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ContinuedfromPage12 soil texture requires ripping or backhoeing to mix the soil to prevent water infiltration problems. Failure to properly break up or mix soil layering will lead to root health problems and premature orchard decline.

Aggressive Pathogens

Aggressive pathogens can also persist in the new orchard. Typically, these should be identified from observing and sampling failing trees from the previous orchard. For example, almond roots infected with Armillaria root rot or oak root fungus (Armillaria mellea) may have white mats between the bark and wood.

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While some pathogens may be identifiable in the field, samples should be sent to labs to confirm a diagnosis. This can be done with help from a local farm advisor or PCA.

The UC Integrated Pest Management Program is a resource that can help identify a problem and provide recommendations on what can be done. Generally, using a resistant rootstock is the best way to mitigate challenges with aggressive pathogens in replanted orchards, so refer to the UC ANR rootstock database or the rootstock comparison chart for more information.

Plant Parasitic Nematodes

A common challenge for replanted orchards is the presence of plant parasitic nematodes, such as root-knot, root lesion and ring nematodes. While many other species of nematodes live freely in the soil and are not detrimental to crops, plant parasitic nematodes can impact root health and development, which impacts tree vigor.

Soil and root sampling can help determine if parasitic nematodes are present in a field and, if present, type and number of nematodes. As nematodes are not distributed evenly across a field, appropriate sampling methods should be implemented to collect a representative sample. Below are some guidelines to help sample for parasitic nematodes:

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Sampling: Ideally, sample in fall as nematode populations are the highest during this time, allowing the best chance of detecting them. However, sampling can also occur in the winter and spring, but is limited to a comparison between “good” and “problem” areas when sampling different sections of an orchard.

Tools: Shovel, soil probe or auger.

Depth: For almonds, collect soil from 4 to 24 inches below the soil surface. Use a shovel to clear away the top four inches of soil and use a soil probe or auger to sample from there to 24 inches deep.

Location: Collect soil from multiple spots in an orchard and then combine these into one sample with a total volume of about 16 oz (or about 2 cups). Sample near trees in order to include roots with the soil sample. Collect soil from at least five locations in one orchard (more for larger blocks). Make sure to sample across the orchard, not just right next to the road. Samples are supposed to represent an orchard’s nematode populations, so take extra effort to sample enough locations. As with physical and chemical soil sampling, you might consider submitting separate samples for “good” areas and “problem” areas in your orchard.

Sanitize: Sanitize your tools before beginning to sample, when sampling a new orchard and between “good” and “problem” area sampled in one orchard. Contaminated tools can lead to lab results detecting nematodes in many fields when they were really just present in one

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field. Remove any stuck-on soil and then use appropriate disinfectants or flame with a propane torch to sanitize tools. Label sample: Clearly mark the sample with the location and date. Location is especially important if you are submitting multiple samples.

Storage: Store samples in a cool location like a refrigerator or ice chest. This helps nematodes in the sample to survive long enough for lab analysis. If using an ice chest, insulate the samples from direct contact with the ice and protect samples from water as the ice melts.

Lab reports show what types of nematodes are present, as well as the number of each type per volume of soil. Sometimes it may be important to request more information from the lab analyzing samples. For example, there is more than one species of root lesion nematode, but only one is considered to be a problem in orchard crops (Pratylenchus vulnus), so be sure to ask the lab to identify which species of root lesion nematode is present in your orchard. Which additional information is needed is dependent on your location and the types of plant parasitic nematodes that may be a concern in that location.

The most effective mitigation strategy will depend on both the type and number of nematodes present. For high nematode levels, pre-plant soil fumigation and the use of resistant rootstock might be the best option. At lower nematode numbers, the use of a resistant rootstock may be sufficient. Consult with a farm advisor or PCA to determine what strategies would be appropriate to help manage your specific nematode situation.

Prunus Replant Disease

Prunus replant disease is a challenge for many almond orchards planted following almond or other Prunus species. The causes are poorly understood but the disease is characterized by reduced tree growth in replanted orchards. It typically occurs in loam, sandy loam and sandy soil textures, and can occur with or without plant parasitic nematodes. Unfortunately, there is no test for Prunus replant disease, but we know that fumigation and anaerobic soil disinfestation can help mitigate the problem. We also have evidence that waiting a year between removing the old orchard and planting the new orchard can help to mitigate this problem.

While removing and replanting an orchard is challenging, it is important to consider what issues the new orchard will face. Trying to fix problems after planting can be costly and difficult, and failing to adequately address these challenges can lead to failure of the replanted orchard. Make the time to identify, mitigate and plan for replant impacts before planting an orchard as this will give your trees the best start toward becoming healthy and profitable.

Resources

Contact the authors: Cameron Zuber at cazuber@ucanr. edu, Jaime Ott at njott@ucanr.edu and Roger Duncan at raduncan@ucanr.edu

UC ANR rootstock comparison tool: fruitsandnuts.ucdavis. edu/rootstocks/rootstock-comparison

UC ANR rootstock comparison chart: fruitsandnuts.ucdavis.edu/almond-rootstock-comparison-chart

UC ANR rootstock database: fruitsandnuts.ucdavis.edu/ rootstocks

UC IPM for Agricultural Pests: ipm.ucanr.edu/agriculture/

UC Davis, NRCS and UC ANR SoilWeb Apps: casoilresource.lawr.ucdavis.edu/soilweb-apps/

USDA Web Soil Survey: websoilsurvey.sc.egov.usda.gov/ App/HomePage.htm

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