Make Mildew on Tart Cherry Take a Powder!

(Adapted from Michigan State Fruit Crop Advisory Team Alert, May 18, 2004 - Vol. 19, No. 6)

Powdery mildew of cherry is most important during summers with hot, dry weather. The fungus may overwinter on diseased, fallen leaves, but it does so more commonly in infected cherry buds, as in the case of apple powdery mildew. When infected buds expand in the spring the new growth becomes completely colonized by the fungus. Warm temperatures without rain, but with sufficient moisture such as high humidity, morning fogs, dews, or intermittent rains, are ideal for rapid increase of the disease. Powdery mildew typically is first seen on the underside of leaves and so orchard scouting is important for initial disease detection. The fungus grows as a white mat on leaf surfaces; continued fungal growth results in spread to upper leaf surfaces. Mildew can cause leaves to become brittle, and these leaves can then be subject to premature drop during harvest.

Conditions suitable for powdery mildew are less favourable for cherry leaf spot and brown rot. Therefore, during hot, dry summers, growers are likely to spray fewer fungicides for disease control, resulting in less suppression of powdery mildew. In years favourable for leaf spot and brown rot, mildew is typically less of a problem in part because growers are spraying more frequently.

Tart cherry growers have to balance fungicide applications to target two major diseases (cherry leaf spot and brown rot) and powdery mildew, a disease of sporadic importance. Mildew control can be initiated at petal fall; this timing would be especially important in orchards with previous significant mildew infection. This spray timing is more critical for leaf spot control, and this disease must take precedence because leaf spot is the most economically important tart cherry disease. Bravo is currently the most important early-season fungicide for leaf spot control both for its broad-spectrum activity and because it lessens the dependence on strobilurin (Pristine & Flint) and SI (Nova, Indar, Topas) fungicides. Remember that Bravo is not registered for use after shuck split. Since Bravo does not control powdery mildew effectively, growers could consider including a mildew control product during the early season, if necessary. One possibility is the use of low rates of sulphur for mildew suppression. Sulphur can be applied throughout the season, but the combination of low rates and long spray intervals may not suppress mildew under conditions conducive to disease. In seasons with reduced powdery mildew risk, low rates of sulphur could be tank-mixed with fungicides throughout the season yielding an overall suppression of fungal diseases.

Research at Washington State shows that secondary cycles drive powdery mildew epidemics. Cover sprays applied after shuck split represent the critical timing for mildew control. Strobilurins (Pristine, Flint) are currently the fungicides most effective in mildew suppression; these fungicides are also excellent for controlling leaf spot and very good to excellent for controlling brown rot. One program, which might be effective, would utilize Flint or Pristine for the 1st and 3rd cover spray, using SI's or another alternative for the 2nd and 4th cover sprays. Remember, the 4th cover spray timing is important for brown rot control as well. The combination of effectiveness of the fungicide and timing will drive the overall success of the program. MilStop, a potassium bicarbonate product, has excellent eradicant activity on powdery mildew. The downsides are that it has no activity against leaf spot or brown rot and it increases pH significantly so it cannot be tank-mixed with other products.