Garden Club of Cape Coral | April 22 | Help – my plant is still sick!
Last month we learned about the three underlying factors that can cause disease in plants, better known as the “disease triangle.” This model explains the interactions between the environment, the host (your plant) and the infectious agent. When vulnerable plants encounter a strong pathogen such as fungi, bacteria or viruses in a favorable environment, disease will occur. You will be better positioned to control or eliminate the disease by removing at least one part of this disease triangle. The most difficult factor to control is the environment, and the wetness and humidity that are so pervasive here in Southwest Florida are two of the most common unfavorable conditions for plant disease. Our challenge is to learn to identify the diseases that are affecting our plants, and to develop a management plan to eliminate and prevent these diseases.
Black spot is a common rose disease caused by a fungal pathogen. The most common symptoms are black spots with irregular or “feathery” borders primarily on the plant’s leaves. The leaves will eventually yellow and fall off, causing the plant to decline. Disease spread can be reduced by removing the fallen leaves and applying fungicides that are designed to control black spot. The most reliable way to guard against black spot is to plant resistant rose cultivars.
Mosaic virus is another common viral infection that can affect cucumbers, peppers, tomatoes and even St. Augustine grass. Although this virus is not always fatal, it will reduce flower and fruit production and diminish the plants’ vigor. Leaf curl and distortion and leaves mottled with green, white and yellow patches are common symptoms of mosaic viruses. Unfortunately, there is no cure or control for viral infections, so infected plants should be removed to prevent disease spread.
Rust is another common fungus that we encounter in the garden. This disease begins as small yellow spots on the underside of leaves followed by rust-colored pustules that form and merge, causing the leaves to turn brown or black and fall off. If you have plumerias in your garden, you have no doubt seen this disease! Spraying your plants early in the season with a copper fungicide can help reduce the occurrence of rust, but the most important practice is to dispose of all fallen leaves to help reduce future infection.
Powdery mildew is another common disease that you may have encountered that is prevalent in spring and fall. This disease begins with powdery white patches on leaves and spreads until the plant is covered by a powdery residue; infected plants will eventually produce stunted leaves and even die. Plantings in shady environments are most susceptible since this virus thrives in moist settings. Controlling irrigation in the area and thinning plants may help reduce powdery mildew, and there are many fungicides that can be applied to control this disease. Note that fungicides are most effective when applied early or preventively; more advanced infections can also be treated with neem oil.
Ganoderma butt rot is a common palm disease, and all palm species are vulnerable. This disease is easily identified by the “conk” or shelf-like mushroom that appears on a palm trunk. Sadly, there is no cure or control for Ganoderma and the diseased palm must be removed. Because the fungus survives in the soil, palms cannot be planted in an affected location again.
Mushroom root rot, or Armillaria, is another usually lethal fungus that decays the root systems of trees and shrubs. This fungus produces clumps of mushrooms in the plant’s root zone in fall, winter and late spring. Since the mushrooms generally appear one to three years after the start of the infection it is often impossible to control the disease once you spot the symptoms. Removing the sick plant and treating the areas with a soil fungicide treatment before replanting are your best options.
As you have probably discerned, many of the diseases that attack our plants here in Southwest Florida are dependent on moisture. Working to eliminate this factor of the disease triangle will help you prevent some of the more common fungal diseases. Good sanitation practices are also important; always remove fallen dead leaves from your gardens to help prevent disease spread. Maintaining clean tools, especially pruners and any other cutting instruments, is also critical to prevent the spread of diseases.
Fungicides also have a place in the process of disease prevention; if you know you have susceptible plants such as plumeria, you can apply fungicides early in the season to help delay or lessen the appearance of fungi such as rust. The most important factor in the equation is your ability to monitor your garden for problems and to act quickly to eliminate issues. Turn your morning stroll through your landscape into a fact-finding mission to identify possible concerns and then consult UF/IFAS online to discover the best solutions to your problems while you sip your coffee!
Cathy Dunn is a Lee County Master Gardener Volunteer, Garden Club of Cape Coral Member and president of the James E. Hendry Hibiscus Chapter. Visit gardenclubofcapecoral.com. Like us on our Facebook page!