Diseases of Ornamentals & Turf
The term "disease" refers to any malady that af-fects a living organism. Thus, many things in addition to pathogenic organisms can cause dis-ease: physical and chemical injury, physiological conditions and nutritional disorders. In this chapter, however, we deal mainly with those disorders that typically fit most people’s idea of disease: pathogens. Plant-pathogenic organisms include fungi, bacteria, nematodes, viruses, viroids and mycoplasmas.
Non-pathogenic diseases include phytotoxicity, nutritional deficiencies, soil problems, graft incompatibilities, various environmental stresses, lightning, physical injury from equipment and other conditions that can cause a plant to decline or die. We will discuss the symptoms of these problems enough to help distinguish them from pathogenic diseases, but this chapter focuses on the latter.
To successfully diagnose plant diseases, you must be a detective and use deductive logic to come up with a list of potential causes and eliminate others. After you narrow down your list of possibilities, you then can perform lab analysis if the situation calls for it. Grounds managers often treat for probable causes using treatment response—or lack of response—as another diagnostic tool. This is not the most efficient approach but can be helpful in some cases. With luck, you can pinpoint the cause using plant symptoms, site history and reference material. If a problem has you stumped, consult an extension pathologist. These experts have vast experience in diagnosis and are helpful in determining the cause of symptoms.
Many hundreds of diseases occur on landscape ornamentals and turf and identifying the exact pathogen is sometimes a baffling process. The first thing you must do is determine the general cause of the symptoms: pathogens, environmental factors, phytotoxicity, nematodes or insect pests. If diagnostic symptoms are present, your task is relatively easy. If the symptoms are more general—leaf drop, wilting or gradual limb dieback, for example—you must investigate further. Information to gather and questions to ask yourself include:
• Define precisely the symptoms causing you concern. This is simple in some cases but not as easy when the plant “just doesn’t look right.” Look closely and determine exactly what isn’t normal.
• Determine the age and species of the affected plant. Host identity and age are important parts of diagnosis.
• Observe the location of the plant relative to nearby features—parks, ponds, streets. Is the plant growing in a low spot or on a slope? Is traffic heavy near the plant? The surrounding landscape may provide clues to problems such as drainage, drought or saline runoff.
• Is exposure to chemicals a possibility? This can be one of the most frustrating problems to diagnose. So many everyday substances can harm plants—cleaners, petroleum products, air pollution—it is difficult at times to determine what the problem might be. Symptoms may be due to exposures you never knew occurred. Pesticide drift from careless applicators on adjacent property (or your own) is a common occurrence. Workers improperly draining tanks of cleaning fluid or other chemicals happens as well. Keep an open mind as to what may have happened. Common phytotoxicity problems include cupping, curling or other distortions of leaves and young shoots, burning or scorch, yellowing (especially interveinal chlorosis) and, in severe cases, branch dieback or death.
• Under what weather conditions did the symptoms first appear? What about during the past winter? Was it exceptionally cold? Did temperatures fluctuate widely? Consider conditions (rain, heat or humidity) that favor pathogens, weather that stressed the plant (making it vulnerable) or simply conditions that caused the symptoms to become visible. For example, diseases that affect vascular systems often do not become apparent until weather turns hot. Low-temperature damage may not be apparent until spring growth starts, when you may notice small shoots that have died. Also look for frost cracks.
• Do nearby plants of the same (or different) species also show symptoms? This may provide clues about whether a pathogen is involved.
• Has construction or a change of grade occurred on the site within the last few years? If so, this may have resulted in compaction or severe root injury. This commonly results in dieback, loss of vigor, early leaf drop and eventual death of the plant.
• How much growth has the plant produced over the last few years? This tells you whether the decline was gradual or sudden, which may help isolate the cause.
• What cultural care has the plant received in the last few years? Improper care may result in nutritional or water-management problems.
• Have you applied fertilizer recently? Careless fertilizing often causes burn. Fertilizing late in the fall can delay hardening, resulting in winter damage.
• Is the plant a recent transplant? Transplanting above or below grade, improper irrigation or inadequate planting holes can cause serious problems that tend to show up quickly.
• What are the soil conditions—pH, soil texture, pan layers, compaction, saturation—on the site? Poor soil conditions often cause visible symptoms.
