Culture clash

Alter the irrigation schedule to promote the recovery of damaged turfs. Lightly irrigate them daily and implement a midday syringe cycle to help reduce heat stress.

As a turfgrass manager, you implement a broad range of cultural practices to maintain turf. While these practices are designed to directly affect the turf, most of them have some impact on disease development. You must understand these impacts to successfully engage in integrated pest management.

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To get the most out of cultural practices, you must first understand their impacts. After you understand the impacts, you can determine how profound the impact will be on your site. If you closely monitor the effects of these practices, you will put yourself in position to use all of the tools available for disease control. After all, you perform most of these practices anyway; why not use them to their fullest?

Fertility

Nutrition is vital to turfgrass health. Therefore, fertilizer applications are one of the most common cultural practices. Nitrogen and potassium often are the two most limiting elements for healthy turfgrass growth. While it is important for you to manage these nutrients to grow turf, you must also be aware of the impact they can have on turfgrass' susceptibility to disease.

  • Nitrogen. Nitrogen plays an important role in plant color, density and growth. It also requires a balancing act to use it for disease management. If you do not apply enough nitrogen, your turf may experience an increase in foliar diseases such as dollar spot, red thread, rusts, anthracnose and melting out. It may also experience an increased susceptibility to the patch diseases of necrotic ring spot and summer patch. Even cultivars that are genetically resistant to diseases such as necrotic ring spot or melting out may be susceptible if you do not apply enough nitrogen.

    However, if you apply too much nitrogen, succulent growth occurs. When this happens, turfgrasses become more susceptible to pythium blight, brown patch, stripe smut, microdochium patch (pink snow mold) and typhula blight (gray snow mold). In order to use nitrogen to affect disease, you must anticipate the time of the year certain diseases occur. For instance, some of the patch diseases are prevalent in summer. Summer applications of N, which cause succulent growth, may contribute to outbreaks.

    Your nitrogen source can also have a significant impact on turfgrass diseases. If you use organic nitrogen sources, you may suppress turfgrass diseases such as necrotic ring spot, summer patch, dollar spot, brown patch, pythium root rot and typhula blight. While slow-release nitrogen produces the most consistent turfgrass growth rates over time, water-soluble ammonium sulfate can reduce the severity of several diseases including summer patch, take-all patch and spring dead spot of bermudagrass. Use quick-release nitrogen formulations to promote rapid recovery from turfgrass disease damage. However, keep in mind that it is possible for succulent growth to occur with these nitrogen sources.

  • Potassium. Potassium is second only to nitrogen in the amount required for turfgrass growth. Potassium helps increase resistance to heat stress, drought stress and winter injury. Some growers also believe that potassium improves disease resistance. For instance, potassium applications may protect bermudagrass from leaf spots and microdochium patch in potassium-depleted soils. These stress and disease characteristics have prompted some growers to increase potassium fertility to a 1:1 ratio with nitrogen to protect against stress and disease. However, be careful; when you increase potassium relative to nitrogen, you may increase your turfgrass' susceptibility to brown patch and increase the severity of dollar spot.

Mowing

Mowing is the most intensive cultural practice that you will perform on turfgrasses. Several aspects of mowing have an impact on turfgrass diseases, and you can manage each one of them.

  • Height of cut. The first aspect of mowing of which you should be aware is height of cut. If you cut turf below its preferred mowing height, the plants' carbohydrate supplies become depleted. This will reduce root growth and result in smaller, shallower root systems. Eventually, the plant may become more susceptible to environmental stresses. Root health also impacts turfgrass' susceptibility to patch diseases, anthracnose, rusts, dollar spot, melting-out and leaf spots. However, if your height of cut is excessively tall, moisture will be prevalent in the grass canopy. This environment can be favorable for diseases like pythium blight, brown patch and typhula blight.

  • Quality of cut. When using dull mower blades, you cause excessive injury to plants by tearing and bruising tissues. This can have an obvious impact on aesthetics and stress, but what is its impact on turfgrass disease? Researchers have found that the incidence of dollar spot on bentgrass can be 10 percent greater when dull mower blades are used. While this is a single instance of the effect of mowing on disease, it may apply to other foliar diseases.

