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Managing Garden Diseases

A plant disease issue can quickly turn a gardening season sour. Battling plant diseases is an unavoidable reality of growing fruits and vegetables. Keep your garden plants as healthy as possible by learning to recognize the symptoms associated with an issue, manage the growing environment and prevent the spread of the causal agent from diseased to healthy plants.

What Causes Plant Diseases?

Leaf spots, fruit blemishes, wilt, stunted or disfigured growth, and unusual colors can all be symptoms of diseases on host plants. The cause of any plant distress can be narrowed down to two categories: living and non-living factors. In the world of disease, living factors mean fungi, bacteria and viruses. Non-living factors refers to environmental issues, which are not always diseases, but can contribute to the decline of a plant, such as a lack of proper nutrients.

For a disease to occur, three factors must be present at a given time:

  1. The plant must be one that is susceptible to the disease. This also means the plant must be in a developmental stage when it is more sensitive.
  2. A disease-causing microscopic organism (pathogen).
  3. An environment suitable for the pathogen to cause disease.

This is often referred to as the “disease triangle,” and a disease can be prevented or managed by disrupting any one factor of this triangle. If you think of it as a three-legged stool, breaking one leg may be enough to prevent the disease development.

Is it a Disease?

Know the plant’s normal appearance, so it is easier to notice when the plant changes. Also, ask these questions:

  1. How does the plant differ from its normal appearance?
  2. Where was the problem noted? On the leaves, stem or fruit?
  3. Is the occurrence showing a uniform or random pattern?

From the answers of these questions, it can be decided if the disease is caused by a pathogen or environmental stress. Sometimes what looks like a fungus, bacteria or virus is actually related to temperature, too much water, too little water or soil conditions. Fungal, bacterial and viral disease symptoms will appear over time and often will be sporadic, showing symptoms in just one area or on one plant first, then spreading to others. Issues caused by non-living factors will appear suddenly, be more uniform across the garden and often only show symptoms in the leaves.

Types of Diseases

Below are just a few of the more common disease symptoms that may be present in a garden setting:

Fungal Disease Symptoms

  • White powder on leaves (powdery mildew)
  • Dark, sunken lesions on stems, flowers or fruit (anthracnose)
  • Plant turning yellow and withering (Fusarium or other wilts caused by soilborne pathogens)
  • Bullseye rings on leaves; browning of stem; leathery, black fruit spots (early blight)

Bacterial Disease Symptoms

  • Blighted blossom, leaf or shoot (bacterial fire blight)
  • Bacterial ooze on stem or fruit (gummosis)
  • Water soaked lesions gradually turning brown with tattered appearance of leaves (bacterial leaf spot)
  • Sudden collapse of whole plant (bacterial wilt)
  • Roughened, crust-like diseased area on the surface of a plant organ (scab)
  • Softening, discoloration and often disintegration of tissue (bacterial soft rot)

Viral Disease Symptoms

  • Mosaic; mottling; necrosis stunting; leaf curling; distortion; vein yellowing; ring spot
  • Yellowing of leaves and stunting growth of plants (tomato mosaic virus)

Non-living Symptoms

  • Wilting (excessive heat, drought or cold stress/frost)
  • Sudden plant decline (root rot from excess moisture)
  • Foliar discoloration/chlorosis (nutrient deficiency)
  • Water soaked spot on tomatoes or peppers (blossom end rot from calcium deficiency)
  • Twisting, stringing, curling or distortion of foliage (growth regulator-type herbicide injury)

Disease Prevention & Management

Strong, healthy seedlings are more likely to outgrow a disease issue. However, that is easier said than done. To prevent disease issues, select disease-resistant varieties whenever possible, and follow cultural practices to keep the garden prosperous, such as the following:

  • Minimize abiotic stresses by providing optimum soil moisture and balanced fertilization.
  • Proper spacing of plants allows good air flow between plants and rows and can keep foliar fungal diseases, such as early blight and septoria leaf spot, under control.
  • Rotate plants each year. Members of the same plant family are susceptible to similar diseases and should see a new spot in the garden each season.

Plant Family

Common Crops

Cabbage

Broccoli, Brussels sprouts, cabbage, cauliflower, mustard greens, radish, rutabaga, turnip

Cucumber

Cucumber, cantaloupe, gourds, muskmelon, pumpkin, squash, watermelon

Tomato

Eggplant, pepper, potato, tomato

Beet

Beets, spinach, Swiss chard

Bean

Beans, peas

Carrot

Carrot, celery, parsnip

Onion

Chive, garlic, leek, onion, shallot

  • Add mulch around the base of plants to prevent soil splash, a common way for soilborne or stubble-borne diseases to infect the lower leaves of a plant.
  • Unless a drip irrigation system is used, water plants in the morning to allow time for the leaves to dry and prevent foliar diseases. Avoiding use of a sprinkler on foliage, especially in the late hours, can also serve the purpose.
  • Remove any diseased plants from the garden to prevent the issue from spreading or allowing the organism to survive from one season to another (overwintering).
  • Biological fungicides, such as Serenade (Bacillus subtilis strain 713), Actinovate (Streptomyces lydicus strain WYEC 108), Prestop (Gliocladium catenulatum strain J1446), Mycostop (Streptomyces griseoviridis), or Root Shield/Plant Shield (Trichoderma harzianum strain T22, Rifai strain KRL-AG2), can prevent disease incidence and severity if used proactively.
  • Use disease prevention materials, such as copper sulfate for organic growers and chlorothalonil, mancozeb or captan + topsin-M for conventional gardens, prior to the prediction of prolonged rainy weather or a disease moving into your area.
Manganese deficiency evident in a leaf.

Manganese deficiency.


Blossom end rot on tomatoes.

Blossom end rot.


Cucumber anthracnose.

Cucumber anthracnose.


Bacterial leaf spot on pepper.

Bacterial leaf spot on pepper.


Spotted wilt on a tomato plant.

Tomato spotted wilt virus.


Tomato cold injury.

Cold injury on tomatoes.


Author: Emily Morrow, WVU Extension Service Agent – Jefferson County

Last Reviewed: July 2020

Recommendations for the use of agricultural chemicals are included as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services does not imply endorsement by West Virginia University Extension Service nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.

Trade or brand names used in this publication are for educational purposes only. The use of such product names does not imply endorsement by the WVU Extension Service to the exclusion of other products that may be equally suitable.