Cause The root-lesion nematode Pratylenchus penetrans is the most important nematode affecting apple production. It is a migratory endoparasite found both in soil and roots. They reproduce better and cause more damage in sandy soils. Extensive feeding on fine roots results in symptom development. Nematode populations are highest in fall, at the end of the growing season. Many weeds are also a host.
An unpublished study in eastern Washington in the early 2000's found that root lesion nematode populations were statistically high-est in M.26 (apple) and pear (OldHome) rootstocks and lowest in peach rootstocks when compared across apple (G.16 and M.26), cherry (Giesla, Mahaleb, and Mazzard), and peach (Citation and Nemaguard) rootstocks.
Symptoms Apple trees (especially younger trees) may exhibit poor growth, stunting, and a gradual decline in yields. Severely infected root systems may lack feeder roots. Nematode damage; however, is not generally obvious.
Sampling It is best to sample in late summer or early fall. 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. Place a handful of soil in one container and a handful of feeder roots in another. It is important to sample the fine feeder roots because nematodes prefer to feed on them rather than the larger roots. 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.
Based on replicated studies, damage may result at 20 to 50 nematodes per 100 g soil. Local soil types, climate, moisture, and other factors vary widely resulting in damage at different nematode densities.
- Plant nematode-free trees.
- The Geneva series rootstocks G11 and G30 support lower populations than Malling or Malling-Merton rootstocks in eastern Washington.
- Control weeds around trees.
- Plant ground cover that is not a suitable host such as tall fescue, red fescue or perennial ryegrass. Yellow mustard (Brassica juncea) contains allyl isothiocyanate, which is active against nematodes. Brassica carinata also has been reported to be effective against nematodes.
- 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 method of application and rate). 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
- MeloCon WG (Paecilomyces lilacinus strain 251) at 2 to 4 lb/A plus a soil wetting agent can be applied to established plants, although it might be better used when applied to plants just before planting. Stable for only days at room temperature, weeks in the refrigerator or for a year if frozen. Unknown efficacy in the PNW. 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.
References Barker, K.R. and Olthof, T.H.A. 1976. Relationships between nematode population densities and crop responses. Annual Review of Phytopathology 14:327-353.
Mazzola, M., Brown, J., Zhao, X., Izzo, A.D., and Fazio, G. 2009. Interaction of brassicaceous seed meal and apple rootstock on recovery of Pythium spp. and Pratylenchus penetrans from roots grown in replant soils. Plant Disease 93:51-57.