  • Time of day. When you mow also has a profound impact on turfgrass diseases. When you mow early in the morning, not only do you prepare your golf course for daily play, you also remove dew from the foliage and reduce the duration of leaf wetness. Most foliar fungal diseases need this moisture. The longer the foliage is wet, the greater the opportunity for infection. Also, about 30 percent of the “dew” that accumulates on turfgrass plants is actually guttation fluid (produced by the plant). These fluids contain nutrients that can promote the development of dollar spot and other foliar diseases. Studies show that morning mowing can reduce dollar spot by as much as 58 percent on golf-course fairways.

While removing dew is a benefit, you are faced with certain risks. Pythium blight, microdochium patch and, to an extent, dollar spot, are aggressive. If you mow when these diseases are actively infecting turf, you can spread the pathogen and cause streaking along the mowing pattern.

Topdressing

The role of sand topdressing in disease development is not as clearly defined as the role of other cultural practices. However, some evidence suggests that sand topdressing may be associated with the development of crown-rotting anthracnose and bacterial wilts. Sand does not contain much organic matter and, therefore, will not support as large of a microbial population as organic topdressings. These other non-pathogenic microbes compete for nutrients and lessen disease severity by limiting the growth of pathogens. Also, these microbes produce antifungal compounds that can inhibit the growth of pathogenic fungi. Frequent sand topdressing also can cause greater wounding of the turfgrass crown and thus increase stress and create openings for pathogens to enter the plant.

Thatch management

An excessive thatch layer does not cause disease; rather, it creates an environment favorable to disease development. Turfgrasses often develop vigorous roots in thatch layers. However, the roots are prone to drought stress because they lack the protective cover of soil. Increased drought sensitivity reduces root growth and allows these plants to be exploited by necrotic ring spot, summer patch, stripe smuts, melting out and rusts. To manage thatch, frequently core aerate and topdress where applicable. These practices also will reduce soil compaction, which restricts root systems and inhibits soil drainage. These conditions can lead to pythium root rot, anthracnose and may induce the formation of black layer.

Irrigation

The obvious value of irrigation is to prevent drought stress. However, the manner in which you irrigate also has a significant impact on the management of turfgrass diseases. Drought-stressed plants are more susceptible to injury by necrotic ring spot, dollar spot, rusts, stripe smut and melting-out. On the other hand, over-watered plants can have other disease problems.

  • When to irrigate. If you irrigate in the evening, you increase the duration of leaf wetness and your turfgrass' susceptibility to disease. A moist environment promotes fungal spore germination and growth, and increases the occurrence and severity of disease. When you irrigate in the early morning or in the afternoon, you reduce the duration of leaf wetness. This limits environmental conditions favorable for the development of turfgrass disease. Early morning irrigation has the added benefit of removing guttational fluids from the leaf surface, thus limiting the availability of the added nutrients to turfgrass pathogens. Afternoon irrigation also cools the turf and thereby reduces heat stress.

  • How much water to apply. The amount of water you apply during each irrigation cycle and the frequency of your irrigations can also have a significant impact on turfgrass diseases. Overwatering is as much of a problem, if not more so, than underwatering. Schedule your irrigation run time and frequency to replace soil moisture that is lost to evaporation and transpiration. If you irrigate excessively, you will promote waterlogging. This results in poorly developed root systems and a greater likelihood of necrotic ring spot, summer patch, anthracnose and black layer.

Alter the irrigation schedule to promote the recovery of turfgrasses with reduced root systems caused by patch diseases. Lightly irrigate these turfs daily and implement a midday syringe cycle to help reduce heat stress.

Plant growth regulators

Plant growth regulators (PGRs) have a remarkable effect on the growth and playability of turfgrasses. While they have become indispensable tools for many situations, you must be aware of their effect on turfgrass and diseases. The main concern with PGR-treated turfgrasses is that the reduced growth of treated turf may limit its recovery time and extend the duration of visible symptoms. While PGRs are not a significant cause of disease development, give PGR-treated turf added attention to prevent the occurrence of foliar diseases.

Just about every cultural practice used to manage turfgrasses can have some impact on the development of disease. Whether you are managing a golf course, athletic field or residential or commercial lawn, you have the opportunity to positively affect disease incidence. Your role as turfgrass manager is to consider the possible effects of these cultural practices on turfgrass disease and integrate them into an overall management program. By recognizing all of the factors attributing to the condition of the turfs you maintain, you will be one step ahead of your disease problems.

Dr. Jon F. Powell is an assistant professor of plant pathology at the University of Minnesota (St. Paul, Minn.).

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