Andreas Westphal, scientist at Kearney Agricultural Research and Extension Center in the San Joaquin Valley, organized a soil plant pathology conference that was held late last month. Growers from as far away as Virginia came to learn about the latest research findings.
Chandler Farms
The first day of the conference was a field tour to three sites and then Kearney itself. After meeting at Kearney, the participants rode on a tour bus to Chandler Farms in Selma. John Chandler, his brother Tom and their father, Bill operate a diversified farm that includes almonds, grapes and citrus.
Soil Fumigants
The Chandlers typically change out one field per year. John lamented the loss of methyl bromide for replanting an orchard or vineyard. He now uses Telone, which he finds “has less effect but still works.”
He always contracts out his Telone application because it requires special training. He has the contractor put tarps over it, beyond the usual seal — to prevent off-gassing — but he says, “It’s not worth it to get acres of plastic.” The tarps add about $1000/acre to the cost.
Westphal explained, though, that Telone is becoming more difficult to use because of a new restriction in the amount of Telone allowed plus a requirement that the fumigant be applied with lots of water. The new regulations say that the soil has to be wet to 2 feet depth. If the soil is bone dry, three inches of water must be added within 72 hours before applying the fumigant.
However, the excess water makes it hard to get a tractor through the field. Also, the fumigant does not move well through water, as it is made to move through air. Lastly, the Telone stays in wet soil longer before vaporizing off, which may make it toxic to plants.
Nematodes
Chandler said that his farm has nematodes — the microscopic roundworms that feed on plant roots. His soil has root-knot, lesion, ring and citrus types of nematodes.
When he can afford it, he fallows the land for a year or two instead of fumigating, but he says that fumigation gives the plants the best chance to get started. Once the crop is established, he notes, there are few tools for nematode adjustment.
In his Ruby Red grape concentrate vineyard, Chandler finds that ring nematodes like the wettest part, whereas other types of nematodes prefer the edges, where they feed on the fresh, new roots. Ring nematodes, he finds, like pure sand, but root knot nematodes thrive most in clay soil with good secondary structure.
The tour participants had many more questions for Chandler, but to stay on schedule, Westphal guided everyone back onto the tour bus.
Burchell Nursery
The second stop was Burchell Nursery in Fowler, where Mauricio De Almeida led the group around the almond, stone fruit, citrus, pomegranate, and retail table grape plants.
What Is Eating the Plant Roots?
Almeida said that, as in all greenhouses, the water, heat and humidity may induce root rot and fungus gnats. The fungus gnat larvae eat soft tissue of the plant roots. The adults do not feed on plants.
Another larva that needs to be managed is that of the European pepper moth. The caterpillars girdle the plant stem below the soil line.
The root rot and insect pests occur even though the potting media — made of peat, bark, wood fiber and sometimes Perlite — comes pasteurized. The nursery uses Nemasys® beneficial nematodes to manage the larval insect pests.
Citrus
Almeida explained that Burchell Nursery started growing citrus in 2010. They grew the citrus indoors to protect the trees from the Asian citrus psyllid. Westphal remarked that if you grow bareroot trees in the field, the soil must be certified nematode-free or else the trees themselves must get tested for nematodes. He also said that you cannot fumigate after pulling up the plants because the fumigant would kill the tree.
Almonds
In almonds, Bright’s Hybrid 5, Hansen and Viking rootstocks and Shasta scions resist root-knot nematode. Viking and Shasta also tolerate ring nematodes. Bright’s Hybrid also has low susceptibility to root lesion nematodes. In susceptible varieties, root-lesion nematodes create lesions that can lead to secondary infections like bacterial canker.
The visitors got to see the Cornerstone cultivar of Prunus rootstock, developed by Burchell Nursery to resist the root knot nematodes Meloidogyne javanica and M. incognita. Used as rootstock for almond and stone fruits, Cornerstone is also less sensitive to crown gall, caused by Agrobacterium tumefaciens.
Unfortunately, in both Kern and Merced counties (but not Fresno County where the tour took place), in 2018 a root-knot nematode from Florida, Meloidogyne floridensis, was found in almond trees on Hansen 536 and Bright’s Hybrid 5 rootstocks, where they caused growth decline of the trees. Now considered an A-rated (“highest concern”), quarantined pest in California, it has since been found in South Carolina and Georgia, after being endemic to Florida before 2018.
Levels of Plant Response to Pathogens
Plants can be divided into three groups of responses to nematodes and other pathogens: sensitive, tolerant and resistant. Tolerance allows for high nematode populations, which gives the nematodes more opportunity to have genetic mutations that enable them to overcome the plants’ resistance. Therefore, nematicides should be used in tolerant cultivars to limit the pest population.
Grapevines
In a vineyard section of the research fields, Westphal showed a poster of nematodes in the soil profile. Of the three types of nematodes represented, ring nematodes were found only in the top foot of soil.
By contrast, citrus and pin nematodes were found in each one-foot segment down to 4-5 feet, the deepest tested. The citrus nematode population peaked at 2-3’ depth and pin nematodes peaked at 3-4’ depth. The population of pin nematodes far exceeded those of ring and citrus nematodes at all depths. Fortunately, pin nematodes do not cause much economic damage in grapes.
Westphal also showed a poster of total sugar yield in 2025 of Pinot Gris cultivated on different rootstocks planted in 2023 in nematode-infested soil. The negative impact of nematodes was measured as sugar content of the fruit.
Of the seven commercial grapevine rootstocks tested, Freedom had the highest sugar content. An experimental rootstock had even higher sugar content. Serving as control groups, Chardonnay and Zinfandel rootstock had the lowest sugar content.
“On sensitive rootstock, even a few nematodes are bad,” Westphal remarked. About 85% of all new grapevine plantings are on Freedom rootstock. Freedom’s downside is susceptibility to quick decline, so the search continues for an even better rootstock.
In sugar beets, the exudate from plant roots helps nematodes find the roots. Westphal and colleagues are looking into the possibility of manipulating plants to exude different chemicals that do not attract nematodes.
Walnuts and Almonds
Walnuts are sensitive to some nematodes and resistant to others, but “every plant has its limits,” Westphal said. Walnut soil is treated pre-plant and at-plant at the planting site. Tank water is used to settle the tree in. Salibro® is added to the tank water to form a concentration that kills the nematodes but not the trees. Westphal cautioned growers to not overwater walnuts.
Among other things, Westphal is testing juice from walnut hulls to see if they inhibit nematodes. He noted that ASD (anaerobic soil disinfestation), created by applying nine tons of rice bran per acre, inhibited nematodes somewhat. However, it would not be worth the $6,000/acre price tag for growers.
Highly resistant rootstocks can grow without fumigation, with less toxic, affordable nematicides applied. As with citrus, nursery trees must be grown in nematode-free soil, or the trees must be certified before leaving the premises.
Westphal showed the tour participants an almond rootstock test orchard, in which nematode resistance is compared among various commercial and experimental rootstocks. Only the most successful experimental rootstocks make it to the next round of trials, along with the commercial ones for comparison.
Westphal then concluded the field tour. The conference continued in an indoor classroom on the following two days.
– By Nancy Power, Assistant Editor