For all the challenges of growing plants in our region, there are probably known more diverse and frustrating to understand as those that arise due to plant disease. While some can be severe, leading in some cases to outright death of plants, most plant diseases are quite harmless and are easily prevented and controlled. In all cases, early detection of diseases is an essential component of your landscape integrated pest management (IPM) program.

The Ecology of Plant Diseases

In order to understand how diseases are controlled, it is essential to have a basic understanding of why they occur in the first place. There are three basic things that all plant diseases require: 1) pathogen, 2) a suitable environment, and 3) a susceptible host. This is called, quite simply, the disease triangle, and when all three components come together, it results in plant disease. Thus, an IPM plan begins with the management of one, or even all aspects of the disease triangle. While you do not need to have a degree in plant pathology, being familiar with disease cycles and knowing how to break those cycles can often help you manage plant diseases.

Cycles essentially start with an infectious inoculum, which can be fungal spores, bacterium, viruses, or nematodes, which colonize and infect plant tissue. Over time, if left untreated or unchecked, these inoculum can continue to spread and infect otherwise healthy plant tissue. If diseases are allowed to survive between growing seasons, which they often do in our mild climate, problems can become worse over several seasons. Unfortunately for the more serious plant diseases, late detection usually means total replacement of plant material.

The majority of biotic (living) diseases are normally caused by fungus. But there also are abiotic (nonliving) factors that may stress plants and open them to infestation by disease-causing agents. A perfect example, and one to which we can all relate, would be the 124 inches of rain that we received last year. Too much water, whether from natural events or from overzealous homeowners can open plants up to infestation of disease. Other examples of abiotic diseases include drought stress, pH imbalance in the soil, wrong plant in the wrong place, and humans. You heard me right. Guilty of often “loving our plants to death,” humans are the number one pest of plants.

Diagnosing Plant Diseases

For every plant there is a veritable laundry list of diseases that can affect it, making diagnoses a challenge. The most important step you can take in diagnosing a plant disease is to first understand what is normal for the plant. A variegated plant leaf may often times appear to have a disease, when in fact that is what is considered normal for that particular plant. Knowing what is right for the plant and continued monitoring of various plant parts helps in determining when something starts to go wrong. Also understanding some common problems of particular plants are will help you in diagnosing plant diseases.

Once you have identified a problem, the next step is to bring in a sample to your local extension office for diagnosis. A sample may also be sent to the Plant Disease and Insect Clinic (PDIC) in Raleigh for analysis. When results return, an agent in your respective county can help recommend control measures for that particular disease.

Current Recommendations for Submitting Potential Disease Samples and COVID-19 Updates

The PDIC is currently not receiving physical samples at their office in Raleigh; however, you can still submit images to your local county Extension Agent for assistance in diagnosing landscape diseases. Below are a few tips for when you submit images:

• Multiple pictures are best, and make sure all of them are in focus.
• Take a picture or two of the whole garden or area affected capturing the entire plant or plants with some of the background included to give the diagnostician some idea of the setting.
• Take a picture or two of plants that are affected – avoid those that are completely dead as they don’t tell the diagnostician anything. Rather, try to capture the range of symptoms.
• If more than one plant is affected, then capture some that are in an early stage, middle stage, and late stage, if possible.
• A close up of a leaf or stem where you see part healthy, part diseased is also good.
  

  Black fruiting bodies from fungal pathogen under magnification.

• If fruiting bodies are present, snap a photo or two.

Managing Plant Diseases

Proper disease management begins with a practical IPM plan which provides a multi-faceted approach that will save time, money, and the unnecessary applications of chemicals. Some of the simplest and most cost-effective controls include things like pruning away infected plant material, using drip irrigation in lieu of overhead watering. Other prevention strategies include selecting disease resistant varieties. Most seed companies, whether you order online or through catalog, will contain disease resistant codes next to select varieties of plants. The next time you look for tomatoes, for example, look for a tiny “V” or “F” that indicates resistance to verticillum wilt and fusarium root rot, respectively.

Learn More!

To learn more about plant diseases, or if you have other gardening-related questions, visit our website, where you can post your questions via the ‘Ask an Expert’ link, or contact your local Extension office. You can also visit the North Carolina Extension Gardener Handbook for more general information on plant diseases, and more! Below you will find links and videos from NC State Extension that will assist you diagnosing and correcting plant disease problems.

NC Extension Gardener Handbook, Diseases and Disorders Chapter.

NC State University Plant Disease and Insect Clinic. Please note that due to COVID-19, offices are closed and they are only accepting digital images.

Watch the series on Understanding Plant Diseases.