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The time for the second application of fungicides on the new ​bentgrass research at the ISU horticulture research station has arrived. Chris Blume and I applied the curative, second treatment of Emerald® (boscalid) and Daconil® (chlorothalonil) today, August 18, 2009. The new bentgrass research is focusing on the susceptibility of different cultivars of bentgrass to the disease dollar spot. In the study, each plot is split, one side receiving no fungicides, while the other is treated with both a preventative and a curative application of boscalid and chlorothalonil. This year has proven to be an excellent year for dollar spot out at the research station and many of our bentgrass areas are spotted with the disease.

The disease dollar spot is caused by the fungal pathogen Sclerotinia homoeocarpa. This disease commonly infects many of our cool-season grasses. Dollar spot is a particular concern on bent/poa greens where the sunken pockets may interfere with putting quality. Dollar spot can develop over a wide range of temperatures (55-80°F) when the dew persists for long periods of time (longer than 8 hours). Dollar spot is also considered a low nitrogen disease and is most severe on nitrogen deficient stands of turf.

The pathogen causes blighted, circular patches of turf which are similar in size to a silver dollar (rarely larger than 2 in. in diameter) on low mown areas, and up to 6 in. or more in diameter on taller mown areas. The blighted spots of turf often occur in clusters and may merge together to produce larger blighted areas as disease development progresses. Active dollar spot infections produce a cottony white mycelia mass that is often evident on the turf during the early morning hours. When viewed under a microscope the mycelium will exhibit y-shaped branching and the presence of septum unlike the mycelia of pythium that are void of septum. In addition to the mycelia mass that may be visible, this pathogen also causes leaf lesions that have light tan centers and a red-brown margin. The leaf lesions of dollar spot are more commonly observed on taller turf species, such as Kentucky bluegrass, where the lesions take on the shape of an hour glass.

There are many ways to control dollar spot. One important management technique is to implement a satisfactory nitrogen fertility program. Nitrogen deficient stands of turf are more susceptible to dollar spot outbreaks. However, over stimulating with nitrogen may increase disease pressure from other, less desirable, diseases. Another option in control is to choose a less susceptible cultivar. Although there are no completely resistant cultivars of bentgrass, there are cultivars that are more resistant than others. For example, the cultivar Crenshaw® is far more susceptible to dollar spot than the cultivar Declaration®. Irrigation practices that limit the duration of leaf wetness will also help limit the occurrence of the disease. There are also fungicides available for the control of dollar spot. These fungicides often require multiple applications to effectively control this disease. Also, fungicides that are classified as site specific inhibitors have a high risk of resistance development and their repetitive use should be avoided. Some effective fungicides are Emerald®, Daconil®, Curlan®, and Tourney® along with many others.

Nick Dunlap

It seems as if summer has finally caught up with us. An unusually mild July has left us and August has brought with it more typical Iowa summer weather. With increasing temperatures and humidity, coupled with the rain we received this past weekend, conditions have once again become favorable for disease development. Out at the ISU Horticulture Research Station we started our week off by discovering an outbreak of brown patch on some of our creeping bentgrass greens.

The turfgrass disease known as brown patch is caused by the fungal pathogen Rhizoctonia solani and can affect all of the cool-season turfgrass species. Brown patch is a summer disease whose development is triggered by hot, humid weather, night time temperatures above 65°F and long periods of dew. During these conditions brown patch may appear overnight. Brown patch is also considered a high nitrogen disease and excessive amounts of nitrogen in your fertility program during the summer can contribute to a brown patch problem.

Brown patch usually produces a circular brown to olive green patch with a grey perimeter giving a ‘smoke ring’ appearance. Often, more than one patch will be evident in an affected area with the appearance of the unique ‘smoke ring’ pattern more clearly defined on low mown turf. Individual leaf blades of the affected turfgrass will show lesions with a chocolaty brown margin. The brown patch lesions are most visible when observed on tall fescue, although they are present on all turfgrass species infected with brown patch.

One of the easiest ways to decrease disease pressure from brown patch is to implement a proper fertility program to avoid excess nitrogen during the summer months. Also, trying to promote shorter dew periods by avoiding late evening irrigation can help reduce the possibility of a brown patch. There are also a number of fungicides that provide brown patch control such as Heritage, Daconil, Medallion, Clearys 3336, and many others.

