Management of Turfgrass Diseases

Joseph M. Vargas

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351434171
• Edition: 2
• Topic: Technologie et ingénierie
• Subtopic: Gestion de l'environnement

This Second Edition of Management of Turfgrass Diseases has been completely revised and updated to provide the latest information on maintaining a healthy turf and identifying turf diseases. Written by a leading international lecturer on turf grasses, the book covers cultural, genetic, biological, and chemical approaches to turf management and provides practical solutions to everyday problems. Fungal, bacterial, and viral diseases; black layer disease; and diseases caused by nematodes are addressed for all major grasses. You'll learn about cool and warm season grasses, growing conditions, new diseases, symptoms, and identification and management techniques. Valuable tips on irrigation, fertilization, and grass culture as well as 72 full-color photographs and more than 100 figures ensure this book will be dog-eared from use. It's almost like having your own private turfgrass consultant sitting right on your bookshelf.
What's New in the Second Edition?
This second edition features new grasses, new diseases, and the latest research findings and practices. You'll have the most up-to-date information available on the major grasses.

CHAPTER 1
Introduction to Turfgrass Diseases

CAUSES OF TURFGRASS DISEASES

There are five groups of organisms that cause plant diseases — fungi, bacteria, viruses, nematodes, and mycoplasma. Fungi are the most important cause of turfgrass diseases; they are followed in importance by the nematodes and the viruses. There is only one major viral disease of turfgrass, St. Augustine decline, and one major bacterial disease, bacterial wilt.

A disease is an abnormality in structure or function that is caused by an infectious agent and that injures the plant or destroys its aesthetic value. Diseases are sometimes classified into two categories, infectious and noninfectious (or physiological), but I prefer to consider noninfectious diseases as injuries. Injury is damage to a plant that is caused by a noninfectious agent and that injures the plant or destroys its aesthetic value. Damage from hail, lightning, nutrient deficiencies, and fertilizer or pesticide burn are examples of noninfectious plant injury.

A pathogen is an agent that causes a disease. Most pathogens are parasites as well. A parasite is an organism that obtains some or all of its nutrients from a living host. (The organism that a parasite lives on is called its host). Some pathogens are obligate parasites; they can obtain their nutrients only from a live host or living tissue. Rusts, powdery mildews, and all viruses are obligate parasites. Organisms that live only on dead organic matter are called saprophytes (slime molds are an example). Organisms that are mostly parasitic but that can, under certain conditions, live as saprophytes are called facultative saprophytes (for example, Typhula, Sclerotinia, and Helminthosporium). Organisms that live most of the time as saprophytes but that can, on occasion, become parasites are called facultative parasites (for example, Rhizoctonia and Pythium). Table 1.1 shows the classification of some organisms that are known to cause diseases in turfgrass, and Figures 1.1 and 1.2 illustrate the developmental stages of the facultative saprophyte Drechslera spp.

In order for disease to occur, three conditions are necessary: a susceptible host, a virulent pathogen, and a favorable environment. These three conditions are called the plant disease triangle. If any one of them is missing, disease will not develop.

The disease process usually involves four steps. They are infection, incubation, symptom development, and inoculum production. Infection is the process by which a disease-producing organism (pathogen) enters the plant. Incubation is the period during which the pathogen inhabits its host without producing visible symptoms. The interaction between the pathogen and its host results in symptom development. Inoculum production is the process whereby the pathogen reproduces propagules for spread and survival. The inoculum can be simply the spore of a fungus, or it can be the entire organism, as it is in the case of a virus or a bacterium.

Table 1.1 A Classification of Organisms That Cause Turfgrass Diseases Based on Life Cycles

Obligate Parasites	Facultative Saprophytes	Facultative Parasites	Saprophytes
Erysiphe graminis	Bipolaris sorokiniana	Colletofrichum	Fairy rings^1
Parasitic nematodes	Drechslera poae	graminicola	Mucilago crustacea
Puccinia spp.	Gloeocercospora sorghi	Gaeumannomyces	Physarum cinereum
St. Augustine	Laetisaria /ucitormis	graminis
virus	Limonomyces rosai penis	Leptosphaeria korrae Pythium
	Microdochium nivale	aphanidermatum
	Pyricularia grisea	Rhizoctonia solani
	Sclerophthora macrospora	Xanthomonas campestris
	Sclerotinia homoeocarpa
	Typhula spp.
	Ustilago striiformis

*Mild infection may occur.

Figure 1.1 A Drechslera poae spore on a fescue leaf with germ tube that has produced an appressorium. A penetration peg will be produced by the appressorium which will infect the leaf.

IMPORTANCE OF TURFGRASS DISEASES

Disease plays a major role in determining the success or failure of a turfgrass stand. It is often the most important single factor limiting the successful growth of a cultivar or species, a fact that you must keep in mind when selecting a turfgrass species or cultivar. For example, spring dead spot (SDS) on bermudagrass limits its widespread use as a fairway grass, especially in the northern regions of the warm-season grass belt; St. Augustine decline (SAD), a viral disease, has eliminated St. Augustinegrass as a desirable turfgrass in many areas; and necrotic ring spot, summer patch, and stripe smut have made it impractical to plant Kentucky bluegrass cultivars like Merion, Windsor, and Fylking. The best-textured, nicest-colored, and fastest-germinating grass, if it is susceptible to a major pathogen, will turn into the worst-looking and poorest-colored grass imaginable when it becomes decimated by a disease, and you will wish it hadn’t germinated at all.

Figure 1.2 Germination of a Drechslera spore and subsequent penetration, incubation, and development of symptoms on a turfgrass blade.

Turfgrass diseases, caused by fungi, are of great economic importance. It is difficult to get exact figures, but the turfgrass industry spent 80 million dollars on fungicides in 1988. More fungicides are used on turfgrass than on any other single crop in the United States.

IDENTIFYING TURFGRASS DISEASES

A little knowledge is said to be a dangerous thing—and it is! Often samples of dead or dying grass are collected and mailed, with little background information, to a diagnostic laboratory. At the laboratory, disease organisms are isolated from the plant material and a diagnosis is made. Under such circumstances a disease may be diagnosed as “Helminth” or as necrotic ring spot, when it is neither. Or what appear to be black Helminthosporium lesions are observed on dying turf, and it is assumed that Helminthosporium is causing the problem, which may or may not be the case. It would be difficult to find a patch of Kentucky bluegrass in the spring that didn’t have Helminthosporium lesions, but does that mean this organism is causing the problem? Maybe. Before you jump to conclusions, look at the other symptoms in the area. Ask such questions as: Were there many black lesions? Were they large? Did the grass show a general, all-over thinning? If the answer...