Nutrients affect turfgrass diseases
Turfgrasses need certain nutrients in the right amounts to grow properly and maintain good health. When these nutrients are deficient or present in excess amounts, diseases can gain an advantage and seriously injure plants. Although well-designed fertility programs by themselves rarely provide complete control of turfgrass diseases, they can reduce or delay the need for fungicides and lessen the severity of outbreaks. Here is an overview of how your fertility program can affect certain turfgrass diseases.
Turfgrasses need at least 16 nutrients for normal growth and development. They require some nutrients in large amounts and others only in minute quantities. Regardless of the amount required, a deficiency of any essential nutrient limits growth and reduces the overall disease resistance of your turf. Thus, a deficiency of calcium can be just as detrimental to your turf as a lack of nitrogen, even though turfgrasses use more nitrogen than calcium. With that understood, realize that some nutrients have a more direct influence on turfgrass diseases than others. Let's look at some specific examples of how nutrients affect disease susceptibility.
Nitrogen-the most influential nutrient Of all the mineral nutrients, nitrogen (N) has the greatest influence on turfgrass diseases. Perhaps the main reason for this is that N affects nearly all aspects of plant resistance to fungal diseases. Plants use N to help manufacture anti-fungal compounds (phenolics, flavanoids, lignins and others) that protect them against invasion by pathogens. In some cases, N enables the plant to produce tissue faster than the pathogen can destroy it. In these cases, the plant is said to "outgrow" the pathogen. N also helps to stimulate activity of microorganisms that are antagonistic to pathogens.
Too much N, conversely, can increase the severity of some diseases. Rapidly growing young tissues resulting from excess N often have less resistance to invasion by certain fungi. Also, too much N leads to high concentrations of sugars and amino acids in leaves and leaf exudates, aiding spore germination and growth of some pathogens. The effects of N fertilization on turfgrass disease depend on the particular disease you are trying to manage. Some diseases are more pronounced in N-deficient plants. These include red thread, pink patch, dollar spot, anthracnose and rust. Others become more severe when N is excessive, especially during high humidity. Such diseases include brown patch, Pythium blight, Fusarium patch and leaf-spot/melting-out diseases. Keep in mind that these relationships are not always clear-cut. For example, leaf-spot/melting-out diseases sometimes are as severe in under-fertilized turf as they are when you've applied too much N.
* Low-nitrogen diseases. Generally, when you supply N to cool-season turfgrasses in the correct amounts at the proper times (spring, late summer and late fall), turfgrasses are better able to resist the low-N diseases. Mid- to late-spring applications of N at rates of 1.0 to 1.5 pounds per 1,000 square feet can help to reduce red-thread and pink-patch diseases, which often damage ryegrass, fescue and Kentucky-bluegrass lawns in the northeastern United States. Typically, these rates are sufficient to maintain good color and growth into June, when red thread and pink patch are most active. Using some slowly available N during spring may help to extend turf color and growth into mid-summer and keep anthracnose from becoming a serious problem.
As days become shorter and nights cooler in late summer (late August to mid-September), cool-season turf-grasses begin to perk up, provided that sufficient moisture is available. In most years, this is an ideal time to apply fertilizer because N will help the plant manufacture food reserves it needs for winter and spring, as well as speed recovery from injury sustained during summer. Because late summer is the peak season for dollar spot and rust-two low-N diseases-the N applications should help to reduce disease injury and the need for fungicides.
Turf managers typically make late-fall fertilizer applications on dormant turf before soil freezes. Because turfgrass diseases are not usually active at this time of year, you won't see any immediate benefits in disease suppression. However, late-fall applications can have a beneficial effect on early spring diseases such as leaf spot/melting out because they reduce the need for early spring N applications, which can overstimulate turf and increase disease susceptibility.
* High-nitrogen diseases. Whereas mid- to late-spring fertilizer applications generally are helpful to turf, high rates (greater than 1.5 pounds N per 1,000 square feet) in early spring may force too much leaf growth and deplete food reserves that the turf will need later in the season to help fend off diseases. Too much N fertilizer at this time also provides a rich food source for fungi that cause leaf-spot/melting-out disease of Kentucky bluegrass and Fusarium patch on golf turf.