Nick Dunlap
GCSAA Campus Representative
Turfgrass Management
Iowa State University
njdunlap@iastate.edu

Recently, I was skimming through a textbook about maintenance reduction practices for turfgrass settings and I stumbled upon a section that discussed the one-third rule of mowing. I’m sure most everyone has heard about this recommendation as it appears in almost every textbook and extension publication. This common rule states that no more than one-third of the plant should ever be removed in a single mowing. However, this rule is commonly ignored. You probably remember many other “rules” from your introductory turfgrass classes that are also commonly violated: irrigate deeply and infrequently, never mow when the grass is wet, avoid fertilization during the summer months, etc… So are there any absolute truths when it comes to turfgrass management practices? The truth of the matter is that turfgrass management often deals in shades of gray. There seems to be exceptions to almost every rule.

Most recommendations stem from research, which is usually a good thing. But sometimes the inference space of the research is extended beyond the original scope of the project. For instance, a study investigating the growth of bentgrass cultivars will only provide information about the growth of bentgrass cultivars. The results of the study should not be applied to other species of turfgrass such as bluegrasses or ryegrasses. The recommendation of the one-third rule is a classic example of a research project being extended beyond its inference space.

The 1/3 Rule is Born
The one-third rule originated from a greenhouse study done by U.S. Department of Agriculture scientists about 60 years ago on forage type Kentucky bluegrass. The goal of the study was to investigate the effect of cattle defoliation on the growth of the roots. Results of the study showed the grass (or roots) didn’t die when more than one-third was removed. The growth of the roots simply paused for a short time before resuming normal growth. The findings from this study were eventually morphed into the creation of the one-third rule.

More Recent Research
A similar defoliation study was recreated in 1986 by a group of turfgrass scientists at North Carolina State University (Shepard et al, 1989). This study was conducted in the field with a “high” and “low” maintenance tall fescue maintained at 3 and 6 inches, respectively. Each grass was allowed to grow 30, 50, 100 and 300% taller than the maintained height before being mowed. The 50% defoliation treatment signified the maximum guideline of the one-third rule – 30% was within the recommended guideline, and the 100 and 300% treatments violated the one-third rule. The researchers found that the tall fescue could grow to double the original height before being defoliated without negative consequences (tall fescue maintained at 3 inches could grow to 6 inches before being cut without serious consequences). The 300% treatment resulted in senescence to a portion of the tall fescue stand. The results of the NC State study indicated that the one-third rule has little merit, yet the recommendation lives on. One reason could be the detrimental effect of excess grass clippings that shade the turf underneath. Either way, the one-third rule is by far an unconditional truth.

So what is the moral of this story? Recommendations should not always be taken at face value. Ask questions and spend some time investigating how they came about before taking action. You may just be surprised when you learn the rest of the story.

NC State University Citation:
Shepard, D.P., J.M. Dipaola, and W.M. Lewis. 1989. Effects of clipping regime on turf quality and mowing requirement. Agron Abstr. p.165

Marcus Jones

Graduate Research Assistant

This week historically brings the first opportunity for frost in a good portion of the state. Fortunately, that doesn’t necessarily mean the terminal end to the growing season for our cool season grasses, unlike many other plants and crops here in Iowa. Frost formation can come in
many levels of severity affected by environmental factors such as low temperature, dew point, relative humidity, wind speed, cloud cover, and elevation. Turf managers only slightly control a few factors such as irrigation and moisture availability, applications of wetting agents, and cutting heights of the turf. Frost formation on turfgrass will begin or advance the entrance into dormancy as the plants prepare for the winter months. Though, the main concern that is always on the mind of the turf manager is damage to the plants due to traffic on frost laden turf. This can be a concern on all turfgrass areas, but especially on grass such as on golf course greens.

Pictures of traffic damage on frosted turf by Zac Reicher, Purdue University

Light frosts that are essentially frozen dew on the upper leaves of the turfgrass plants as the temperature approaches 32 degrees. This typically occurs close the coldest point of the night usually around sunrise, or in golf course terms right when you would like to mow. As the ice forms on the plant it begins to freeze the plant cells in the tissue of the turgrass leaf. This weakens the cells and makes them vulnerable to permanent damage, kind of like cracking an egg. In general, most damage primarily occurs on the leaf tissue, but in heavier frost events and on lower mowed turfgrass the damage can extend down into the crown. If the frost traffic occurs and cells near the growing point are damaged, the plant could be a complete loss.
This issue of the frost setting in causes not only delays in your maintenance system, but delays in golf play or other turf activities. The key is that time is money. So, we all want to limit the time our employees are at the shop, or golfers are holding up tee times, etc.
What can we do to limit these time delays? One practice that is commonly used is a light irrigation cycle which contains water that is warmer than 32 degrees to melt off the frost and warm up the surface. The issue with this is that it must be timed properly to not form more ice on the system. The other thing to remember is that the irrigation will only knock the frost off of areas it can reach, so cart or foot traffic damage can occur leading to the irrigated areas.
I have included a chart referenced from a past issue of Horticulture Home and Pest News authored by Richard Jauron. As you will see the data is somewhat dated, but after conversations with State Climatologist Harrry Hilaker the fall dates still are very accurate. The spring dates and growing degree days may change slightly.
 

