Researchers at New Mexico State University continue to learn more about a root-infesting nematode associated with dieback of new growth and decline of pecan trees.
First found in five Texas orchards in 1996 and shortly thereafter in an orchard in Dona Ana County, N. M., the pecan root-knot nematode (PRKN) (Meloidogyne partityla) has since been confirmed in orchards in Arizona, as well as Oklahoma, Georgia and Florida.
“Most likely, it has been here forever and can be a pretty damaging pest,” says Stephen Thomas, a NMSU research nematologist.
Unlike the more common rootknot nematodes, PRKN has a very limited host range and can only reproduce on trees in the pecan family, which includes walnut and hickory.
“This nematode spends nearly all of its life inside pecan roots, where it transforms root cells into specialized feeding sties and induces external knots or root galls,” Thomas explains. “These processes interfere with the ability of roots to absorb and move water and nutrients into the tree. The feeding sites also act as nutrient sinks, casing the tree to redirect photosynthetic products away from new shoot growth and developing nuts to feed the nematode.”
In the Southwest, PRKN has been found in isolated locations. In the winter of 2005-06, researchers surveyed 5,939 acres of pecans in New Mexico’s seven major producing counties and found this particular nematode in 7 of the 10 orchards surveyed. However, infestations affected only 3.2 percent or 189 acres of the acreage surveyed.
The absence of native pecan, walnut or hickory in areas of the Southwest where pecan orchards have been established should help restrict establishment and spread of the parasite.
The PRKN probably causes more damage than the more common species of rootknot nematode, Thomas says.
“This parasite has to be impacting yields because the infested trees aren’t producing well.”
In fact, the symptoms of PRKN are often confused with a nutrient deficiency. “If pecan trees won’t respond to treatment with supplemental zinc or nickel, then pecan root-knot nematode is a possible cause,” he says.
“The symptoms become obvious in spring when the tree starts to leaf out. Some branches may not leaf out completely, or the tree may leaf out and then lose some of the leaves on newer growth. Usually, there is dead wood in the tops of the infected trees.”
No chemical controls are available for PRKN. Keeping trees well-pruned and encouraging growth of the root system won’t control this parasite, but it can reduce stress on the tree, Thomas notes.
“Pruning the canopy back somewhat to reduce the need for nutrients and water may allow the tree to produce more roots and live with the parasite.”
The idea is to avoid putting the tree in a high-nutrient stress by preventing a heavy nut load. Encouraging a lot of top growth that the roots cannot support could throw the tree into decline and cause more damage, Thomas says.
He also recommends growers keep equipment clean to prevent transporting nematode-infected soil to other areas.
“For all practical purposes, the PRKN won’t survive unless it’s introduced into an area where pecans are growing. Without this host, the parasite can’t live more than one or two years.”