Spring: A Key Time for Plant Diseases and Disease Control
From the time the buds begin to open until leaves and branches are fully expanded, a plant is highly susceptible to most of its pathogens. Young expanding leaves and twigs of dogwood, ash, oak, maple, and sycamore are vulnerable to infection by the various fungi that cause anthracnose and other leaf spots. Likewise apple scab, rose black spot, Volutella blight of pachysandra, Ovulinia petal blight on azalea, leaf spots on viburnum, Entomosporium leaf spot on hawthorn, and black rot on Boston ivy all begin in the spring as new leaves are forming.
Bacterial diseases too, such as fire blight on the Rose family of plants, cause new infections during flowering in the spring. The needlecasts on Douglas-fir (Rhabdocline and Swiss) and spruce (Rhizosphaera and Stigmina) infect the current year's needles as the needles form in the spring. The cedar rusts that overwinter on junipers form spores in the spring that are blown to their broadleafed hosts. In the case of herbaceous plants, gray mold (Botrytis) and downy mildews (not powdery mildews!) begin as young plants emerge from the ground or are moved from the greenhouse to the landscape.
Although the pathogens involved have been lurking about on plant parts that were infected last year or were dormant in plant debris not cleaned up at the end of the previous season, they were not active during the winter. They were in a dormant state, highly resistant to cold and wet or dry conditions. Because they are not active, they are not sensitive to any chemicals applied to them during their dormancy. That is why there are no 'dormant sprays' for plant pathogens. The physiology of the pathogen must be active in order for it to be disrupted by fungicides or bactericides.
During plant and pathogen dormancy is when other things are done to manage disease including pruning infected plant parts and cleaning up and removing plant debris...physically removing the pathogen from the vicinity of what will become susceptible plants in the spring. Spring is when temperatures favor the activity of most plant pathogens and when the wetness on leaves and twigs that is required by almost all plant pathogens for attack is supplied abundantly by Mother Nature.
For these reasons, if you plan to use fungicides or bactericides to manage plant diseases and expect them to give you effective control, application during flowering and leaf and twig expansion in the spring is when these chemicals should be applied.
Later in the summer when the symptoms are obvious but the pathogens have slowed their activity because of high temperatures or lack of wetness, applying a fungicide may make you and your client feel good but the pathogens have had their day and are saying, "Bring it on. You're too late!"
Plant pathogens 'spring' into action as buds open and leaves and twigs expand. That's your signal to spring into action too.
Gary W. Moorman, Professor of Plant Pathology