Landscape and Nursery - Black Knot of Prunus

The following is an article on the Black Knot disease of Prunus species from University of Maryland Extension.

Black knot of Prunus is a disease caused by the fungus Apiosporina morbosa and is common on many Prunus species in Maryland (and Delaware). Examples include plums, cherries, and flowering almond. Black knot is very easily identified and often can be seen along on the roadside on native black cherry.

Symptoms: Wart-like galls form along the twigs and stems of susceptible plants. Galls may be large (greater than a foot long) and are often easily visible. Early symptoms of black knot may be overlooked because they appear as small light brown swellings on current season’s growth. As the galls age, they darken to olive green (2nd year infection) and eventually become black. Mature galls can be very large and may compromise the structure of the tree on larger stems.

Disease Cycle: In the spring, fungal fruiting bodies form ascospores on the surface of mature black) knots. Wet, rainy weather causes the asci to rupture and eject spores. The spores are splashed and wind blown to newly emerging stems and germination occurs. The first signs of infection appear in autumn as swellings on current season’s growth. Galls are perennial and enlarge every year.

Monitoring: Scouting for this disease should be done in autumn or winter after leaf drop because older knots are easily visible.

Cultural: Remove unwanted volunteer black cherry trees and any severely infected ornamental trees to reduce inoculum. Desired trees with light infections of black knot should also be pruned during the winter. Pruning should be done prior to budbreak and knots should be pruned 6-8 inches below visible symptoms. All resulting debris should be removed from site or burned. Resistant varieties should be used for ornamental plantings, especially if black cherries are nearby.

Chemical Management: Fungicides are only recommended at sites when numerous trees need to be managed (as in a nursery) or if susceptible varieties are located near heavily infested trees that may act as sources of inoculum. Efforts made to correct the situation may take 2-3 years. For most landscapes, cultural practices are usually an adequate control strategy. Spray programs consist of several treatments beginning just prior to budbreak (delayed dormant); applications are repeated as growth emerges. Fungicides may be applied preventively prior to rain when temperatures are above 60 F (conditions favoring the pathogen). Products labeled for control of black knot include chlorothalonil (various trade names), copper hydroxide (various trade names), copper hydroxide + mancozeb (Junction®), and thiophanate-methyl (Topsin®). Remember to rotate modes of action; always read and follow fungicide labels.

Extracted from the March 23, 2007 edition of the TPM/IPM Weekly Report for Arborists, Landscape Managers & Nursery Managers from University of Maryland Cooperative Extension. Go to http://ipmnet.umd.edu/07Mar23L.pdf for the full newsletter.