Summer brings a host of happenings to our lives: summer cookouts, family vacations, July 4th celebrations, and yes, turfgrass diseases on the golf course. While “three out of four” might be good enough in most other life scenarios, it’s not when it comes to this one.
There are many inputs and actions responsible for the development, suppression and explosion of the various pathogens that insult golf turf, but several of them rise to the top. Some are proactive and others are reactive, but all are important.
Perhaps the best place to start with management of summer diseases is with scouting, commonly referred to as having “a good look-see.” Most golf turf maintenance workers know what the desired turf is supposed to look like, whether it’s bentgrass, bluegrass, ryegrass, Bermuda, bahia or Zoysia. When turf doesn’t look quite right, it should raise a red flag. The problem might be disease, insect damage or any of a number of abiotic causes such as hydraulic fluid leaks, heavy thatch or hydrophic conditions or a nutrient deficiency.
There are many good references to help you get a good handle on the initial symptoms of turf problems. They can’t all be addressed in this article, so superintendents should consider typing a phrase such as “golf turf disease diagnosis” into their favorite search engine. Several good textbooks and mobile device apps are available as well.
It’s best to start with the ones in your state or region, as some diseases can look different from place to place. The ones that include color pictures of early and full-blown symptoms tend to be the most helpful.
Observation of the initial symptoms of a pathogen’s presence is of key importance to keeping them in check. When noticed early in the life cycle, before disease pressure is high, management techniques and several options for control may be available. In some situations, the lower recommended rate can be utilized rather than the higher end of the range, saving money on products as well as putting less fungicide into the environment and increasing the chances for success.
Scouting will help you spot differences in turfgrass appearance, but sometimes taking a closer look is required. Sometimes the best option involves digging to examine the roots, considering recent cultural practices and weather events that took place in the area in recent days, and comparing turf in one location on the course to another. Taken as a whole, that series of individual scouting events is called monitoring.
Another difference between the scouting and monitoring is that the latter usually involves consulting course records to look for notes on outbreaks of diseases, their extent, when they occurred and what control measures were taken.
At a minimum, scouting should be done before and after a fungicide application to confirm the presence of the pathogen, as well as to determine the effectiveness of the control attempt. Depending on the weather conditions and fungicide used, more frequent inspections might be necessary.
Scouting and monitoring are frequently done in one of two ways:
- with dedicated scouts whose primary job is to closely inspect various parts of the course on a regular (if not daily) basis, or
- by dividing the function among everyone who moves around the course, asking them to look for red flags as they go about their work.
Each approach has its pros and cons, so it’s up to the superintendent to choose the best path.
Summer patch (Magnaporthe poae) and necrotic ring spot (Ophiosphaerella korrae) are two of the most damaging of all root diseases. Generally, necrotic ring spot occurs during the spring and fall, whereas summer patch symptoms appear during the hot summer months. The symptoms of these diseases appear to be quite similar but lab work can determine which one is causing the problems.
Affected turfs show 6- to 12-inch circular, semicircular or serpentine patches of light tan grass blades. A “pockmarked” or donut-shaped appearance often develops, with brown, matted turf plants that contain a tuft of apparently healthy turf in the center. This is sometimes referred to as the “frog-eye effect.” Plants on the edges of the patches are often unthrifty due to slowed function of the roots, a symptom caused by both diseases.
There are three main Pythium diseases in golf course turf: Pythium blight, Pythium root dysfunction (PRD) and Pythium root rot (PRR). Pythium blight is favored by high humidity (80 percent or higher) and heat (above 76 degrees Fahrenheit at night). The disease is most severe on turf grown in native soils with poor drainage. Symptoms include spots or patches of turf that are dark and water soaked. Large areas of turf can be killed when conditions are conducive.
Pythium root rot and Pythium root dysfunction are soil-borne diseases. With PRR, the roots are completely rotted, whereas with PRD the root hairs are lacking. Symptoms of PRR typically develop in low-lying areas as irregular patches ranging from 6 to 20 inches in diameter. The patches may have a water-soaked appearance initially and quickly transition to large areas of necrotic turf. The crowns and roots are very necrotic as well.
Usually, the symptoms are most severe when root zones become anaerobic. PRD develops in areas with good drainage and high areas on creeping bentgrass putting greens. The symptoms develop as patches ranging in size from 6 to 24 inches in diameter, and unlike PRR, the existing patches rarely get larger. That is because the PRD pathogens infect the turf during the spring and fall when soil temperatures are between 55 degrees and 75 degrees.
