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BLACK KNOT….DISEASE AFFECTING MAYDAY AND SCHUBERT

Symptoms

The disease is characterized by the presence of thick, black, irregular swellings on the twigs (Fig. 1). The presence of these symptoms is often first noticed in the winter season when they are unobscured by leaves. However, the fungal disease-causing agent has been present for quite some time. The pathogen’s presence disrupts the normal growth of the twigs and a tumor-like growth forms at the infection site. Infections may take place as much as a year or more prior to the development of these characteristic “knots”, therefore, the swellings are normally not noticed until the winter of the second season of infection. It takes a keen observer to notice the subtle, initial symptoms present during the first season of infection.

The first symptoms appear as small, light brown swellings of the current or previous season’s growth.. By the next season the swellings turn olive-green in color with a velvety texture. Over this growing season the knots darken and appear to have a hard, brittle texture (Fig. 2). The hard, black knots are the typical symptoms associated with the disease.

Knots vary in size from approximately 1-30 cm (0.5 to 12 in.) in length and from minute measurements to 5 cm (2 in.) in circumference. The infected twigs often appear bent at the tips because of extra cellular growth on one side. Trees with heavy infections may contain numerous knots. Some of the older knots may appear white or pink in color. This discoloration is often seen in late summer and is caused by the fungal parasite, Trichothecium roseum.

Disease Cycle

The fungus overwinters in the knots. About the time of bud emergence in the spring, the first ascospores are forcibly discharged from the ascostromata following a period of warm, wet weather. Apparently very short periods of wetness (only a few hours) are enough to prompt ascospore discharge. Temperatures between 16 and 27oC (60-80oF) are ideal for the dissemination, germination, and infection of new plant tissue. Recent studies have confirmed and concluded that rainfall and temperature are the key factors in the release of spores and that the duration of the rainfall or wet period is not a factor.

The ascospores are spread by air currents and rain splashing. Mainly the succulent green shoots and, occasionally, wounded tissues are most susceptible to infection by ascospores. Ascospore discharge continues to occur for 2-3 weeks after bloom. Infections take place during this time but may continue for a longer time period if susceptible host plant tissue is available. The germinating ascospores have the ability to penetrate unwounded surfaces of elongating, green shoots directly.

Figure 3. The early stages of knot formation results in only small galls. (provided by Dr. Phil A. Arneson, Cornell University)
Figure 4. As knots develop, they become very soft in texture and are covered with olive-green conidia. (provided by Dr. Phil A. Arneson, Cornell University)

The knots develop very slowly, and by the end of the summer they appear only as small galls that might easily be overlooked (Fig. 3). Further development does not occur until the following spring when the knots enlarge very rapidly. They initially are quite soft in texture and become greenish-brown in color as conidia develop over their surfaces (Fig. 4). The conidia are disseminated by wind and splashing rain but probably do not figure as prominently as the ascospores in establishing new infections.
By the second summer after infection, the knots have enlarged considerably and begin to change to a hard, coal-black structure. The old knots enlarge every year by advancing at the margins. The fungus mycelium can also spread internally and give rise to new galls some distance from the original knot. The central, older portions of the knot eventually break down and are invaded by boring insects.

Black knot disease cycle. (provided by Dr. Wayne Wilcox, Cornell University, NYSAES, Geneva, NY)

Management Strategies

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 15-20cm (6-8 in.) below the knot. Winter is also a good time to look for and remove sources of inoculum in nearby wild Prunus species in hedgerows and woodlots. 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.