GPS-guided, shank-applied, spot treatments with fumigants are among experiments by USDA and University of California scientists seeking solutions to long-standing problems with replant disease in almond and stone fruit orchards.

Without soil fumigation and/or other steps before planting, severe Prunus replant disease (PRD) kills or prevents growth in half or more of replacement almond or stone fruit trees in California. Even surviving trees remain stunted with poor production.

Much is unknown about PRD, but it is associated with a complex including soil conditions, fungi, bacteria, and molds from the preceding crop. Parasitic nematodes can also play a role.

Greg Browne, USDA research plant pathologist at UC Davis, detailed current PRD research progress at a recent gathering at the Kearney Agricultural Center, Parlier.

He leads the Pacific Area-Wide Pest Management Program for Integrated Methyl Bromide Alternatives, which is investigating fumigant and crop rotation-based approaches for preventing PRD at various sites in the state. Research is being supported by the almond and tree fruit industries.

“We have physical and chemical and chemical aspects to the problem, and we sometimes find aggressive pathogens such as Armillaria, Phytophthora, or Verticillium,” Browne said. “Sometimes we find root feeding insects, such as ten-line beetle, and nematodes, especially ring nematodes, on sandy soils, but even in the absence of all these, we are left with PRD. It is almost universal on loamy or sandy-loam soils.”

Another sign of PRD is darkened, smaller fine roots. “The action really seems to be in the roots, and we see in fumigated Lovell peach, for example, a significant increase in fine roots compared to non-fumigated.”

No soil tests to predict the extent of PRD are currently available, but Browne said for the time being local experience of growers and field trials can be very useful in predicting the risk and severity of the disorder on a given soil series with a given crop history.

Browne’s team of plant pathologists, soil and water specialists, agricultural engineers, growers, and others is probing an array of practices to find alternatives in view of regulations on methyl bromide, Telone formulations, and other fumigants.

Methyl bromide, classified as an ozone-depleting compound, is now restricted to temporary critical-use exemptions. Use of Telone products is limited for each township.

In California, air-quality control limits have been set on agricultural use of materials that release smog-forming volatile organic compounds into the atmosphere. Soil fumigants are also subject to buffer zones based on the amount of them applied in a field.

“Given the high cost of soil fumigation and the dynamic nature of regulations governing it,” Browne said, “it is important for growers to get maximum benefit from every pound of fumigant used.”

The multi-year research in orchards at Parlier, Firebaugh, and Madera, he added, has revealed a number of findings that may contribute to solutions. Results, based on tree growth indicated by trunk diameter, include the following:

– Chloropicrin and mixtures of it and iodomethane (not registered in California), Telone, or methyl bromide are more effective than the latter three alone.

– Rates of 300 pounds to 400 pounds per treated acre of chloropicrin or mixtures of it with Telone or iodomethane appear optimal for prevention of PRD. Product rates and labels are subject to local regulations.

– Short-term rotations with Sudan grass, wheat followed by mustard, or a single season of fallowing can reduce effects of PRD.

– Over or under-irrigating almond trees replanted without pre-plant soil fumigation after removal of almond on peach rootstock can worsen PRD.

– GPS-controlled, shank treatments applied to tree sites before planting with chloropicrin or Telone, or drip irrigation-applied spot treatments with a formulation of the two chemicals, appear nearly as effective as strip or broadcast treatments with the same fumigants.

The spot-fumigation system, developed by Shrini Upadhyaya, agricultural engineer at UC Davis, in cooperation with TriCal, Inc., is basically a conventional shank-rig mounted on a John Deere 7400 tractor.

It is controlled by a GPS computer for automatic and precise injection of fumigant in a prescribed grid of rectangular areas where trees will be planted. The method saves considerable material compared to a broadcast application.

The rig is scheduled to be used in additional commercial trials this year. “We have to do some proving, but with everyone testing and evaluating our findings as we go, I think there’s a lot of potential,” Browne said.

Another speaker at the Parlier gathering was Bruce Lampinen, orchard irrigation specialist at UC Davis and member of the research group. He warned of keeping close watch on irrigation in replanted almond orchards.

“In early season water management — April through June period — the first year is really critical,” he said.

“Without fumigation, careful irrigation management is much more important, and either over or under-watering can really reduce growth and long-term productivity. Careful irrigation can overcome some of the problems associated with lack of fumigation, but not all of them.”

Fumigation, he added, is essentially an insurance policy to offer some leeway in irrigation accuracy with young trees and their smaller root systems. “If you shut down these trees, especially on non-fumigated soils, it’s really difficult to get them growing again.”

Information on pressure chamber use to determine stem water potential and related guidelines for irrigation of almonds is available at ucmanagedrought.ucdavis.edu/.