Integrated Pest Management

IPM Manual for Landscape Pests in British Columbia

Chapter 7: IPM for Diseases in Turf

Learning Objectives

When you have completed this lesson, you should be able to:

Classify common types of turf sites into three categories according to maintenance level.
List five practices that help prevent fungal diseases in turf.
Know where to look for help in identifying turf diseases.
Describe visual inspection and counting methods used in monitoring turf diseases.
Describe the use of mowing and microbial soil amendments in managing turf diseases.
Describe two main groups of turf fungicides and list general guidelines for using fungicides to control turf diseases.
Describe the role of evaluation in an IPM program for turf diseases.

Introduction

Nearly all turf diseases in BC turf are caused by fungi. Common diseases in BC are red thread (Laetisaria fuciformis), anthracnose (Colletotrichum graminicola), brown patch (Rhizoctonia solani), Pythium root rot (Pythium spp.), Fusarium patch (Microdochium nivale), Typhula snow molds (Typhula spp.), take-all patch (Gaeumannomyces graminis), as well as localized dry spot and fairy ring, leaf spots and rusts. Necrotic ring spot (Leptosphaeria korrae) occurs in parts of the BC interior. Most fungal disease problems occur in highly managed turf (such as golf greens), especially in coastal areas or in years with higher than normal rainfall.

Turf management, especially with respect to diagnosing and managing diseases on highly managed turf, is a complex, technical field. Therefore, this chapter is intended to provide the general framework of an IPM program for turf diseases. For more information, consult the references listed under Further Reading at the end of this chapter. In particular, anyone managing large areas of turf should get a copy of Turfgrass Disease and Pest Management Guide for Professional Turfgrass Managers in BC. In addition to identification keys and colour photographs, the guide includes management and treatment recommendations for over 35 turf diseases in BC conditions.

As described in Chapter 2, using a system for categorizing sites according to maintenance level can help in planning an IPM program.

Facility Categories

Examples of turfgrass sites that might be included in each category include:

Class A - High level of service: fine ornamental lawns, golf and lawn bowling greens, irrigated sports fields.
Class B - Moderate level of service: general park areas, residential and commercial lawns, boulevards, recreational fields, golf fairways.
Class C - Low level of service: meadows, picnic areas, rough grass, undeveloped and naturalized areas.

The following section describes the general steps in an IPM program for turf diseases and gives examples of techniques currently available to turf managers. Given the number of different diseases that may occur, however, this is general information, intended only as a starting point.

Prevention

Prevention is the main approach to managing turf diseases. Before any disease can occur, three elements must be present: a susceptible plant, the disease agent (called a pathogen) and favourable environmental conditions for development and spread of the pathogen. If any parts of this "disease triangle" are missing, disease does not develop, therefore prevention of turf diseases is based on removing one or more of these elements.

Prevention for all turf pests involves using optimum site design and management. When designing and planting turf areas, plan for long-term disease management by starting with the correct drainage, soil preparation and choice of turfgrass cultivars adapted to the conditions. Follow this with turf management practices that ensure healthy, vigorous turf (see Chapter 5 for more management methods that contribute to healthy turf).

Conditions that predispose turf to most fungal infections include overly lush growth (i.e., from over-watering or over-fertilizing with quick-release nitrogen) and prolonged periods of leaf wetness (i.e., through natural weather conditions or poor irrigation practices). Other contributing factors are: mowing too short, compacted or poorly drained soils, temperature stress (too hot or cold for the type of grasses), drought or waterlogged conditions, under- or over-watering and nutrient deficiencies and imbalances.