• Are the roots healthy? Dig up some small feeder roots and see if they are white and actively growing or brown and rotting. The latter indicates a root pathogen or soil conditions that are killing the roots.
• What, if any, symptoms show on the trunk and large limbs? The presence of lesions, cracking or sloughing bark, fungal fruiting bodies or even mechanical wounds often indicate the cause of the disease.
• Slice open affected shoots with a knife and inspect wood for discoloration or rot. Internal symptoms such as these indicate vascular disease or wood rot.
• Inspect leaves closely. Foliar symptoms often are diagnostic.
Gathering this type of information may lead you to the direct cause or at least help you narrow it down to some type of pathogen, if not the exact one.
A final point to remember is that pathogens that are present may be secondary diseases. That is, they may have moved into the weakened or wounded plant after an initial infection or other malady. For example, saturated soil often kills roots. The dying tissue serves as an entry site for soil fungi that normally would not affect healthy roots. In this case, improving drainage would be the best treatment. Plants weakened from any cause are more susceptible to infection, so you must always ensure good plant vigor.
IDENTIFYING THE PATHOGEN
The following are the major groups of plant pathogens and some of the common symptoms they produce.
--Fungi. Fungi are the most common plant pathogens. Fortunately, many are relatively easy to treat as well. Fungi are single- or multi-celled organisms that feed on a variety of materials. In fact, most fungi are harmless— feeding on dead and decaying organic matter—and many are beneficial, forming symbiotic relationships with plant roots that increase plant growth and vigor. A minority of fungi cause diseases.
It is difficult to generalize about fungal symptoms, which take many forms. However, when a plant has a fungal infection, you can learn a great deal about its identity by observing where and what type of symptoms the plant exhibits.
• Leaves and young shoots may show lesions, scorching or discoloration on the surface and these often are diagnostic.
• Branches and trunks may display fungal fruiting bodies or cankers. Soil and roots at the base of the trunk also may display fruiting bodies, such as mushrooms.
• Roots may be dying back. Internally, some fungal infections discolor conducting tissue (wood or phloem). Mycelium—the fungal “body” typically composed of white tissue in the form of sheet, strands or a cottony mass—may be visible under bark or on roots.
--Viruses. In established landscapes, viruses most often spread as a result of two things: Either they were vectored by an insect host that fed on the plant or they spread by contaminated pruning tools. Occasionally, however, plant material brought in from nurseries may already be contaminated. Unfortunately, it does not matter how a virus was spread—no cure exists. Occasionally you successfully can prune out affected branches and eliminate the virus completely, but typically you must remove and destroy the entire plant.
Viruses cause peculiar discolorations and patterning— mosaics, mottling or yellow rings—in many plants. Odd distortions of newer growth, generalized yellowing and stunting also occur frequently.
--Mycoplasmas and viroids. These have recently come to light as plant pathogens, though it is unclear how significant they are as such. Mycoplasmas are intermediate in size between bacteria and viruses and may be the cause of diseases such as aster yellows and some witch’s broom diseases, formerly thought to be viral.
Viroids are smaller than viruses, and it probably is a stretch to call them living organisms. As with mycoplasmas, it is unclear how significant viroids are as plant pathogens. However, they are infectious and apparently cause some plant disorders that scientists previously attributed to viruses.
--Bacteria. These are single-celled organisms. Some of them are significant pathogens and most are difficult to treat. Bacteria must have some means of entry into the plant—they cannot enter through an unbroken cuticle. Therefore, wounds from pruning or mechanical injury are a frequent means of infection. Flowers are another common infection site and stomata or other openings also apparently are routes to bacterial invasion.
Bacterial infections cause three general types of symptoms:
• Bacterial wilt diseases result from infection of the plant’s water-conducting tissue.
• Necrotic blights, rots and spots result from death of parenchyma tissue. Fireblight is a notable example of this, as are some rots of tubers and rhizomes.
• Abnormal growths, such as crown gall, are a third type.
--Nematodes. These organisms are small—usually microscopic—worms that feed on plant roots. Not all nematodes are parasitic, but those that are can cause serious problems for ornamentals. Above-ground symptoms are usually subtle, such as lack of vigor. Root symptoms are more diagnostic and include knots or other swellings of root tissue.
