Powdery mildew appears as superficial, white powdery growth on leaves and shoots that results in the stunting and distortion of young growth. Right: ruit like this Jonathan apple, when infected with powdery mildew, are stunted and russetted, and fruit set may  be reduced.

Powdery mildew appears as superficial, white powdery growth on leaves and shoots that results in the stunting and distortion of young growth. Right: ruit like this Jonathan apple, when infected with powdery mildew, are stunted and russetted, and fruit set may be reduced.

BRITISH COLUMBIA MINISTRY OF AGRICULTURE

A good year for apple scab is usually a bad year for powdery mildew, and vice versa.

Apple scab thrives under cool, damp conditions, while powdery mildew likes hot, dry weather. But the two apple diseases develop about the same time, early in spring, and coexist, with weather determining which will be the more severe ­problem.

In the past, all that hasn’t mattered too much, says Kerik Cox, a fruit pathologist at Cornell University. Widespread use of sterol-inhibiting (SI) and quinone outside inhibitor (QoI) fungicides to control apple scab has kept mildew in check, he said. Unfortunately, things are changing.

Apple scab in the Northeast is becoming resistant to SI fungicides like Rally and Vintage and to the QoIs like Flint and Sovran, Cox said. Apple growers are having to rely on protectant programs early to manage

SI-resistant apple scab populations, using mixtures of mancozeb and captan. This combination is ineffective against powdery mildew, Cox said.

There is also a possibility that powdery mildew is becoming resistant to SI fungicides, just as is apple scab, Cox said.

“Since powdery mildew can’t be grown in culture, it is extremely difficult and costly to determine whether it has become resistant or not,” he said. “To date, there has not been a single mildew control failure that can be attributed entirely to fungicide resistance, but we do know that SIs don’t seem as effective at low rates. The newer SDHI (succinate dehydrogenase inhibitor) and QoI fungicides aren’t as effective on powdery mildew as the DMIs (demethylation inhibitors) were ten years ago. We may need to apply fungicides specifically designed to ­manage powdery mildew.

“In the future, growers may need to have separate fungicide programs for each of the two diseases, or for cultivars of differing susceptibility,” Cox told growers he spoke to at the Mid-Atlantic Fruit and Vegetable Convention in Hershey, Pennsylvania.

“Powdery mildew has historically not been a serious concern in apples,” Cox said. Some varieties, however, are highly susceptible—Cortland, Idared, Ginger Gold, Jonagold. The disease can reduce yields from aborted blossoms, poor return bloom, and suppressed shoot growth, he said. The white powdery blight infects young shoots and leaves and can russet fruit.

The two diseases, scab and mildew, respond differently to different fungicides within the DMI fungicide class. DMIs that have the most activity against powdery mildew tend to have the least activity against apple scab, and vice versa, Cox said.

The new SDHI fungicides that are now being developed for use against apple scab appear to have some  activity against mildew.

Several companies are also developing other novel fungicides with mildew activity. One forthcoming fungicide is Torino SC, from Gowan, which will be marketed exclusively for powdery mildew control and will be something to consider for growers with mildew-susceptible varieties, he said.

In New York, the 2009 season started cold and wet and was the worst in many years for apple scab, Cox said, so growers prepared for heavy pressure in 2010, focusing on fungicides that are strong against scab and protectants. The 2010 season, however, was hot and dry early, leading to light scab pressure but severe mildew problems.

2011

What’s his advice for 2011? “To manage both apple scab and powdery mildew successfully, growers must ­tailor the early season program to fit the seasonal weather and the susceptibility of their varieties,” he said.

If weather this spring is hot and dry from tight cluster to pink, and cultivars are susceptible to mildew, he ­recommends Rally 40WSP (myclobutanil) or Topguard (flutriafol) plus mancozeb to second cover.

If the season is cool and varieties are scab susceptible (like McIntosh), he recommends using Inspire Super (difenoconazole), Indar (fenbuconazole), or Flint (trifloxystrobin) WG with a low rate of mancozeb to second cover. An alternative is to use a half rate of mancozeb plus captan and sulfur repeatedly to second cover.

“Middle ground” programs would fit scab-susceptible cultivars in a hot, dry year or mildew-susceptible cultivars in a cool, wet year. Growers might choose one of these three: Rally or Topguard plus mancozeb to second cover; or Inspire Super, Indar, or Flint WG with a low rate of mancozeb to second cover; or the old standard program of captan plus a half rate of mancozeb plus sulfur (applied often) to second cover.