What can farmers do when two of the world's worst weeds are in cahoots with one of the world's worst roundworm crop pests, reducing yields up to 40 percent in chile peppers and 25 percent in cotton?
The weeds in this case are purple nutsedge and yellow nutsedge and the worm is the southern root-knot nematode. All three share a vast range on five continents, from southern South America, Africa and Australia northward into Asia and the southern portion of the U.S. - including southern New Mexico.
Farmers and researchers have long recognized all three as significant pests, but the symbiotic relationship between the nematode and the two nutsedges did not become apparent until recently. Much of this understanding has come from work done at New Mexico State University by researchers in weed science and nematology.
The key NMSU investigators have been Jill Schroeder, professor and interim department head in NMSU's Department of Entomology, Plant Pathology, and Weed Science; Steve Thomas, also a professor in EPPWS; and research associates Cheryl Fiore and Jacqueline Beacham.
Statistical design has played a significant part in the analysis of the data. Leigh Murray, formerly an NMSU faculty member and now a professor at Kansas State University, is the main collaborator on this facet of the research. Other NMSU collaborators have included Ian Ray, a professor of alfalfa genetics in the Department of Plant and Environmental Sciences, and Jim Libbin, professor of agricultural economics and an associate dean in the College of Agricultural, Consumer and Environmental Sciences.
In southern New Mexico, southern root-knot nematodes infect the roots of chile and cotton plants, just to name two very commercially important plants. The nematodes cause bulges known as galls that interfere with water uptake to the leaves and fruit. According to Thomas, who directs NMSU's nematology lab, the worms also send a chemical signal to their host plants that essentially says, "I'm a fruit," tricking the plants into rerouting photosynthates from the leaves to the roots, thus depriving the chile pods and cotton bolls of nutrients.
The purple and yellow nutsedges are pests in their own right. Like typical weeds, they compete with crops for space, water and soil nutrients. In addition, they provide a willing host for these nematodes, as the NMSU researchers have found. But these sedges are not merely unaffected by the presence in their roots of southern root-knot nematodes - these weeds actually thrive in a symbiotic relationship with the worm.
"The nematode actually makes the yellow nutsedge produce more of these tubers and that's how it winds up to be a win-win system for both the nematode and the nutsedge," Schroeder said.
Understanding in detail how this "pest complex" relationship works and how we can use that knowledge to the producer's advantage has been a priority for Schroeder, Thomas and colleagues for a number of years.
Among the important insights that the group's research has produced are the following:
The nature of the symbiotic relationship between the nematode and the nutsedges is such that there is a positive correlation between the density of nutsedge plants in an area of a field and the level of concentration of the nematode. More nutsedges mean more nematodes. Murray's statistical modeling has proven effective in predicting nematode populations based on nutsedge population.
Because these nutsedge varieties have a grass-like root system and propagate underground, merely getting rid of the individual stalks in a field will not keep the weed at bay. The tubers - similar to potato tubers - will remain, continuing to produce new plants and offering safe haven to the nematodes, which reproduce and spread to the susceptible crops.
Except for the short period that newly hatched nematodes spend finding a new home, they normally hide in the nutsedge tubers, where they are shielded from the effects of fumigant pesticides normally used to control soil pests prior to planting. There are currently no environmentally safe, readily available pesticides that are effective against this nematode once inside a plant, according to Thomas.
As mentioned above, chile peppers and cotton, both widely grown in southern New Mexico, are seriously affected by the southern root-knot nematode. Some farmers rotate chile and cotton, which actually exacerbates the problem.
Certain plants are both resistant to the southern root-knot nematode and competitive against the nutsedges. These plants can actually suppress the nematode population by crowding out the nutsedges.
Rotating a resistant variety of non-dormant alfalfa with chile can result in more productive chile plants. Unfortunately, the researchers have found that it takes three years of alfalfa production in a field to effectively reduce the nematode and nutsedge threat, and the result is only one year of nematode-free chile cultivation. This might seem like a high price to pay for someone who is predominantly a chile producer.
Where nutsedge is not a problem, NMSU chile breeder Paul Bosland has found it effective to rotate certain varieties of marigolds with the chiles as a defense against the nematodes. He has been employing that strategy effectively at the Chile Pepper Institute's demonstration garden in Las Cruces. Tilling under one year's marigolds keeps the nematode population down for the following year. The down side of this approach for commercial producers, however, is that marigolds aren't a cash crop, so they would only have income from a field every other year.
Thomas has suggested several possible directions for future research:
Identifying other crop varieties that resist the nematodes, can compete against the nutsedges, and give growers more flexibility than three years of alfalfa. Pearl millet may have potential as a summer forage crop for local dairies, according to Thomas.
Incorporating CT scanning technology, which would allow researchers to see into the nutsedge tubers and learn how the two pests affect each other's development in hopes of finding weak points.
Exploring how to use winter accumulation of soil heat units and irrigation timing as tools to determine when to disrupt the early season impact of nutsedges and root-knot nematodes on young crops.
The value of the latter two research directions would be in better understanding of the relationship of nutsedge germination and nematode emergence. More precise prediction of these stages would allow producers to be more precise with their treatment efforts.
Beyond the knowledge gained about how this specific pest complex functions, the project has led to the development of strategies for identifying and investigating other pest complexes.
"We have started a collaboration with Soum Sanogo, an associate professor in EPPWS, to understand the interactions of three annual weeds—spurred anoda, Wright groundcherry and tall morning glory—with the nematodes and the Verticillium fungus," Schroeder said. "The research is based on observations made by Sanogo in the Deming area in fields infested with all these pests.
"The root-knot nematode and Verticillium pathogens infect all three weeds but the weeds are not damaged. As a matter of fact, the tall morning glory grew better when infected with these pests that are devastating to chile. We are beginning to think that these predominant annual weeds in Southwestern chile pepper production systems may actually enhance populations of both pathogens, reinforcing the importance of effective weed management for effective management of the disease. We plan to continue research on the biology of this interaction."