Septoria leaf spot on Rudbeckia (Photo by S. Guiser)
The activities of the organisms that cause plant diseases are governed by the temperature, relative humidity, moisture and other environmental factors as well as the physiology of the plants they infect. The most important time of the year to consider all these factors is the Spring. At bud break, the plants produce succulent leaves and twigs and the pathogens come out of dormancy to attack those highly susceptible tissues. These are the plant parts that must be protected from those first infections.
An effective disease management plan is designed to prevent those first infections rather than reacting after infections have already occurred. You can design a disease management plan before Spring gets into full swing if you know what diseases affected the woody ornamental or herbaceous perennial last year because that same pathogen will probably be there again this year.
Go back to any plant disease clinic reports you received last year, sit down with your crew to review what was observed and assess what was done at a particular site. If you haven't kept written records, plan on beginning to do so this year in order to better manage plant diseases in the future. But if you have records or a good memory, use that information this year.
Ideally, you have the following information in hand. But if this is the first season you've thought along these lines, here is what you need:
- Inventory of the plants at the site. Mapping their location and noting which plants had disease problems can save time because those same plants will probably have those same diseases each year.
- The important characteristics of the site such as exposure to wind, proximity to road, walks, drainage pattern. A record the date of any site changes such as excavation, paving, removal of over-story trees.
- A record of the general weather conditions at the site during the year including unusual events such as drought, flooding, and winter damage.
- A record of the general appearance and health of each important plant being maintained. Especially note unusual characteristics such as smaller than average leaves, unusually light green leaves, smaller than average internode length, and mechanical injuries. Photos are worth a thousand words, especially if the photos are dated.
- Record the use of insecticides, fungicides, herbicides, fertilizers, or any other chemicals on or near the individual plants. Note the chemical, formulation, rate and method of application, and weather conditions at the time of application as well as the time of day the material was applied.
- Notes on the date of the first occurrence of the diseases you most want to control. You will discover that in most cases, that will be a date in the Spring as new leaves and twigs are developing. If you don't recognize the disease, get a sample to the Extension Educator in your county for a diagnosis. If that person is uncertain as to the cause of the problem, they will forward the sample to the Plant Disease Clinic at Penn State or give you the information you need in order to submit a sample directly.
The most important time to begin a disease management regime, for most diseases, is in the Spring at bud break. The diseases most threatening at that time, the ones you want to prevent from happening in the first place, include:
- anthracnose on dogwood
- apple scab
- fire blight on hawthorn, pear, crabapple, and related plants
- gray mold on peony
- leaf and flower gall on azaleas and rhododendrons
- needlecasts on pine, Douglas-fir, and spruce
- Ovulinia petal blight on azaleas and rhododendrons
- rose black spot
- Septoria leaf spot on Rudbeckia
- Volutella blight on pachysandra
All of these, except fire blight, require the application of a fungicide to protect the new leaves and twigs. That means, the fungicide must be on the plant as the new leaves and twigs expand. Usually, more than one spray is required. In the case of fire blight, pruning of branches that were infected in previous years must be done before bud break occurs and must be done during dry weather.
Sprays can be applied to suppress new fire blight infections of open flowers but such sprays do nothing against the fire blight that is already in the wood. Once the damage is done by these diseases, the symptoms will be visible for the rest of the season regardless of how much you spray. The bottom line is, if you prevent disease initiation in the Spring, the rest of the season is much easier. So, be ready for Spring because the plant pathogens certainly will be ready.