By Saad L. Hafez and Mahesh P. Pudasaini, University of Idaho
Plant parasitic nematodes are minute, microscopic, worm-like animals found in soil that feed on plants, reducing crop growth and yields. They possess a stylet in the mouth, which they use to puncture plant cells and feed on cell contents. They commonly attack the roots producing symptoms of general root insufficiency: unthrifty growth, leaf yellowing and wilting. All crops are attacked by one or more nematode species, but particular nematodes attack only certain kinds of crops. Crop varieties can vary from highly susceptible to highly resistant. Crop damage is related with the nematode population level at planting.
It is essential to manage the nematode population in the field if it is above the economic threshold level (ETL). ETL is the population level at which nematodes cause economic damage to the crop. Therefore, nematode analysis is necessary before planting the crop to estimate possible damage and to help decide the suitable management options (e.g. resistant varieties or rootstocks, soil fumigation or treatment with a nematicides, or green manure crops as bio-fumigation, etc.).
Nematode infestations in Idaho result in yield decline and/or reduction in quality, thereby contributing economic loss to the growers and industries. Since 1982, the University of Idaho Nematode Diagnostic Laboratory at Parma has been continuously analyzing soil samples and advising on nematode management.
The collection of soil samples and plants are the first step in the diagnosis of crop disorders caused by nematodes that attack root systems. An accurate diagnosis depends on proper collection and processing of samples. Improper collection and handling of samples may lead to the dismissal of nematodes as part of the problem; hence any management strategy developed to alleviate the problem will be deficient.
Information such as crop and cultivar, previous cropping history, history of other known or suspected problems, irrigation or rain fed, and previous applications of soil amendments (organic or any pesticides) are needed along with samples to assist in the diagnosis of the problem. The location of the samples (field ID, town, district, county and state) is important. This information will permit comparisons with other problems reported previously from the region, or indicate if the samples represent the first report of a nematode species from the area.
Several nematodes species such as cyst nematodes (Heterodera schachtii), root-knot nematodes (Meloidogyne chitwoodi, M. hapla), root lesion nematodes (Pratylenchus spp.) and stubby root nematodes (Trichodorus and Paratrichodorus spp.) are the most serious problems in major crops and a constant threat for the growers and industries in Idaho and Eastern Oregon. Fields cropped repeatedly to the same crop have experienced significant losses depending on the initial population of these nematodes present in the field at planting time. This is evidenced by the increase in the number of soil samples containing high populations of nematodes submitted to the University of Idaho Nematode Diagnostic Laboratory at Parma.
Additional evidence is provided by the high percentage of acreage of several crops that are fumigated for nematode management. If nematode populations (based on soil sampling) are high enough to warrant applications of a nematicide, treatment is generally done preceding or at planting of desire crop. This application is costly and requires careful management because of health and environmental concerns.
Growers and industries should take soil samples regularly for nematode testing, particularly before planting and at harvest. Samples should be analyzed in the fall or early spring to determine ETL so that best management option can be applied. Growers should keep records of population levels in different blocks, in different crops before planting (spring), reduction of nematode population due to treatment, etc. to assess the effectiveness of management programs and such practices further reduce the input of the growers. Since adoption of nematode management practices are need-based, growers are advised to treat the soil only if they have identified the specific nematode that cause damage to that particular crop.
Number of samples and Sampling pattern
Distribution patterns of plant parasitic nematodes in the fields are non-uniform and non-random. They are irregularly distributed across the field in aggregated clusters. Thus, estimates of nematode population densities are inherently imprecise and subject to a high degree of variability. Therefore, it is essential that soil samples be composite of multiple sub-samples and the numbers of sub-samples and the area that one composite sample has to cover should be optimal in terms of minimizing the variation and optimizing time and effort.
Once the area to be sampled has been identified, sub-samples can be collected using a systematic sampling procedure such as a zigzag, `W', or `X' pattern. A sample composited from two sub-samples per acre and an area of no more than 25 acres per composite sample is optimal. This means 50 sub-samples per 25 acres produce one composite sample. At least one quart soil should be taken from the composite sample after well mixings and sent to University of Idaho nematode laboratory for nematode analysis.
If the field is larger than 25 acres, multiple composite samples should be collected. Large fields should be subdivided based on differences in soil type, crop history and previous yields. Portions of a field with a history of high yields are unlikely to have a serious problem and need not be sampled intensively, whereas portions of the field with a history of low yields should be samples more intensively.
When collecting samples from a current crop, it is still necessary to collect composite soil samples. One should avoid collecting sub-samples from plants that are dead, but instead concentrate on living plants that are exhibiting a range of symptoms. Separate samples should also be taken from around plants that appear to be healthy, for comparison. Sampling fields outside the cropping season or when the soil is very dry will yield very few active nematodes when extracted. In all cases, it is important to collect sub-samples from the root zone of the crop. Typically this means collecting the soils from beneath the crop canopy.
The depth of the sample taken will vary somewhat with the crop. The vertical distribution of nematodes is usually proportional to the vertical distribution of the root system, but it is seldom necessary to collect samples from depths of greater than 15 to 18 inches or discard the upper 2 inches of soil, which will contain few nematodes. The usual range is from a depth of 12-15 inches. Occasionally, more shallow samples are sufficient when plant roots grow near the surface.
While collecting soil samples, it is also best to collect root samples to aid in the diagnosis. When doing so, one should dig up the plant so as to obtain as many of the fine feeder roots as possible. If the plant is pulled from the ground, most of these feeder roots will be lost. If sampling a perennial crop, it is also important to collect feeder roots specifically from the current year's growth rather than larger and older roots. It will be difficult to make an accurate diagnosis from a sample that only contains large roots.
Of course, some nematode parasites are rarely found in the soil or roots but are found primarily in the bulbs, corms, stems or foliage. In such cases, care must be taken to collect the appropriate symptomatic tissues. Again, samples should not be taken from long dead plants, as the parasites may be difficult to detect in such samples. It is best to collect samples from a number of live plants that are exhibiting a range of symptoms.
There are different tools to collect soil, such as soil probes, trowels, hoes, narrow-bladed spades or shovels. However, they are collected most efficiently with sampling tools designed for the procedure, such as a standard Oakfield soil probe with a diameter of one inch. If using a shovel or spade, it is best to collect only a narrow column of soil from each shovelful of soils to avoid excessive sample volumes. The multiple sub-samples should be thoroughly mixed together in a large bag or bucket and a final sample of 1 to 2 liters of soil placed in appropriately labeled plastic bags.
Care of samples after collection
The biological vitality of the sample should be preserved after the collection of plant and soil samples as extraction and identification procedure needs live nematodes to achieve the best accuracy. Samples should be delivered or shipped to the diagnostic laboratory without delay. Samples should be protected from extremes of temperature, i.e. freezing (less than 32 degrees F) or temperatures above 95 degrees F. Thus, they should be packed in insulated containers and kept in a cool environment. Refrigeration (storage at 40 degree F) is not required if the sample is being processed within a day or two, but is helpful if the samples will be stored for longer time period. It is usually not necessary to pack samples in ice for shipment, but shipping over a weekend or holiday period should be avoided. This will reduce the possibility of the samples being left unprotected on a loading dock or in a warehouse for several days. A good rule to follow is to treat the samples like perishable food that one wishes to consume in three to four days.