Can water applied to cherry trees for frost control lead to problems with powdery mildew?

After being asked the question by tree fruit growers, Dr. Gary Grove, plant pathologist with Washington State University in Prosser, believed this was worth looking into and did some research.

The powdery mildew fungus over­winters as cleistothecia in the fissures of tree barks. The cleistothecia contain the mildew spores, and free water is required to liberate the spores. Just 0.1 of an inch of moisture is needed at a temperature of 50°F for the spores to be released, and this must occur between bud break and pit hardening.

For a primary infection to occur, the spores must be released, and then they must germinate on the leaf. In his experiments, Grove focused only on whether water applied for frost protection caused spore release.

The experiment was conducted in two orchards at Prosser that were several hundred feet apart on a night when there was a high risk of frost. In one, water was applied for frost control. In the other, no water was applied. He used spore ­samplers to catch whatever was in the air and used a polymerase chain reaction technique to identify the mildew spores.

Where the water was applied, mildew spores were detected, indicating that the water was causing spore release.

"Whether or not this translates to primary infection and the establishment of mildew in the orchard, we don’t know yet," Grove said when he explained the research during the annual Stone Fruit Day in Wenatchee.

Grove noted that when water is applied for frost control, the temperature is ­probably too low for the spores to germinate. However, there’s the possibility that after cold, clear nights, the temperature could warm up rapidly during the day, perhaps to temperatures conducive to germination.

"We think this is worth looking at in more detail," he said.

Resistance management

A model for cherry mildew is available on WSU’s Decision Aid System, which is linked to data from the AgWeatherNet. Growers can learn when a mildew epidemic began, the current disease pressure, and the forecast for the next five days. The model is cross-referenced to a fungicide table that lists the compounds by trade name, common name, and class. The table also shows how the products have performed in efficacy trials and how high the resistance risk is.

Grove said a decade ago, growers had few fungicide options—just sulfur and two DMI (demethylation inhibitor) fungicides, Rally (myclobutanil) and Rubigan (fenarimol). Now, many more fungicides are available, and in recent years, there’s been an upsurge in the use of the strobilurins Flint (trifloxystrobin), Pristine (pyraclostrobin and boscalid), and Cabrio (pyraclostrobin), that have a particularly high risk of resistance development. Disease resistance has been seen in pear orchards in California where 15 to 20 applications of strobilurins have been made since the compounds were introduced, Grove said. Resistance to DMI fungicides has also been documented.

He recommended that growers keep track of their spray applications and count how many times they’ve applied strobilurins. "If you’ve applied them 15 to 20 times, it’s time to rotate out to another chemistry, at least for several years," he said. If critical compounds lose effectiveness because of resistance, growing ­cherries will become more challenging.

He advises growers to avoid applying more than two strobilurins in sequence and to never make more than three applications from the same class of fungicides in one year.

To manage resistance, use multiple modes of action and rotate strobilurins, DMIs, and quinolines with sulfur. Products with a low risk of resistance include carbonates, sulfur, and oil.

"Now we have a full chemical toolbox, we can be good stewards of the fungicides we have," he said. "Ten years ago, this was not the case."

New fungicides

Two new, unnamed compounds are scheduled to be released soon, Grove said. What class they’re in is not yet known, but he said they appear to be as effective as Pristine or Quintec (quinoxyfen).

Oil can be used for mildew management. It might be safe to use on a Bing orchard in the early season, but on late-maturing or sensitive varieties the risk of phytotoxicity of the foliage is sufficiently high that he would recommend using some of the other chemistries. "With all the options available now, there are other choices to make. "

Sulfur can be a very good fungicide, but the weather needs to be moderately warm for it to work well, he said. If the weather is cool, it is not effective, and if it’s too hot, it can damage foliage and fruit, particularly in Rainier cherries.

Always spray in good conditions to be sure of good coverage, Grove advised. Disease pressure can be minimized by keeping the orchard mowed to improve air circulation and remembering that when water is applied during the growing ­season to improve fruit size, it will also increase powdery mildew pressure. "What you’re doing on the one hand to grow a profitable crop might also increase your disease pressure," he warned. Grove said he believes the long-term solution to controlling powdery mildew is the development of resistant cherry ­varieties, not fungicides.