Black knot is a serious fungal disease that affects several varieties of fruit tree. When black knot fungus first infects a tree, usually in summer or spring, light brown wart-like or corky swellings appear on the new season’s growth. As the fungal infection progresses, the swellings grow into knots, turn green or tan and develop a velvety texture. By their second year of growth, the knots turn coal black and woody in texture. These black knots may grow to 6 inches in size and may completely encompass and distort a fruit tree’s limbs. To cure your fruit tree of this disease, prune black knots as soon as they emerge.
Prune away all small, non-essential infected branches. Ideally, this should be done in late winter before the start of the new growing season when black knot fungus discharges its spores. Prune these branches back to the main branch, leaving as little stub as possible. Sanitize your tools with alcohol after each cut. This will prevent the accidental spread of the fungus.
Extract black knots from major branches or the tree's trunk. Carve the knot out of the tree, and take at least 4 inches of the healthy tree tissue that surrounds it. The fungus may have colonized the surrounding tissue. Sanitize your saw with alcohol after each cut.
Move all excised black knots and pruned branches from the area and burn them. If they cannot be burned, bury them far away from any trees under 1 foot of soil.
Prevent the spread of the fungus by spraying the trees with a fruit tree fungicide containing benomyl in spring when the first buds appear. Respray two to three more times at seven- to 10-day intervals.
Fertilize the fruit tree at the beginning of the growing season. Black knot fungus puts the fruit tree under considerable stress.