By Mike Jiggens
More fungicide goes into dollar spot treatment than that of the next top five turfgrass diseases combined, those attending the 12th annual IPM seminar series sponsored by Master’s Turf Supply were told in March.
Dr. Bruce Clarke, chairman of the department of plant biology and pathology at Rutgers University in New Jersey, told his audience of golf course superintendents at Milton, Ont.’s Granite Ridge Golf Club that suppressing dollar spot through reduced fungicide inputs is a challenge.
Rutgers, Ohio State University and North Carolina State University have spent the past six years collaborating on dollar spot research, studying isolates from around the world. The objective of the research is to determine the true causal agent of dollar spot.
Clarke said there are actually six distinct fungi which can cause dollar spot.
“We’re not dealing with one pathogen,” he said.
Clarke said this might explain to some extent why there seems to be a significant variability in fungicide control and why some golf courses may have resistance to certain fungicides and not others. He added it might also explain some of the differences seen in the benefits of certain cultural practices aimed at suppressing the disease.
Now that it’s known there are six distinct causal organisms, laboratory tests can be developed to identify which of the six agents is present. Clarke said they are “reinventing the wheel” to a degree after all these years of dollar spot presence, knowing there are now six different organism triggering dollar spot.
“Essentially, we’re with dollar spot where we were 12 to 15 years ago with anthracnose,” he said, adding science is just beginning to understand what it is up against.
Dollar spot was first identified in the United Kingdom in 1937. According to pathologists, it is not a true sclerotinia. It has no spores and is purely mycelium contact from plant to plant. It is the most common disease among cool season grasses in North America. It survives well in all types of temperatures and is an opportunist, attacking plants while they are in a weakened state.
Dollar spot overwinters as minute sclerotia, resuming growth when temperatures reach about 15 degrees Celsius.
“Dollar spot ebbs and flows throughout the summer.”
Clarke said it shows up in the spring, goes through summer and into the fall. In more recent years, it has occurred later and later into the season. He said that when he started at Rutgers about 30 years, dollar spot was seen as late as the end of September or early October, but now it’s been identified in November and even into December if the conditions are right.
When a golf course has thick thatch in excess of 11/2 to two inches, an increase in the disease is likely “because this is where the fungus hangs out.” Thatch provides protection against ultra-violet light.
Low soil moisture, periods of extended dew and low fertility are other conditions conducive to dollar spot.
Clarke said it is one of those pathogens that thrives in dry soil, mainly because of the weakness of the plant. During periods of drought stress when greens are running dry, often a cycle of wilt stress followed by pulling it back with syringing and more wilt stress followed by pulling it back with syringing may occur. That kind of fluctuation can weaken the plant, he said. By running greens a little too dry, the lack of soil moisture can enhance a disease like dollar spot.
The fungus is leaf-infecting and requires extended periods of dew or high humidity to thrive. If there are at least nine hours of consecutive leaf wetness and high humidity of 95 per cent or more in the leaf canopy, an enhancement of the disease will be observed.
Clarke described dollar spot as being bi-polar. It needs dry soils yet extended periods of leaf wetness.
In situations of low fertility, one of the best things to have happened is an increase in spoon feeding, he said. When talk about anthracnose control began, superintendents increased their spoon feeding and discovered other benefits, including better control of such stress-related diseases as dollar spot.
Some common sense-type practices can be employed to help fight off dollar spot such as balanced fertility through the application of proper amounts of micro and macro nutrients. If nitrogen amounts are sufficient, it will promote better suppression of both dollar spot and anthracnose, Clarke said.
Spoon feeding will help relieve some of the pressure from the disease and reduce some of the severity of the infections. He advised to do this with light, frequent applications when dollar spot and anthracnose are active.
The use of soil moisture probes involves additional labour, Clarke admitted, but they will allow superintendents to better detect dry spots. More importantly, the superintendent will have a relative number he can use to either increase or decrease his water usage.
“It takes the guesswork out of it.”
He can detect dry areas and syringe those and not necessarily the areas that have sufficient soil moisture. It allows him to tailor his syringing program to the drier spots and leaving the others alone.
Most foliar fungi, including dollar spot, need at least nine hours of consecutive leaf wetness or high humidity. If the superintendent is wetting down his foliage at about dusk and humidity is high enough where the turf will not dry off, it will ensure there will be at least nine hours of consecutive leaf wetness.
Clarke said there is nothing wrong with irrigating during the day, but when dusk is approaching and there is high humidity, it should be backed off at least until the sun sets and dew begins to form. He said once dew has formed and it is knocked off through the irrigation cycle, not only is the dew removed but also the guttation fluid which contain the sugars exuded from the leaf blades. The fungi requires those sugars, he said, which makes knocking off the dew a real benefit.
