Vive la resistance!

Turfgrass breeders have made real progress in improving disease resistance over the years and will continue to do so in the future.

Every year, golf course mangers spend millions of dollars on fungicides to control turfgrass diseases. This has made golf courses targets of individuals and groups concerned about pesticide use in the urban environment. Consequently, golf course managers are searching for alternative means of controlling diseases. In response, turfgrass breeders have intensified their efforts to find turfgrasses with improved disease resistance. The following is a brief overview of disease resistance in turfgrasses, with emphasis on how to choose the most resistant grasses for your golf course.

Sorting out disease resistance in turfgrasses

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More than ever, golf course managers are barraged with information from seed companies and consultants about the strengths and weaknesses of the new turfgrasses for golf courses. In addition to reports of improved turf density, growth habit, mowing tolerance and wear tolerance are claims of resistance to one or more diseases. Given the number of new cultivars on the market and the differing opinions from experts regarding those cultivars, sorting out claims of disease resistance is a formidable task. This is mostly because the word “resistance” can have different meanings to different people. Varying degrees of disease resistance occur in turfgrasses, ranging from highly resistant to extremely susceptible. Whereas one person may consider a cultivar “moderately-resistant” to a disease, another could view it as “somewhat susceptible.” Understand that complete resistance — in which a cultivar does not get a disease known to occur on other cultivars of the same species — probably does not exist in turfgrasses.

Ideally, a breeder should be able to place a cultivar's level of resistance to a disease into a discreet category such as high, moderately high, moderate or low, and then relate that category to the amount of damage you can expect. Unfortunately, no such system exists, and claims of “high” or “moderate” disease resistance do not have much value. Without a meaningful system of categorizing resistance, we can only say that disease resistance is relative. This means that a cultivar's level of resistance is determined by comparing its reaction to a particular disease with that of other known cultivars growing under the same conditions at the same site. In fact, such comparisons are made routinely by planting numerous cultivars in small plots and rating disease when differences in disease reaction among cultivars become apparent. If enough tests are done in this manner, we can get an idea of a cultivar's resistance relative to other known cultivars. Such tests are regularly conducted at universities through the National Turfgrass Evaluation Program (NTEP) and by other public and private research programs.

The NTEP, in cooperation with seed companies and universities, coordinates evaluations of hundreds of experimental turfgrass selections and commercial cultivars at numerous locations across the United States and Canada. They evaluate the same turfgrasses at each location, so that they can offer a breeder information on cultivar performance over different environments and a golf course manger information on performance in his or her area. If a disease occurs in one of these tests, the evaluator determines the amount of turf injury on each experimental selection and cultivar caused by the disease. These data, along with data on other aspects of turf performance, are compiled, analyzed and reported by the NTEP each year. Realize that the NTEP system is not perfect. For some diseases, data may be highly variable, especially among locations. Disease data from certain selections and cultivars may vary from one year to the next at the same location. Also, data may be unavailable for certain diseases because it did not occur at any of the test sites. Despite these limitations, the NTEP is about the only means of obtaining unbiased, comparative information on relative disease resistance of commercial turfgrass cultivars.

The race card

When gathering disease-resistance information from NTEP or other cultivar tests, don't draw conclusions prematurely based on data from just one location or from only one year. The reaction of a cultivar to a disease may vary among different environments, management conditions and, possibly, strains or races of the pathogen. Turfgrass pathogens frequently have different races that affect cultivars differently. For instance, race ‘1’ of a fungal pathogen may predominate at one location and cause more injury on cultivar ‘A’ than cultivar ‘B’. At another location, race ‘2’ of the pathogen may be more common and cause more injury on cultivar ‘B’ than cultivar ‘A’. For this reason, disease data from different locations will often vary. In some cases, more than one race may be present at the same location and data may vary between successive ratings. When searching for disease-resistant cultivars, try to determine if resistance trends among cultivars are consistent at two or three of the test locations nearest to you. Also, make sure that the disease data from these locations is similar over the duration of the test. Although there is no guarantee that a new race of the pathogen will not overcome a cultivar's resistance to a disease, you can at least avoid choosing cultivars that are known to be susceptible to the disease pathogens in your area.

Don't forget the other diseases

Another key point to consider when assessing a cultivar's resistance to a particular disease is its reaction to other diseases. Just because a cultivar has good resistance to one disease, it does not mean it has good resistance to all diseases. Colonial bentgrasses, for example, usually show excellent resistance (relative to the creeping bentgrasses) to dollar spot and gray snow mold. However, a quick glance through recent NTEP data reveals that, in many regions of the United States, colonial bentgrasses are highly susceptible to brown patch. Most grasses have special strengths and weaknesses to certain diseases — it's up to you to sort them out.

