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Disease Cycle and Epidemiology

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BLACK KNOT DISEASE CYCLE
Courtesy Wayne F. Wilcox

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°C and 27°C (60-80°F) 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 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.

The knots develop very slowly, and by the end of the summer they appear only as small galls that might easily be overlooked (figure 10). 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 (figure 11). The conidia are disseminated by wind and splashing rain but probably do not figure as prominently as the ascospores in establishing new infections.

The beginnings of knot formation show only small galls.

Figure 10

As knots develop that become very soft in texture and are covered with the asexual olive-green conidia.

Figure 11

Ascospores are not produced until the spring following the second winter after initial infection. By the second summer after infection, the knots have enlarged considerably (figure 12) and begin to change in appearance to a hard, coal-black structure. The old knots enlarge every year by advancing at the margins (figure 13). 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 become riddled by boring insects (figure 14).

Figure 12	Figure 13
Figure 14

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