To overcome the replant problem in replacement almond and tree fruit orchards, a University of California, Riverside nematologist has developed five steps for what he calls a “starve and switch” strategy.

Michael McKenry described the method during a series of talks and field demonstrations for growers and PCAs at the Kearney Agricultural Center at Parlier dealing with alternatives to the fumigant methyl bromide, the use of which is under increasing restriction.

After 15 years of research with almond, tree fruit, grape and other crops, McKenry concluded that the replant problem, or the persistent failure of replacement trees or vines to grow properly without preplant fumigation, is not a disease per se, but made up of four components.

“There is something very general that occurs in the new planting following removal of a Nemaguard root system,” he said.

The components are the “rejection component,” soil pests and diseases, the soil physical and chemical composition, and nutritional elements at planting and afterward.

“The rejection component is the complete biology that lived off the previous root system and leaked out of it for many years,” McKenry noted.

Prunus replant problems can be solved on 65 percent of sites, he said, by:

  1. identifying prevailing soil problems

  2. applying Roundup and waiting one full year

  3. preparing the soil to remedy physical and chemical problems

  4. replanting with a rootstock having different parentage

  5. applying very small amounts of starter nutrients.

McKenry learned he could destroy old roots by spraying Roundup on the trunks after the final harvest of an orchard. The systemic herbicide, applied by mid-November, can kill 95 percent of Nemaguard roots in about 60 days.

“As a side benefit, if the root system is infested with ring knot nematodes they will die off with the roots. Root lesion nematodes, however, can survive for two years,” he explained.

Following the root kill, the acreage is fallowed for one full year to “starve out” the surviving pests to destroy 85 percent of the biology of the rejection component.

In his almond trials he found that by replanting a Nemaguard site with Hansen 536 rootstock after the above procedure, the trees were uniform in development, as opposed to replanting with Nemaguard, which showed uneven growth.

“The Roundup is important, but switching the rootstock is equally important in Prunus,” he said.

Thus, in almond, a “switch” from Nemaguard to Hansen 536, he continued, can be done without fumigation. “Unless,” he cautioned, “you have ring nematodes, because Hansen 536, particularly in sandy soils, can't handle them.”

Almond growers need rootstocks with nematode resistance, but they also need a different parentage than Nemaguard to avoid the rejection component. But much depends on the species of nematodes and the soil textures of the site.

McKenry searched the globe for 41 potential rootstocks for Prunus. He found 30 to be resistant to root knot nematode, but only two of them, the dwarf types for tree fruit, Krymsk 1 and Krymsk 2 from the Ukraine, have resistance to root lesion nematode.

“We need more rootstocks than what we have available,” he said. Among new rootstocks not yet thoroughly researched are the HBOK (Harrow Blood x Okinawa) series, which has shown protection against ring nematode and potential tolerance to the rejection component.

MCKenry also discussed his separate research on the movement and fate of Telone II fumigant in the soil, noting that the soil texture and type govern, to a large extent, the amount of moisture and air spaces in the soil.

“The most far-reaching diffusion patterns in mineral soils are those obtained in soils whose moisture content is nearest the wilting point of plants, or 15 bars moisture tension,” he said.

“As the moisture content of the soil is increased, the diffusion pattern gradually becomes more limited. This effect is most striking when fine-textured soils have moisture contents in excess of half-bar moisture tension at the 1-foot depth.”

Soil air space and size of pores are important because the chemical moves primarily in the vapor phase and the smaller pores are most easily blocked when water is present, McKenry explained.

That's why it is so important for fumigant applicators to thoroughly seal the soil surface and chisel shank “chimneys” after an application.

Failure to do so will cause significant losses to the atmosphere, especially if the subsoil is in a moist to wet condition, he added.

Soil temperature also influences the results. Warm soil at about 77 degrees allows for faster and farther diffusion. Soil at 41 degrees, on the other hand, makes for slower diffusion, and although the chemical persists longer, the diffusion distance of an effective dosage is decreased.

“Fumigation is all about how well you distribute the product to get to the nematodes,” McKenry said.

Replant problems also exist in walnuts, and Bob Beede, Kings County farm advisor, said fumigation needs to be made on a broadcast, not strip, basis.

“And most likely, I'll be recommending addition of chloropicrin to Telone because of research showing improved results with the combination,” he said. Both materials have county regulations on amounts.

Citing McKenry's findings, Beede said new walnuts following removed walnuts require fumigation for a change in the orchard's soil ecosystem to eliminate “the bugs” and the rejection issue.

More importantly, walnut growers have to think in terms of an orchard lasting 40 years and not the six to eight year lifespan of a peach orchard.

He also warned that removal will become more difficult and costly over time, with recent reports of $800 to $1,000 per acre to remove old walnut roots.

“Other crops have discovered sufficient genetic diversity for rootstocks to overcome the replant issue, but none exists for walnut,” Beede said, adding that Butternut is being researched as a potential rootstock.

Own-rooted Serr may have some potential for staving off certain nematodes. Clonal Paradox-family rootstocks are also being investigated, but breeders say the proving process takes 20 years.

Beede listed challenges in removing walnut orchards, ranging from anticipating the greater size of walnut roots to meeting requirements for moisture and dirt content of chipped orchard residue by cogeneration plants.

For growers replacing walnut orchards, Beede said, “We recommend complete root removal and modification of the soil profile to a depth of 6 feet. We highly recommend slip-plowing, even in fairly uniform soils, because walnuts love oxygenated soil and this is your only opportunity to make the decision. Once they are in the ground, you'll have to live with them for another 40 years.”