Seeing the unseen: how to scout for turf nematodes
Often, nematodes are overlooked pests in turfgrass. The above-ground symptoms that result from their feeding are extremely variable, including lack of wear tolerance, slow recovery from aerification, heat and drought stress, nutrient deficiency and increased susceptibility to disease. Nematodes rarely kill turf outright, but instead cause turf to be more susceptible to these stresses.
Below ground, nematodes reduce the depth, density and health of turfgrass roots. Certain nematodes, like root-knot, cause distinct galls on the roots that can be seen with a hand lens or low-power microscope. However, most nematodes do not cause distinct symptoms; instead, the roots will be sparse, unbranching and dark with fungal infections. In fact, it is very common for nematode problems to be initially diagnosed as a disease.
Caption: Affected roots may be stunted or necrotic.
If you have areas that are constantly weak or susceptible to stress, nematodes may be a contributing factor. The first step to diagnose a nematode problem is to collect and send a soil sample to a laboratory for analysis. It typically costs $20 or less to have a nematode sample analyzed, making it one of the most inexpensive diagnostic tests that can be done for a golf course. Visit Nematode Knowledge and:
- watch a step-by-step tutorial with Dr. Billy Crow collecting and shipping soil samples
- locate laboratories in your state
- download a sampling guide
- watch a nematode webinar and earn GCSAA education points
Caption: Collect 15 to 20 samples, avoiding severely damaged areas.
The nematode lab will extract and count the number of each nematode species per volume of soil. These numbers are then compared to “damage” or “action” thresholds established for each nematode species on each turfgrass species. If the population is above the threshold, turf is more likely to benefit from a nematicide application. If the population is below the threshold, turf is less likely to benefit from a nematicide application.
The phrases “more likely” and “less likely” are used intentionally, as assay results do not necessarily provide a definitive diagnosis. Thresholds are a rough guideline, and how many nematodes your turf can tolerate depends on other factors like growing conditions, management inputs and other stresses. Nematode assay results are often inconclusive, with populations just above or just below the threshold. For instance, results may find a combination of several species that are below threshold, but collectively may be causing damage.
In these inconclusive cases, the best way to definitively diagnose a nematode problem is to apply a nematicide, such as Divanem® nematicide, and leave an untreated area for comparison. It is recommended to make at least two applications of Divanem at 12 fl. oz./A on a 28-day interval (follow all label instructions on soil surfactants and watering-in). If Divanem helps alleviate the problem, then nematodes were a contributing factor. If not, other factors may be limiting turf quality to a greater extent than nematodes.
It is crucial to leave an untreated area for a side-by-side comparison of turf quality and root development. Avoid relying on nematode assays to determine if an application helped, unless you sample untreated areas at the same time. Nematode populations fluctuate widely during the season, so it’s impossible to tell how a nematicide influenced the population without an untreated area for comparison.
Nematode feeding and fungal diseases often occur together — the wounds created by nematodes are prone to infection by a wide variety of fungi. Root diseases like take-all root rot, summer patch and Pythium root rot are often more severe where nematode populations are high. When submitting samples for nematode analysis, it’s also recommended to submit samples to a disease diagnostic lab. If significant disease activity is observed, consider tank-mixing Divanem with a fungicide like Heritage® Action™ or Velista®. These tank-mixtures often improve turf quality more quickly, and to a greater extent than Divanem alone, by controlling two factors that are limiting to turf health.
For more information about identifying and controlling nematodes, visit GreenCastOnline.com/NematodeKnowledge.
©2018 Syngenta. Important: Always read and follow label instructions. Some products may not be registered for sale or use in all states or counties and/or may have state-specific use requirements. Please check with your state or local extension service to ensure registration and proper use. Divanem is a Restricted Use Pesticide. Divanem®, Heritage® Action™, Velista® and the Syngenta logo are trademarks of a Syngenta Group Company.