Harmful Diseases and Insects in the Summer Garden – Symptoms, Prevention and Treatment

Nothing frustrates the avid gardener more than the presence of pathogens and harmful insects in the summer garden—disfiguring leaves, ruining fruit production, stunting growth and disrupting overall aesthetics. To further complicate matters, different pathogens can produce similar symptoms while some insect infestations can even mimic signs of disease. Take heart, however, because many diseases and insect problems can be treated, controlled or prevented with some initial planning and routine monitoring along with a basic knowledge of plant diagnostics. 

Plant diseases are typically categorized as viruses, bacteria, fungus, nematodes (tiny worm-like organisms) or parasitic plants. They can be present in the soil, over-winter in perennial weeds or distributed by wind, water, insects and humans. They can occur from nutrient imbalances, unfavorable growing conditions or environmental factors like temperature extremes, frost damage, excessive moisture or drought.

Bacteria can invade plants through open wounds or natural openings in bark and leaves by way of soil, water, infected seeds, unsanitized pruning tools or insects. Viruses, on the other hand, affect plants on a molecular level, disrupting their cellular process. While no treatments are available to cure viruses many plant diseases can be controlled with fungicides and bactericides. Symptoms to look-out for include wilting leaves, abnormal foliage color, defoliation, premature leaf drop, cankers, galls and stunted growth. Unfortunately, by the time signs of disease appear, it is often too late to reverse the damage.

Ideally, the best solution for preventing the spread of harmful diseases and insect infestations in the garden is to manage outbreaks before they become a serious problem. Properly planning, preparation and planting is the first step in creating a healthy, stress-free garden. Other useful tips include:

• Know your plants, their cultural requirements and nutritional needs and locate your plants in the garden accordingly. Avoid siting sun-loving plants in the shade or squeezing a hybrid tea roses in a confined space. Read the label before buying to determine if a plant is right for the intended location and be willing to remove a plant with chronic problems.

• Prepare the proper environment for a healthy, pathogen-free garden. Specifically:

  • Incorporate compost and organic matter into the soil on a regular basis to improve drainage, release nutrients slowly for plant intake and improve the ability of soils to hold water

  • Fertilize as needed to maintain plant vigor 

  • Apply a layer of mulch top-dressing to maintain even soil moisture levels and prevent weeds 

  • Purchase your plant material from a reputable garden center, like Reynolds Garden Center, and focus on purchasing plant cultivars specifically bred for disease and insect resistance

  • Plant new plant material at the correct time and avoid planting during periods of extreme stress (ie. high heat or drought)

  • Water newly planted trees and shrubs deeply and slowly and invest in an automatic irrigation or soaker hose system to ensure consistent water. Avoid watering in the late afternoon or evening to prevent fungal diseases from breeding during hot, dark nighttime summer conditions

  • Remove weeds as soon as possible – they rob plants of nutrients and water and can harbor diseases and insects

  • Prune back trees and shrubs to increase air circulation. (This is especially crucial for rose bushes to prevent black spot and mildew diseases.)

  • Maintain a healthy, sanitary environment in the garden by removing and disposing of diseased or infested plant parts immediately; raking up diseased leaves and fruit; sanitizing all garden and pruning tools after use; and cleaning up all garden debris in the fall to ensure no opportunity exists for pathogens to over winter in the garden

• Encourage ‘beneficial’ insects, mites and nematodes into your garden and learn to distinguish and recognize beneficials from pests. For example:

  • Ladybugs are predators to harmful aphids, mealybugs and other small insects

  • Lacewings—characterized by large, transparent and finely networked veins—are predictors of aphids and other small harmful insects

  • Praying Manthis and Ground Beetles (bluish-black in color) are both large general predators that will eat anything (including members of their own species)

  • Dragonflies and Damselflies are predictors to aphids

(Note: To attract beneficial insects to the garden, incorporate the following plants in the garden as a food source for them—carrots, yarrow, dill, fennel, parsley, zinnias, marigolds, daisies, black-eyed Susan, mints, and salvia.)

If it does become necessary to treat an insect or disease infestation on a plant, it is best to start with the least toxic treatment first then, if necessary, proceed to a more highly concentrated chemical control. A few of the more common plant diseases likely to be encountered during the summer months include:

• Powdery Mildew: A fungal disease that affects a wide variety of plants including roses, hydrangea, zinnia, dahlia, verbena and the foliage of many vegetable plants. It is characterized by a powdery white coating on the upper side of leaves. To prevent powdery mildew for forming in the first place avoid high-humidity, shady environments and provide adequate air circulation around plants. Always buy mildew resistance cultivars when possible. 

For a holistic treatment, combine one tablespoon of baking soda and one-half teaspoon of liquid non-detergent soap with one gallon of water and apply liberally to plants. Fungicides are also available at Reynolds Garden Center, however, always read the label directions thoroughly before application.

• Black Spot: is another fungal disease that affects ornamental and garden plants, particularly roses. Like the name suggests, it produces black spots on the surface of the leaves and, if left unchecked, can be quite damaging. It is prevalent in times of hot, humid weather so avoid watering the plant foliage (apply water to roots) and choose a location with plenty of air circulation. Always try to space your roses 3 to 4 feet apart for proper circulation.
  
  Black Spot can be treated holistically by mixing a couple tablespoons of baking soda with a few drops of bleach-free dish soap to a gallon of water and applying to both sides of the leaves weekly and after rain events. Again, fungicides to treat black spot are also available at Reynolds Garden Center.  

• Aphids: Aphids are small, sap-sucking insects mostly invisible to the naked eye. While they can cause stunted growth and curling, distorted leaves, they are most damaging as a transmitter of viruses that can kill a plant. They are a common pest on many garden plants and prefer the tender, leafy shoots of new plant growth. A manufactured insecticidal soap can be purchased at Reynolds Garden Center or a homemade remedy can by made by mixing a few teaspoons of dish soap with one quart of water sprayed routinely to both sides of the leaf surface.

Despite the damage and disfigurement unwanted pests and pathogens can cause in the garden, with proper planning and preparation along with careful monitoring and routine sanitary practices, most infestations can be prevented and controlled before serious consequences result.

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To learn more about disease resistant plant material or to purchase chemical controls for a disease or insect problem, visit Reynolds Landscaping and Garden Shop--open 7-days a week from 9:00am to 6:00pm. For assistance identifying a disease or insect problem, bring a sample of the affected plant to your local county Cooperative Extension Service.