Tree Fruit Leader, Vol. 5(2) Nov. 1996

Nematodes - A Big Factor in Replanting?

by Dr. Thierry Vrain, Agriculture Canada, Summerland 
and Mike Sanders, Tree Fruit Specialist, Kelowna 

Poor growth of apple trees replanted in old orchard soils is one of the biggest problems facing British Columbia orchardists. Factors known to contribute to this problem include poor weed control, unfavorable soil conditions, nutrition, moisture stress and various soil pathogens. Nematodes also contribute to poor growth of young trees.

Nematodes are microscopic worms that feed on plant roots restricting root growth and opening up wounds that permit entry of soil pathogens. High levels can severely limit growth of young trees whereas lower levels result in stress to trees, which when coupled with other stresses cause reduced tree growth. Mature trees can also be affected.

There are a number of nematode species that attack plants. The most common species in the Okanagan is Pratylenchus penetrans, commonly called the root lesion nematode.

To determine the extent of Pratylenchus penetrans nematodes in new plantings and to determine their possible involvement in poor tree establishment, a survey was done in June and July this season. The work was done by Michael Andres, a summer student funded by OVTFA.

Soil and root samples were collected from orchards planted between 1993 and 1995 in the Vernon and Kelowna areas. Orchards sampled had growth ranging from poor to good. Soil samples were analysed in the Kelowna BCMAFF office and root samples were analysed at the Agriculture Canada Research Station in Vancouver.

The results show that nematode levels in about 50% of the plantings surveyed were above threshold levels.

The conclusion of this survey is that nematodes can be a significant factor in the replant problem. Growers should do nematode analysis prior to planting. The sample should include both soil and root samples (from the existing trees). Due to the complexity of nematodes, analysis should be done by a reputable lab. 

Nematode analysis should not be done instead of a replant analysis but should be done in addition to the replant analysis.

If the replant and nematode analyses are not done prior to replanting, the best bet by far is to fumigate. Use the rate suggested for replant disease (generally the maximum label rate). Lower rates control nematodes but not replant disease.

In most cases fumigation is beneficial prior to replanting. Not all growers have been completely satisfied with their results of fumigation, however. The key with fumigation is thoroughness of site preparation and application. Where good results have not been obtained there may have been problems with soil preparation, rate, application, weed control and cultural management.

Regardless, both replant pathogens and nematodes can restrict growth of newly planted trees and fumigation helps to control both.

For more information on fumigation, pick up the newly revised publication "Soil Fumigation for Orchards: An Overview", published by OVTFA. Also available from OVTFA is a video showing fumigation techniques.

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