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Disease Management
Cultural Management
Cultural management strategies are important in black knot management. Sites containing Prunus species should be monitored on a scheduled basis for possible infections. The main strategy to lower disease incidence is the removal of sources of inoculum. All shoots and branches bearing knots should be pruned out during the winter. This pruning should be completed before ascospore discharge begins in the spring, usually about the time that the buds first break. To be sure that even the unseen internal mycelium is removed, the cut should be made at least 15-20 cm (6-8 in.) below the knot (figure 15). Winter is also a good time to look for and remove sources of inoculum in nearby wild Prunus species in hedgerows and woodlots (figure 16). The knots are capable of producing ascospores for some time after removal from the tree. Therefore, they should be burned, buried, or removed from the site regardless of the time of year the pruning takes place.
 Figure 15 |
 Figure 16 |
Genetic resistance
When creating a new planting, consider selecting varieties with known resistance. Varieties may vary in their ability to tolerate or resist an infection.Black knot resistance is as important as fruit characteristics, tree size, and flowering time. Up-to-date listings of varieties with high levels of resistance are often available from local extension offices.
Site selection
Consider the site location. Avoid planting new trees near areas with known problems such as abandoned orchards or where wild varieties have been observed with the disease. When possible, remove wild varieties of the trees from the area.
Biological control
Interest in biocontrol agents is increasing because of the loss of certain fungicides registrations and the fact that applicators would prefer to reduce their exposure to pesticides. A possible biological control agent for black knot may be the fungal parasite, Trichothecium roseum, introduced in the Symptoms and Signs section.
Chemical Management
Fungicides are normally only recommended for sites with valuable trees and/or severe infections levels. Fungicides work best as a protectant and are ineffective if cultural practices are not also employed.
If a fungicide application is required, remember the biology of the organism. The applicator should pay particular attention to the weather conditions and inoculum levels when determining the timing and frequency of fungicide applications. Sites with severe black knot infections may require protective applications from early spring around bud break through summer (figure 17). In some seasons the sprays can be terminated earlier if monitoring determines that inoculum is no longer available. Sites with low levels of inoculum may only need protection during the most susceptible period when ascospores are abundant and released in the spring. Regardless of frequency, fungicides have been found to be most effective when applied prior to a rain event and temperatures are above 16°C (60°F).
 Figure 17 |
Copyright © 2002
by The American Phytopathological Society