Thinning out treed areas to promote better air circulation or installing fans to move air around a little bit will reduce the possibility or probability of infection, Clarke said, adding it can play a significant role in reducing dollar spot development.
Aerating, reducing compaction and decreasing thatch will help with dollar spot reduction, he said.
Mowing has been shown to reduce dollar spot by 30 to 80 per cent when done early in the morning. Studies show the practice to be more effective in fairways than on greens, helping to break the cycle of leaf wetness. Early mowing dries up the leaf wetness, disrupting the cycle before it reaches nine hours.
By drying the leaf wetness before it reaches nine hours, a significant dropoff in infection will be realized. Waiting until later to mow, when there might be 10 or 11 hours of consecutive leaf wetness, an increase in the disease is apt to occur.
When dew is removed through mowing on a daily basis, a reduced incidence of dollar spot can be realized than when mowing on alternate days. Both mowing and dragging as dew removal methods are helpful.
Not taking measures to remove leaf wetness within the critical nine-hour period will promote a 20 to 30 per cent increase in dollar spot, Clarke said.
Many organic fertilizers and compost materials have been reported to effectively reduce dollar spot. It has been questioned whether it’s the added microbial organisms or the increase in nitrogen which is to account for the effectiveness, but Clarke said research suggests it’s mainly due to the increased nitrogen.
He said the key with biocontrols is that they need to be applied on a preventative basis and must be compatible with other synthetic fungicides being used.
“Dollar spot has been the poster child for fungicide resistance.”
Clarke said looking back over the past 50 years, easily 90 per cent of all reports of fungicide resistance on North American golf courses have essentially been on dollar spot. He said it’s because there is tremendous variability in the dollar spot fungus, knowing now that there are six different organisms that cause the disease.
A fungicide is applied to the plant, it’s absorbed into the fungal mycelium, it goes in and attacks the plant at one point. What it’s trying to suppress in terms of the cell division and microtic growth in the fungus actually is control in the fungus by one gene, he said.
“It’s very easy to overcome that type of tolerance.”
Clarke said that when building a program, the superintendent mentally wants to know when the major diseases are occurring and what fungicides he has available as well as their strengths. He should then time those fungicides to hit the strengths they’re designed for and reap the side benefits, if there are any.
The recommendation among many is to go out after the second true mowing of a fairway—where clippings are actually picked up—with a strong dollar spot fungicide with the idea that it will knock back the population of the fungus and would delay the onset of the disease by several weeks or, if it did develop, it was much less severe.
Clarke said the bottom line with regard to dollar spot is that we’re starting to know that we don’t know as much about dollar spot as we thought we knew.
Topdressing has been shown to not only effectively suppress anthracnose, but also dollar spot.
“It’s looking like from our research that you can use much finer sand than you were told you could use for topdressing programs.”
Medium-coarse sands are OK, but don’t incorporate as well, he said, adding there is a lot of mower pickup and they ding the blades. Also, golfers don’t like it, and superintendents tend not to want to topdress as frequently because of those issues, particularly during the summer.
Research into topdressing with medium-fine or medium sands is that they not only incorporate better, but they don’t seem to clog pore spaces and they show some real benefits in suppressing diseases such as anthracnose and dollar spot.
Clarke said he wasn’t encouraging anyone to go out and buy finer sands, but was simply stating the USGA is taking a second look at some of Rutgers’ research to perhaps recommend some finer sands.
“If that’s the case, I think it will make it a lot easier for superintendents to go on a more frequent topdressing program because the incorporation issues are not near what they are with the more coarse sands.”
If water pH is in the 7s or 8s and the superintendent isn’t buffering his tank or adjusting the pH, he’s probably doing himself a disservice, Clarke said. That could reduce the disease suppression ability of a fungicide by 20 to 30 per cent.
On the other hand, if the pH in the tank is changed, control could be increased by 20 to 30 per cent, he said.
Clarke asked the question of how water volume and dew removal affect fungicide efficacy. When using a contact fungicide at 1.8 ounces per 1,000, better control of dollar spot is realized when dew is removed in the morning vs. when dew wasn’t removed. When the leaf is sprayed when dry, the fungicide acts as a protectant, coating it to prevent infection. The contact fungicide is not getting into the plant, nor is it curing existing infections. Its purpose is to prevent infection from occurring.
When a plant is coated in dew and the spray hits the dew, it simply rolls off and goes to the base of the plant and into the thatch. If it was a penetrant fungicide, it wouldn’t be much of a problem because it is absorbed into the plant, be it the root, stem or leaves, and then it moves upward. A contact fungicide would hit the dew, roll off and not provide its intended protection. It may suppress the population in the thatch, Clarke said, but if it’s not doing that, the plant is getting either poor or no control.
He said when using a contact fungicide, it’s better to mow the grass first, drag the dew off or let it dry off before applying the fungicide.