Large doses of N in summer can lead to increased development and severity of brown patch and Pythium blight, two hot- and humid-weather diseases. Because both these are high-N diseases, you should fertilize minimally or not at all during the hot and humid conditions of summer.
Natural-organic nitrogen sources In recent years, fertilizers made from plant or animal by-products-the so-called "natural-organic" fertilizers-have made a comeback in the turf market. This is partially due to their improved N-release properties. Another reason for the success of natural organics is that some of them suppress certain turfgrass diseases.
Research at several universities has shown that certain natural-organic fertilizers dramatically suppress dollar spot on creeping-bentgrass putting greens. Some researchers suggest that these fertilizers promote the buildup of microbes that compete with or antagonize the dollar-spot pathogen. The results of these studies are so encouraging that research is now focusing on the synthetic-organic (urea and urea-based fertilizers) and inorganic N sources to determine if they can provide similar dollar-spot suppression.
Recently, researchers at The Pennsylvania State University set out to determine if natural-organic fertilizers were superior to synthetic-organic N sources for dollar-spot suppression. Three years of field tests revealed that urea, a synthetic-organic N source, provided equal or better control of dollar spot than five commercial fertilizers derived mostly from natural-organic sources. Of the natural-organic N sources, those that provided the fastest release of N gave the best disease suppression. During most of the test, disease was less severe on turf showing the darkest green color-a common turfgrass response to N uptake.
This research showed that N protects turfgrass plants against dollar spot, regardless of whether it comes from urea or natural-organic fertilizers. However, it does not necessarily mean that this protection is only due to N's influence on plant resistance (production of anti-fungal compounds and ability to outgrow the pathogen). Some of the N from fertilizer also is available to microbes in the foliage, soil and thatch and can help to increase their populations. These microbes may have a direct role in suppressing diseases such as dollar spot. Recent studies from the University of Guelph (Ontario, Canada) confirm that ammonium nitrate (an inorganic N source) and several natural-organic fertilizers increase bacteria and fungi in the turf/soil system, and these microorganisms may have a role in suppressing dollar spot.
The question of how much disease suppression comes from increased plant resistance vs. microbial suppression is academic. The bottom line is that N helps to protect turfgrasses from dollar spot. For best suppression, try to time N release from fertilizers with peak dollar-spot periods, but be careful not to overdo it.
Ammonium fertilizers-remedies for root diseases Take-all patch is a root disease that has plagued creeping-bentgrass fairways and greens for decades. Because fungicides are only moderately effective in controlling this disease, golf-course superintendents often rely on cultural methods for take-all-patch management. One cultural practice that has been effective in suppressing take-all patch is the application of ammonium-containing fertilizers, especially ammonium sulfate. The main reason for the success of ammonium sulfate appears to be that it makes soils more acidic-a condition that inhibits this disease.
For years, researchers have tried to determine why acidifying soils with ammonium fertilizers suppresses take-all patch. Although theories abound, most have not stood up to scientific scrutiny. However, a new theory is gaining acceptance among plant pathologists and involves an unlikely candidate-themicronutrient manganese.
After years of researching the influence of nutrients on take-all of wheat (essentially the same disease that affects creeping bentgrass), scientists at Purdue University proposed that acidifying the soil surrounding roots promotes the buildup of microbes that convert manganese to a form that plants can take-up and use while, at the same time, suppressing microbes that prevent manganese uptake by plants. Manganese is involved in the synthesis of compounds that protect grasses from take-all disease and also affects root development. The scientists say that in high-pH soils (especially soils that have been recently limed) the microbes that prevent manganese uptake are more prevalent than those that convert manganese to a form that plants can use. Hence, bentgrasses may suffer from a manganese deficiency under high-pH conditions even though this micronutrient may be plentiful in the soil.