Specific preventative practices for turf diseases (also called "cultural controls" in some references) are:

a) Reduce periods of leaf wetness

Reduce the time dew spends on leaves. "Dew" on turf includes guttation water, which leaks out of grass blades, carrying with it sugars and other nutrients. This gives fungi a food supply and also allows the pathogens to move on a film of water to nearby plants. Sweep or drag light hoses or poles across the turf to disperse dew early in the day or irrigate for a short period early in the morning (i.e., for 2-3 minutes, 2 hours after sunrise) to dilute and wash off the nutrient rich dew.
Time irrigation for early morning, rather than evening, to allow leaves time to dry thoroughly during the day. Use manual overrides on automatic watering systems so they can be readily adjusted to avoid overwatering in wet weather or use automatic rain delays to shut off irrigation in rainy weather.
Prune nearby trees and shrubs to reduce shade and increase air movement across the turf, which will speed drying of the leaf blades.

b) Plant disease resistant turfgrasses

Some species and cultivars of grasses are tolerant of, or resistant to, some diseases. Planting mixtures of species and cultivars, rather than monocultures, helps slow the course of infections and reduces the chance that plants will be infected. Where the same diseases are a serious problem every year, renovating or overseeding with particularly resistant grasses may be advisable. Contact seed suppliers for local recommendations.

Mowing lawn c) Use correct mowing practices

Mow at the correct height for the turfgrass species. Keep all mower blades sharp to reduce bruising and tearing of leaves that open the way for fungal attack.

Avoid cutting turf when blades are wet as this increases the probability of spreading diseases.
Clean clippings from the underside of mowers between jobs to reduce spread of disease.
Allow turf to grow longer before the dry summer season or when a disease is observed. This depends on the disease, as some leaf diseases, such as red thread, can be managed by mowing, which removes the infection.

d) Manage traffic on turf

Schedule maintenance procedures to avoid working in weather and turf conditions that favour the spread of diseases. Manage use of playing fields and sports greens to reduce damage and excessive wear, especially during wet, cool weather.

e) Remove snow and ice

To reduce diseases caused by snow moulds, turf can be protected with snow blankets or straw mulches. Some managers apply peat, soot, well-aged manure or a dark-coloured fertilizer (e.g., Milorganite®) to speed melting of snow and ice. Ice can also be broken up with some types of aeration equipment.

Identification

Healthy lawn Correct identification of diseases is essential because management methods must be tailored to fit the pathogen. Diseases are often difficult to diagnose because different diseases have similar symptoms. Also, symptoms may be mistaken for injury caused by poor growing conditions or physical damage. Diseases with similar symptoms may need to be positively identified by a plant pathologist.

To aid in diagnosing the problem, knowledge of the weather conditions in the preceding weeks is usually critical. See MacDonald & Gerber (1996) (under Further Reading at the end of this chapter), for a key to diagnosing turfgrass diseases based on recent weather and readily visible symptoms.

Other important factors are:

when and what kind of fertilizer or soil amendments have been used
irrigation amounts and timing
whether pesticides (including fungicides) have been used, what products and when
wear and use patterns
physical characteristics of the site, such as low-lying areas, north side of slope, etc.

Accurate diagnosis usually requires careful collection of affected turf roots, stems, leaves and crowns and the use of a microscope or hand lens. The plant material must be taken from the area where the pathogen is still active. This is important because fungi found in dead turf may be decomposers and not the pathogen that killed the plant. The area of active infection is usually in the zone of turf between healthy and diseased plants, such as at the edges of patches, rings or spots. Carefully separate individual plants and tease out the roots to try to determine which part of the plant is affected (i.e., roots, crown, leaves). Rinse roots in water to check whether they are white and healthy or yellow, brown or rotted. Some leaf diseases may be secondary problems on turf that has a root disease or is growing poorly due to other factors.

Check soil in the root zone for a "black layer" (caused by lack of oxygen due to compaction and poor drainage) or a "dry layer" (water does not penetrate to the root zone).

Compare with photographs in identification guides using:

signs of the disease, such as fruiting bodies, spores, mycelium
symptoms of disease, such as wilting, colour of leaf, leaf spots, shape, colour and size of affected patches

Identification resources include:

reference books, plant guides (suggestions are listed at the end of the chapter)
resources on the internet
staff at garden centres, community colleges, botanical gardens
professional diagnostic services.