PREVENTION AND TREATMENT
Plant pathologists teach that three conditions must exist for disease to occur: The pathogen must be present, the host must be present and conditions must be appropriate for the pathogen to infect and develop. This is the so-called disease triangle (see figure). Thinking of disease in these terms is helpful in several ways. Of course, if disease has already arisen, it’s a given that those three conditions existed. However, identifying the conditions that contributed to the disease illustrates changes you can make to prevent future outbreaks or even cure an existing one. For example, realizing that a proper host is necessary forces you to think of alternate plant choices that may not be susceptible to prevalent diseases. Another example: An outbreak of a pathogen that thrives in saturated soil should prompt you to improve drainage.
--Preventive measures vary according to the pathogen. However, all plants are less susceptible to diseases when they are healthy and vigorous. Thus, proper cultural practices are the No. 1 preventive measure. Ensure that plants are sited properly, correct any soil and drainage problems and pay attention to ongoing irrigation and fertility management.
Air circulation and light penetration are important for reducing diseases that spread via wet plant surfaces, so avoid over-planting, which reduces air circulation. Further, you may need to alter your irrigation scheduling or change from overhead to ground-level irrigation to eliminate wetted foliage.
An important practice is sanitation. Pruning crews commonly move from plant to plant with little regard for the cleanliness of their tools. Thus, many pathogens spread in this manner. If you are pruning plants that have had any history of disease, disinfect your tools with a chlorine solution (see “Sterilizing tools”) between cuts and before moving to another plant.
Another aspect of sanitation involves removing dead or infected plant material from the landscape. Dead plant material, especially that which was infected while alive, may harbor pathogens that can then spread back to living plants. Thus, you should remove such material from the site entirely or burn it. Even leaves that drop from infected trees in the fall can be a problem. Site sanitation can be as important as tool sanitation. Nursery stock occasionally is infected with pathogens— it pays to thoroughly inspect plants before you buy them. This not only ensures a healthy transplant, but safeguards existing ornamentals to which pathogens could spread.
Insects commonly vector pathogens from plant to plant while feeding. In theory, controlling these insects also should control spread of the pathogen. In practice, this often is impossible, and you should avoid indiscriminately spraying the entire landscape with insecticides to prevent diseases unless you are following accepted recommendations for a specific disease.
Bulbs or other perennial plant parts you store for later planting are subject to certain rots. You can prevent these with good air circulation during storage. Fungicide dusts are useful as well.
--Treatment of disease is practical in many cases. However, you should accompany any treatment with a change in environmental or cultural factors that may have contributed to the disease in the first place.
Fungicides are available in several formulations, and you apply them according to the type of disease you’re treating. Treat leaf diseases with a foliar spray. Vascular diseases are more difficult to treat, but trunk-injection fungicides are available for some pathogens. Root and soil-borne diseases also are difficult to treat, but you can apply certain fungicides as a soil-drench treatment.
Modern, synthetic fungicidal compounds are widely used. However, a few old remedies remain effective for treating fungi. Products containing lime, copper or sulfur are effective in preventing many types of fungal (and bacterial) infection. They do not have significant curative properties, however.
Fumigation kills all living organisms in the treated soil and is an option when the site has a history of soil-borne fungi or nematodes and you want to protect new transplants.
Bacterial infections are not easy to eliminate. You can prune out infected branches, which may help, and antibiotics are available in injectable or sprayable formulations.
Viruses require you to remove and destroy affected plants unless you can successfully prune out infected limbs. More often, however, you can count on losing the plant.
Your treatment options for nematodes include just a few products. However, plants can tolerate moderate levels if they are vigorous. Fumigation before planting is wise on heavily infested sites.
COMMON DISEASES OF ORNAMENTALS
This section lists some common fungal, viral and bacterial diseases of ornamentals, grouped by the part of the plant most affected. Many other plant diseases occur, and a good reference is helpful both for diagnosis and treatment recommendations. Again, use the services of a diagnostic lab or an extension specialist if you need help with diagnosis. Suppliers can help you select products that are registered to control specific diseases. For a discussion of disease management in bedding plants, see Chapter 6.
