Plant diseases are usually caused by four major types of living organisms: fungi, bacteria, viruses and pathogenic nematodes. Diseases are an imbalance of microorganisms and sometimes hard to identify since the results of infection are more visible than the organisms themselves.
All organic products help control disease. When soil is healthy, there is a never ending microscopic war being waged between the good and bad microorganisms, and the good guys usually win. Disease problems are simply situations where the microorganisms have gotten out of balance. If allowed to do so, the good guys will control the bad guys. When pathogens are brought into their proper proportions they are no longer troublesome. In most cases they become beneficial at that point. Proper drainage is a key ingredient for the prevention of diseases.
As with insects, spraying for diseases is only treating symptoms, not the major problem. The primary cause of the real problem is usually related to the soil and the root system. Therefore it is critical to improve drainage, increase air circulation, add organic material, and stimulate and protect the living organisms in the soil. Here are some of the most common diseases.
Serious lawn diseases are fairly rare. Only during periods of high humidity does the potential for a disease problem exist. The high humidity needed for the development of most diseases can be produced when the lawn is watered too frequently.
Many times a change in the weather or a reduction in watering frequency can "cure" a lawn disease. In other words, drying a lawn out can stop a disease. The exception is St. Augustine Decline (SAD). Choosing a SAD resistant variety of St. Augustine is desirable.
As with all pest problems it is necessary to identify the disease before a chemical control can be selected. If it becomes necessary to use a fungicide the local Cooperative Extension Office can help identify the disease and make control recommendations if necessary.
Cool weather fungal disease of St. Augustine. Brown leaves pull loose easily from the runners. Small spots in lawn grow into large circles or free forms that look bad and weaken the turf but rarely kill the grass. Potassium bicarbonate is a curative spray; soil health, drainage, and low nitrogen input are the best preventatives. Common disease of At. Augustine, although it can be a problem on bermudagrass. Leaves of the grass plants are killed in a circular area. The dead giveaway for this disease is that the affected leaves can be easily pulled off the runners. Usually not fatal. Mostly present in spring and fall during periods of frequent rain and high humidity. It is not uncommon for areas that have had brown patch during the previous fall to be the victims of winter kill.
These appear as circles, or arcs, of dark green grass in any lawn. The mushrooms, or fruiting bodies, of the soil borne fungi that cause fairy ring may or may not appear. The dark ring is caused by the breakdown of organic material deep into the soil by the fungi. There is no chemical control or cure for this disease. A good fertility program usually will “mask” the rings and they shouldn’t be as evident.
Gray Leaf Spot
This is another disease that can infect St. Augustine. It’s much like helminthosporium in that the infected spot on the leaf is surrounded by a dark margin. Here again, a few spots won’t necessarily do any harm, but a lot will.
This is a fairly common leaf spot disease, usually found on bermudagrass, tall fescue and ryegrass. The small spots on the leaf blade are brown in the middle with a dark ring. One or two of these infections on a plant’s leaf will not seriously hurt it, but as more develop, the leaf’s ability to produce food is reduced and the plant becomes weaker. A lawn with a serious infection of “helminth” will slowly thin out. This disease also has been called “fading out” or “thinning out.”
This is a disease primarily of St. Augustine. It affects the stolon, or runner. A small lesion develops that looks much like the leaf spot diseases. The spot grows larger and larger until it completely encircles the runner. This results in the death of all the new plants between the lesion and the end of the runner. It can be a very serious disease that certainly can be promoted by over watering.
Pythium can be a serious disease of both perennial ryegrass and bermudagrass. It can develop under warm and very wet conditions, especially in low areas of the lawn. The grass takes on a wilted, greasy look at first. Later some spots may have a cottony appearance and, for this reason, the disease may be called cottony blight. The spots may be small circles or they may be streaked. Over watering may be one of the reasons this disease develops.
This is a disease that can be found on most turfgrasses, although zoysia may be the most severely affected. The rust develops orange or brown pustules on the leaves. If you get enough of these on a leaf, the plant’s ability to manufacture food is reduced and the turf thins out.
Turf fungal disease that is mostly cosmetic. Slime mold spore masses coat the grass and look like cigarette ash on the surface of the blades. The spores can be easily wiped off. Remove the mold spores from the grass by rinsing with water during dry weather, or mowing and raking at any time. Baking soda spray, potassium bicarbonate will kill it. So will cornmeal. These molds can cover the above ground parts of the plant with a dusty dark gray mass. While slime molds are not too common, it is not uncommon to find them growing on bermudagrass seed heads. There is no chemical control and they usually disappear when the weather becomes drier. They tend to develop during wetter weather.
Spring Dead Spot
These are simply circular spots of bermudagrass that do not green up in the spring. The grass in these spots died sometime during the winter. The organisms which cause this problem have never been identified. The turfgrass will be slow to spread back into these areas. It may take all of the next summer for the dead spots to fill in completely. Usually, seeds will not germinate in these areas for a year or so. Lawns that get spring dead spots usually have been on very high fertility programs, especially those programs that are high in nitrogen.
Soil Borne Diseases
Soil borne diseases are controlled by increasing the biological activity in the soil. When microbes are plentiful, the beneficial organisms control the pathogens. Pathogens themselves are not pests when in their proper populations. The best products for stimulating life and the balancing of biological activity in the soil and thus controlling soil borne diseases include cornmeal, garlic, molasses, compost and mycorrhizal fungi products.
A broader scale soil borne diseases can be controlled by improving and maintaining drainage, keeping bare soil mulched, and planting native and adapted plants. Common sense is the basic answer.
St. Augustine Decline
Virus in common St. Augustine grass that causes a yellow mottling. The grass slowly dies away. The answer is to replace turf with a healthier grass and to plant a mix of native grass, wildflowers and herbs. This is a serious viral disease of St. Augustine. There is no cure. The lawn will more than likely go through a long, steady decline as more and more plants are infected. Leaves take on a spotty or mottled appearance. A good management program will prolong the life of the affected lawn, but the end is inevitable. The only realistic way to deal with St. Augustine Decline is to begin to introduce SAD-resistant varieties into the lawn. Those that have demonstrated resistance are Floratam, Seville and Raleigh.
Take-All Patch (Bermuda Decline)
A disease that can attack several species of grass. It is caused by the fungus Gaeumannomyces graminis var. graminis, and is mostly found in St. Augustinegrass but can also cause problems in Bermudagrass. It is most active during the fall, winter and spring especially during moist weather. The first symptom is often yellow leaves and dark roots. Area of discolored and dying leaves will be circular to irregular in shape and up to 20 feet in diameter and thinning occurs. Unlike brown patch, the leaves of take-all infected plants do not easily separate from the plant when pulled. Stolons will often have discolored areas with brown to black roots. Regrowth of the grass into the affected area is often slow and unsuccessful because the new growth becomes infected. Controlling take-all patch is said to be difficult but isn’t with organic techniques. Good surface and subsurface drainage is important. Cut back on watering and fertilizing. Use only organic fertilizers. If soil compaction exists, aeration will help to alleviate this condition and allow the grass to establish a deeper, more vigorous root system. Prevent Take All Patch by maintaining healthy soil. Control the active disease by aeration, cornmeal and compost and the Basic Organic Program.
View photos of diseases.