Creeping bentgrass and dollar spot

Dollar spot is the most common disease of creeping bentgrass throughout much of the northeastern and midwestern United States. Golf course mangers spend more money to control this disease than any other. Although most control measures focus on cultural practices and fungicides, progress is being made towards dollar-spot resistance in some new cultivars.

Among the commercially available creeping bentgrasses that were evaluated in the most recently completed NTEP putting green test, L-93, Penn A-1 and Pennlinks showed the best dollar-spot resistance. This is based on an the average of data collected from 11 locations ranging from Texas to Rhode Island over a period of 4 years. Although the data varied somewhat among locations and years, the trends were remarkably consistent. This appears to indicate that differences in disease resistance among certain cultivars are relatively stable over many environments, management conditions and, perhaps, among different races of the pathogen. Even though occasional fungicide applications are required on L-93 and Penn A-1 greens (resistance is not complete), fewer applications presumably are required compared with some of the more susceptible cultivars.

Crenshaw and 18th Green are two cultivars that are susceptible to dollar spot relative to L-93 and Penn A-1. However, they were developed primarily for use in regions where dollar spot is not as much of a concern. Crenshaw tends to perform best in southern regions of the United States where good heat tolerance is needed. 18th Green has excellent resistance to gray snow mold, a significant problem on bentgrasses in western Canada. As new bentgrass cultivars enter the turf market, be aware of their strengths and weaknesses so that you choose the grass that is best adapted to the environment, management conditions and diseases that prevail at your course.

Kentucky bluegrass and leaf-spot resistance

Forty years ago, Kentucky bluegrass was a dominant fairway grass in the northern United States. Over the ensuing decades, the popularity of Kentucky bluegrass waned and golf course managers began to use more creeping bentgrass and perennial ryegrass. One of the reasons for the demise of Kentucky bluegrass on fairways was its susceptibility to leaf-spot diseases caused by Drechslera poae or Bipolaris sorokiniana.

The development of Merion Kentucky bluegrass set a new standard among Kentucky bluegrasses for resistance to leaf-spot diseases. As a result, it was planted extensively in the northeastern United States during the 1960s and 70s. Although Merion was a big success at first, it eventually lost favor among turf managers due to problems with another disease: stripe smut.

Over the past two decades, breeders have further improved leaf-spot resistance of Kentucky bluegrass (and also stripe-smut resistance). Without these improvements, Kentucky bluegrass would not be considered for use on golf courses today. Leaf-spot resistance, coupled with improvements in density, close-mowing tolerance and low, compact growth habits, have allowed Kentucky bluegrass to make a resurgence as a fairway turf in several regions of the United States.

Despite the improved leaf-spot resistance in many Kentucky bluegrass cultivars, there are still many commercial cultivars with poor resistance. Therefore, be sure to consult turf disease experts or NTEP results before choosing cultivars for fairways use.

A new challenge: Finding gray leaf spot resistance in perennial ryegrass

Perennial ryegrass has gained acceptance as a fairway grass in many northern regions of the country over the past 20 years. During this time, many improvements were made, including better tolerance to mowing, herbicides and winter injury. Although breeders have made some improvements in disease resistance, perennial ryegrass is still considered one of the more susceptible species to brown patch, Pythium blight and dollar spot. Because bentgrasses are also susceptible to these diseases, there hasn't been a lot of incentive for golf course managers to convert from ryegrass to bentgrass (at least from a disease-management perspective). A newly recognized disease of perennial ryegrass called gray leaf spot may change all of that.

Gray leaf spot (caused by Pyricularia grisea) was first recognized as a serious problem in perennial ryegrass fairways in the early 1990s. Although it does not cause major damage every year, when epidemics do occur, the result can be devastating. Epidemics in 1995 and 1998 caused severe damage to perennial ryegrass fairways from the mid-Atlantic region to New England and in several Midwestern states. At least three university research programs are engaged in a search for gray leaf spot resistance in perennial ryegrass. Although there are some hopeful signs of success, it will take much more work before resistant cultivars reach the market.

In the meantime, many golf course managers are converting fairways from ryegrass to bentgrass or at least considering this option. For many, gray leaf spot is the last straw with respect to disease problems on perennial ryegrass. If this species is going to continue as a dominant fairway grass, breeders and plant pathologists will have to meet the challenge of finding resistance to gray leaf spot and other diseases — soon.

A work in progress

Turfgrass breeders have made real progress in improving disease resistance over the years and will continue to do so in the future. You probably will not see a turfgrass with complete resistance to all of the major diseases in your lifetime, but you should be able to find cultivars with good resistance to certain troublesome diseases. When choosing species and cultivars for a new establishment or an overseeding program, make sure you look for disease-resistance data and whether the cultivar is well-adapted to the environmental and management conditions at your course. Also, consider using mixes and blends to create a more genetically diverse stand.

