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Powdery mildew is a tough disease to control for Pacific Northwest cherry growers, but it’s even tougher for organic growers who have limited fungicides to use. With so few organic tools available, an integrated approach is needed, says a Washington State crop consultant.

Understanding the life cycle of cherry powdery mildew, components of your orchard, and what strategies and timing are effective are needed when putting together an integrated program, said Tom Pitts of Washington’s Cascade Agriculture Service during statewide horticultural talks in Wenatchee last December.

It’s key to know when a primary infection is going on, Pitts said. Overwintering cleistothecia survive on the bark, crevices, leaves, and debris on the orchard floor, and wait for free moisture in the spring to release ascospores and initiate primary infection, he explained. Close observation is needed to see the small yellow patches on the upper leaf surface or delicate webbing on the underside that indicate primary infection.

After the ascospores are released, white mildew colonies are formed on the underside of leaves near the center of the trunk or close to main scaffold limbs. These primary mildew colonies then produce chains of spores called conidia, giving the powdery appearance and spreading the infection throughout the orchard.

A powdery mildew model has been developed for Northwest cherries by Washington State University and is part of WSU’s Decision Aid System (http://das.wsu.edu) that can help growers predict critical disease periods and time cover sprays.

“If you can find mildew in your orchard around petal fall, then you know it’s going to ramp up as the season goes on,” Pitts said.

Orchard components

Tree structure, orchard location, varietal sensitivity, and irrigation and frost management all play roles in the development of powdery mildew.

Pitts noted that some tree structures, like the compact Spanish bush, may be more likely to encourage powdery mildew than the central or steep leader systems.

Orchard locationsloping or flat landcan also impact airflow and humidity, he said. Early irrigation can also play a role. “While we have no control over rain events, are we creating our own problem from spring frost irrigations? By delaying irrigation in the first few weeks, you could save one or two mildew sprays later on.”

Additionally, cherry varieties can make a difference when it comes to mildew. Oregon growers with Regina cherries have had few mildew problems, while growers have found varieties like Bing, Rainier, Skeena, and Lapins to be more susceptible, he said. “Everybody I’ve talked to says that Sweetheart cherries are their worst problem for mildew.”

And, timing of harvest can influence disease severity. Late varieties have more time to develop secondary infections than early season cultivars.

Control strategies

Pitts reminded growers that an integrated approach to mildew uses a variety of strategies:

Cultural: Are you keeping trees open and removing root suckers, which can be a source of mildew? Is your cover crop low or high?

Data logging: Do you track temperatures and degree-days from a nearby weather station? Do you use WSU’s computer mildew model to make management decisions?

Monitoring: Are you actively looking for signs of mildew in the orchard?

Management: How much time do you have for the cherry orchard? Do you have enough equipment to apply fungicides in a timely manner? Are you following your consultant’s recommendations?

Chemical controls: Are you willing to use chemicals, if needed?

Fertilization: Are your trees vigorously growing? Putting on 100 to 200 pounds of nitrogen per acre to achieve tonnage will encourage vigorous tree growth, which may in turn encourage disease.

Rain covers: Use of rain covers will increase humidity within the orchard and provide conducive conditions for mildew.

Organic materials

Pitts warned that the approved list of organic materials for mildew is a moving target and must be carefully checked before using a product. If the crop is to be exported, materials used must also be on the foreign country’s list of approved pesticides. (A list of maximum residue limits by country can be found on the Northwest Horticultural Council’s Web site at www.nwhort.org.)

Organic growers must be prepared to spray, and spray often, on a seven-day schedule if disease pressure is moderate to severe, he said. Choice of materials include oils, sulfurs, bicarbonate-based chemicals like Kaligreen (potassium bicarbonate), biological agents like Agra Quest’s Sonata (Bacillus pumilis strain QST 2808) and Serenade (Bacillus subtilis strain QST 713) and Acadia AgriTech’s Stimplex (kinetin and benzyladenine), which is reported to have a synergistic effect when used with fungicides, and a few miscellaneous products.

“Be wary of new products if they have not been well tested,” he cautioned. Most organic products act as eradicants, he said, but oils seem to have a protectant effect on leaves for five to seven days.

With application costs averaging $20 per acre, organic mildew chemical control will cost between $700 to $1,000 per acre for the season or $1,000 to $1,200 per acre for growers with late varieties, he estimated.

Biggest problem

Late varieties, such as Sweetheart, grown under dense canopies, seem to have the biggest problems with mildew, he noted. “Even in orchards with good management, with growers doing all the right things, the late varieties are barely holding on.”

Early sites can usually get by without major mildew problems, but midseason locations, especially those with susceptible varieties and dense tree structures, often “just get by” in terms of acceptable levels of damage from disease, Pitts said.

Trees on dwarfing rootstocks, like Gisela 6, usually have acceptable levels of disease because growers are getting good coverage with their sprays, he said, adding that spray coverage tends to be much poorer in big, tall, dense trees.

“This past year, if you had small fruit and mildew, with the poor market, your crop just didn’t get picked,” he said.