Corky root disease

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This disease is caused by the nematode Xiphinema bakeri. Most plant-parasitic nematodes are small (0.5-2 mm long) vermiform animals belonging to the phylum Nematoda. Those that damage conifer nursery seedlings are soil inhabitants and feed on non-woody roots and mycorrhizae. Corky root is the only nematode-caused disease that has been important locally. Both the pathogen and the disease are confined to coastal, bareroot nurseries. Corky root normally occurs in nurseries established on recently cleared forest lands. Xiphinema bakeri is indigenous to most coastal forests and when these forests are converted to nursery production, the pathogen numbers increase. The nematode populations rarely reach damaging levels on the first crop; however, subsequent crops are severely damaged, especially if cropping is continuous.

Hosts and damage

Locally, the disease affects seedlings of the spruces (Sitka, white, Engelmann), western hemlock, and Douglas-fir, the latter being the most sensitive. Symptoms (Figures 26-28) first become noticeable on Douglas-fir about midway through the first growing season, when the secondary needles of random seedlings become somewhat chlorotic and the shoot slightly stunted (Figure 26). Diseased taproots have few if any laterals, are dark, swollen and often club-tipped, but they are not rotten. Patches of affected seedlings become evident as the symptoms develop and eventually these patches coalesce (Figure 27) to form patches 5-65 m in diameter.

Seedlings affected in their first growing season fail to recover and are culled when lifted. Two-year-old stock (Figure 28) and transplants may also become diseased if X. bakeri populations are large. When diseased, small-rooted seedlings such as spruce often frost heave. The fungus Cylindrocarpon destructans frequently invades diseased roots, but the nematode is the primary pathogen. Corky root disease prevails in soils that are sandier, less fertile, and lower in nutrient-holding capacity than disease-free soils. The sandier soils are physically better suited for X. bakeri movement. Thus, the two prerequisites for optimum disease development are large numbers of the pathogen and low soil fertility.

Life history (Figure 29)

The life cycle consists of eggs, four juvenile stages, and adults. Only the latter are sexually mature; males are rare. The last three pre-adult stages and adults feed on roots. Eggs are laid in the summer, total populations peak in early fall, and the nematodes overwinter primarily as eggs and juveniles. Almost all of the nematodes are present in the upper 10 cm of soil around the roots.

Management

Nematode numbers are reduced to non-damaging levels by starvation, heat, and desiccation. Therefore, losses from corky root can be eliminated by bare fallowing, surveying fallow fields in June or July to detect X. bakeri populations, and disking or rototilling infested soils during the hot, dry part of August and September. To prevent introduction of the pathogen into disease-free nurseries, fill and other potentially infested soil amendments should be checked for X. bakeri. Seedlings, especially diseased ones, should not be transferred among nurseries.

Pre-plant nematicides or soil fumigation can be used, but they are expensive and often adversely affect seedling growth. Because local soil temperatures normally limit application to late summer or early fall, the use of nematicides often results in loss of seedling production for a year.

Selected References

Sluggett, L.J. 1972. Corky root disease of Douglas-fir nursery seedlings. Environ. Can; Can. For. Serv., Pac. For. Res. Cent., For. Pest Leafl. 53, Victoria, B.C.

Sutherland, J.R. 1975. Corky root disease: population fluctuations of Xiphinema bakeri nematodes, and disease severity in forest nursery soil cropped with different seedling species. Can. J. For. Res. 5: 97-104.

Sutherland, J.R. and L.J. Sluggett. 1974. Time, temperature, and soil moisture effects on Xiphinema bakeri nematode survival in fallow soil. Phytopathology 64: 507-513.

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Summary

Corky Root Disease

Figures

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    Figure 26. Corky root symptoms on 1+0 Douglas-fir.

     Figure 27. A field of 2+0 Douglas-fir showing corky root symptoms.

     Figure 28. Corky root (two right-hand seedlings) on 2+0 Douglas-fir.

     Figure 29. Life history of corky root disease.