Figure 1. Soybean cyst nematodes females (yellow arrows) and nodules (white arrows) on soybean roots. Photo credit: Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org
Nematodes are thread-like roundworms that live in a wide range of environments including soil and fresh and salt water. There are species of nematodes that feed on fungi, bacteria, protozoans, other nematodes, and plants. They can also parasitize insects, humans, and animals. Nematodes that feed on plant parts are called plant parasitic nematodes (PPN) and are ubiquitous in agricultural soils. The life cycle of a nematode includes eggs, juveniles and adults, and they can overwinter at any of these stages. Crop damage is the result of a complex interaction of the environment, initial nematode populations at planting, the pathogenicity of the nematode species and the ability of the plant to tolerate nematode feeding.
Most PPNs feed by piercing and killing root cells with needle-like structures called stylets. Nematodes that utilize this type of feeding include lesion, lance, needle, sting, stunt, and sting nematodes. Some of the most economically damaging nematodes like the root knot nematode (RKN) and soybean cyst nematode (SCN) enter roots and establish a permanent feeding sites where they complete their life cycles without killing the cells around them.
Symptoms associated with nematode infection are similar to those caused by impaired root growth and function, therefore they may resemble abiotic stress like drought and nutritional deficiencies as well as biotic factors like stem and root rots. General symptoms from nematodes include yellowing, stunting, and wilting, accompanied by a yield decline. In the case of SCN, signs of infection are white-to-pale-yellow female bodies present in roots that can be seen with the naked eye (Figure 1). Nonetheless, above ground symptoms are not always obvious and infections can go undetected until populations are well-beyond economic thresholds. The RKN causes root galling, however, the degree of galling may depend on the interaction between the plant and the RKN species.
Nematode assays from soil samples are the only way to confirm the presence and population density of PPNs in your field. Once a crop is planted and roots start to grow, it will serve as the food source to support nematode reproduction and population growth. As such, the best time to monitor nematode densities in your soils is right before harvest or at harvest when nematode populations reach their highest levels. These results can be used to predict potential nematode problems in future crops. However, it should be noted that sampling can be conducted during the growing season if there is suspicion of nematodes affecting your crop. Samples collected in the spring will normally have reduced levels of nematodes due to unfavorable conditions for the pathogen and lack of host crops, and in some cases species like the RKN may not be detected.
Sampling for nematodes: Collect 1-inch-diameter soil cores to a depth of 8 inches. For sample collection follow a zigzag pattern and take soil cores from within the rows and between plants. Collect the soil cores in a bucket and mix them together to create a composite sample. Place the composite sample in a plastic bag, keep it protected from direct sunlight or heat, and store in a refrigerator until submission. Obtain at least 20 soil cores for areas of 20 acres or smaller. For patches of stunted and yellow plants, samples should be collected from the margin of the affected areas, avoiding the area where plants look heavily damaged. Additionally, you can dig out plants and send specimens to a laboratory for a more accurate diagnosis. If you intend to send plants, please contact the laboratory about sampling submission procedures. There are many public and private laboratories that conduct nematode assays.
The number of nematodes required to cause crop losses varies according to the nematode and plant species, therefore the implementation of management practices to reduce PPN impacts will based on the correct interpretations of the nematode assay results. Approaches for nematode analysis interpretation include:
- Action thresholds: the PPN population at which management practices must be implemented to prevent economic losses.
- Risk level: nematode populations are classified as low, moderate, or high according to the likelihood of causing crop damage.
Based on the results of the nematode assays, most laboratories will include management guidelines in their reports. However, it is important to note that thresholds and risk levels apply for samples taken at the end of the growing season and those can differ from one state to another according to specific crops, soil types, local research, and the laboratory’s own experience.
Thresholds levels and recommendations from different laboratories also vary according to the amount of soil used for assays. While our results were based on 500 cm3 of soil, the available action thresholds are based on the number of nematodes in 100 or 250 cm3 of soil. The number of nematodes must be standardized per unit of volume of soil to make an adequate interpretation.
Nematode sampling in Pennsylvania
As part of a state-wide research project aimed to study soybean yield limiting factors, as well as our commitment to the national SCN Coalition, samples for nematode assays were collected in the spring of 2018 from more than 30 farms across PA. Current results indicate the presence of lesion, sting, spiral, stubby-root, lance, and dagger nematodes. Some of these PPN have been reported in soybean, corn, or both. The fact that RKN and SCN were not detected in the spring samples should not be interpreted as if they are not present in fields, since as previously stated, populations in the spring may be too low to account for all PPN and management guidelines are typically based on population levels from samples taken at harvest. We will compare the results from the spring samples (baseline data) to monitor how PPN populations changed during the growing season. Additional analyses to identify nematodes at the species level may be required to make a better interpretation of the results.