I once read a statement that some golf courses/country clubs spend up to $50,000 per year on fungicides. Obviously that figure implies there are some serious diseases of turf caused by fungi. But, what about nematodes? They too can cause serious problems but are often ignored. I’m often fond of saying, “Ignoring a problem won’t make it go away.” Nematodes are probably the cause of some symptoms observed on turf and are going undiagnosed. I base this assessment on the fact that until the past two or three years we typically received fewer than a dozen commercial turf samples per year in Diagnostic Services for nematode analyses. Things have changed as I think some superintendents now have me on speed dial.
Over the past few years, nematode problems appear to have become more prevalent on golf course greens in our region. The cause for this phenomenon is unknown but if I am to think out loud, I wonder if it has to do with a movement away from traditional insecticides used to control cutworms and other insects in turf to materials of different chemistries. Nematologists have observed this in corn as many growers have stopped using traditional insecticides for corn rootworm control in lieu of genetically modified, stacked, seed-treated corn hybrids. The organophosphates and carbamates used in the past probably helped to keep nematode numbers in check but with their discontinued use, nematode populations have had the opportunity to increase in densities. Could this be occurring in turf? Roughly 75% of all golf course samples processed in Diagnostic Services this year have contained high to severe numbers of plant-parasitic nematodes. At these levels, infected turf will exhibit symptoms that many superintendents say mimic those caused by summer patch. And if summer patch is suspected, my guess is nematode control is not objective number one.
Cool-season turfgrass species are hosts to at least 10 genera of plant-parasitic nematodes. Typically, samples collected from greens and tees on golf courses contain at least three genera. In general, it appears, our turfgrasses tolerate nematode feeding fairly well. However, there have been situations in Michigan and our neighboring states where nematode feeding has resulted in serious symptoms. I’ve visited some greens where the turf has been thinned so severely that only sand exists especially after top dressing. One thing to keep in mind about the impact of nematodes is their feeding does not result in the production of any characteristic above-ground (secondary) symptoms. In controlled studies at MSU, the only quantitative effects nematodes had on the above-ground growth of creeping bentgrass were reductions in the numbers of tillers and leaves produced. The distribution of symptoms often gives a clue as to whether nematodes are involved. Nematodes tend to be aggregated in their distributions (especially cyst and root-knot nematodes), so the symptoms appear as patches. However, other soil-borne plant pathogens, especially fungi, can and often do have similar distributions.
The only way to properly diagnose nematode problems is to collect soil and plant tissue samples and send them to a nematode lab for analyses. To avoid problems, this should be done preferably in the spring. Roots of cool-season turf grasses grow most vigorously when soil temperatures are cool and due to this phenomenon, nematode numbers tend to rise as more feeding sites are available. To avoid problems, control tactics must be implemented at action threshold levels before nematodes reach damage thresholds. This is an important principle of pest control but is better understood for insects than nematodes. The key strategy is to keep population densities of pathogens and pests below levels where they are expected to cause damage.
One significant issue when learning of a plant-parasitic nematode problem in turf is the lack of chemical control options. Nemacur was an effective nematicide for use on creeping bentgrass greens but is no longer available. Other products exist but have not been evaluated in Michigan. Any organophosphates or carbamates should provide some control of nematodes but those marketed as insecticides will not be extremely effective. For example, nematode populations have been shown to rebound very quickly on greens where chlorpyrifos had been applied.
Typically, nematode control is not necessary on new greens and tees because sand mixes are used for their establishment and these materials should ideally be free (or nearly so) from nematodes. But, this is not always the case as some superintendents have experienced severe symptoms due to nematode feeding on newly established greens from contaminated sources. Usually, however, nematode numbers increase over time as they migrate from aprons, collars and fairways, from deeper in the soil or are transported on plugs from core cultivators. New greens and tees should be sampled for nematodes roughly five years after establishment. Plant-parasitic nematodes are much easier to manage, or to alleviate symptoms of their feeding, if control tactics are implemented when their numbers are low. Early detection is important, remedies are limited.
If you suspect you have a nematode problem, in addition to collecting a sample for a nematode analysis, I also suggest you also collect soil and grass clippings for nutritional analyses. Because chemical control options are limited, cultural controls are the first tactics to consider. If nematodes are recovered at levels below damage thresholds, then the evidence indicates they are not the causal organisms if symptoms are present. However, if high population densities are recovered, action should be taken to reduce their population densities or alleviate the symptoms they cause. What approach to take? Well, the soil and tissue samples will help to determine the course of action. If the soil and tissue results indicate less than adequate levels of nutrients, especially potassium, obviously an additional fertilizer application(s) is necessary. However, if the soil results indicate nutrients are above critical levels but tissue tests suggest otherwise, these results indicate root dysfunction. The nutrients are available in the soil based on the test result(s) but the roots are not capturing them, therefore the plants are growing poorly. What pathogens reduce root volumes and weights? Nematodes. Additional steps now need to be taken to improve plant health and this may involve reducing the population densities of plant-parasitic nematodes. However, proper fertilization, using synthetic or non-synthetic fertilizers, using a different watering program and aeration should go a long way toward alleviating the symptoms caused by nematode feeding. If nematodes are present, the most important concern is the health of the turf roots. Reducing nematode numbers may not be necessary. Implement any cultural tactics that improve root health. After all, is the owner or member of a country club going to be more interested in the numbers of plant-parasitic nematodes in the soil or that the greens appear healthy and look good?07.31.14