Root rots are common fungal diseases of woody plants. It can be difficult to diagnose the exact pathogen because the initial symptom—decline in vigor—is common to several pathogens and can result from numerous other factors as well. The decline can last for several years and affected plants are difficult to save. Improving drainage and maximizing plant vigor are the best ways to prevent infection. Avoid damaging root and trunks.
When you remove a plant that died from root rot, eliminate as much of the old root system as possible. Then, fumigate or use a soil-drench fungicide at the site before replanting.
--Armillaria—shoestring root rot. This fungus is one of the most common and widespread causes of root rot in ornamentals. Oaks, maples, firs, pines, rhododendrons and dogwoods are common hosts. After infection, parts or all of the affected plant decline in vigor. Leaves may be dwarfed, pale and may drop prematurely. The best way to distinguish this pathogen is by the thin sheets of white mycelium growing under the bark or on the main roots. Speckled brown mushrooms often grow from the soil around the base of infected trees. After a tree dies, dark-colored rhizomorphs—the root-like “shoestrings”—develop under the bark, over infected roots and outward from the tree. A distinct mushroom odor normally is present.
Affected plants usually die, but healthy plants can be fairly resistant. Therefore, good cultural practices are the best way to prevent Armillaria. Pull mulch and other material away from the base of the trunk to keep bark dry.
Mushroom root rot, Clitocybe, is a related disease prevalent in the far South and in tropical climates. It affects plants similarly but does not develop rhizomorphs.
--Phytophthora root rot. Phytophthora is prevalent in poorly drained, infertile soils and affects many ornamentals including both broadleaf and conifer trees and shrubs. Symptoms start with declining vigor and growth. Later, branches die back and sprouts may arise from the base of the trunk. Leaves tend to be small and drop prematurely, often curling and appearing scorched in hot weather. Affected plants eventually die.
The best way to discourage this soil-borne fungus is to improve drainage. Saturated soil kills roots, which provides the fungus with entry sites. Soil-drench fungicide applications are helpful before planting in infested soil.
--Ganoderma root rot. This is another soil pathogen that can affect numerous ornamental species and also produces symptoms similar to other root rots. Reddish-brown conks—shelf-like fruiting bodies—appear near the base of the trunk in the first few years of infection. Prevention is the most important aspect of managing this disease as no practical cure exists. Practices discussed under previous root-rot diseases apply to Ganoderma. Occasionally, you can cut out diseased roots before the pathogen spreads.
--Wilts. Several fungal pathogens cause wilt in many woody ornamentals. Verticillium, oak wilt (Ceratocystis), Dutch elm (also Ceratocystis), mimosa wilt (Fusarium) and persimmon wilt (Cephalosporium) are the most serious and widespread. Most trees affected by wilt do not survive. Their leaves turn yellow or brown, wilt and then drop prematurely. This can continue for several years, or trees may die suddenly.
The infamous Dutch elm disease is one wilt for which treatment is sometimes successful, if expensive. Prevention involves severing root grafts between adjacent trees, eliminating breeding sites of the elm bark beetles that vector the fungus and pruning out affected limbs before symptoms spread. Trunk-injected fungicides are available for Dutch elm and some other wilt diseases.
The best strategy is to use wilt-resistant species recommended for your area. Fortunately, breeders have developed wilt-resistant varieties of American elm, and you should use only these for new plantings.
--Pine wilt. Nematodes that colonize in conducting tissue in pines cause this disease. Some pine species are more resistant (notably white pine) than others, but none are immune. The nematode is spread by wood-boring beetles. Needles fade from their normal color and ultimately turn reddish brown upon death, which may take a few weeks to several months. Laboratory diagnosis is necessary to confirm pine wilt, but you should act promptly anytime you suspect this infection. Do not keep the wood of infected trees you’ve removed for firewood, as this may harbor borers that can then spread the disease to other pines.
--Wood rots. Decay of the inner wood occurs in most trees sooner or later. This does not necessarily portend an early death, however. Affected trees can live many more years, though some may slowly lose vigor and decline. Decay fungi may produce fruiting bodies visible on the surface of the trunk. Keep trees as healthy as possible and avoid unnecessary wounding. If practical, control borers, which open entry sites for decay fungi.