Active in spring and summer, symptoms produced by dollar spot (Sclerotinia homoeocarpa) are straw colored, somewhat sunken patches; infected blades express tan leaf spots with reddish brown borders. Dollar spot is most severe when temperatures are above 55 degrees and average relative humidity remains above 70 percent for at least four to five days. It can occur from late spring to early autumn.
Dollar spot is most severe when nitrogen is limiting because the plants cannot recover from dollar spot damage. On close-cut golf turf, spots range from 1 to 1.5 inches (hence the name dollar spot), while infections in higher-cut turf are usually 3 to 5 inches across. Individual leaf blades within the affected area develop with lesions up to an inch long, light tan with reddish brown margins. The lesions usually span the width of the leaf blade.
Gray leaf spot
Gray leaf spot (Cercospora zeae-maydis) usually occurs with the onset of hot and humid weather. The disease is most severe in newly established stands of perennial ryegrass and is typically more severe in the first year of establishment. The potential for disease development can be reduced by implementing several cultural practices. Key symptoms include the rapid dying of taller turf, the presence of twisted leaves that resemble a fish hook, water-soaked lesions on leaf blades, and blighted leaves that appear matted and greasy. In high humidity, leaves usually turn gray and fuzzy with sporulation structures. Gray leaf spot can be difficult to diagnose because it is sometimes confused with Pythium blight symptoms.
Brown patch produces symptoms that include light brown grass blades (usually not matted down), with irregular blotches of tan lesions with dark markings on the leaf blades. Patches of affected turf range from 6 inches to 2 feet in size. The lesions will not always go across the leaf blade, as they do with dollar spot.
Brown patch (Rhizoctonia) is often induced by heavy midsummer applications of nitrogen. The most influential factors in brown patch development are periods of high heat and high humidity, especially when they last more than 24 hours.
Although each disease encountered in summer has its own set of unique management techniques, there are some that are common to all. The first is to maintain healthy turf through proper cultivation, irrigation, fertilization and landscape design factors. These keep the turf vigorous and able to naturally resist active pathogens. Making an accurate diagnosis is important for control, as specific cultural and chemical control agents are usually linked to efficacy. For example, fungicide X may be very effective for disease Y, but only marginally effective for disease Z. In this way, the best fungicide can be selected based on local fungicide trials at land grant universities.
Timing is everything. Applications are much more effective during early sign than spraying after the infection is full-blown. The active ingredients of the fungicide must reach the pathogen in order to be effective, so it’s crucial to put the product where the pathogen is, whether it be in the roots, in the thatch or on the shoots.
Uniform coverage of the target site is important, so be sure to avoid over- and underapplication. Underapplied formulations often result in breakthroughs of the disease symptoms, while overapplied products are simply a waste of money.
Fungicide resistance is a looming problem with many implications: loss of possible control agents, environmental activist pressure, and less-than-aesthetically-pleasing and functional turfgrass. There are many facets to the acceleration of resistance, beginning with the mode of application of the product. The Fungicide Action Resistance Committee, a group of scientists that actively study this issue, has categorized active ingredients into more than 40 groups according to their modes of action. One strategy for avoiding resistance is to rotate between classes of fungicides for each application.
In addition to mode of action, fungicides are often grouped by their capacity to attach to either one or multiple sites in the cells of the pathogen. In terms of resistance, multisite fungicides are generally less susceptible. Using preventative applications when weather conditions are conducive and history of the site indicates a likelihood of infection as well as curative applications in a pre- and post-outbreak routine, leads to fewer incidences of resistance.
The phytomobility – how the fungicide moves within the plant – is a key component in resistance prevention, because the more we know about its movement, the better our approach will be. The deposition of the application factors such as rate, interval, timing and volume are important, as deviations from the ideal in any of those areas can lead to less-than-effective results, and therefore the potential for resistance. In addition to applying insufficient product to the target site, other factors that lead to increased resistance include biological or photodegredation, metabolism transformation, and removal of product during mowing and aerification.
While you’re at it
Summer isn’t only for looking down at turf; it’s also a great time to look up at trees to inspect for tree diseases as well. The same principles and protocols apply for woody plants as for turf: early detection, scouting/monitoring, accurate diagnosis and the careful consideration of management options.
As for turf maladies, it’s critical to have a few workers who know the basics of tree disease symptoms and can spot them on trees and shrubs. Common pathogens include a variety of cankers and leaf spots, anthracnose of many species, and pathogens that tend to be specific to a particular group of plants such as cedar apple rust, apple scab and fireblight. Abiotic factors including stem girdling roots, chlorosis, deep planting, over/under watering and mower blight should be investigated as well.