Sampling for Plant Parasitic Nematodes

Many growers are facing the question of replanting orchard blocks. This aspect of orchard maintenance is critical and essential in maintaining an orchard business. Outside of the general tree removal, soil and ground preparation, there are 3 key biological replant problems that must be taken in consideration when replanting. These include plant parasitic nematodes, aggressive soil pathogens and pests, and replant disease of almond. This blog will focus on the first mentioned aspect of replant problems: nematode sampling and nematode control.

In almond production, there are three main parasitic nematodes that are of concern – Ring, Rootknot, and Lesion. Depending upon your soil type, planted rootstock, and previous cropping history, the populations of these nematodes can vary within the soil. It is important to keep in mind that plant parasitic nematode populations are directly related to tree production – in other words, the higher the number present, the more tree damage to expect. A previous blog describes the affect of nematodes on tree growth.

Orchard Sampling to Determine Nematode Populations.
To determine nematode populations, soil samples collected from the root zone area of a tree should be sent off to a laboratory for processing. In collecting samples, the following procedure should be used:
1. By using a soil map, identify soil texture changes within the orchard block,
2. Time the sampling to occur after the year of cover cropping or fallow period – if one occurred,
3. Attempt to sample from different soil textures and areas of previous tree performance. A good sampling strategy for a 40 acre block can be seen in figure 1. Please note the sampling in different soil textures and random samples throughout the field,
4. Use a soil tube and sample the 4″-16″ depth of the profile – this is the area of most active root growth,
5. Place the soil in a PLASTIC bag, label with location, and place in a refrigerator until sent to a diagnostic lab. It is important to keep the soil out of the direct sunlight and extreme temperatures,
6. A minimum of 4 sample for a 20 acre block, 6 sample for a 40 acre block (Keep adding 2 more samples for every additional twenty acres) should be taken to give an accurate idea of the populations present.
Figure 1: Proper spacing for nematode sampling within a 40 acre block. Note the inclusion of different soil textures and areas of the field.

Interpreting the results.
Once the results are received back from the labs, the first question to answer is if nematodes are present or absent. If the populations are not detected, fumigation for nematodes may not need to occur. If they are detected, the population density will be used to determine which method of fumigation to use. It is important to note how the results are reported – the amount per liter of soil, per 250 mL of soil, etc. In order to use the following guidelines, please convert all results to nematodes per liter of soil.

Nematode Reduction for Orchard Sites.
Medium to Low Population Density (10-100 Nematodes per 1 Liter of Soil):
With a medium to low population density, many cultural practices can be used to help reduce nematode populations. These include an aggressive weed control program for 1-2 years before the orchard is removed, and taking a year off to cover crop with True Sudan. The principle behind these pratcices is to remove any nematode hosts (weed control) and plant a crop that nematodes feed on in order to “starve them out.” These methods will only serve to reduce the population, not eradicate.

In the new block, choose rootstocks that have genetic resistance to the nematodes found. A list of rootstock performance in nematode-infested soils can be found in Dr. Michael McKenry’s report in the 2008 Proceedings of the Almond Board of California.

If fumigation is possible, plan to treat a 6-8′ row strip with Telon-II at 33 gallons per treated acre.

High Population Densities (>100 Nematodes per 1 Liter of Soil):
Plant to follow the same cultural and rootstock methods as outlined above. The only change is that fumigation is strongly encouraged, and a broadcast application of Telone-II at 33 gallons per treated acre should be used.

These practices will help reduce nematode populations and the corresponding damage to young almond trees.

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