Key Plants--Key Diseases--Key Times for Disease Control

Of all the plants used in the landscape or grown in the nursery, certain ones usually have diseases each year. These can be called “key plants.” These key plants should be scouted and monitored on a regular basis in order to detect the onset of disease. Although each plant is susceptible to many diseases, only certain diseases are very common and occur each year. These “key diseases” are the ones that the nurseryman and landscaper should especially learn to identify, understand their biology, and know what control measures must be implemented for effective control. For each disease, there are “key times” for control when disease management practices are most effective. The occurrence of key diseases and the timing of important disease management activities can be roughly grouped into four different categories because of the activity of the plant or general weather conditions.

1. Dormant season: late autumn through winter
   1. Conduct inventories to determine what plants are present at the site. Identify the key plants.
   2. Run soil tests to check pH and fertilizer status.
   3. Mulch to protect roots.
   4. Protect evergreens from drying winds, salt sprays, and ice damage.
   5. Prune dead, cankered twigs and branches.
   6. Rake and destroy fallen leaves around trees and shrubs that had leaf spotting diseases, especially rose black spot, apple scab, and anthracnose.
   7. Examine the plants for galls such as those caused by cedarapple rust (on juniper), white pine blister rust, pine-pine gall rust, black knot on plum and cherry, and crown gall. Remove infected branches or remove severely affected plants entirely.
      
      
2. Bud break: spring through early summer
   1. Spray to protect emerging leaves of plants that have a history of severe anthracnose, leaf spots, or twig blights, or are at high risk for these diseases. Diseases of primary concern include:
      • Sphaeropsis tip blight Phytophthora dieback
      • Juniper twig blight Volutella on pachysandra
      • Apple scab Douglas-fi r needlecasts
      • Rose black spot Cedar-apple rust
      • Dogwood anthracnose Pine-pine gall rust
      • Ovulinia petal blight Fire blight
   2. Pick off and destroy any gall or gall-like tissue, such as cedar-apple rust galls from junipers and leaf and fl ower galls from azaleas.
   3. Apply soil drench fungicides to azaleas and rhododendrons, which are at risk for Phytophthora root rot.
      
      
3. Summer
   1. Apply fungicides during wet weather periods, to prevent the further spread of diseases including:
      • Rose black spot
      • Volutella on pachysandra
      • Cease spraying when the weather is dry.
   2. Apply soil drench fungicides to continue the Phytophthora protection. Do this at the recommended interval noted on the product label.
   3. Apply fungicides to control pine and spruce needlecasts.
      
      
4. Cool weather: late summer to autumn
   1. Spray to control powdery mildew on highly susceptible plants, particularly during periods with cool nights and warm days.
   2. Spray to protect the new autumn growth from twig blight infection on juniper.
   3. Prepare new planting sites that should be fumigated and treat them while soil temperatures remain above 55°F and soil moisture is 50 to 85 percent of field capacity. Allow the site to aerate several weeks before planting or cover the treated area with a plastic tarp and plant in the spring.
   4. Update the site inventory and be certain you have recorded all the diseases that developed on each plant.
      
      

   In addition to these four key periods of disease control, times during which plants are under stress (such as drought or defoliation) should be shortened as much as possible through watering and controlling insects. Stresses weaken plant vigor, rendering them more susceptible to weak plant parasites that can cause cankers and root rots.