Dagger Nematodes, the Unseen Enemy

The most notorious of the various plant parasitic nematodes that inhabit the soil in our region is the dagger nematode (Xiphinema americanum). Dagger nematodes are the primary vector of tomato ringspot virus in the orchard. Tomato ringspot virus causes stem pitting in peaches and cherries and brown ring union necrosis in apples and plums.

Xiphinema americanum is a large nematode that feeds as an ectoparasite on plant roots, resulting in a slight swelling of the root tissue. When plant root tissue swells, the roots stop functioning properly, resulting in a reduction in water and nutrient uptake by the plant. The symptoms of nematode injury in the orchard may be subtle initially, appearing as a general stunting of the plant with reduced vigor and plant growth.

One grower that I had worked with in Maryland would replant a peach tree in the same spot where one had died the previous season. The newly set tree would look good at first, but by year three, the peach tree would be in an advanced state of decline. Nematode assays submitted by the educator revealed that there were high populations of dagger nematodes in this field. Close examination of some of the older declining peach trees at this site revealed stem pitting, which was indicative of a tomato ringspot virus infection. Faced with this reality, this grower decided to push out the entire orchard and began taking steps to remediate the site so that he could replant the orchard in a few years.

Old orchard sites that are replanted to tree fruit without a long crop rotation or biofumigation often harbor high populations of plant parasitic nematodes. Fruit trees planted in old orchard sites often fall victim to a disease complex known as Orchard Replant Disease, which is caused by a combination of both root pathogens and plant parasitic nematodes. Unfortunately, fruit trees planted at old orchard sites have been known to die within the first three growing seasons due to the Orchard Replant Disease complex.

Fruit growers replanting old orchard sites or who are establishing an orchard at a new site should assay the soils for nematodes two to three years prior to planting or replanting the orchard. Nematode assays are conducted in advance of the scheduled planting date so that remediation or management options may be deployed by the grower to reduce nematode populations. Nematode assays should be conducted in early September after the summer heat has passed and before the ground freezes. After the soil samples for the nematode assays have been collected, they should be packaged as directed by the laboratory and sent overnight to the nematology lab to maintain the viability of the nematodes in the soil samples while in transit. Most nematology labs will process the soil samples very quickly and will forward the results to your address within 5–7 days.

Upon receiving the results of your nematode assay, please consult with a member of Penn State Extension's tree fruit team to review the action thresholds and treatment options for plant parasitic nematodes. If a nematode issue is confirmed, it is critically important to develop a management plan that considers crop rotation, biofumigation, and/or the use of labeled nematicides.