--Phloem necrosis of elm. Often mistaken for Dutch elm, a mycoplasma causes this disease, which spreads by root grafting or insect vectors. Leaves of affected trees curl upward, turn yellow and then wither and drop. After the onset of symptoms, trees may die in just a month or two. There is no practical cure, and you should promptly remove diseased specimens from the landscape. To distinguish this disease from Dutch elm and other similar maladies, look at the inner bark near the trunk base—it will be yellow or butterscotch colored and may contain dark flecks.
--Cankers. Cankers are lesions (dead spots) that develop in the bark of twigs and branches. Cankers often develop in open wounds and can enlarge until they girdle entire limbs. Fungi cause nearly all cankers, although a few bacterial cankers occur as well. With few exceptions, chemical treatments are ineffective. Cankers can girdle entire branches and ultimately will kill the host. If you spot cankers on smaller branches, prune these limbs out, making the cuts at least 6 inches below any sign of the canker. On large branches or trunks, it is sometimes possible to eliminate cankers with surgical removal. Use a sharp knife to cut away the cankerous tissue, making the cuts within healthy tissue just outside the infected area. Disinfect the tool between cuts and immediately cover the wound with shellac. Although cankers usually infect tissue just inside the bark, the fungi sometimes penetrate into the underlying wood. In this case, remove the discolored wood as well.
Severely affected plants are probably lost causes, so cut them out and remove their debris from the landscape entirely.
--Wetwood. The primary symptom of wetwood is oozing of slime flux—slimy, dark, often smelly sap that drains from trunk wounds and wets the bark below. This chronic condition can result in a slow decline of the tree. Elms and poplars are the most commonly affected trees, though numerous other hardwoods are susceptible.
Wetwood results from bacterial infection of spring-wood parenchyma cells. No practical treatment exists, but fertilizing with nitrogen may help trees overcome the effects of wetwood.
--Witches’ brooms. These peculiar growths are the plant’s physiological reaction to a fungus, virus, bacterium, insect or even mistletoe. Witches’ brooms are dense masses of dwarfed shoots that are especially conspicuous in dormant deciduous trees. To control them, simply prune them out.
--Galls. These growths often are the plant’s reaction to attack by insects but also result from many other pathogens. No treatment exists for galls, and controlling the agents that promote them is a strategy that is not often effective. Fortunately, galls seldom cause life-threatening injury to ornamentals and the best approach is tolerance. Though not attractive, they are not typically lethal, and you can do little about them anyway, except to prune out those that are particularly unattractive.
--Crown gall. This gall—whitish at first, then turning dark—is somewhat different than most galls that affect leaves and young shoots. Crown gall causes large tumor-like growths on stems, trunks and even roots of many species of plants. The causal bacterium is Agrobacterium tumefaciens and persists in soil for several years. Therefore, do not replant susceptible species in the same spot where an infected plant grew previously. If possible, prune out branches that display galls, sterilizing tools between cuts.
Leaf diseases generally do not pose a serious risk to trees and shrubs. They may cause early leaf drop, and several successive years of this can deplete food reserves to some extent. However, this usually is the most serious consequence, and trees generally tolerate it. Infection ends as soon as leaf fall occurs in late summer or fall. Evergreens are more vulnerable, because leaf drop may remove several years of leaf growth. Therefore, you should give evergreens higher treatment priority than deciduous plants, with which landscape managers usually tolerate foliar diseases.
--Powdery mildew. This disease is common to a great many ornamentals, though it rarely is a serious threat to plants. It is, however, unattractive—its superficial white, powdery growth on leaves is easy to spot. Powdery mildew is most frequent in shaded or crowded plantings with poor air circulation. Improving this factor alone often alleviates mildew problems. Otherwise, several products easily control powdery mildew. Anti-transpirants apparently can reduce outbreaks of this disease.
--Fireblight. The bacterium Erwinia causes this disease, and it is a serious problem for plants of the rose family. Affected leaves, flowers, shoots and fruits suddenly turn brown or black and look as if they’ve been scorched by fire. Cankers also may appear on twigs and branches. The pathogen spreads by splashing water, wind or contaminated pruning tools.
Prune out affected shoots 6 to 12 inches below any sign of infection, disinfecting tools between cuts. Streptomycin is an effective bactericide for use against fireblight.
