Cherry Powdery Mildrew




Powdery mildew of sweet and sour cherry is caused by Podosphaera clandestina, an obligate biotrophic fungus. Mid- and late-season sweet cherry (Prunus avium) cultivars are commonly affected, rendering them unmarketable due to the covering of white fungal growth on the cherry surface.


Season long disease control of both leaves and fruit is critical to minimize overall disease pressure in the orchard and consequently to protect developing fruit from accumulating spores on their surfaces.

Initial symptoms, often occurring 7 to 10 days after the onset of the first irrigation, are light roughly-circular, powdery looking patches on young, susceptible leaves (newly unfolded, and light green expanding leaves). Older leaves develop an age-related (ontogenic) resistance to powdery mildew and are naturally more resistant to infection than younger leaves. Look for early leaf infections on root suckers, the interior of the canopy or the crotch of the tree where humidity is high. In contrast to other fungi, powdery mildews do not need free water to germinate but germination and fungal growth are favored by high humidity (Grove & Boal, 1991a). The disease is more likely to initiate on the undersides (abaxial) of leaves but will occur on both sides at later stages. As the season progresses and infection is spread by wind, leaves may become distorted, curling upward. Severe infections may cause leaves to pucker and twist. Newly developed leaves on new shoots become progressively smaller, are often pale and may be distorted.

Treatment


The key to managing powdery mildew on the fruit is to keep the disease off of the leaves. Most synthetic fungicides are preventative, not eradicative, so be pro-active about disease prevention. Maintain a consistent program from shuck fall through harvest. Consider post-harvest preventative measures (e.g. application of sulfur, prevent trees from pushing for leaf growth) to obstruct the fungus from overwintering. Preharvest fungicide applications should be based on the residual period of the product. Rotate between modes of action (MOA) in order to avoid resistance (Table 1). Also, visit the Fungicide Resistance Action Committee web page for updated fungicide information (http://www.frac.info/) and download the fungicide mode of action poster (http://www.frac.info/docs/default-source/publications/frac-mode-of-action-poster/frac-moa-poster-march-2017f19b282c512362eb9a1eff00004acf5d.pdf).



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