As if the southern root knot nematode wasn't bad enough, an even more aggressive relative could pose an even more serious threat to Southern row crops.

The guava root knot nematode, first reported in China, reported in Florida in 2012, identified in the Carolinas in 2013 and identified in a Louisiana sweet potato field in 2018, is more difficult to manage than the southern root knot nematode, a close relative, says Josielle Rezende, LSU AgCenter.

The GRKN is a bad pest, Rezende said at the recent Louisiana Agricultural Technology and Management Conference in Marksville, La. The Louisiana Agricultural Crop Consultants sponsor the annual conference.

"The GRKN was first reported in China, but we do not know the origin of this nematode," she said. "We do not know if this nematode is from China."

The GRKN reported in Louisiana was detected on sweet potatoes brought from North Carolina, Rezende said.

The Louisiana Department of Agriculture and Forestry enacted an external quarantine for Florida, South Carolina, and North Carolina to restrict entry of items that can potentially spread the guava root knot nematode (M. enterolobii).

The GRKN is not a widespread problem yet, but Rezende said it poses serious threats to many Southern row crops, including soybeans, sweet potatoes, cotton, tomatoes, and peppers. The GRKN has been known to cause complete yield loss.

Related:Root knot nematodes demand multi-pronged attack

Difficult to identify

"It has wide distribution, a wide host range, and overcomes plant resistance. It is also difficult to identify in the field or the laboratory without significant effort," she said.

Differentiating the GRKN from the SRKN requires microscopy and molecular techniques to extract DNA, Rezende said. "That adds to time needed to identify."

The GRKN has been reported in Asia, Africa, South America, Central America and now North America.

"This is one of the most aggressive root knot species in the world," Rezende said.

Infected plant symptoms include galling on roots and bumps and cracks on sweet potatoes and black spots inside. It can overcome resistant plants.

"One nematode may lay as many as 400 eggs," Rezende said.

Louisiana survey

She said Louisiana is currently running a survey to determine if GRKN populations exist in fields other than the one in Morehouse Parish identified in 2018. The survey focuses primarily on cotton, soybeans and sweet potatoes.

"We found no GRKN last year," Rezende said. "We will expand the survey in 2020 and hope we find no more."

In the meantime, she said LSU AgCenter researchers are screening plants for resistance. "We have limited information available for Louisiana crops."

She recommends that growers look for suspicious symptoms and report them to the nematode advisory service at the LSU department of plant pathology and crop physiology. Check www.lsuagcenter.com/nas to download a reporting form.

Symptoms to look for include damaged roots, "especially if you see stunted plants. Look for large galls on roots. Look for bumps and cracks on sweet potatoes.

"GRKN symptoms are much more severe than SRKN symptoms," Rezende said.

Management options include rotation, resistant varieties (none have been identified so far), weed control (several weed species are good hosts for root knot species) and "make certain to use certified seed and disease-free plant materials."

The GRKN, although not identified in Louisiana last year, poses a serious threat to numerous crops grown in the state, so Rezende recommends growers watch closely for symptoms, which could be much more severe than SRKN damage.