Keep on Top of Cherry Leaf Spot and Black Knot Regardless of Crop Load

Spring frosts had a significantly negative impact on sour cherries in some parts of Ontario this year. However, with a reduced crop or even no crop, disease management remains important throughout the year. If no fruit is expected, growers may cut back somewhat on insect pest management, but should continue to be vigilant against diseases such as leaf spot and black knot. Those two diseases have important impacts on the long term health of trees.

Cherry leaf spot symptoms first show up a few weeks after petal fall as small, purplish spots on the upper surfaces of leaves. These spots eventually turn brown and may coalesce to form large patches or fall out giving a "shot-hole" appearance. Leaves with lesions will quickly turn yellow. Later in the summer the disease will be most noticeable in the tops of trees (yellow leaves and/or premature defoliation) where spray coverage is often inadequate. Premature defoliation weakens trees making them more susceptible to winter injury. Trees may be stunted or even killed if defoliation occurs in successive years.

Although pruning and burning infected branches is the most important first step in black knot management, protective fungicides are also critical in controlling this disease. Infections that occur this year may not be apparent until next spring. The more knots present in an orchard, the more spores produced that can initiate new infections. Some sour cherry orchards have no history of black knot, so can rely on visual checks for occasional knots at pruning time.

To keep on top of these diseases, consult the cherry calendars in OMAFRA Publication 360 -Fruit Production Recommendations and do not skip any fungicide applications. There is a wide range of products registered for leaf spot control and only a few that are effective against black knot. Note that a new product in the calendar this year - Mission 418EC is the same chemical (propiconazole) as Topas 250E. Be sure to rotate between different fungicide families through the season for resistance management.

It is important to protect the trees from premature defoliation (caused by leaf spot) even after harvest time so that the trees go into the winter in a healthy and hardy state. Use full label rates and ensure excellent coverage by using sufficient water volumes and spraying every row. Even with a small crop or no crop this year, you should not skimp on any aspect of disease management.

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