Strawberry powdery mildew model readied

The grape powdery mildew index model to effectively time disease control measures developed by University of California plant pathologist Doug Gubler and others is now used on more 200,000 acres of grapes in California.

Now, Gubler has developed a similar model for coastal strawberry producers that will be offered soon to producers there.

Powdery mildew is a temperature-driven disease, Gubler told strawberry growers at the recent California Strawberry Commission Monterey Bay Academy strawberry research field day near Watsonville, Calif., recently.

For strawberries, Gubler said the optimum temperature for powdery mildew development is 61 to 80 degrees for six to eight hours during the day.

Using in field weather stations to record temperatures, Gubler's model assigns points based on whether conditions are optimal for disease development.

This allows producers to better time powdery mildew applications to control the disease before it can get a foothold.

Powdery mildew can be so destructive, producers manage it with scheduled treatments of either fungicides or sulfur. With Gubler's model, growers have a better idea of how the disease is developing and can schedule fewer and more effective treatments.

Stretch spray intervals

By adding or subtracting points based on the weather station readings, growers can stretch the intervals between sprays. For example, with a powdery mildew of 0 to 30 in grapes, growers can stretch sulfur treatments to 14 days. When the index reads 60 to 100, the sulfur dust interval is cut to seven days. After each treatment, the grower sets the index for a particular field back to zero and begin recalculating the points. Strawberry growers now have a similar model.

“What the model does is offer the potential to effectively reduce the number of fungicide sprays,” said Gubler, plus it also allows producers to better pinpoint when to apply the all-important first and second powdery mildew control applications to achieve good early season control.

Gubler said the strawberry model he developed has effectively reduced powdery mildew applications on strawberries from a dozen to only three or four.

The grape model was picked up by Western Farm Service in conjunction with the weathers stations it markets and is provided to its growers and others. WFS is also picking up the strawberry model for coastal growers and offering on its Web site sponsored by Syngenta, according to Cindy Bishop, marketing assistant for Western Farm Service.

Also offered by WFS will be a botrytis disease model developed by Gubler for strawberries. It is similar to the Botrytis bunch rot model Gubler developed for grapes.

“The model we have developed so far has been used fairly effectively, but we would like to have it a little better,” said Gubler.

Sets risk levels

It is based on daily hours of leaf wetness and temperature and can help producers determine the amount of risk they want to assume before applying a fungicide.

“In some years, using the model to time fungicide applications has reduced the number of fungicide applications while maintaining a level of control comparable to a 14-day calendar spray,” he said. However, in other years Gubler said there was either no reduction in fungicide use or control was compromised.

Gubler believes the fruit rot level at harvest is due to a combination of conducive conditions at flowering or shortly thereafter. He has determined that factors other than leaf wetness in the week prior to harvest may play a role in predicting fruit rot at harvest. He has tagged flowers to correlate weather at flowering and fruit rot at harvest in an effort to “fine tune” the Botrytis model.

Wide ranging research continues to find viable alternatives to methyl bromide, which is due to be phased out by 2005 and reduced 70 percent by next year.

It is a perplexing problem since there are several diseases that affect strawberries and available alternatives do not always control all of them. Weed control is another casualty of the methyl bromide ban.

Growers are resorting to bed-applied fumigants and fumigants applied through drip lines as alternatives to the long used flat fumigation to reduce methyl bromide use for now. Beds are also being covered in different colored tarps to evaluate weed and disease control and yields.

UC Davis graduate student Mark Johnson discovered that white, yellow, brown and green control weeds as well as black tarps. Plant growth did not differ significantly among colors. However, yields across an array of colors were off compared to clear plastic, although red gave acceptable yields.

Verticillium wilt

UC Davis plant pathologist John Duniway reported variable results with methyl bromide alternatives for control of verticillium wilt.

However, at a federally sponsored methyl bromide trial at Salinas, Mike Nelson of Plant Sciences, an agricultural consulting and research firm from Watsonville, said in the trials to find alternatives to control root nibbling fungi, not verticillium, there are promising alternatives.

One is Dow AgroSciences InLine, a combination of Telone and chloropicrin. In trials at Salinas and Oxnard, InLine applied through a drip system on beds yielded as well as the check, flat fumigated and bed shanked methyl bromide and chloropicrin.

Watsonville, Calif., strawberry producer Richard Uyematsu has tried InLine and said it was “pretty good.”

Uyematsu is like many strawberry growers, however, and remains concerned and skeptical about the future without methyl bromide.

“There are still a lot of questions about these alternatives that look good. Time will tell how they will work in the long run,” he said. “There is a positive residual effect from methyl bromide, and we don't know what will happen with these other products. We could see other diseases and problems emerge that we have not had with methyl bromide.”

While he is uncertain about the alternatives, he is certain about one thing:

“We are going to have to change the way we farm without methyl bromide, and I don't think the end results will be very good,” he said.

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