Note: Professional services and companies can send samples to the provincial Plant Diagnostic Laboratory, Ministry of Agriculture and Lands, 1767 Angus Campbell Road, Abbotsford, B.C. V3G 2M3. Phone: 604-556-3126 (there is a charge for this service). Contact the laboratory for specific instructions on how to collect and package turf samples to ensure that active disease organisms are present. When sending samples to a lab provide as much background information as possible about weather conditions, turfgrass species, cultivars, any pesticide use, fertilizing, irrigation and other management practices.

Monitoring

Monitoring for turf diseases starts with keeping records of weather conditions, such as temperature, rainfall, relative humidity and light levels. Inspections for diseases should be conducted during the seasons and the periods of weather that favour disease development. It is a good idea to train all staff involved in managing turf to know what to look for and how to recognize early symptoms of the turfgrass diseases most likely to appear on the site. Monitoring methods for turf diseases include:

Visual Inspections

Walk over the turf areas, noting damaged areas and symptoms of damage, such as shape, colour, size of patches and where the damage is in relation to soil or other conditions (shade, waterlogged or dry areas, etc.). Record information on a map for future reference.
Use a hand lens for close visual inspection and a soil probe or knife to search deep in the thatch layer and root zone. Inspect turf leaf blades under a hand lens or magnifying glass to detect early signs of disease. Check roots as described under Identification (above).

Counting Methods

Carefully collect a pre-determined number of leaf blades (e.g., 30-40 blades). Count the number of blades with disease symptoms, such as spots, reddish coloured streaks or other discolourations. Determine what percentage of the total number of leaves collected are diseased.
Measure the width of active disease rings to use as indicator of how fast the disease is spreading. It can be useful to mark the boundary of diseased areas to show changes over time.
Count the number of disease patches in transects or grids using the same methods described for weed monitoring (see Chapter 5).

It is important to keep written records of all counts, as well as notes, photographs and maps of damage found during visual inspections for future reference.

When and How Often: The monitoring schedule depends on the category of the site, on how susceptible the turfgrass is to diseases, and on weather conditions, which are critical in influencing the growth of fungi. Monitoring carefully and often during expected infection periods makes it possible to catch disease infections early. For example, on highly managed turf (i.e., Class A sites) monitoring several times a week, or even daily, may be necessary during damp weather, but would be done less often at other times of the year. Familiarity with past disease problems on specific sites will help you in planning when and what to look for in the current year.

Class C turf may not require any monitoring as disease is seldom a problem on this type of site.

Action Decisions

How much injury from fungal diseases is tolerable depends a great deal on the particular disease and the susceptibility of the turfgrass. It also depends on whether conditions favouring the disease are likely to continue (such as spring rains) or whether they are under the control of the turf manager (such as irrigation practices) and can be corrected quickly. The timing of treatment actions also depends on how the treatment affects the pathogen. For example, most fungicides work by preventing the pathogen from infecting healthy turf, rather than by killing the pathogen directly. This means that in high risk sites such fungicides would be used when there is a sound basis for expecting infection to occur. Injury levels also depend on the category of site. For example:

Class A Sites: There is usually low tolerance for disease due to potential for swift spread on highly managed turf.

Class B Sites: Some damage may be tolerable, especially if turf is likely to recover with good management, a change in irrigation practices, or if weather is expected to change to conditions unfavourable to the pathogen.

Class C Sites: Generally, turf diseases are not a concern.

Treatments

Although fungicides have been the most common treatment for most diseases, not all diseases can be controlled with fungicides. A few other controls are available, such as mowing as a mechanical control for red thread and biological control products that show promise for suppressing other pathogens.

Mechanical Controls

Mowing: Red thread (Laetisaria fuciformis) is a leaf pathogen that takes 10-14 days to develop after the grass is infected. Regular mowing (and removal of the clippings) during the period the pathogen is active can control it by removing the infected leaf blades before the disease develops. Recommendations for managing red thread usually include increasing nitrogen applications if the turf is deficient in nitrogen. This helps the turf grow out of the infection with successive mowing.

