Landscape and Irrigation is read by decision makers throughout the landscape and irrigation markets — including contractors, landscape architects, professional grounds managers, and irrigation and water mgmt companies and reaches the entire spetrum.
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www.landscapeirrigation.com leaf thoroughly. One problem that turf managers often have is that they have only one sprayer, and setting it up for both herbicide and fungi- cide applications can be time consuming. The time it takes to refill a sprayer tank also has to be taken into consideration when deciding on spray volumes for fungicide applications over large areas, but the large volumes are on the label for a reason. Nozzle design also can have a large impact on the effectiveness of fungicide applications. In general, nozzles that produce many smaller droplets or droplets that are designed to shatter into many tiny droplets on impact (flat fan or air induction type nozzles) give better results than raindrop type nozzles designed to produce fewer, large droplets. How- ever, smaller droplets also drift much more easily. Air induction nozzles may offer the best combination of reduced drift and good coverage. Some fungicides can be absorbed into a leaf and diffuse around dif- ferent parts of a single leaf, but they do not enter a plant's vascular sys- tem and so cannot be transported from leaf to leaf. These are called local penetrant (sometimes also "locally systemic" or "translaminar") fungicides. Local penetrants, by entering a leaf and diffusing through it, reduce the need for absolutely perfect spray coverage, although they are not able to move down from a plant's leaves to the roots and so, like the true contact fungicides, are not effective against root diseases. With both true contacts and local penetrants, the recommended re- application intervals are relatively short, on the order of five days to two weeks depending on the individual product and disease pressure. New grass leaves that formed since the last application are not protected, and the fungicide coating can be susceptible to being washed off the leaves or degraded by sunlight. The tradeoff for a relatively short window of protection is that contact fungicides usually are the cheapest. A fungicide that is able to move throughout an entire plant is called systemic. Systemic fungicides are generally very useful in pre- ventive applications, because they are able to be absorbed by and re- main present in a plant for several weeks. Re-application intervals for these types of fungicides are generally in the two- to four-week range. However, most systemic fungicides can only move upward in a plant. If they are absorbed by the roots they will be moved to leaves, and they will move from lower leaves to newer leaves, but they will not move from leaves down to roots. The only exceptions to this rule are the phosphite fungicides. Because these fungicides are actively taken up by plants, they can be effective against root diseases — provided that there are actually roots there to absorb the fungicide. The problem with root diseases is that above-ground symptoms often don't appear until the root system is almost totally destroyed. In some cases, such as spring dead spot, symptoms don't appear until months after the initial infection. It is much, much better to use a preventive application than a curative ap- plication to fight root diseases in almost every case. It is very difficult to predict where a root disease will occur with- out complete and accurate records. With most root diseases, the fun- gus stays in the soil year after year and disease occurs in the same areas over and over again when the weather is favorable for the fun- gus. So keeping good records of a disease occurrence will allow the proper preventive applications to be made before the next outbreak. When making fungicide applications to prevent a root disease, re- member that the fungicide has to move down through the canopy, through the thatch and into the soil before a root can absorb it. Many times a preventive fungicide application will fail because it wasn't sprayed in enough water to wash it through the canopy (or it wasn't irrigated in after application), or because the fungicide be- came bound to organic matter in an excessively thick thatch layer. Again, following label directions for spray volume and irrigation and managing the thatch layer are critical factors in using fungicides suc- cessfully against root diseases. Even then, sometimes root diseases defy fungicides. Anyone who has ever fought a fairy ring knows how difficult it is to control with a fungicide. The hydrophobic soil fairy rings produce does not allow for good fungicide penetration. New formulations, wetting agents, aggressive aerification and the like can help, but sometimes very little seems to work. Mode of action and fungicide resistance Mode of action refers to the specific biochemical processes in a fungus that a fungicide interferes with in order to kill it, or at least stop it from growing. There are many different modes of action avail- able in the fungicides labeled for use on turfgrass. Some fungicides interrupt a fungus' cell division; some interfere with cell wall or cell membrane synthesis; some disrupt a fungus' ability to make DNA, RNA or proteins; some stop energy production; and some have more than one mode of action. One mode of action relatively new to the turfgrass market is the activation of plant defense responses. Chemicals that do this are not toxic to fungi, but they "fool" plants into activating their array of physical and biochemical responses to infection before they are ex- posed to a fungus. This, in turn, boosts the plant's ability to resist in- fection and reduces disease incidence and severity. As noted above, however, this only works if the defense response activator is applied before any infection takes place. This type of fungicide does not work as a curative application. Landscape and Irrigation 27 Dead patches of ryegrass on overseeded baseball field that won't grow back until the bermuda greens up.