI), was developed in the mid-1990s to predict development of the disease and guide fungicide treatments. Temperature and humidity readings are provided either online, via paid subscriptions, or from individual vineyard weather stations.
In its original form, the index has provided participating growers greater, although somewhat conservative, accuracy of timing to reduce mildew sprays by two or three per season. However, Gubler said revised guidelines from lab trials, which are being verified in the field, are expected to reduce powdery mildew sprays by up to four or five per season after an early-season treatment with highly-refined mineral oil. Monitoring for signs of the disease continues to be an essential part of control.
Disease spores are released from canes and cordons by spring rains during temperatures of 50 F or higher. “Then, if there is green tissue,” he said, “the epidemic is initiated. Infection occurs within about 12 hours, and in some areas infection is 80 percent to 90 percent in seven days after spring rainfall.”
The fungus is ignited in conditions of three consecutive days having six continuous hours of temperatures in the 75 to 80 F range. The RAI uses a system of points to calculate disease pressure and guide timing of treatments.
“We are now leaning toward first allowing the spores to release and then using JMS Stylet oil or PureSpray Green oil. These infections can be completely eradicated, even two weeks after infection,” Gubler explained.
He added that the former recommendation was that wettable sulfur be applied as a protectant for 95 percent control at disease onset. “We now know the oil will do the same thing, but it will also eradicate the pathogen.”
The former thinking was the disease was killed by ambient 90 F temperatures. However, the new information shows that it survives until about 99 F, at which it begins to rapidly die off. This will be incorporated into software for the index weather stations.
Leaf-surface temperatures for the model are expected to be gathered with more-sensitive instruments in upcoming research.
Gubler said the new data makes the RAI less conservative, and, by increasing the interval between treatments and reducing their number, it will delay disease resistance to strobilurins, sterol inhibitors, and other fungicides, which has emerged in other grape-growing areas around the world.
Also on hand for the program was Michael McKenry, UC Riverside nematologist, who described progress with his trials with the systemic insecticide Movento, which is also active against nematodes in grapes and other perennials.
Since 2000, he's been searching for an alternative to the venerable, but now unavailable Nemacur. Sifting through a number of materials, he found that foliar-applied Movento moves within days into the roots of grapevines to counter the root-feeding pests.
It offers broad-spectrum control of several sucking insect species. As one of the new chemical class of tetramic acids, it attacks immature stages of the pests and also interferes with reproduction in exposed adult females.
“At times the impact of Movento on nematodes is lethal, and at other times they are only sickened. We don't understand why,” he said. It shows some activity against all species of nematodes and, at higher rates, is active against phylloxera.
“Phloem transport of molecules having relatively subtle effect on nematodes will require a greater understanding of application timing relative to nematode development, as well as environmental and prevailing field conditions,” he reported.
In his 2007 trials, Movento showed 25 percent control for two months, not enough to improve yields. But in the following year, he learned that irrigating too soon after an application reduced Movento's performance. He is focusing on timing of irrigations to learn more.
“Currently,” McKenry noted, “spring/fall timings are associated with avoidance of post-treatment irrigations rather than toward the date of root flush.”
That strategy, he added, will change, depending on the crop and the method of irrigation. For the moment, two well-timed irrigations per year provide a starting point toward a better understanding of the pest management complexities when multiple target pests are involved.
In table grapes at Delano, he found that Movento-treated vines had no greater yield, although they did produce better first-pick quality.
McKenry's work with walnuts showed that a November treatment was not a significant improvement, and now he plans to make an earlier application in mid-October. He said Movento requires an adjuvant to penetrate the plant. He plans to continue observations with the product for the next two years.
USDA soil scientist Dong Wang is studying nematode control through soil fumigation in a vineyard of own-rooted Cabernet Sauvignon at the USDA station across Riverbend Avenue from KAC.
The vines were planted in 2007 on land that had previously been in grapes showing damage from nematodes and soil pathogens.
Wang explained that with the ban on methyl bromide as a fumigant, Telone, with or without chloropicrin, is being used as a replacement to curb the soilborne pests.
Due to air quality restrictions, these materials are subject to buffer zones and limits on use in each township.
“Ongoing discussions,” he said, “suggest that even more stringent regulations on fumigant emissions are likely to be issued in the near future to further protect air quality and reduce human exposure.”
His project is to determine potential emissions of the chemicals under shank injection of fumigants without tarping.
At the same time, he is evaluating subsurface drip application and tarping techniques for their ability to retain the materials in the soil, as well as their efficacy in controlling the underground pests.
For comparison, the trial includes shank-fumigated, bare soil plots and plots planted to a mustard cover crop.
Wang's preliminary results showed no nematodes (rootknot, citrus, or pin) survived the fumigated plots. Although sufficient control was seen for Pythium species, evidence of surviving Fusarium was found.
Virtually impermeable film tarping significantly reduced fumigant emissions, and the subsurface drip application plots also illustrated potential for keeping the material in place in the soil, Wang said.
The project will continue for the next several years and will involve growers in field trials.
Next year yields at the Parlier vineyard will be recorded to compare effects of the various treatments.