Biological Controls

Healthy soils, turf roots and thatch are home to many beneficial fungi, bacteria and other microorganisms that suppress pathogens in the soil. For example, take-all patch (Gaeumannomyces graminis) can be severe on newly established greens, but in 3-4 years symptoms are likely to disappear. It is thought this may be due to the build-up of disease suppressing microorganisms in the soil. These microorganisms may suppress pathogens by using up their food supply or making key nutrients unavailable. Others produce antibiotic compounds in the surrounding soil or may attack the pathogens directly. Researchers at Guelph are in the process of applying for registration for a promising product containing the fungus Typhula phacorrhiza for control of snow mold diseases caused by pathogenic Typhula spp.

Turf management practices that promote healthy conditions in the root zone also favour the native beneficial microorganisms. In contrast, broadcast applications of fungicides and other pesticides can wipe out these natural populations and leave turf open to infections. Research on ways to increase populations of beneficial microorganisms in turf has looked at using composts and commercial soil amendments containing beneficial organisms as a top dressing. For example, studies on control of brown patch (Rhizoctonia solani) and necrotic ring spot (Leptosphaeria korrae) have shown promising results using microbial amendments. This is an experimental area, however, and more work is needed to find out how well such products work under BC conditions.

Chemical Controls

As a group, fungicides differ from other pesticides in that most do not kill the target pest. Most fungicides work by preventing spores from germinating or by stopping the fungi from growing. They are used to stop the spread of diseases by preventing new turf from being infected. Fungicides for turf can be divided into two main groups:

Contact Fungicides: These work by providing a protective shield over the surface of the leaf that prevents infection. The disadvantage is that they do not provide long-lasting protection because they break down or wash away and are eventually mowed off as the grass grows.

Systemic Fungicides: These also provide a protective shield around the surface of the plant, but they are also absorbed into the plant and moved along to other tissues. They last longer than contact fungicides and provide protection from root and crown diseases that would not be reached with contact fungicides. The major drawback, however, has been the quick and widespread development of resistance to systemics as compared to contact fungicides. For example, resistance to the benzimidazole fungicides (e.g., benomyl, thiophanate methyl) can occur in dollar spot (Sclerotinia homoeocarpa) after just two or three seasons of use.

Managing Fungicide Resistance

Development of fungi with tolerance or resistance to fungicides is an increasing problem. Although the most serious problems have been with resistant strains of dollar spot, which is not (yet) a common problem in BC, the potential for developing resistance is present for other pathogens. Overuse of fungicides contributes to this problem. Popular strategies for managing fungicide resistance, including using higher application rates and more frequent applications have not been shown to be effective. In some cases, combining systemic and contact fungicides or alternating fungicides with different modes of action may delay resistance. Results have not been consistent, however, and more research is needed.

To delay or avoid the development of resistance, it is essential to use preventative (cultural) methods as much as possible and to only use fungicides when monitoring shows they are necessary, rather than routinely.

Choosing Fungicides

It cannot be emphasized enough that it is essential to correctly identify the disease to be able to choose effective fungicides. This is because modes of action and effects on fungi vary. Choosing the wrong fungicide may make matters worse. For example, using benomyl may increase the levels of Pythium infections rather than help control it. If a fungicide treatment is required, select the product according to the following guidelines. Make sure the product selected is:

effective and registered for the specific pathogen
chosen to minimize the risk of developing pesticide resistance
chosen to minimize the risk of damaging the turf or increasing infections from another pathogen
applied at the right time to provide protection or have the greatest effect on target species
applied only to those areas requiring treatment to limit the damage to disease suppressing micro-organisms in the soil

When an infection occurs that must be treated, the manager should re-evaluate the management program for the site to determine how to improve turf health and how to prevent the disease in the future.

For more precautions and guidelines for using pesticides in landscapes, see Chapter 11.

Evaluation

At suitable intervals after any type of treatment, use visual inspections or counting methods to assess the effect. Follow-up inspections include checking for:

reduction or elimination of disease symptoms
restoration of turf to normal vigour

Use this information, along with other monitoring records, observations from field staff, feedback from clients, budget records, etc. to assess the effectiveness of the turf IPM program and find ways to improve it for next year.