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Cherry (Prunus spp.) - Black Knot
Cause The fungi Monilinia fructicola and M. laxa can incite a blossom blight, a twig and branch dieback, and a fruit rot of several Prunus spp. including many ornamental and fruit trees. Fungi survive year to year on infected twigs, branches, old flower parts, or mummified fruit. Wind and rain blow spores (conidia and ascospores) to healthy blossoms in spring to begin the infection process during wet weather. Infection does not occur below 50°F and will occur for M. laxa above 55°F. Flowers can be blighted any time floral tissue is exposed but are most susceptible at full bloom. More spores can be produced on this tissue, initiating several more disease cycles during the spring. Under severe conditions, non-flowering shoots or leaves can be infected directly.
Some infections may be symptomless until fruit begins to ripen. The risk of these latent infections is highest from bloom through pit hardening, declines to a low risk at embryo growth then begins to increase as fruit ripen. Ripening fruit are highly susceptible to infection, and more disease cycles can occur near harvest through new infections or (infected) fruit to (healthy) fruit contact. Peach, nectarine, and prune fruit that fall to the ground due to lack of pollination, thinning, or overripeness can significantly increase inoculum and the amount of fruit rot at harvest. Fruit infected in the orchard may not show symptoms until it is in storage or transit. High nitrogen fertilization also is associated with increased levels of brown rot.
Both fruiting and ornamental cherries, peaches, nectarines, almonds, apricots as well as prunes and plums are susceptible. Pome fruit, including quince, can be susceptible under high disease pressure years. The disease is more of a problem west of the Cascade Range.
Symptoms Infected flower parts turn light brown and may develop areas of buff-color (M. fructicola) or gray (M. laxa) spores. Infected petals may look water soaked, which can be mistaken for frost injury. Flowers generally collapse as the fungus invades through the pedicel. Infected flowers often adhere to twigs and spurs through harvest or even winter.
Depending on the fungus and plant infected, the disease may continue into twigs or spurs. Lesions may remain discrete or girdle the twig, causing all distal portions to die. Profuse gumming also may occur in these areas. Again, buff or gray spores (in sporodochia) may develop on these necrotic twigs.
Fruit symptoms begin as small, dark spots that enlarge rapidly. Fruit remains fairly firm and dry relative to a watery rot caused by Rhizopus sp. Production of masses of buff-color spores is equally rapid in the necrotic area.
Cultural control These must be supplemented by chemical methods especially in the wettest areas such as west of the Cascade Range.
- Remove and destroy infected twigs and branches in summer.
- Shorten large trees and prune out shaded limbs in summer for a more open canopy that dries out quickly.
- Remove and destroy all mummified fruit in and around the tree. Cultivating or burying old fruit before the growing season will not reduce the risk of this disease. However, removing fallen fruit (due to thinning or lack of pollination) can significantly reduce the amount of rot at harvest.
- Control insects that could wound and injure fruit.
- Avoid wounding fruit during harvest.
- Cool oe dry fruit rapidly after harvest.
- Use moderate amounts of nitrogen fertilizer.
- A short (2.5 minute) soak in hot water (122°F) has reduced postharvest decay in nectarines and peaches. Some additives have increased efficacy of this treatment. Microwaves used to increase fruit temperature for a short period have also been successful.