Lime sulfur is a fungicide. It is popular for use on cherry trees, because it can be a deterrent, as well as a solution to an existing problem. If your cherry trees appear to be suffering from black knot or black rot, both of which are fungal infections typified by warty, black knots that form on the trunk and branches of the tree, then lime sulfur can save your tree and prevent reinfection.
Sulfur is toxic to fungi. However, it can also harm some plants. Lime is added to the sulfur to make it safer for the cherry tree. When you spray your cherry tree with lime sulfur, the sulfur kills the black knot fungus--and other fungi--on contact, while the lime prevents the tree itself from sustaining injury.
Lime sulfur should be sprayed on the tree either while the tree is dormant, to prevent infections, or during the growing season to combat existing infections. For dormant applications, apply the lime sulfur when temperatures are above freezing, but before there are many leaves on the plant.
You can apply lime sulfur during growing season, but do so in the morning before bees are active to avoid hurting pollination and to prevent leaf-burn in high temperatures. Once the buds on the tree have broken, you cannot use lime sulfur.
While lime sulfur will kill black knot fungus, it is best to supplement your efforts with pruning. Remove infected branches as soon as possible and dispose of them far away from your trees and other crops. Do not compost them. Some gardeners burn the branches, while others simply place them in garbage bags and send them to the landfill.
Lime sulfur will kill black knot fungus. It may also burn foliage if it is applied during the middle of the day when the cherry tree gets a lot of sun. If the black knot fungus has taken over the tree, killing the fungus with lime sulfur may not save the tree itself, since the tree may be weakened beyond recovery or too disfigured to survive.
Take safety precautions when applying lime sulfur. Wear long sleeves and pants, safety goggles and gloves. Do not apply it when the wind is up and avoid inhalation since it is corrosive and can damage your lungs. If your cherry tree has fruit or open buds, then you cannot apply lime sulfur without destroying your crop and rendering surviving fruit unfit for consumption.