Plant Diseases

   Just like people, plants can get sick too. Sick plants do not grow well or produce good quality food. The microorganisms that cause diseases in plants are similar to those that can cause diseases in humans. These include pathogenic viruses, bacteria, fungi, protozoa, and nematodes. Plant pathogenic microorganisms often favor plants that are stressed. Stresses for plants can include too many or too few nutrients, moisture, and light. Plants also experience stress from toxic chemicals, air pollution, and competition with other plants (usually weeds). Damage to plants via insects, animals, or harsh environmental conditions can leave open wounds for pathogens to enter the plant and cause disease as well.

    There are many common practices that home gardeners can do to reduce pathogen presence and spread, including:

  • Purchasing certified disease free seed
  • Selecting cultivars resistant to common diseases
  • Proper fertilization and irrigation
  • Reducing tillage to increase the beneficial microorganisms in the soil
  • Removing and destroying weeds
  • Removing plant debris and infected plant parts
  • Rotating with plant species that are immune to problem pathogens in the area
  • Controlling insects and other vectors of pathogens 
    There are a number of common diseases that a home gardener in Pennsylvania (PA) may face in their lifetime. The following is a list of five commonly found plant diseases with links to University Extension publications that will provide more information on the disease and management of each pathogen:

Septoria leaf spot on tomato
1. Septoria leaf spot caused by Septoria lycopersi
Septoria leaf spot is a very common disease of tomato often confused with early and late blight. On tomato plants, Septoria lesions tend to develop on the bottom leaves first. It forms tan lesions with small brown/black dots inside. Septoria lives in infected plant debris and will overwinter in PA. This Ohio State University Fact Sheet provides more information on Septoria leaf spot. 

2. Early blight caused by Alternaria solani
Early blight is a common disease of tomato and potato in PA. Early blight lesions begin as brown circles with yellow halos. As they expand, they may form concentric rings (target spot pattern). Early blight may spread to unripe fruit forming dark, sunken lesions. More information on early blight can be found at the Penn State Tomato and Potato Early Blight Update.

3. Late blight caused by
 Phytophthora infestans
Late blight is a very well known disease due to its ability to cause complete destruction of potato and tomato plants if left unmanaged. The late blight pathogen requires living tissue to survive. The disease is usually introduced to PA through infected transplants, or the aerial spread of the pathogen from the southern United States. USA Blight is a national project created to reduce the impact of late blight and track the progression of the disease. To find out if late blight has been reported in your area, visit the Late Blight Map. More information on late blight can be found at this Penn State Extension Fact Sheet

4. Bacterial wilt caused by
 Erwinia tracheiphila
Bacterial wilt of cucurbits is a vascular disease. Symptoms begin as the wilting of a single leaf. As the bacteria spread throughout the plant, the wilted leaves and stems turn brown and die. Spotted and striped cucumber beetles vector the bacteria. This Cornell University Fact Sheet provides more information on bacterial wilt.  

5. Powdery mildews
There are a number of powdery mildew species in PA which cause disease on a wide variety of hosts. Powdery mildew presents as a white powdery fungus on the upper surface of leaves. The fungus often causes the deformation of leaves and reduction in yield. More information on powdery mildews can be accessed at this Penn State Fact Sheet. A common powdery mildew found in the garden is cucurbit powdery mildew, described here by Penn State.


    If you believe the plants in your Community Garden plot may be suffering from a disease, it is important to take the proper control measures to prevent this disease from spreading to your neighbor’s plants. Often problems like nutrient deficiencies can be mistaken for a plant disease, which is why it is important to properly diagnose problems on your plant before taking action. Remember, the Penn State Community Garden is open to the public so it is important to use organic methods of pest management. If you need help in diagnosing a disease on your plant, you may submit a sample to the Penn State Plant Disease Clinic for diagnosis. A representative sample of the problem, and thoroughly completed specimen information form is critical to diagnosis. More information about the Plant Disease Clinic and submitting samples can be found at the Penn State Plant Disease Clinic website

    The information does not stop here. If you want to learn more about plant diseases, Penn State and other Universities provide plenty of trustworthy information on the subject. 

Happy Gardening!

Authored by: 

Jennie Diehl Mazzone- Master's Student, Department of Plant Pathology and Environmental Microbiology, Penn State

Images credited to:

Beth K. Gugino- Extension Vegetable Pathologist, Penn State

Jim Jasinski- Ohio State University Extension,