Dogwood anthracnose : light intensity and drought stress as predisposition factors and heat treatments as a therapeutic control measure

Light intensity and drought stress were studied as predisposition factors for dogwood anthracnose. Two-year-old potted dogwood trees (Cornus florida L.) were placed in one of the following light treatments on July 17, 1991: 100, 50, 10, or 2% ambient light. Trees in 100% light were set in plots in full sunlight. Trees in the other three light treatments were placed in tents constructed of 50% light transmission cloth, 10% light transmission cloth, or 2% light transmission cloth. In June of 1992, trees from each light treatment were inoculated with Discula destructiva Redlin sp. Nov., the causal agent of dogwood anthracnose. Trees from each light treatment were inoculated naturally by placing them under infected dogwood trees. The remaining trees from each light treatment were inoculated artificially by placing them under a plastic grid which held infected branches of C. florida leaves. Water was misted above the infected branches from dawn to dusk and trees were sprayed once with a suspension of D. destructiva conidia. After inoculation, the trees were returned to the appropriate light treatments and some of the trees from each light treatment were subjected to drought. Every fourth day, disease progression on leaves was monitored on each tree. The method of inoculation (natural or artificial) did not significantly affect disease progression. Disease progression was low in the 100 and 50% light treatments, greater in the 2% light treatments, and greatest in the 10% light treatments. Water deficits greatly accelerated the development of disease among the 50, 10, and 2% light treatments. Evaporative potential increased with light intensity, and a combination of high light intensity and high evaporative demand apparently suppressed disease progression in the 100% light treatment despite drought. Chlorophyll concentrations increased with each decrease in light intensity.

Heat treatments were studied as a therapeutic control measure for dogwood anthracnose. Detached dogwood leaves infected with D. destructiva were heat treated at 25, 40, and 45 C for periods of 12, 24, 48, 72, and 96 hrs. Discula destructiva spores were confirmed on leaves after temperature-time treatments of 25 C for 96 hrs, 40 C for 96 hrs, and 45 C for 48 hrs. Numbers of leaves with viable spores declined with increased temperature and incubation time. When similar temperature-time treatments were applied to infected dogwoods with fully expanded leaves, D. destructiva sporulation was not affected by the heat treatments. However, temperature-time treatments at and above 45 C-48 hrs resulted in damage to the trees. Healthy dormant seedlings were able to withstand the temperature-time treatments with no apparent adverse effects.