NEIGHBORHOOD GARDEN FOR MAY 25, 2013

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PLANT DISEASE TRIANGLE

BY GEORGE EDGAR

          All of us have had problems with disease in our garden or lawn at some time. It may be black spot on roses, blight on tomatoes, powdery mildew on a lilac hedge, fungus on the lawn, or tip blight on Austrian Pines. I have had all of the above at one time or another. I used to go to the Extension Office or a garden center and find out what fungicide, if any, I could use to treat the problem. This article is about another way to look at these disease problems and what you can do before you apply that chemical.

          As the title of this article suggests, there is a combination of three crucial factors that must be present for a plant to get a disease: (1) a susceptible host plant; (2) the disease pathogen; and (3) a favorable environment. All three must be present.

ENVIRONMENT

                   HOST PLANT                                    PATHOGEN

Now let us look at these three factors:

          HOST PLANT: To get brown patch or summer patch on my lawn I have to have the type and variety of grass that is susceptible to the disease. Lawn and turf specialists have been working and doing research to try and develop grass cultivars that are disease resistant. Each year new cultivars come on the market and are made available to the homeowner. So, one way to prevent having this disease is to plant cultivars that are resistant to disease. If you are buying a crab apple tree, make sure you get one that is resistant to cedar apple rust. Remember, they are disease resistant not disease proof.

          PATHOGEN: A pathogen is an organism that causes a disease such as Septoria Leaf Spot (fungus) and Fusarium Wilt that attacks tomatoes. We can greatly reduce the pathogens in our gardens by removing debris, weeds, and dead material in the fall where many pathogens survive from year to year. Make sure you clean up dead material every fall so your plants do better.  Rotating where we plant our vegetables and flowers will also help reduce disease problems as some pathogens survive year to year in the soil and if we plant the same crop in the same place each year the disease will continue. Another preventative measure is to control insects that carry pathogens to plants. For example the cucumber beetle carries a blight pathogen which is able to infect a cucumber plant and then the blight can develop to the point where the plant dies.

          Some pathogens are carried by the wind, some by insects, some from your tools that have not been sanitized, and some live over on old dead plant material, and some survive from year to year in the soil. Learn how and when a disease inoculates a plant, how it is transmitted, and how and where it survives over the winter. If you do this, you are on the way to finding a good treatment solution.

          ENVIRONMENT: This is a critically important factor. Probably one of the easiest ways to prevent diseases is to change the environment. Even the most susceptible plants, exposed to huge amounts of a pathogen will not develop a disease unless the environmental conditions are favorable. Pathogens seem to attack plants under stress, so preventing stress from over or under watering, from over or under fertilizing, and by planting the right plant in the right place will go a long way in preventing disease. Too many homeowners mow their lawns too short and this causes stress in the grass so disease, insects, and weeds move in.

          The disease that causes tomato blight is in old infected plant material and in the soil. You can prevent this disease by cleaning up old plant material in the fall, by rotating where plants are grown in the vegetable garden, by staking or caging the tomatoes so they do not touch the ground, and by pruning off the lower 8 to 10 inches of leaves when the plant gets about 3 feet tall. Also you can mulch so the rain does not splash the pathogen onto the lower leaves.

          Very crucial in the prevention of disease is how and when you water. Most disease pathogens need a drop of water on the leaf, in the cool of the evening, in order to inoculate a plant. Therefore, outside never water after 2:00 p.m. so the plant leaves are dry before evening. This is true of tomatoes, cucumbers, zinnias, roses, and even lawns. I have seen many people out in the evening watering their lawn, or roses, or tomatoes, and then wondering why they have fungus, or tomato blight, or black spot, or another disease. Also, you can change the environment by making sure that plants are not too close together and get good air flow.

          Applying a fungicide to a plant, including a lawn, usually only hides the symptoms and may not cure the disease. It is like taking an aspirin for pain when you have a sore muscle. It helps deal with the pain but does nothing to cure the problem. If you are having problems with a plant, instead of applying a chemical, get an accurate diagnosis and then specifics about the disease. Also, look around to see what the environment is in the lawn, in the vegetable garden or in the rose bed. Are you over watering or not watering at the right time in the right way? Are you applying too much fertilizer so the plant is stressed and thus more susceptible to disease? In the vegetable or flower garden, are your plants so close together they don’t have good air flow? Are you planting the right plant in the right place? Did you find out what kind of growing conditions that nice looking plant desires before you planted it? Maybe it needs to be moved to a more environmentally favorable place. Also see if there is too much or not enough sun for the plant. All of these factors are involved in preventing plant disease problems.

          Remember, if you want to cure or prevent a disease, all you have to do is remove or alter one or more of the three critical factors in the disease triangle. Make sure you are preventing problems rather than creating them and good gardening. 

Copyright 2013