Part of the fumigant decision-making process for new and replanted almond orchards should include a thorough assessment of current nematode populations. That means conducting proper soil sampling coupled with proper laboratory examination, as well as an understanding of differences in soil type and texture.
Observations of specific pest pressures in the field and those in neighboring areas also are useful when making decisions where to sample and whether to fumigate before planting almonds.
University of California Agriculture and Natural Resources Assistant Nematologist Andreas Westphal at the Kearney Agricultural Center in Parlier, CA, says: “It’s always important to know what crop(s) were there in the past because that can be a first indication of whether to expect nematode problems.
“Grapes, for instance, are a good host for nematodes,” Westphal says. “So while you may have less of a replant disease problem, you may very well inherit a nematode problem from previous crops.”
Sampling Always Comes First
Sampling should be done not only before planting, but before any treatment or management decision is made.
When and where to sample depends on previous plantings and length of fallow. If sampling an existing older orchard, it is a good idea to sample from the feeder root zone near the drip line where nematode populations are likely to be most active. Uneven growth in an existing orchard can also provide a guide for where to sample for possible nematode hot spots.
When almond follows almonds or other Prunus species, particular attention needs to be paid to replant disease, as well as nematodes and other soilborne problems. Replant disease describes a general growth suppression of young almonds.
The actual cause of replant disease is poorly understood, but work by Greg Browne, USDA-Agricultural Research Service, supports the current hypothesis that it is the result of microbial communities left behind by the previous crop. The good news for growers is soil fumigation is also the key remedy for replant disease.
Ideally, growers should leave orchards fallow or cover-cropped for a year to minimize the impact of replant disease. How long the field has been fallow should guide how it is sampled, Westphal says.
“The longer the field is fallow, the deeper you want to sample,” he says. “A lot of nematodes occur quite deep in the soil. Many labs that offer the sampling service go 18 inches deep and in many instances that’s enough. The orchardist needs to be aware, though, that large populations can occur at 3 to 4 feet or deeper.”
In fallowed ground, preplant sampling is best timed in early fall. In the extreme cases of very deep rooting soil and long fallow periods, a thorough sampling scheme calls for an auger to sample at 1-foot depth increments down to 5 feet deep. At least 10 core samples per acre may be necessary, depending on variability in soil texture, to provide an adequate representation. In the average cases, a sampling to 18 inches will probably be sufficient.