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  • Marlia Bosques-Martínez

Are Your Plants Sick? Plant Disease Basics

By: Marlia Bosques Martínez

If you are a new grower, have indoor plants or a garden, this beginner guide may help you understand what is killing your plants. 

First, let's not confuse diseases and pests. "Pests" is commonly used to describe insects that attack plants, and entomologists study these. On the other hand, plant disease is defined as "anything that prevents a plant from performing to its maximum potential." This broad definition includes abiotic and biotic plant diseases. Biotic diseases are living organisms (pathogens), including fungi, oomycetes (fungal-like organisms), bacteria, viruses, and nematodes. Abiotic disorders are chemical or physical factors like poor soil drainage, nutrient deficiency, unfavorable climate, among others. Plant pathologists study all of the diseases mentioned above. The main goal of Plant pathologists is to identify the organism causing complex diseases and create management practices to combat them (click here to read about careers in plant pathology).

Biotic Plant Diseases 

As previously mentioned, organisms that make plants sick are called pathogens. Plant pathogens have very similar biology to those that cause disease in humans and animals. Pathogens can spread from plant to plant and may infect any part of the plant, including leaves, shoots, stems, roots, fruit, and seeds. 

  • Fungi and fungal-like organisms (oomycetes) collectively cause the most plant diseases than any other plant pathogens. A fungus is a eukaryote that usually grows as threads or strands called hyphae. A single hypha can be only a few inches long or miles long! Most fungi reproduce by producing spores that may spread long distances by air or water, or soil. Common fungal disease symptoms include root and stem rots, shoot and leaf blights, leaf spots, cankers, vascular wilts, and postharvest storage rots.  

Bruce Watt, University of Maine/Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo
  • Nematodes are tiny microscopic roundworms that generally live most of their lives in the soil. All plant-parasitic nematodes have a needle-like mouthpart called a stylet used to pierce the plant cells and feed on the cell contents. Typical root symptoms indicating nematode attack are root knots or galls, root lesions, excessive root branching, injured root tips, and stunted root systems.

Scott Bauer, USDA Agricultural Research Service/Ulrich Zunke, University of Hamburg,Elizabeth Bush, Virginia Polytechnic Institute and State University/Jonathan D. Eisenback, Virginia Polytechnic Institute and State University
  • Bacteria are microscopic single-celled organisms with no nucleus that reproduce asexually by binary fission (one cell splitting into two). Most bacteria associated with plants do not harm and are actually beneficial, such as decomposing nutrients! They can be spread by rain splashing or carried by the wind, birds, or insects. Common symptoms include overgrowths, wilts, leaf spots, specks, blights, soft rots, scabs, and cankers.

Paul Bachi, University of Kentucky Research and Education Center/ David B. Langston, University of Georgia/Ulla Jarlfors, University of Kentucky/Howard F. Schwartz, Colorado State University
  • Viruses are intracellular (live inside the cell) DNA or RNA particles with a protein coat that infects other living organisms. They replicate in the hosts they infect by inducing host cells to form more virus particles. Viruses are transmitted by vectors (insects, nematodes, etc.) and vegetative propagation (cuttings, grafting, etc.) Typical symptoms include stunting, mosaic, or ring spot patterns on leaves and fruit.

Whitney Cranshaw, Colorado State University/Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo/Florida Division of Plant Industry, Florida Department of Agriculture and Consumer Services

Learn more about biotic diseases here. 

Abiotic Plant Diseases

Abiotic factors are divided into two categories: chemical and physical. 

Chemical factors: 

  • nutrient excesses/deficiencies 

  • air pollution and damaging gases

  • chemical injuries (herbicides, fungicides, insecticides)

  • plant growth regulators 

  • de-icing salts

Physical factors: 

  • soil pH

  • soil structure 

  • moisture extremes

  • Water stress

  • temperature extremes 

  • mechanical injury 

  • storm damage 

  • equipment injury 

  • animal damage 

To learn more about these abiotic disorders, click here. 

Keep in mind that each pathogen or abiotic factor may present distinctive symptoms, but it's not always the case. Additionally, the plant may have more than one disease, making it even harder to diagnose. 

Disease triangle 

To fully understand plant diseases and manage them, we must learn about the plant disease triangle. For a disease to develop, three factors must be present at the same time. The three elements are a susceptible host, a virulent pathogen (one that can cause disease), and a suitable environment. In infectious plant diseases, practices that favor plant growth over pathogen activity tend to decrease the amount of disease observed. In this respect, plants that are fertilized and watered correctly will be less likely to develop the disease. Thus, altering elements of the disease triangle allows you to manage your disease. For example, if you have a zucchini with powdery mildew, consider changing to a cultivar with resistance to this disease. 

To learn more about the disease triangle, click here. 

Why should we care?

Everything you eat (i.e., fruits, vegetables, grains), indoor plants, and gardens can be affected by diseases. Diseases can drastically reduce the yield, value, and overall appearance of plants. Farmers are continually battling diseases and lose half of their harvest because a devastating disease infected their crops more often than desired. This situation means that the farmer not only loses money but that the prices of the produce may increase. On top of that, they just waited tons of energy and resources. For example, in the US, the disease Citrus Greening (bacterial pathogen) leads to a drastic reduction of citrus production and the increased price of all citruses. In worse cases, inadequate management of diseases has led to hunger. Throughout the African continent, epidemics of Cassava mosaic disease (viral pathogen) that started in the 1980s resulted in significant economic loss and devastating famine. Thus, diagnosing diseases is critical for efficient management and preventing losses. 

