- 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 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.