MTC Turf News – David McCall, Ph.D., Assistant Professor, Virginia Tech Turfgrass Pathology
We are all living in unprecedented times. Our daily routines have been thrown out the window in favor of Zoom meetings, trying to follow lesson plans for now-homeschooled children, and scouring the planet for the two-ply holy grail we used to take for granted. The novel coronavirus causing COVID-19 has flipped our world upside down as we live in our safe, sanitary, and socially distanced bubbles. But someone forgot to tell the grass (and associated pests)!
Just because the majority of humans around the planet have modified their daily lives does not mean that any other biological organisms are going to follow our rules. The grass is still going to grow and the associated pests are still going to be a nuisance. This means that many of us in the professional turfgrass management industry are still doing “business as unusual” because it is essential. Sure, most of us are having to get by with smaller budgets, smaller staffs, staggered work shifts, and greater mental stress, but we’re still in the business of keeping grass alive. Some tasks are more time critical than others as we try to keep our heads above water until we are able to resume normal activities. I’m a pathologist so I’ll give a few thoughts on managing turfgrass diseases during these times.
When I completed my Ph.D. in 2016, my wife got me a shirt that said “Trust me, I’m a doctor.” It was a fun shirt making light of the new letters attached to my name suggesting that because of these letters, people were suddenly supposed to take anything I say as gospel. Spend two minutes with me and you’ll quickly realize that I am not a great philosopher with the likes of Aristotle, Socrates, or the legendary Houston B. Couch. One thing I did learn about while completing my Ph.D. was how diseases spread in space and time. Plant disease epidemiology was probably my favorite course, taught by my co-advisor and a mentor, Dr. Anton Baudoin. We learned about disease progress curves, factors that influence the rate of spread, and ways to flatten the curve. Sound familiar?
Today, the news is riddled with talk of flattening the COVID-19 curve. Strategies include washing your hands frequently, avoiding groups of 10 or more, practicing social distancing when in public, disinfecting common surfaces frequently, and keep your Amazon packages in the garage for three days. Strategies to reduce disease. See the connection with turfgrass diseases? Sure you do! All disease epidemics have a beginning phase, a spreading phase, and an ending phase. How each particular pathogen (turfgrass or otherwise) spreads depends on the life cycle, reproduction rate, surrounding environmental conditions, and the host’s ability to defend itself. This is more commonly referred to as the Disease Triangle and is part of any pesticide recertification class when discussing plant pathogens.
We track outbreaks with disease progression curves, and not all diseases are created equal. Let’s take dollar spot and gray leaf spot as examples (Figure 1). Both can cause a lot of disease, and turfgrass managers should take both seriously, but the progression of symptom development is very different. For many in Virginia, dollar spot can appear in the spring when days are warm, nights are cool, and fog is heavy. The progression can last throughout the summer but often holds steady or tapers off during the warmest periods before resurging with a vengeance in the fall. The pathogen that causes gray leaf spot, on the other hand, will sit dormant until conditions are just right for an explosion of spore production, infection, colonization, and widespread death. But then the epidemic quickly ends, sometimes even faster than it began, after widespread distribution. Most spores and resulting infections do not result in complete plant death but many do because the pathogen is so prolific. COVID-19 acts more like gray leaf spot, from what the data shows so far.
There are two ways to slow an epidemic: delay the infection rate of disease and reduce the inoculum load. For COVID-19, social distancing delays the infection rate and proper sanitation reduces the spread of inoculum (virus particles). Many of the tactics that we promote for turfgrass management do one or the other, and sometimes both. A well-timed fungicide application can not only delay the start of an epidemic by protecting the plant but can also reduce the initial number of spores that may be just gearing up to do their thing. Strategies like disinfesting equipment and/or removing clippings can reduce inoculum load with highly prolific spore producers like Pythium species and the gray leaf spot pathogen. These practices aren’t as beneficial with less abundant pathogens, like those causing red thread, brown patch, large patch, and fairy ring. For these types of diseases, delaying the epidemic with a good growing environment for the turf and proper fertility may help get you through the rough patch by limiting the overall amount of damage before conditions are less favorable for disease development. We can even practice social distancing with our turfgrass selection to aid in disease management. Studies have shown that a mixed stand of Kentucky bluegrass and tall fescue can reduce overall brown patch development by acting as a physical barrier between susceptible tall fescue leaves.
Another factor to consider is the latency or incubation period from the time of initial infection until the time symptoms first start to develop. For COVID-19, early indications suggest an incubation period of 5-10 days. For a disease like gray leaf spot of tall fescue, that period may be 48-72 hours and management decisions need to be made immediately to mitigate the damage before it is too late. For spring dead spot of bermudagrass, that latency period is more like six months and the window to manage for this is the previous fall, much earlier than when symptoms first appear. When you see spring dead spot symptoms, you cannot “cure” the disease, you can only promote recovery with fertility (Figure 3). The flip side is that fertilizing will require additional mowing and you may not have the resources at this time. Again, not all diseases are created equal.
Managing for turfgrass diseases will be different in 2020 than it has been in the past. It has to be because of the limited resources (both labor and financial) that COVID-19 has forced on most facilities. Every approach and every facility will be different. I suggest taking a moment to think about the diseases that have plagued your facility in the past. Are they slow-spreading nuisance diseases that may not make for the most attractive surface but do not detract from the safety and uniformity? If so, maybe place management of these diseases on the back burner and focus your efforts elsewhere for the time being. Or are the diseases ones that will cause rapid, widespread death and inevitably force re-establishment? If so, pay close attention to weather forecasts so that you can make appropriate fungicide applications that will reduce both the rate of spread and inoculum load.
I hope that you all stay safe, healthy, and appropriately socially distanced as we continue to learn more about the COVID-19 pandemic. The safety of yourself, your employees, and your clientele take priority over the safety of your turfgrass, but a little thought into the spread of diseases may help determine how to minimize the damage.READ THE ISSUE