Sting nematodes are one of the most problematic turfgrass pests in warm environments, especially in sandy soils. It doesn’t take many of the little critters to cause damage, and threshold populations of 10 to 20 per 6 cubic inches (100 cubic centimeters) of soil are commonly used as a warning level throughout warm-season grass regions. Although we do have chemical control options (some of which are new), they are often limited in some way — the use of the product is restricted to a certain number of applications, an application requires specialized equipment, or continued use may eventually lead to resistance.
The hunt is thus always on to identify biologicals that can control or suppress sting nematodes, such as, for example, plant extracts that could be used with minimal safety issues in a wide range of locations for nematode control. The folks at Clemson University (graduate student Campbell Cox and turfgrass faculty members Bruce Martin, Ph.D., and Bert McCarty, Ph.D.) set out to find this sweet spot of nematode control products, and they focused on extracts from various plants.
Using a combination of personal observations (for example, why is spurge always growing in sting nematode-infested turf sites?) and previous research, the scientists chose spurge, poinsettia (which is in the same family as spurge), lantana, tall lettuce, goldenrod and Brassica species. (Brassica species have long been known to suppress nematodes because they produce isothiocyanate-derived compounds from glucosinolates.)
Except for the Brassica, which was a prepared seed meal, the selected plants were either grown in the greenhouse or collected from the field. The roots and shoots of each plant were separated, run through a blender followed by a juice extractor, and then filtered to create solutions that would not clog sprayers and were free of fungal spores and bacteria.
Soil infested with sting nematodes was collected from a South Carolina golf course, and the nematodes were removed from the soil. Plant extracts were placed in tubes, and 20 nematodes were transferred to each tube. After periods of 24, 48, 72 or 96 hours, nematodes were removed from the extracts, repeatedly placed in tap water, and then poked with a dental tool to determine whether they were alive. Live nematodes would react to being poked (wouldn’t you?). The entire experiment was repeated. Further experiments were conducted in the greenhouse, where the Brassica seed meal or poinsettia shoot extracts were sprayed to saturation into the root zone of bermudagrass growing in soil infested with sting nematodes. Nematode mortality was determined as described previously.
What worked? In the laboratory tubes, the Brassica extracts were most effective, causing up to 99% nematode mortality. Extracts from goldenrod and tall lettuce were completely ineffective, resulting in no nematode deaths. Spurge and poinsettia were in the middle, with nematode mortality of 25% to 98%, their shoot extracts more effective than their root extracts, and increasing mortality over the 96-hour period. Similar results were observed in the greenhouse study, with nematode mortalities of 61% to 100% when Brassica seed meal or poinsettia shoot extracts were sprayed. If the extracts didn’t kill the nematodes, they made them sick, as treated nematodes were slow and lethargic.
So, why don’t we apply Brassica seed meal and extracts from poinsettia shoots to our greens? One reason is that the Brassica seed meal, when allowed to dry on the leaf blade, caused phytotoxicity. Another issue is that the concentrations of the poinsettia extract may not be strong enough to be effective in the field. Regardless, these extracts do show promise, and additional work is needed to determine effective field rates, the best delivery methods, and turfgrass tolerance to the extracts.
Source: Cox, C.J., L.B. McCarty, J.E. Toler, S.A. Lewis and S.B. Martin. 2006. Suppressing sting nematodes with Brassica sp., poinsettia, and spotted spurge extracts. Agronomy Journal 98(4):962-967 doi:10.2134/agronj2005.0235
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., editor-in chief for the American Society of Agronomy, and president-elect of the Crop Science Society of America. She is a 20-year member of GCSAA.