By Dr. Jesse Benelli
From the August 2023 Issue
Turfgrass is an integral component of many landscapes. However, the prospect of battling disease makes maintaining healthy turfgrass a challenge. Plant diseases are caused by various types of pathogens including fungal, bacterial, or viral organisms. In turfgrass, most diseases are caused by fungal or fungal-like organisms. Damage from turfgrass disease can range from mild (superficial damage) to severe (complete loss of the turfgrass stand). Understanding when to implement cultural and chemical management strategies can save your operation time, labor, and money.
Understanding when to implement cultural and chemical management strategies can save your operation time, labor, and money.
The Disease Triangle
The disease triangle is a model used to explain the development of plant disease. It illustrates the interrelationship between three main factors that contribute to the establishment and progression of disease. These factors are:
- Host. The host refers to the plant species or variety that is susceptible to the pathogen. Different plants have varying levels of susceptibility to specific pathogens. Healthy and stressed plants may also respond differently to pathogen invasion.
- Pathogen. The pathogen represents the disease-causing organism. Pathogens have specific requirements for growth, including favorable environmental conditions and a susceptible host.
- Environment. The environment encompasses various abiotic factors that affect the development and severity of the disease. The primary factors include temperature, humidity, and soil moisture.
The disease triangle emphasizes that all three factors must be present for disease to occur. This is an important concept since we can use the triangle to not only help confirm the presence of a disease, but also to rule out certain diseases or disease all together. Believe it or not, in most turfgrass disease diagnostic labs the ‘no disease’ diagnosis is normally the most common.
Innocent Until Proven Guilty
Not everything that negatively affects turfgrass health or causes abnormalities in its appearance is necessarily a turfgrass disease. Some common non-disease factors that can affect turfgrass include:
- Nutrient deficiencies or imbalances. Inadequate or excessive levels of nutrients can result in yellowing, discoloration, or stunted growth.
- Improper watering or irrigation practices. Overwatering or underwatering can have detrimental effects on turfgrass. Excessive moisture can lead to root rot or shallow root development, while insufficient water can cause wilting and dormancy.
- Compaction and poor soil conditions. Soil compaction from heavy foot traffic or machinery can restrict root growth and reduced vigor.
- Physical damage. Turfgrass can be damaged by factors like improper mowing techniques, mechanical injury, sports activities, and animal activity.
- Environmental stress. Extreme temperatures, drought, frost, excessive heat, or cold can stress turfgrass and cause a decline in turfgrass appearance and quality.
When diagnosing turfgrass issues, it is essential to consider all possible factors and not solely focus on disease. Understanding the specific symptoms, observing environmental conditions, considering maintenance practices, and ruling out non-disease causes can help determine the underlying problem accurately and guide appropriate management strategies.
Signs & Symptoms Of Disease
In the context of turfgrass diseases, signs and symptoms refer to different aspects of the disease and can be distinguished as follows:
Signs are the visible evidence of the disease-causing agent. In the case of turfgrass diseases, the most common sign that we readily see is the presence of fungal mycelium on the leaf surface. Diseases such as brown patch and dollar spot are known to produce abundant aerial mycelium during the nighttime/morning hours when active.
Symptoms, on the other hand, are the visible effects or responses of the turfgrass to the pathogen. The most common symptoms of turfgrass disease may include:
- Discoloration. The turfgrass may exhibit patches or areas that are yellow, brown, or off-color compared to the healthy surrounding turf.
- Leaf spots or lesions. Small or large spots, blotches, or lesions may appear on the turfgrass blades.
- Patches or rings. Circular or irregularly shaped areas of abnormal turfgrass growth or decline may develop.
Recognizing signs and symptoms is crucial in successfully diagnosing turfgrass disease. If you are unsure, it is always recommended to send a sample to your regional diagnostic clinic for confirmation. The diagnosis is the first step in implementing effective management strategies.
Cultural Management Strategies
Managing turf disease in residential lawns requires a combination of cultural practices aimed at creating a healthy and resistant turfgrass environment. Here are some cultural management strategies you can implement to prevent and control turf disease:
Watering. The best time to irrigate home lawns is during the early morning hours. This is when winds are calm and the turf would already be wet because of dew formation. Watering in the evening can extend the duration of leaf wetness which extends the infection window. It’s also important to turn off automatic irrigation systems after significant rainfall events since excessive soil moisture can contribute to unhealthy turf and give rise to root rot.
Fertilization. It is important to maintain adequate nitrogen fertility throughout the Summer months. Recent research has shown that applying monthly applications of nitrogen during June, July, and August at 0.5 lb of nitrogen fertilizer does not enhance brown patch severity and may help the turf grow out of existing symptoms. Similar findings were observed with large patch disease on zoysiagrass, where research showed that Spring and Fall nitrogen fertilizer applications did not enhance disease severity.
Thatch management. Thatch accumulation can have a significant influence on turfgrass disease development. While a thin thatch layer can provide some benefits, excessive thatch can create an environment conducive to disease. Core aeration during the spring and fall on cool-season grasses is a great time to manage thatch. On warm-season grasses the best time to manage thatch is during late-Spring through early Summer.
Plant genetics. Many of the newer cultivars of various turfgrass species have been developed for improved disease resistance. Overseeding these new cultivars into your existing lawn will help keep diseases at bay.
Promoting sunlight and air movement. Selective pruning of trees and shrubs will help improve air circulation to help reduce the duration of leaf wetness.
Fungicides are an effective tool in controlling turf diseases in lawns, but it’s important to note that their use should be part of an integrated approach that includes cultural practices and proper disease management.
Fungicide applications perform much better when used on a preventative schedule compared to curative applications. Applying fungicides preventively will help reduce the fungal inoculum. Curative applications will typically require higher product rates and shortened application intervals.
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It is important to know where exactly the fungicide needs to be when making an application. If foliar disease control is desired (such as for brown patch, dollar spot, or leaf spot) then the application should be made at a time where it can dry on the leaf surface. Conversely, if you are targeting a root-affecting disease (such as summer patch, necrotic ring spot, or take-all root rot) then applications should driven into the soil. This can be done by using higher carrier volume (>4 gallons per 1000 square feet) or by watering the turf immediately after application with at least 0.10” of irrigation.
Dr. Benelli is based in Greenville, SC, and is the regional Green Solutions Specialist for Envu covering the Southeast region. He is a graduate of the Pennsylvania State University Turfgrass Program and received his MS and PhD degrees from the University of Tennessee. Prior to joining Envu, Dr. Benelli was the Director of Turfgrass Programs for the Chicago District Golf Association, where he provided agronomic consulting with more than 400 member clubs throughout the region.
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