If the manganese theory is true, all a superintendent would have to do to prevent take-all patch from causing problems on bentgrass is apply a manganese-containing fertilizer. The Purdue scientists suggest that this approach may not be that simple. Applications of manganese to high-pH soils are not effective in disease suppression because most of it is in the form that can't be taken up by roots. Foliar sprays can correct manganese deficiencies to grass foliage but most does not move into roots where take-all-patch infection occurs. Frequent applications of ammonium sulfate at low rates is the best cultural approach to managing take-all patch because this fertilizer will lower soil pH and allow plants to take up manganese.
Researchers at Rutgers University have used the concept of soil acidification with ammonium fertilizers to control summer patch, a serious root disease of Kentucky bluegrass and annual bluegrass in the Transition Zone and the northeastern United States. They found that ammonium sulfate reduced summer patch in Kentucky bluegrass up to 80 percent compared to nitrate sources of N using rates of 4 pounds of N per 1,000 square feet, split into several applications over the course of a growing season. Treatments of ammonium sulfate reduced the soil pH from 6.6 to 5.8 over a 2-year period. Although Kentucky bluegrass is best adapted to soil-pH values between 6.0 and 6.5, acidification may be justified if summer patch is starting to cause problems in your turf. Be careful not to reduce soil pH to below 5.5-such a highly acidic condition can lead to phosphorus deficiency and aluminum toxicity in plant roots.
Other nutrients Compared to N, other turfgrass nutrients have a more subtle influence on diseases. Because their effects on disease are not as dramatic, researchers do not study these other nutrients as much. Despite our lack of detailed knowledge, we can make some generalizations about some of these nutrients and their effects on turfgrass diseases.
* Potassium. On occasion, applications of potassium fertilizers suppress dollar spot, leaf spot, take-all patch and stripe smut. Although our understanding of the conditions under which this occurs is sketchy, the susceptible plants presumably are in need of this nutrient. Because potassium regulates many plant functions-including cell-wall thickness and the plant's water content-it is easy to see why a deficiency can lead to weaker and more disease-prone plants.
* Calcium. Turf managers usually apply calcium to turf as lime (calcium carbonate) to raise soil pH. However, calcium also is an essential plant nutrient that plays a crucial role in cell division and cell-wall formation. The turfgrass disease that appears to be most influenced by calcium is Pythium blight. Pythium species possess pectolytic enzymes that dissolve plants' cell walls, causing a rapid disintegration of leaf tissues. Calcium inhibits the activities of pectolytic enzymes, thereby protecting cell walls. This does not mean that calcium controls Pythium blight. However, you may reduce its severity by avoiding a calcium deficiency.
If you apply too much calcium (in the form of lime) to take-all-patch-infested bentgrass, it can increase the amount and severity of this disease. Do not apply materials that raise soil pH when take-all patch is a problem.
* Sulfur. In some regions of the Northwest, sulfur applications show dramatic effects on turfgrass diseases. In certain cases, applications have provided significant control of Fusarium patch and take-all patch. While sulfur applications may provide take-all-patch suppression in other regions of the country, ammonium sulfate usually is more effective.
* Micronutrients. For years, turf managers have used micronutrients to enhance turf color and, to some extent, growth. Now, studies are shedding more light on their disease-suppressive properties. Research at The Pennsylvania State University has revealed that applications of two micronutrients, zinc and manganese, can lessen the severity of dollar spot and red leaf spot when these diseases are present at low levels in creeping bentgrass. Although more research is needed to determine the exact conditions under which this occurs, it does provide golf-course superintendents with another tool to manage these troublesome diseases.
Until more-precise information becomes available on disease management with nutrients (other than N), the best approach is to periodically conduct soil or tissue tests and fertilize according to test recommendations. Soil and tissue testing should give you enough information to avoid nutrient deficiencies. Avoiding nutrient deficiencies results in stronger, more disease-resistant plants and helps reduce the need for fungicide applications and turf repairs.
Dr. Peter Landschoot is associate professor of turfgrass science at The Pennsylvania State University (University Park, Pa.).
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