Cause The dagger nematode (Xiphinema spp.) transmits the Tomato ringspot virus to apples causing Union Necrosis and Decline. These nematodes also transmit Cherry rasp leaf virus causing the Flat Apple disease. Both the nematode and virus are required for either disease to occur. Damage caused by the virus affects fruit production. The nematode is an ectoparasite found only in soil. Nematode populations are highest in fall, at the end of the growing season. Xiphinema was overall the third-most prominent genus of plant-parasitic nematode found in the Okanagan of BC, being found in 59% of apple blocks.
Symptoms No known symptoms occur from nematode feeding alone. Union necrosis results a dark necrotic line that forms at the graft union flanked by spongy orange tissue. Fruits on Flat-Apple-infected trees are smaller, flatter, and the calyx end tends to be open.
Sampling It is best to sample in late summer or early fall when nematode populations are higher than in early spring. At the tree dripline, remove the top 2 to 3 inches of soil and, using a shovel, sample to a depth of 1 ft or to the point that feeder roots are present. Put a handful of soil in the bucket. Take 10 to 20 subsamples from a given area, mix soil thoroughly, and combine into one sample. When sampling single trees, take two to five subsamples depending on the size of the tree.
Cultural control Practice good weed management as many weeds are reservoirs of virus.
- Preplant soil fumigation in fall is highly recommended; however, spring fumigation is effective provided criteria for fumigation conditions are met. Generally, soil temperatures and moisture for fumigation is better in fall than in spring. In addition, nematode populations are more active in fall and, therefore, more susceptible. Also, cool, wet conditions in spring slow the fumigant's diffusion rate, thus delaying planting.
- Basamid G. Avoid application when soil is over 90°F. Do not apply within 3 to 4 feet of growing plants or closer than the drip line of larger plants. Do not harvest within one year of application. 5-day reentry. Restricted-use pesticide.
- Dominus at 10 to 40 gal/A. Apply as a shank injection to an entire field or raised beds when soil temperatures are above 60° F but not greater than 90°F. 5-day reentry.
- Metam sodium (refer to label for rates and application method). Restricted-use pesticide.
- Telone II at 27 to 35 gal/A broadcast, or 24 fl oz per single-tree planting site delivered 5 ft deep. 5-day reentry. Restricted-use pesticide.
- Ecozin Plus at 25 to 56 oz/A is registered for use after planting through a drip irrigation system. Label suggests using additives to aid penetration into the soil and to make applications in the morning. Efficacy in the Pacific Northwest is unknown. 4-hr reentry. O
- Velum Prime at 6.5 to 6.84 fl oz/A can be used after planting using drip, trickle of micro-sprinklers. Do not apply within 7 days of harvest. Group 7 fungicide. 12-hr reentry.
Reference Jaffee, B.A., Harrison, M.B., Shaffer, R.L., and Strang, M.B. 1987. Seasonal population fluctuation of Xiphinema americanum and X. rivesi in New York and Pennsylvania orchards. Journal of Nematology 19: 369 to 378.
Forge, T., Munro, P., Midwood, A. J., Philips, L., Hannam, K., Neilsen, D., Powers, T. and Zasada, I. 2021. Shifting Prevalence of Plant-Parasitic Nematodes in Orchards and Vineyards of the Okanagan Valley, British Columbia. Plant Health Progress. https://doi.org/10.1094/PHP-10-20-0079-RS