Nematodes on the March (or Wiggle)…
Dr. Nathaniel Mitkowski
Chair, Dept. of Plant Sciences and Entomology
University of Rhode Island
It’s about this time of year when superintendents start asking me, “When should I start sampling my greens for nematodes?” This is a difficult question to answer because it’s a moving target. There really is no specific answer. Firstly, ever year is different. Some springs are warm, some are cold, some are wet and some are dry. If there is still snow on the ground, it’s probably not worth sampling. But that’s only because you can’t treat over snow. When the soil is not frozen, nematodes will be feeding, even under snow. Usually this is at a low level and not a problem but residual fall populations can remain very high throughout the winter, even in frozen soils. Last year, Russian scientists reported resurrecting nematodes that had been frozen in the Siberian tundra for 41,700 years. So nematodes are pretty hardy. If we get an early thaw or a very warm winter, sampling earlier is better. When we talk about insects, we think about “generations”. ABW might have 3 generations over the growing season. Nematodes do not usually have generations. Nematode life-cycles are temperature dependent. In turf soil, they just reproduce and feed continually, in direct proportion to the amount of heat in the soil. The warmer it is, the more feeding damage they cause and the more eggs they lay. Eggs usually hatch quickly after being laid so populations can grow rapidly with warm temperatures. Unfortunately, this can even happen with nematicides in place, especially towards the end of an application interval.
Secondly, if you have sampled before and have never had a problem, you may not need to track as regularly. If you did a late fall application, your spring numbers are likely to be low and you may not need to sample until the end of May or June. And if you have made a spring application without sampling, you may not need to formally check your numbers until the summer (although it never hurts to be prudent and check earlier). Sadly, it turns out that even after years of research, we don’t have a good handle on why some courses have perennial nematode problems and others do not. We do know that push-up greens are more susceptible to nematodes. We also know that many nematodes prefer Poa and velvet bentgrass. And older courses usually have more nematodes than newer courses. But newer courses are built on sand with lots of creeping bentgrass so each one of the variables we identify is strongly linked! It’s difficult to separate these factors experimentally. In general, new sand based greens rarely even see significant nematode populations within the first 10 years.
So to answer the question, my standard reply is “the end of March or beginning of April”. But that could shift earlier or later, based on the weather. If is it a very wet spring, nematodes are not going to do very well and populations will remain lower for longer. But in February 2018, I tested greens from a Rhode Island golf course that had stunt nematode levels reaching 21,000 nematodes/100 cc soil. You read that correctly: 21,000. The threshold for this sample was approximately 2,000 nematodes. And at 21,000 stunt nematodes, rooting will be minimal into April and turf will be lost by July. We always hope that the winter will result in some nematode mortality but as mentioned previously, nematodes are very hardy. We don’t always see a significant drop in numbers even in the coldest winters. Water may be more important than temperature.
Sampling technique is as important as sampling timing. It is important to note that a single sampling or single green is not going to tell you much. Nematode populations change over time and they are not the same on every green. My recommendation is to pick three greens. They can be the worst quality greens but they should not be the best quality greens. Or they can all be mediocre quality greens. Avoiding the best quality greens is important because these are likely to have the lowest nematode populations or have the best environmental conditions, making them more resistant to nematode damage. Quality isn’t always easy to define but you know it when you see it and you’ll have past seasons to tell you who the troublemakers are. Once you have your greens picked, do three annual samplings. Start with late March/early April. If nematodes are low, great. If not, you can treat and your root mass will respond accordingly. Then check again in mid/late May. Once again, nematode populations are increasing. If they were low in April, they could still be low. Or not. Do at least one more sampling in the end of June or beginning of July. This is when populations are really going to climb. If they are low, you are probably good for the rest of the year. If they are high, treat. If the fall is very warm and long, like in 2017, additional sampling in September or October may be required. But it all depends on which chemicals you have applied and how your roots are growing. We have seen populations that were treated in June bounce back by October but this matters less if you have an actively growing root system in the fall.
While cup-cutters can be used to suggest a nematode-related issue, they are not the best strategy. If you are going to track your nematodes regularly, you need the data to be repeatable and comparable. The only way to get this type of reliable data is to take composite samples. For each green you want to sample, you need to take 20-25 cores and combine them in one bag. This is an average, or composite sample. Cores should be taken an approximately equal distance from each other (5 foot centers is common) and they should form a grid. The more cores you take, the more reliable the data. Thirty cores is a very good number of cores, 20 is a minimum amount. In addition, every time the green is resampled, the cores should be taken from the same area of the green. Repeatability is important in comparing numbers across time.
There isn’t one specific formula for tracking and treating nematodes. Almost everything happening on a golf course is primarily influenced by weather conditions and nematodes are no different. But I do not recommend treating preventatively. We treat fungal disease preventatively because we know dollar spot is going to pop up or that Pythium blight will be a problem in a certain area after a string of 90° days. But nematodes are more fickle. If you spend a lot of money treating in April, your nematicide may no longer be working in July. And you might have to do it again. And you may not have needed it in the first place. Ultimately, you may be spending a good piece of your budget and be putting an unnecessary application into the environment. The only way to know if you need to treat is to track your nematodes. And once your numbers are determined, we can talk about the right strategy based on the numbers, your budget, greens conditions and time of year.
Finally, most of what I’ve outlined really only applies to nematodes in the northern Unites States. South of the Mason-Dixon line, nematodes go from being a manageable problem to being an outright scourge. If you are fighting sting nematodes and growing bermudagrass, all bets are off. Fortunately, both of these organisms are still dissuaded from taking root in the North because of our cold winters.