As with all issues involving the weather it is hard to tell what might actually happen. Good luck finishing up the growing season!

Neric Smith,
Commercial Horticulture Program Specialist
Iowa State University Extension

Germination characteristics such as speed, synchrony, and viability determine the success of newly seeded areas. Traditional establishment from seed involves sowing seed into bare soil or turf which have been treated with non-selective herbicides. In either case, the newly emerging seedlings face little competition from surrounding plants. While germination characteristics are important when using traditional establishment methods, superior germination characteristics may be necessary when using non-traditional seeding methods such as interseeding.

Interseeding poses an interesting dilemma. The main attraction of converting through interseeding is the ability to convert to a new turf without taking the area out of play and facing the resulting economic losses. Research shows that the success of interseeding is related to the level of disruption created and the speed of germination of the interseeded species. As the level of disruption increases, the competition from surrounding plants decreases allowing the seedlings an opportunity to establish. However, the amount of disruption necessary for successful interseeding may not be conducive with a smooth uniform playing surface. Because minimal disruption is needed so as not to disrupt play, superior germination characteristics are needed.

We conducted a germination study at Iowa State with 'Penncross' and 14 improved cultivars of creeping bentgrass. The cultivars ‘L-93’, ‘T-1’, ‘Apha’, ‘Penn A-1’, ‘Penn A-4’, ‘Crystal Bluelinks’, ‘Pennlinks II’, ‘Pencross’, ‘Tyee’, ‘007’, ‘Mackenzie’, ‘SR1150’, ‘Memorial’, ‘Independence’, and ‘Declaration’ were evaluated in this study. Each cultivar was represented by two to four seeds lots from production year 2007. Standard germination tests were conducted according to the rules established by the Association of Official Seed Analysts (AOSA). The objectives of the study where to determine if improved cultivars of creeping bentgrass possess superior germination characteristics compared to 'Penncross' and if there was a relationship between seed weight and germination characteristics.

Results

Significant differences existed between 'Penncross' and the improved bentgrass varieties for germination speed (MGT), germination synchrony (T10-90), and viability (FGP) but not weight (table 1). These results indicate that improved varieties of creeping bentgrass possess greater viability and germinate faster and with greater synchrony compared to ‘Penncross’. Differences between ‘Penncross’ and each of the improved varieties can be viewed in table 2 (significant differences are highlighted in red). ‘T-1’ outperformed ‘Penncross’ in all three parameters. While some of these differences appear small numerically, it is important to remember that these tests were conducted under optimum conditions. The differences would likely be greater under field conditions when the seeds are subject to environmental stresses.

Our study also revealed a significant correlation between seed weight and speed of germination (MGT). Seeds germinated faster as the weight of the seeds increased. This information could be used to predict the germination speed of other cultivars without conducting tedious germination studies.

What does it all mean?

This study clearly shows that improved cultivars of creeping bentgrass have superior germination characteristics compared to ‘Penncross’. So is it possible that the success or failure of interseeding could depend on cultivar selection? Past interseeding research supports this very idea. Attempts at interseeding ‘Penncross’ into an established annual bluegrass putting green were largely unsuccessful (Gaussoin et al., 1989). However, in a similar study, researchers were able to establish over 70% ‘L-93’, and ‘Penn A-4’ into an annual bluegrass putting green (Henry et al., 2005). These results could be the explained by the differences in germination characteristics that we observed in our study. Clearly, not all cultivars of creeping bentgrass exhibit similar germination characteristics. And while the success of interseeding will not result from a single practice or strategy, proper cultivar selection will play an integral role as part of an interseeding conversion program.

Literature Cited

Gaussoin, R.E. and B.E. Branham. 1989. Influence of cultural factors on species dominance in a mixed stand of annual bluegrass/creeping bentgrass. Crop Sci. 29:480-484.

Henry, G.M., S.E. Hart, and J.A. Murphy. 2005. Overseeding bentgrass species into existing stands of annual bluegrass. HortScience 40:468-470.

Marcus Jones
Graduate Research Assistant