Dr. Peter Landschoot is associate professor of turfgrass science at The Pennsylvania State University (University Park, Pa.).

BLENDS AND MIXES: INCREASING THE ODDS OF SURVIVAL THROUGH DIVERSITY

A basic concept in disease management is to use blends of different cultivars or mixes of different species when planting turfgrasses. This practice reduces the chances of a severe outbreak of a disease that is particularly damaging to a certain cultivar or species. In theory, by increasing genetic diversity, the odds of the entire sward withstanding a disease attack are improved.

Using blends and mixes is especially important if you plan to establish cultivars with a narrow genetic base (when all or most plants are genetically alike), such as Kentucky bluegrass cultivars. Many Kentucky bluegrasses produce a high percentage of seed through a process called apomixis. This type of asexual reproduction results in viable seed production without pollen from other plants. It produces seed that is genetically identical to the mother plant. Exclusively using this apomictic seed for a new establishment results in a very uniform stand. The stand may also be a ticking time bomb — waiting to be attacked by a pathogen that has a particular taste for that cultivar.

For turfgrass cultivars with a broader genetic base than Kentucky bluegrass (i.e., perennial ryegrass, fescues and bentgrasses), blending and mixing is probably not as critical for reducing disease. However, using blends and mixes of these grasses is still considered an important disease-management strategy that can yield benefits in reduced damage and fungicide costs over time.

Blending of creeping bentgrass cultivars for golf course plantings is increasing in popularity. Part of the reason for this is that there are now more cultivars to choose from, along with increased availability of packaged blends. Whereas blending is commonplace in fairway plantings, it is still somewhat controversial for new putting greens. On putting greens, uniformity of the putting surface is of paramount importance. A blend of grasses with incompatible characteristics (texture, density and growth habit) will segregate over time into patches, compromising uniformity. Thus, choose grasses with similar textures, colors and growth habits while, at the same time, including those with superior disease resistance. Unfortunately, comparative information on bentgrass-blend performance is limited, because NTEP does not evaluate blends. Blend performance should be judged over long periods (years).

Conventional wisdom suggests that there is little value in including a disease-susceptible cultivar into a blend in hopes that other, more resistant and aggressive cultivars will overtake it. This “survival of the fittest” approach does not always work. Some research suggests that disease-susceptible cultivars may linger in a blended stand for long periods. Don't cut corners — choose only the cultivars with the best performance characteristics and the best disease resistance.

TIPS ON ACCESSING AND INTERPRETING NTEP DATA

With hundreds of cultivars on the market, it's tough to make general statements about which ones have the best disease resistance. Fortunately, you can access NTEP disease-resistance data by logging onto the NTEP web site at www.ntep.org. You can obtain individual “hardcopy” reports for the different tests by writing or calling:

The National Turfgrass Evaluation Program
BARC-West, Bldg. 001, Rm. 245
Beltsville, MD 20705
(301) 504-5125
FAX (301) 504-5167

In these reports, data on disease resistance (and other performance criteria such as quality, density and texture) are arranged in tables that supply values corresponding to a disease rating for each cultivar. Diseases are rated on a scale of 1 to 9, with 9 indicating “no disease” and 1 signifying “very severe disease damage.”

At the bottom of each column in a table is a number called the LSD (Least Significant Difference). Differences between two cultivars are meaningful only if the LSD value listed at the bottom of each column is exceeded by the numerical difference between two cultivars. For example, if cultivar ‘A’ is 3.0 units higher in a disease rating than cultivar ‘B,’ then this difference is only meaningful if the LSD value is 3.0 or less. If the LSD value is greater than 3.0, then the numerical difference between the two cultivars may be due to inherent variability in the test area or some other element of chance.

How to determine meaningful performance differences among cultivars using the LSD value

Below are disease-injury ratings for three cultivars. Try to determine if they are different from one another by using the LSD value at the bottom of the column.

Cultivar Disease rating
A 9.0
B 8.3
C 5.0
LSD value 1.0

Now let's see if you interpreted the data in this example correctly. Disease ratings for cultivars A and B differ by 0.7 units (9.0 - 8.3 = 0.7). Hence, the difference does not exceed the LSD value of 1.0. This means that cultivars A and B are not different from each other when inherent variability of the test site and other elements of chance are taken into account. On the other hand, cultivar C differs from A by a factor greater than 1.0 (9.0 - 5.0 = 4.0). Thus, it can be considered more susceptible to the disease than cultivar A.

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