Fire blight can decimate an orchard, starting in early spring as soon as blossoms develop; it spreads blossom-to-blossom via rain, wind and pollinators. When average temperatures reach 60º F or more, and the relative humidity is 60% or more (or rain occurs), these bacteria can enter a new host and cause infection. Infections can continue through the growing season as the causative bacteria multiple and move via a tree’s vascular system.

Apple scab, which also prefers wet weather, is most active during periods of cool, moist spring weather, when primary fungal spores are released and carried by wind and rain, spreading infection. This disease can also occur throughout the growing season as secondary spores cause leaf and fruit infections during rainy periods.

Growers in regions where weather conditions have historically favored fire blight and apple scab development may not be as familiar with powdery mildew, which prefers high relative humidity (above 90%) and warmer spring temperatures (optimally in the mid-60s to low 70s), and whose spores will not germinate during rain events. Powdery mildew has historically been a bane of West Coast growers. But things are changing, thanks to climate change and changing cultivar preferences, making powdery mildew a growing concern in some regions.

Powdery Mildew Basics

The fungal pathogen Podosphaera leucotricha is the causative agent for apple powdery mildew. The good news is the fungus cannot survive without its host, but the bad news is that there are numerous hosts, including lilac, hydrangea, oak, magnolia, dogwood and crabapple.

The 2021 growing season in regions of the Northeast did favor powdery mildew development, as a mild winter did not kill the overwintering pathogen, which hides out in bud tissues of its host. Bud tissues die off with sustained temperatures below -11º F, so the pathogen dies too in cold winter weather. Mild winters followed by dry springs – instead of the more typical wet spring in the Northeast – are a recipe for powdery mildew infections. If prolonged periods of warm, dry weather occur in spring or summer, secondary infections can spread.

Dr. Kerik Cox and David Strickland, Ph.D. student in Cox’s lab at Cornell AgriTech who has focused on powdery mildew research, were featured in a Perennia podcast discussing powdery mildew infections in apples and their research on disease control. They explained the disease lifecycle and what growers can do to control the pathogen. The podcast is available at tinyurl.com/rb5tkk7h.

The fungus finds its way to bud tissue as the tree enters dormancy in midsummer, where it overwinters. The protection of the bud tissue allows the pathogen to survive mild winters. In spring, as bud growth occurs, the pathogen will emerge and colonize new plant tissue as it grows in what is known as the primary infection. The fungus spreads over infected flag shoots in a white, hazy pattern and will result in aborted blossoms. Any flowering buds will be deformed or killed, and any fruit formed from these buds will be abnormally shaped or russeted.

Both primary and secondary infections will continue to grow, and spread, throughout the growing season as conditions permit. Secondary infection happens when overwintered pathogens release spores (carried by the wind) which then land on the leaves and start new infections in the canopy, on new trees and can even spread well beyond the orchard.

If not treated, newly developing buds will eventually be infected by these secondary infections, and the pathogen will overwinter in these buds, starting the primary infection cycle again in spring. Infected buds open later than normal, so new tissue is readily available to continue the pathogen’s spread.

Powdery mildew damage to an apple. Photo courtesy of Clemson University USDA Cooperative Extension Slide Series, Bugwood.org

But powdery mildew spores will not germinate if there is any free water available. So when apple scab is a problem, powdery mildew is not. Most apple disease requires free water, but when conditions are dry, powdery mildew thrives.

The traditional means of controlling powdery mildew is fungicidal applications. Strickland’s research has shown that using weather-based factors when applying fungicides for powdery mildew is as effective as calendar-based fungicide applications that do not account for weather or disease pressure. But the weather-based application system utilizes between 50% and 83% less mildew-specific fungicide applications, reducing the cost, time and labor needed, and reducing the risk of developing resistance.

Removal of twigs, shoot buds and flowers that have been infected is another means of control, as it reduces the primary inoculum available for the next spring. Cox recommended that growers avoid overfertilizing the crop, as free nitrogen is “protein material for the fungus,” and excess leaf growth will add susceptible surfaces for the pathogen to colonize. Promoting good air circulation in and around orchard trees and using resistant cultivars are also means of reducing powdery mildew pressures.

Resistant cultivars include Red Delicious, Braeburn, Gala, Fuji, Jonafree and Winesap. Highly susceptible varieties include Honeycrisp, Cortland, Granny Smith, Ida Red, Stayman Winesap and Rome Beauty.

Powdery mildew does impact yield, but it does not always happen right away. It can take several years of increasing infections to see the impact, Cox said.

“Over time, you might realize your yield is a lot less,” he said. “It’s not negligible; it’s harder to notice.”

Likewise, it will take several years of management to get powdery mildew under control. Strickland emphasized that there is an action threshold of 20% or higher for disease pressure in the orchard.

Another concern is that climate change means some regions will be having less precipitation than normal in summer, leaving it warm and humid and dry – perfect for powdery mildew growth. Even with scattered, heavy afternoon thunderstorms, high temperatures will dry the free moisture and result in conditions of high relative humidity, which are preferred by the powdery mildew pathogen. Combined with a reduction in the number of infected buds being killed as winters become milder, and with apple trees exiting dormancy earlier in spring, the fungus will have more time to establish itself during each growing season.

“For years, it’s been fire blight and apple scab,” Strickland said, “but maybe powdery mildew will have its day.”

Knowing what to do if the forecast is ripe for apple powdery mildew will prevent that warm, fuzzy fungus from getting the upper hand.