Some years ago I was invited to a high school to help diagnose a problem on the infield of the school’s baseball field. Half of the turf was brown, with patches of dead grass scattered throughout the entire infield area. The immediate, knee-jerk reaction was to assume it was disease-related, since the symptoms were similar in nature to disease symptoms and it was August, when many diseases, such as brown patch and dollar spot, are active.
The first thing I do when diagnosing a turf problem is to identify the turfgrass. The dead/dying grass on the infield was Kentucky bluegrass, and the healthy looking grass was perennial ryegrass. Kentucky bluegrass is susceptible to summer patch disease, but perennial ryegrass isn’t. However, the symptoms didn’t look like summer patch, which show as circular patches or rings from 6 inches to 3 feet in diameter. The problem on the infield was more uniform and mottled. Upon close inspection of the turf, its crown and underlying roots, it quickly became apparent that the damage was caused by bluegrass billbugs, which lay their eggs in the crown of bluegrass plants, but not ryegrass plants.
In order for a disease to become a turfgrass problem each of these three components – turfgrass host, the right environment, pathogen – must be present.
Identifying a turf problem as a disease is challenging, since the disease organism is typically very small. Working through a series of clues, like a detective, will usually result in narrowing it down to the exact problem.
The first step is turfgrass identification, the next is an evaluation of environmental conditions, and the last is a learned knowledge of local diseases that could be the cause. These three steps are often referred to as the “disease triangle” (Figure 1), because in order for a disease to become a turfgrass problem each of these three components – turfgrass host, the right environment, pathogen – must be present.
Identifying the turfgrass plant is important because not all turfgrasses are susceptible to all diseases. Some turfgrasses are prone to only one or two diseases, and some, like perennial ryegrass, are prone to many (Table 1). Since so much perennial ryegrass is used on sports fields, a sports turf manager must always be vigilant, particularly during warm and humid weather. Perennial ryegrass is also used frequently as an overseeding tool, further exacerbating its susceptibility to seedling diseases like damping off (Pythium, Rhizoctonia and Fusarium spp.)
Temperature, humidity and leaf wetness/moisture are the environmental factors that drive disease incidence, and disease pathogens are active within certain known temperature ranges. Prediction models have been developed that predict when environmental conditions are right for a disease to occur. These prediction models are based predominantly on local weather data, namely temperature and humidity.
Turf damaged by billbugs is often mistaken as turf damaged by disease.
PHOTOS COURTESY OF PAMELA SHERRATT
For example, brown patch disease is most severe when the sum of the daytime and nighttime temperature exceeds 150, with 10 hours of continuous leaf wetness. If those environmental conditions exist and the cultivars of tall fescue on a sports field are particularly susceptible to the disease, then there is a high probability that brown patch will occur.
Dollar spot appears on fields that have low and deficient nitrogen.
In addition to the right temperature and humidity, diseases also need water to grow, infect and spread, and continuous leaf wetness offers the right environment for disease development. Turfgrass that stays wet because it’s watered too much, or covered regularly with plastic tarps, or growing in an environment with poor air circulation, has a greater risk of disease infection.
Another environmental factor that influences disease severity is soil fertility, particularly in regard to nitrogen status. There are diseases that occur if an excess of nitrogen has been applied. Examples include brown patch and Pythium blight. If there’s a history of brown patch on a field, it may be that too much nitrogen fertilizer has been applied.
Pythium blight can occur if nitrogen content is too high.
Conversely, some diseases, such as rust, red thread and dollar spot, occur on fields that have a low or deficient nitrogen status. Turfgrass that’s deficient in nitrogen is also unable to grow out of the disease symptoms and recover quickly. In most incidences these diseases can be prevented with a sound nitrogen fertilizer program.
Pathogen signs and symptoms
Turfgrass diseases are typically caused by fungal pathogens that spread by airborne or waterborne spores or vegetative hyphae. They’re generally microscopic and difficult to see. However, it’s possible to see signs of the disease. These include features like mycelium (the mass of white fungal threads produced by dollar spot), spore masses (rust) and sclerotia (hard mass of mycelium caused by gray snow mold). These signs are seen first thing in the morning, before the dew has dried, although some, like red thread and rust, linger a little longer into the day.
It’s much easier to identify the fungal pathogen by the symptoms it causes rather than the signs. Symptoms include leaf spots, blight, root rot, leaf tissue collapse and/or death. From a distance, these symptoms will look like patches or mottled areas of discolored turf. They’re generally circular in nature and do not spread in straight lines or blocks.
Examples of disease symptoms are:
- hourglass-shaped lesions on leaves caused by dollar spot disease;
- discolored spots on leaves and leaf twisting caused by gray leaf spot disease;
- circular patches of brown grass caused by brown patch disease; and
- frogeyes, arcs and rings caused by summer patch, necrotic ring spot or fairy rings.
Some symptoms are best viewed close up, possibly with a hand-held magnifying glass, while others are more obvious from a distance. Some diseases have unique symptoms, like the blue smoke ring seen around a brown patch of grass in the early morning (brown patch disease) or the obvious frogeye of summer patch disease.
Heat can take its toll on Poa annua and cause several turf diseases.
PHOTOS COURTESY OF PAMELA SHERRATT
Each of the diseases has a unique sign and symptom that help to identify it from the others. If there are no visible signs and symptoms are difficult to diagnose, it’s best to send a sample away to a diagnostic lab. Check to see if your local extension service or university turf program has one.
This is what gray leaf spot can do to your field.