Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Apple -- Nematode (Root-lesion)
 
Cause: The root-lesion nematode Pratylenchus penetrans is the most important nematode affecting apple production. It is a migratory endoparasite (see Nematodes) found both in soil and roots. Damage is caused by extensive feeding on feeder roots. Nematode populations are highest in fall, at the end of the growing season. Many weeds are host for P. penetrans.
Symptoms: Apple trees (especially younger trees) infected with P. penetrans 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.

Damage may result at 20 to 50 nematodes per 100 g soil; however, that range of numbers is only a suggested damage level. Numbers are based on replicated studies, but local soil types, climate, moisture, and other factors vary widely, and so our numbers may not be representative.
Cultural control:

  1. Plant nematode-free trees.
  2. Control weeds around trees.
  3. Plant ground cover that is not a suitable host.
Chemical control:

  1. Preplant soil fumigation in fall is highly recommended; however, spring fumigation is effective provided criteria for fumigation conditions are met. Generally, soil temperature and moisture for fumigation is better in fall than in spring. Also, 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.
    1. 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. 24-hr reentry plus ventilation.
    2. Metam sodium (refer to label for method of application and rate).
    3. Telone II. 5-day reentry. Restricted-use pesticide.
References:
Barker, K.R. and T.H.A. Olthof. 1976. Relationships between nematode population densities and crop responses. Annual Review of Phytopathology 14:327-353.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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