Water stress and suceptibility of Cornus florida and Cornus kousa to dogwood anthracnose

Before dogwood seedlings can be screened for resistance to dogwood anthracnose, a reliable inoculation technique is needed. Furthermore, the effects of water stress on disease resistance should be known before any cultivar is developed and marketed as disease resistant. Leaves of forty Cornus florida seedlings were wounded with a pin-prick device and inoculated using one of two isolates (collected from different locations, and one maintained on artificial media for one year longer than the other), and one of two fungal propagules (conidia or vegetative hyphae), then enclosed in a moistened plastic bag for one of four periods of time (0, 2, 4 or 7 days). Trees were incubated in either a humidified, air-conditioned greenhouse chamber or an environmental growth room. Lesions were observed and measured weekly for five weeks. No significant differences were found between the isolates or propagules, but bag time was an important factor in lesion development. Lesions on leaves bagged for 7 days were almost five times larger than those on leaves enclosed for 0, 2, and 4 days. Based on the results of this experiment, we chose the inoculation technique to use in subsequent experiments. Inoculum chosen was conidia of the isolate from Catoctin Mountain Park. We enclosed wounded, inoculated leaves in moistened plastic bags for seven days. Water availability was manipulated to determine its effect on dogwood anthracnose severity on Cornus kousa and C. florida trees. Experiments were conducted (summer, 1995) and repeated (fall, 1995) in the greenhouse chamber and environmental growth room. Neither severe drought [rewatering when soil reached 35% soil relative water content (RWC)], moderate drought (rewatering when soil reached 45% soil RWC), nor flooding (watering daily with no drainage allowed) caused symptoms more severe than in controls (watering daily with drainage) on either species. In the first trial of this experiment, dogwood anthracnose symptoms were more severe than in the second trial. It is possible that environmental conditions were more disease conducive during the second trial. Lesion sizes on the two species depended on the trial. In the first trial, C. florida trees had more severe dogwood anthracnose symptoms than did C. kousa trees. In the second trial, severity of dogwood anthracnose symptoms were similar on the two species. Our results suggest that dogwood anthracnose resistance is not consistent throughout the C. kousa species