Also, plants vary widely in their susceptibility, so use resistant varieties.
--Anthracnose, leaf blight, leaf blotch, leaf spots, shot hole and scab. These diseases, caused by a variety of fungi (a few bacterial leaf spots also occur), can affect nearly every woody ornamental and are quite common. No fundamental distinction exists between most of them; they all cause necrotic areas of various sizes and shapes. These affect leaves and, often, entire young shoots. This is not to say that symptoms are not distinct enough to be diagnostic—many are.
Symptoms of scab, leaf spot and shot hole include round, oval or angular spots with raised or sunken centers which may fall out, creating “shot holes.” Anthracnose, leaf blight and leaf blotch produce larger, more irregular dead areas that m ay extend to give entire shoots a scorched appearance. Little basis exists for differentiating between the three, and the terms often are used interchangeably.
Though unattractive, most of these diseases are not particularly injurious to trees and shrubs. Therefore, it is difficult to justify control measures except for valuable specimens. Sanitation, such as removing fallen leaves in the fall, may reduce the incidence of these diseases.
Anthracnose is practically an annual occurrence in many areas. It causes much fretting by grounds managers concerned about the defoliation that this disease causes, sometimes two or even three times in a single season. While this can weaken trees, fungicidal treatments are expensive and not financially justified in most cases because trees generally recover adequately once weather becomes warm and drier. However, particularly valuable trees or shrubs may warrant treatment, for which several effective products are available. Consult with local extension pathologists for recommendations.
--Needle blight, spot or cast. These consist of spots or lesions on the needles. Needle cast is the term that describes the advanced stages when needles actually drop. Not fundamentally different from leaf spot on broad-leaved plants, these diseases (many pathogens cause them) are not usually serious unless they advance to the point that needles begin to drop in large numbers.
In this case, treatment may be necessary to prevent defoliation which, as stated previously, can be more serious for evergreens than deciduous plants.
--Rust. This is a peculiar group of fungi, some of which require two different hosts to complete their life cycle. Yellow, orange, red, brown or even black pustules develop on the undersides of affected leaves, which may drop early. The spores are wind-borne, and the disease—though it is not typically serious—can stunt or even kill a plant outright in severe cases.
In cases where the fungus requires an alternate host, destroying the alternate host plants growing in the general area often suffices for control. The classic example of this type of situation is cedar-apple rust. A common disease of apple and crabapple trees, this rust also requires Juniperus spp. to complete its life cycle. Thus, if you can eliminate all nearby juniper hosts—such as the weedy eastern red cedar—within several hundred yards of the trees, the disease should be much less severe. Unfortunately, this measure often is impractical. Fungicidal treatments work well but may require 4 or 5 applications. Therefore, many landscape managers simply tolerate the disease.
--Sooty mold. This “disease” results from mold that grows on honeydew secretions of insects such as aphids or whiteflies. This creates black or gray patches on the leaf surfaces. The fungus is not parasitic to the plant, merely unsightly. If, for some reason, you must eliminate sooty mold, the best strategy is to treat for the insects themselves.
--Leaf blister or curl. Blister diseases are similar to leaf spots and anthracnose in the sense that they affect many plants but rarely in a serious way. Symptoms include wrinkled, curled or puffy “blisters” of various colors on leaf surfaces. Affected leaves usually drop early. Again, treatment is possible but not usually warranted.
--Mosaic and ringspot. The symptoms of mosaic and ringspot—mottling and circular lines or other odd markings—result primarily from viral infection. The foliar symptoms of these two diseases can be quite distinctive. A general decline in vigor and stunting often occur in conjunction with foliar symptoms or alone. No practical treatment exists, though some grounds managers attempt to prune out infected shoots before the virus spreads, with occasional success. The usual action is to remove severely infected plants. Virtually all plants can develop some type of viral infection, but viruses are not one of the major plant-disease problems most landscape professionals face.
--Lethal yellowing. This disease has killed large numbers of palm trees in Florida since its introduction in the 1970s. It is caused by a mycoplasma-like organism that infects conducting tissue. Initial symptoms include fruit drop and necrotic flower parts. Foliar symptoms follow and may include the appearance of bright-yellow leaves in the middle of the crown or, in some species, leaflet browning or general leaf discolorations that range from gray to brownish red.
