Searching for resistance to nematodes
Resistant rootstock may be the only real answer in replant situations.
Mike McKenry believes "starve and switch" (killing nematodes on old roots and replanting with a nematode-resistant rootstock) will be the only way to replace methyl bromide.
Although there are no easy ways to deal with tree fruit replant and nematode issues, a University of California plant pathologist hopes that resistant rootstock may help address the problems.
Dr. Mike McKenry, stationed at the UC Kearney Agricultural Center in Parlier, has studied different components of replant issues: physical and chemical soil properties, nutritional needs and physiology of the tree, rootstocks resistant to nematodes, cover crops, nematicides and fumigants, and root physiology. No magic formula has been discovered yet, but he's identified a rootstock that shows promise for nematode resistance.
McKenry's soil work has taken on a more urgent note in the last decade with the phasing out of the soil fumigant methyl bromide and more stringent controls in California on the remaining fumigants. Vapam (metam sodium) is effective, but the "complexity of putting it on right is what makes it hard," he said, adding that Telone (1,3-dichloropropene) label restrictions requiring "serious" additions of water reduce the viability of the product. Soil fumigation is also expensive, around $650 per acre for Telone.
"One of the biggest problems in a replant situation is rejection," he said. "The plant [tree] starts to grow and then stops growing in June. You have uneven growth in the field until the fall or following June when the plants suddenly take off and grow," he explained. "What's happening is that the root systems are getting rejected, and you lose a year of growth. "
"Nematodes are usually mixed in with the soil and weaken the trees," McKenry said.
McKenry has established a Prunus rootstock trial to evaluate resistance to nematodes like rootknot, root lesion, and ringworm. Some 45 rootstock selections are in his trial, coming from around the world. Resistance is defined as a rootstock in which nematodes don't reproduce; tolerance would be a rootstock that is still standing in McKenry's trial 20 years later. Rootstocks are planted in fumigated and nonfumigated rows that have been treated with nematodes to compare their growth under fumigated and nonfumigated conditions.
He reported that Krymsk 1 grew equally well whether or not it was fumigated. Marianna was not tolerant to nematodes and did poorly, as did Cadaman and Viking. "Most rootstocks didn't tolerate all the nematode species completely."
After the fumigation trial, the second step is to take the rootstocks and try them under a replant situation, testing them in fumigated and nonfumigated treatments. Under the replant test, he found promise in Hansen 536 because it had no problems when planted after Nemaguard rootstock without fumigation, but he cautioned against overdoing Hansen 536 in sandy soils.
Unfortunately, 44 out of 45 rootstock selections couldn't handle root lesion nematodes, he noted.
Starve and switch
McKenry subscribes to the "starve-and-switch" theory as being the only sure-fire way to replant in nematode-infested soil without using methyl bromide. Under this regime, the old trees are sprayed with the full-strength herbicide glyphosate to kill the roots and then removed.
"Killing the tree drives the nematodes out of the roots but not out of the soil," he said. The switch comes by planting a rootstock with nematode resistance. "Walnuts have a nematicide inside the root system, but the root tips are like a lightning rod for nematodes," he said, adding that the new root system is highly susceptible. "We can't get away from methyl bromide until we can find rootstocks to do this."
McKenry has studied a number of different cover crops with the hope of suppressing nematodes. "But we're putting them at the wrong places," he said. "They need to be at the roots, not in the middle of the rows."
Sudan grass helps dry out the ground, he said, adding that some cover crops are deterrents to nematodes. "But they are not enough, and you'll still see replant problems."
Nematicides and elixirs can help reduce the number
of nematodes, but not eliminate them completely, McKenry concluded.