Dormant sprays for disease control

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Dormant sprays of lime sulfur, Bordeaux mixture (copper and lime) and heavy dormant oils used to be an important part of fruit culture. With the rise of synthetic pesticides, dormant sprays became less common. More growers are using these sprays to reduce pesticide costs, target problem diseases or because they are accepted organic sprays.

Lime sulfur is not lime and sulfur, but calcium polysulfide. It is a caustic material and after application this compound breaks down, releasing sulfur and burning exposed tissues. This burning action is very effective against diseases that overwinter on the host. The list of diseases controlled by these treatments is very comprehensive and growers should remember that this list dates from the days when there were no other treatments for these diseases. Lime sulfur is also effective against many insect pests that overwinter on the plant.

Lime sulfur is registered for use on most fruits, but is most commonly used on small fruit. Lime sulfur is used on brambles to control anthracnose, spur blight, and cane blight. In blueberries, the diseases controlled include anthracnose and phomopsis. Annemiek Schilder of MSU Plant Pathology, has done research in grapes showing that lime sulfur is effective against black rot, powdery mildew and phomopsis. Lime sulfur is also used in apples and pears to reduce scab and in peaches to control peach leaf curl.

Copper sprays especially Bordeaux (copper sulfate with lime as a safer) are commonly used on tree fruit. Bordeaux mixtures are described by the ratio of copper sulfate to lime in 100 gallons of water. For example, an 8-8-100 Bordeaux is 8 pounds of copper sulfate, 8 pounds of hydrated spray lime in 100 gallons of water. The lime is added to the copper to reduce the phytotoxicity of the copper. Plants differ in their tolerance to copper. Spray oil is often used as a sticker spreader in the dormant sprays. After bud break when the green tissue is exposed, the copper rate is reduced to ratios such as 4-4-100 or 2-6-100, increasing the relative lime amount, as more and more green tissue is exposed.

Copper is a common material for dormant sprays in peaches, cherries, apples and pears. Copper sprayed on the trees will be redistributed during subsequent rains and the high copper levels reduce the amount of bacteria growing on the trees. This is the main reason it is used on stone and pome fruit. Using high copper rates in early sprays reduces the need for additional sprays. Some growers will put on a single large application early, while others will use several sprays at a lower rate.
Copper is used on cherries to reduce bacterial canker. Sweet cherries are very sensitive to copper and dormant applications reduce the risk of injury to the host. Once green tissue appears in sweet cherries, we do not recommend the use of copper. Tart cherries are quite tolerant of copper and it is used as a control for cherry leaf spot during the growing season.

Copper is used in apples and pears to reduce the inoculum for fire blight and often as a green tip spray to control scab. Higher rates of copper can be used before the green tissue appears, but these sprays have little effect on scab infection periods that need exposed green tissue for infection. Copper is believed to provide some protection against fire blight. The fire blight bacteria over winters inside the tree and the copper has no effect on this population, but as spring progresses and fire blight cankers begin to ooze, the bacteria on the outside of the tree is exposed to the copper residue reducing bacteria population. Copper sprayed on the exposed flowers in the expanding flower buds can cause russeting of the fruit so rates are reduced as the buds open and copper is not recommended after tight cluster.

Peach leaf curl is a sporadic peach disease in Michigan. This disease of peaches and nectarines can defoliate trees. Infections take place in the early spring as the buds open. The fungus infects peach buds from bud swell to bud opening under wet conditions. Copper is a very effective material against this disease and has the benefit of suppressing bacterial spot as well.

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