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Garden Guru: The three sides of plant diseases

By John Porter
CHARLESTON, W.Va. -- As the heat and humidity of summer churns along and the weather offers only brief respite from rain, gardeners should be on the lookout for diseases and other garden issues.Many of the fungal and bacterial diseases that invade the vegetable garden and landscape thrive on moisture, whether it is in the form of rain, humidity, or even applied watering. Coupled with the heat, plants can be overcome with blights, wilts, rots and all manner of unsavory maladies.Do not fear; there are still ways to reduce the likelihood and severity of diseases in the garden. Vigilant gardeners who scout regularly for diseases can catch problems early and control, if not eliminate, them.But gardeners should also remember the words of wise old Ben Franklin-- "An ounce of prevention is worth a pound of cure." Taking steps to reduce diseases in the garden, especially when conditions are so ripe for disaster, can be key in keeping problems at bay.First, you should know that the bacterial and fungal organisms that cause most plant diseases have to have three conditions just right to infect the plant. These conditions are often referred to as the "disease triangle." If one is missing, then the chance of disease is lessened. So gardeners have three chances to reduce the likelihood of disease in their gardens.The first side of the triangle is the pathogen, or disease-causing organism, itself. Some diseases will find their way into your garden -- blowing in, etc. -- but gardeners can unknowingly introduce pathogens and start the downward spiral to disease and doom.Buying seeds and plants from reputable sources are a start. Seeds sold by garden companies should be inspected and certified, and there should be a certification stamp somewhere on the seed display or packet. When shopping for new plants or transplants, give them a good inspection for spots, wilting, or yellowing leaves. Also, make sure that any mulch, soil, seed starter, or anything else you use is disease free.Reducing plant contact with possible sources of pathogens is also part of this side of the disease triangle. Keeping plants from touching the bare ground, where pathogens can be living on soil particles or garden debris, is a must. Staking, caging, or trellising vining or large floppy plants is a must.
Fresh mulch also goes a long way in keeping water from splashing soil on the bottom leaves of plants, a common place where infections start. In the vegetable garden, I prefer to use shredded newspaper or straw; in the landscape, mulched leaves or wood chips do fine.Removing debris from the garden can reduce the amount of pathogens that over-winter to infect plants the following year. Gardeners should prune out diseased plant parts or completely remove diseased plants from the garden; the longer they stay, the more diseases they can pass on to their neighbors. Rotating crops to new spots in the garden each year can also reduce the build-up of pathogens in the soil.The second side of the triangle is an environment conductive to allowing the disease to grow. We can't control rainfall, but we can make conditions less favorable for diseases to take hold. Adequate spacing between plants improves air flow, reduces the localized humidity and moisture around plants, and keeps airborne pathogens from settling in one space for too long.Healthy soil reduces the likelihood of diseases. Improving the texture of heavy soils (with which most of West Virginia has been blessed) can keep soil from staying too wet, which causes root damage and allows for root rots and wilts to invade plants.Increasing the number of good fungi and bacteria in the soil makes it more difficult for the bad guys to take hold, much like the good bacteria (say from yogurt) can help improve health in people. Feeding the good guys with lots of organic matter and using winter cover crops in the vegetable garden are keys in keeping the good guys healthy. Crop rotation will also keep them happy.The third side of the triangle is the plant itself. The pathogen has to be able to infect it to cause a disease. There are several ways to make sure that the plant and pathogen are not compatible.Rotating crops will help keep plants and their potential pathogens separated (I see a pattern emerging). Making sure the plant is healthy, with proper fertility, light, etc. goes a long way in helping the plant resist or recover from disease.
But perhaps the best way to make sure that the plant and pathogen are not compatible is to select disease-resistant varieties. While it may be too late to do so this year, doing so next year will help reduce disease instances if there were problems this year.John Porter is the WVU Extension Service agent for agriculture and natural resources in Kanawha County. He may be reached at or at 304-720-9573. Twitter: @WVUgardenguru.
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