Nursery, Landscape, and Turf - Moisture and Diseases

Moist conditions favor disease development in landscape plants. The following is an article on the subject.

In Mid-May, there were days with persistent rain, mist, fog, and humid, overcast weather in many parts of Delaware. Wet weather increases disease pressure for most foliar fungal and bacterial diseases of plants in the garden and landscape. Many fungal pathogens require free moisture on the host plant surface for germination of their spores. The longer it is that leaves remain continuously wet, the better chance it is that fungal spores on the host will succeed in infecting the host plant. At average temperatures of 60-65 F, the apple scab fungus needs 9 consecutive hours of wet leaves for successful infection, the grape black rot fungus needs 8-9 hours, the rose black spot fungus needs 7 hours, shade tree anthracnose fungi need 6-12 hours, and the cedar-apple rust fungus needs only 3-4 hours. Following infection promoted by wet weather, disease symptoms typically appear on the plant a week or two later.

The two other components of the plant disease triangle, the susceptible host and the virulent pathogen are needed for a disease outbreak. For example, diseases such as apple scab, oak anthracnose, cedar-apple rust, and rose black spot are already present on susceptible host plants in many landscapes. These infections occurred during wet periods this past month. Depending on where susceptible plants are located, recent disease-favorable wetness periods have lasted anywhere from 9 hours in one day to 45 consecutive hours over two or three days. Thus, for the rest of this spring and even into summer, expect these infections to produce symptoms and more spores to continue the cycle of disease every time a prolonged wet period occurs.

Adapted from COMPLETING THE PLANT DISEASE TRIANGLE - DISEASE-FAVORABLE ENVIRONMENT By John Hartman in the May 19 edition of the Kentucky Pest News from the University of Kentucky, College of Agriculture.