Plants can suffer from diseases just like people. Trying to identify what is wrong with your plant can be challenging, but you can learn to be your own “plant doctor.” Start by learning the most common diseases in your area. Plant diseases can be fungal, bacterial, or viral. There are also nematodes that can cause disease. Recognizing the differences can help you properly treat your plants.
It’s important to know what your plant is supposed to look like when it is healthy. A plant’s appearance can give us clues as to what type of pathogen might be the culprit. A plant disease symptom is defined as a visible effect on the plant. This could be wilting or yellowing leaves, dying branches, spots on fruit or stunting of the plant. A sign of plant disease is actually seeing some physical evidence of the disease such as bacterial ooze from a canker or the white powder of powdery mildew. Signs and symptoms can look very similar for some diseases.
Pathogens are moved around in different ways. Wind and splashing water or rain moves fungi, bacteria, and water molds. Lawnmowers, insects, birds, even cats and dogs can be responsible. Working in the garden when plants are wet can move diseases around. Propagating plants that are infected with a bacteria or virus can spread the disease. Pathogens can be moved from state to state or even to another continent when infested seeds or infected plants are transported.
Fungi and fungal-like organisms cause more plant diseases than any other group. Fungi do not contain chlorophyll and cannot produce their own food. They get their nutrients from other living things. Fungi are made up of thread-like structures called hyphae. A collection of hyphae is referred to as mycelium. Most fungi feed on dead and decaying matter aiding in decomposition and returning nutrients to the soil. Some common fungal diseases that are destructive include blights, botrytis, clubroot, damping off, leaf spots, mildews, and wilts.
Start with disease-free seeds and plant material (bulbs, tubers, transplants, sets) to keep from introducing fungal diseases into the garden. Varieties that are resistant or tolerant to fungal diseases are available for many of the major vegetable crops and plant breeders are continually developing more. Examine plants thoroughly for signs of leaf or stem disease. Remove leaves and fruit that are suspect. Pull out plants that are badly infected. Fungicides are typically more effective when applied before the onset of disease symptoms rather than after. At the end of the growing season, clean up all vegetable crop residues. Diseases can over winter in debris and may infect new plants the following season. Do not buy diseased plants, even at bargain prices. You do not want to bring it home and introduce it to your garden.
Bacteria are single-celled organisms that you need a microscope to see. They reproduce very rapidly. Plant bacterial diseases are frequently named for the first plant they were discovered in. Bacteria need a way to get inside the plant, such as a wound or natural opening, like a stoma. Once inside, they kill the host plant cells by various means in order to grow. They have a variety of symptoms such as galls, wilts, leaf spots, rots, and cankers. Sometimes bacterial ooze is present which is very diagnostic. Another thing to look for is a chlorotic (yellow) halo around a lesion on leaves. Some common bacterial diseases include fire blight, bacterial speck and spot of tomato, bacterial leaf scorch, crown gall and bacterial soft rot.
Bacterial diseases of plants are difficult to control so put your efforts into preventing them. Use resistant varieties when available. Sanitation in the garden is a key component to prevent their spread. Disinfect pruning tools and shears between plants. Bacteria can survive unfavorable conditions by going dormant in plant tissue, soil, water, or an insect. Controlling those insect vectors can help.
The first virus ever described was tobacco mosaic virus in 1898. You can only see a virus with an electron microscope. They require a living host to multiply and a way to enter a plant cell, usually through some kind of wound. Insects, especially those in the Homoptera order, are known for transmitting viruses by their piercing, sucking mouthparts. This group includes aphids, whiteflies, leafhoppers, and mealybugs. Some plant viruses can also be spread via seed or pollen. Plant virus names usually refer to the first plant in which they were discovered such as rose, cucumber and tobacco mosaic viruses, impatiens necrotic spot virus and tomato ringspot virus. Viruses usually have a very wide range of host plants.
Viruses are hard to identify just by their symptoms since they can vary plant to plant. Things to look for are a mosaic or mottling pattern on leaves. The leaves can also be deformed, puckered, or cupped under. Fruit can be smaller than normal, deformed, mottled, discolored, or have rings on the skin. Flowers can have streaks of white or be all green (when they shouldn’t be). The only way to be positive a plant has a virus is to get it analyzed at a lab. However, if you have a plant exhibiting these symptoms, you may just need to assume it is a virus.
Unfortunately, once plants are infected with a virus you can’t eradicate it. The plant is systemically infected, from roots to shoots. The best thing to do is to cut your losses and remove the plant from the garden. While viruses rarely kill a plant outright, it will act as a reservoir and could be the source of infection to other plants in your garden and your neighbors.
Scout for symptomatic plants, including weeds, and rogue them out. You can use insecticides to control insect vectors but its best to eliminate the source. Sanitation is also important as viruses can be spread by tools. Wash your hands after handling infected plants. If you are propagating or dividing plants only use healthy plants. Purchase clean seed and use resistant varieties when they are available.
It is important to regularly scout your plants for any changes in appearance and vigor. The earlier a problem is accurately identified, the more likely it can be successfully treated or controlled. This will also help prevent its spread to other plants in your garden.
Resources: Michigan State, Ohio State, University of Minnesota,
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Join CCE for Garden Talk on Aug. 5 at noon via Zoom. The topic will be “Introduction to Beneficial Insects.” Most of the insects that live in your garden or landscape do little or no harm to you or your plants. Many of these good guys provide free pest control for you. Who are these allies? Join us to learn about the beneficial insects that might be in your garden. This program is free, but registration is required to get your Zoom link. Visit the events page at the CCE Genesee website: genesee.cce.cornell.edu/events.