One important thing to remember about nematodes is that eradicating the pest is just about impossible, but they can be managed.

That was the clear consensus of a recent panel discussion for agricultural technology professionals at the North Carolina Biotechnology Center in Research Triangle Park, where experts discussed ways to control such troublesome pests as soybean cyst nematode and guava root knot nematode by utilizing  breeding, cultural  practices, resistance management and  other tools.

The challenge is that nematodes are very adaptable creatures. They have many hosts and can adapt to different geographies and varying environments. And the beneficial nematodes far outnumber the more troublesome ones.

Jennifer Riggs, research and development manager, seed treatment for BASF Corporation, stressed that the focus must be on management rather than eradication.

“We all think it’s easy to get rid of something that shows up. This is a pest that survives in soil; it can survive season to season on a weed and multiple hosts. The things that could be used to eradicate it also are not very sustainable in today’s environment when you think about the type of tools we have today,” Riggs said at the forum. The problem with chemical control is that chemicals also kill the beneficial nematodes.

4 of 5 animals are nematodes

Riggs shared an interesting fact that four out of five animals on earth are nematodes. She noted that a nematode is an animal, albeit very, very tiny, not an insect. She pointed out that only 10% to 15% of nematodes are considered plant pathogenic nematodes. There are a lot of beneficial nematodes out there that are a necessary part of the ecosystem.

Riggs noted that the soybean cyst nematode is the No. 1 pest of soybeans in the United States, and resistance management has proven successful in helping control the pest. She said the soybean cyst nematode can provide valuable lessons in managing the guava root knot nematode that is becoming more troublesome in sweetpotatoes and other crops.

“Fortunately for soybean cyst nematode there has been some native resistance out there for 20 some years that we see still working, but there’s been some shift in the effectiveness of that. There are many other tools that have gone along with that from cultural practices as well as crop rotations. The difference between the guava root knot nematode and soybean cyst nematode is the host. You do have crops you can rotate to with soybean cyst nematode and the nematode won’t reproduce or survive from one year to the next,” she said.

Riggs said it is critical for farmers to be educated and supported in their efforts to manage nematodes since the they are the ones who have to deal with the pest on their farms.

“They have to be able to understand it. Nematodes you can’t see. You see the symptoms of the nematode and you can get under a microscope and see them, but a grower can’t walk out in a field for the most part and see a bunch of dead plants like you can with some other pests,” she explained.

“You have to be a trained nematologist to see some of the damage you see out there. A nematode can’t kill its plant because it needs the plant to survive, to feed to reproduce, so it becomes very difficult for growers without an educational program to really understand the impact these pests have on their particular farms and in their particular environments,” she said.

Breeding resistant sweetpotatoes

As program leader of the sweetpotato and potato breeding program at North Carolina State University, Craig Yencho says 60% of the university’s sweetpotato breeding work is now focused on finding varieties that are resistant to guava root knot nematode. He acknowledged it is a daunting challenge, but is optimistic solutions will be found for farmers battling the pest.

Yencho says  the challenge with sweetpotatoes and guava root knot nematode is that the sweetpotato is asexually propagated from cuttings, not a seed. Yencho noted that seeds are generally cleaner and don’t harbor as many pathogens as cuttings.

A challenge in sweetpotato breeding is that the sweetpotato is a complex hexaploid, making it different than maize or soybeans. Yencho points out that the sweetpotato is not an inbred or a diploid.

“Every cross I make results in extreme segregation. Every new variety is an F1 hybrid. My challenge as a breeder is when I make a cross it’s segregated for a myriad range of traits. Since sweetpotato is a food crop that people pick up off the grocery shelf and judge it by its appearance, or its quality, the size of the storage root and the potential flavor, I have to package all of those traits and it’s roughly about 40 plus traits into every new sweetpotato variety,” Yencho said.

“If I had back crossing, if I could put it in an inbred, and I could just use a single gene and I could put that in a nice inbred parent and then make a hybrid and get that in using back crossing and hybrid selection, that would make my job easier. But that’s not my life,” he said.

Yencho noted that his program is now using genomic selection for the first time in breeding sweetpotato varieties. He is optimistic the technology will help his team find a variety that’s resistant to guava root knot nematode. He believes gene editing can be used to help improve the process.

“We can build good parents and sort of stack the deck if you will when I make that cross and I can pull out what I say is a royal flush so I can stack the deck using molecular markers. Because this trait is single gene, I can maybe build parents using our markers that have multiple doses of that trait in it and then make the cross and the probability of getting resistance in my progeny is much higher,” Yencho said.

In the meantime, using biologicals to control the guava root knot nematode still presents many challenges. Wiseborn Danquah, senior field development specialist with Novozymes BioAg, explained that many biological control products are living organisms that work in in- vitro conditions, but often won’t show that same performance when taken to the field.

“Most of the assays for biological controls that most scientists have developed are not scaled for field conditions, but for lab conditions,” Danquah said. “When you put them against any challenge, they are able to effectively overcome those challenges in a lab condition. However, when you go into the field there are so many other factors that  interact with these biological organisms that we are not going to be able to go in the field and put in as many of the interventions that we put in under in vitro conditions in a field.