Diagnosing a plant disease: step by step

Sometimes we want to go straight to control the disease. Still, the most crucial part of successfully managing the disease is through correct diagnosis. An accurate diagnosis means identifying the cause and name of the disease. Imagine applying a fungicide when, in fact, the plant has a bacterial disease! The same concept applies if we get sick. If we drink antibiotics when we have a viral disease, it won't help at all. On top of that, you lose money, and the disease may get worse. Therefore, to correctly identify a plant disease, there are critical steps that need to be followed. 

1. Notice what is normal. Recognizing healthy plant appearance and proper plant identification (common name, scientific name, cultivar susceptibility) is the first note you should record. If possible, look for pictures of the variety you grow and compare it to your plant. Certain cultivars have resistance to specific diseases, so knowing what pathogen the cultivar has resistance will help you avoid diseases.

2. Observe symptoms and signs. First, let's define some essential terms. Symptoms are changes in plant growth or appearance in response to biotic or abiotic factors. This could be an alteration of normal appearance, necrosis or death of plant parts, overdevelopment of tissues or organs, and underdevelopment of tissues or organs.

American Phytopathological Society /Jerzy Opioła
Andrej Kunca, National Forest Centre - Slovakia/ Tim Broschat, USDA APHIS PPQ
Howard F. Schwartz, Colorado State University/ Rebecca A. Melanson, Mississippi State University Extension Yuan-Min Shen, Taichung District Agricultural Research and Extension Station/ Margaret McGrath, Cornell University
Howard F. Schwartz, Colorado State University/Don Ferrin, Louisiana State University Agricultural Center
Jennifer Olson, Oklahoma State University/ James W. Amrine Jr., WVU
Robert L. James, USDA Forest Service/ Timothy Schubert, Florida Department of Agriculture and Consumer Services
Clemson University - USDA Cooperative Extension Slide Series/ Howard F. Schwartz, Colorado State University
Donald Groth, Louisiana State University AgCenter/ Howard F. Schwartz, Colorado State University
European and Mediterranean Plant Protection Organization/ J. Dunez
Jonathan D. Eisenback, Virginia Polytechnic Institute and State University/ Manfred Mielke, USDA Forest Service/ Joseph OBrien, USDA Forest Service
Joseph OBrien, USDA Forest Service/ Andrej Kunca, National Forest Centre - Slovakia
Whitney Cranshaw, Colorado State University/ Parthasarathy Seethapathy, Tamil Nadu Agricultural University

Signs are observable evidence of damaging factor(s). This could be bacterial exudates, mycelium, spores, fruiting bodies, rhizomorphs, insects, and insect frass.

Scott Bauer, USDA Agricultural Research Service/ Mary Burrows, Montana State University
Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo/ Edward Sikora, Auburn University
T.W. Mew, International Rice Research Institute/ Rebecca A. Melanson, Mississippi State University Extension

In this step, you should try to identify what characteristic symptoms you observe and look for signs of pathogens. It is also essential to take note of what parts of the plants are affected. Is it the whole plant? Is it only the stem? Some diseases, such as nematodes, only infect the roots, so if you suspect the pathogen is infecting your plants, you should take out the whole plant and look for symptoms. 

3. Check for patterns. Check for the distribution of signs and symptoms. If a disease is biotic, it usually progresses to different parts of the plants. If the plant is affected by an abiotic factor, symptoms typically don't progress unless its water or nutrient deficiency. It's essential to also check for host specificity symptoms. Host specificity refers to some pathogens' capability to cause disease only on particular plant species or only on some members of a specific plant species. In other words, knowing what crops are and are not infected gets you a step closer to identifying your problem. 

4. Ask questions! You need to question what activities are done around your plants. Have you over irrigated your plants? What pesticides have been applied? How much fertilizer did you apply to your plants? Always keep record management practices such as watering, fertilizing, pruning, temperature, etc. Also consider investigating the weather patterns, analyzing plant nutrients, and looking for drainage and compaction. For example, wilting is a prevalent symptom caused by several pathogens but can also be due to a lack of water! These details will also help you differentiate between biotic and abiotic diseases. 

Ask fellow gardeners and farmers for advice because most of them might have gone through similar problems. Most importantly, each country has its own plant disease diagnostic center and excellent online resources. Depending on where you live, you can send plant samples to a plant disease clinic and receive a diagnosis for free or for an affordable price. If you use online resources, be sure to use trustworthy sources such as university extension service websites.

To learn more about disease diagnostics, click here. 

Suppose you decide that diagnosing diseases is not for you, no problem! However, all of the previous information is necessary when you fill out a form to send your plant sample to a plant disease clinic. The more information you provide, the higher the chances for a correct diagnostic. 

Identifying diseases is not an easy task, but it is necessary to decide what management tactic to apply. Plant pathologists are here to help! Even we have a hard time knowing what disease is making your plants sick. Make sure to follow these tips the next time you have a sick plant. Management is a whole other conversation but remember, if you ever apply pesticides, always read the label and wear the proper protective equipment. 

Stay tuned to learn more great tips about plant management, soil health, and much more! 

Some fantastic resources you should check out: 



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