Your Plants Look Sick? Now What?

What are plant diseases?

Among the many challenges farmers face when growing crops, we have plant diseases, which reduce the quality and quantity of the food, fiber, and ornamental crops we produce. Plants are considered "diseased" when some of their basic functions are impaired. Keep in mind that for a plant to vigorously grow, all plant organs must be healthy. For example, when plants are affected by pathogens causing root rots and decay, the impaired water supply to the above ground of the plant will reduce leaf photosynthesis, and hinder plant growth.

A plant disease of historical importance and will always be remembered because of its devastating effects is Potato late blight. Phytophthora infestans, the microorganism that causes Potato late blight, was introduced into Europe in 1845. In Ireland, the impact of this disease was so devastating that about a million Irish died of starvation during what is known as the "Irish Potato Famine" or "The Great Hunger", and one to two million men, women, and children were forced to migrate into other cities in Great Britain and North America. In commemoration of this tragedy, several monuments have been built in various cities around the world. Once a year, the Scottish football team Celtic wears the Famine Memorial Crest on the club's jersey.

What causes plant diseases?

Fungi, bacteria, nematodes, and viruses that cause plant diseases are known as plant pathogens. Pathogens can spread to other plants through various mechanisms, including wind, water, soil, pruning scissors, or insects. If the plant’s defense mechanisms do not work against the pathogen, and when there are optimal weather conditions, infection and colonization of the plant will occur, and disease will develop.

Nevertheless, plants can also look sick even in the absence of pathogens. Nutrient deficiencies, excess heat, herbicide damage, flooding, and drought can also impair plant growth and cause similar symptoms to those caused by pathogens. In this case, since there are no microorganisms involved, there will be no transmission from plant to plant. 

How do we identify plant diseases?

A good start is by looking at symptoms and signs. You have likely seen plants with yellow leaves, leaf spots, and blotches, or roots that disintegrate when we want to pull a plant from the ground. All these abnormalities are termed disease symptoms (Figure 1), and can be associated with different parts of the plant, like leaves, stems, roots, flowers, and seeds.

The remaining question is, what is causing those symptoms? A fungus, a virus, a bacterium? Most microorganisms are difficult to observe; however, in some instances, either the pathogen or structures produced by the pathogen are visible, and these are called disease signs. Examples of disease signs on various crops are shown in Figure 2 and include fungal growth in the form of tufts or spore masses on leaves and other plant parts, sclerotia (hard fungal masses that serve as survival structures), and a combination of cotton-like growth and spore masses.

The correct identification of the pest or pathogen is the number one step of integrated pest management in a production field, the greenhouse, and even at home. It is important to remember that many different symptoms can be associated with environmental factors, pesticides, and insects, and not due to plant pathogens. Another confounding scenario is when plant tissues or entire plants die as a consequence of a disease or an environmental factor, and the affected tissues are rapidly colonized by other fungi that are not pathogens but love feeding on decomposing organic matter. In this case, trying to diagnose the problem only by considering the visual observation of symptoms and signs may be misleading. Some plant diseases are easy to identify, while others require submitting a sample to a diagnostic laboratory. If you are a homeowner or a field crop grower who needs help with plant disease identification, contact your local Penn State Extension Office. They will assist you by contacting the proper educator in Agronomy, Horticulture, or the Master Gardener program. If further help is required, remember that the Penn State Plant Disease Clinic provides a free diagnostic service for people within Pennsylvania.

Here are some considerations before you send a sample to the diagnostic clinic:

1. Ensure the samples are representative of the area showing a problem.
2. Collect plants that show a range of symptoms, ranging from healthy to very diseased. You should include enough plants showing how most plants in your field/greenhouse/pots appear.
3. When possible, include whole plant specimens. If your sample contains roots, make sure to place the root in a bag and tighten the bag around the base of the stem to keep the root and soil separated from the top part of the plant.
4. Wrap the samples in a dry paper towel and place them in bags or boxes for shipping.

To make things easier for clients, the Plant Disease Clinic is currently accepting digital sample submissions via email. You can take pictures of the diseased and healthy plants and send them to the clinic via the following email: PlantClinic@psu.edu. If you plan to send a sample or an image to the Plant Disease Clinic, make sure you fill out and send the Specimen Submission Form along with the samples. Check the Plant Disease Clinic website for detailed instructions on packing the samples and current drop-off recommendations.

References

• Irish potato famine. 2017.
• Official Celtic Football Club Website.