You can extend the life span of—but you cannot cure—infected palm trees with oxytetracycline injections.
However, it may be better to remove infected trees so the disease does not spread to other palms. Some landscape managers use oxytetracycline as a preventive measure for valuable palms growing in infested areas. The best strategy, however, is to use palm species and cultivars resistant to lethal yellowing.
Most turfgrass diseases are caused by pathogenic fungi that invade the leaves, stems and roots of plants, thereby causing various symptoms such as leaf spots, root rots or death of entire plants. Sometimes these fungi produce visible structures such as mushrooms, white powdery mildew or a fluffy, moldy growth. These fungi are normally present in most lawns, but disease only occurs when environmental factors favor growth of the pathogen and increase the susceptibility of the grass host. This relationship between the environment, host and pathogen are the key factors in disease causation and control. Turfgrass management practices alter the environment and therefore have a major impact on disease development. These management practices include mowing, irrigation, fertilization, thatch control, traffic and soil pH.
Mowing favors infection and disease by creating wounds through which a pathogen may enter the plant easily. Mowing also spreads fungal spores and mycelium. Height of cut is a major factor in disease susceptibility. Close mowing predisposes turf to Helminthosporium diseases, rust, powdery mildew, brown patch and dollar spot disease. The continuous removal of the youngest, most photosynthetically productive tissues when mowing below recommended heights causes depletion of food reserves in the grass plant. These reserves are needed for active disease-resistance processes in plants, and plants also use them to recover from injury.
Irrigation provides moisture critical to spore germination and fungal growth. The timing, duration and frequency or irrigation may greatly affect disease intensity. Light, frequent irrigation discourages root development and predisposes turf to injury when extended periods of drought occur. Subjecting turf to drought stress appears to favor Helminthosporium diseases, stripe smut, powdery mildew, summer patch, dollar spot and fairy rings. Excessive irrigation also restricts root development and encourages disease.
Turfgrasses grown under wet conditions develop succulent tissues and thinner cell walls that pathogens presumably more easily penetrate. Algae and mosses thrive in waterlogged soils, particularly where turf density is poor. Morning or afternoon irrigation is often recommended during summer to ensure that plant tissues are dry by nightfall. This practice helps minimize the intensity of Pythium blight and brown-patch disease. Use of proper soil-fertility programs improves the vigor of plants and their ability to resist disease. Excessive use of nitrogen promotes tissue succulence and thinner cell walls, which as previously mentioned, pathogens more easily penetrate. Conversely, turfgrasses growing in nutrient-poor soils are prone to invasion by dollar spot, red thread and rust diseases.
Application of nitrogen to diseased turf under low-fertility conditions stimulates growth at a rate that exceeds the capacity of the fungus to colonize new tissues, thus reducing the level of disease injury. Many turfgrass pathogens survive as resting structures or as saprophytes (organisms living on dead organic matter) in thatch. Thatch also provides fungi with moisture. Fungal pathogens such as Helminthosporium spp. produce enormous populations of spores in thatch, particularly when the thatch is subject to frequent wetting and drying. Stripe smut, Helminthosporium and summer patch are diseases that appear to be favored by excessive thatch accumulation.
Traffic, like mowing, produces wounds that some fungal pathogens easily invade. Compaction caused by heavy traffic impedes air and moisture movement into soil and eventually restricts root function, causing a decline in plant vigor and disease resistance. Soil pH also may affect disease development in turfgrasses. For example, extremes in soil pH result in reduced plant vigor and, therefore, a reduction in plants’ ability to resist disease.
As with ornamental plants, you diagnose turf plant diseases using signs and symptoms. Signs represent the visible parts of the pathogen—for example mycelium, fruiting bodies, resting bodies and spores. Symptoms are the outward expression of a plant that is suffering from a disease. Symptoms include, for example, leaf spots, tissue blighting, rots, yellowing, wilting and stunting. Symptoms of most turf diseases take the form of leaf-spot lesions, blighting of leaves, water-soaking of leaves, and crown and root rots. A symptomatic key to common lawn diseases is provided here. Control of diseases by the use of fungicides may become necessary in some situations.