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Pythium root rot

Pythium root rot can affect bareroot and container seedlings in British Columbia. This is particularly true where: (i) soil or growing medium drainage is poor, (ii) temperature is unfavorable for root development, and (iii) the same seedling species is grown in a field over several years.

Hosts and damage

Pythium species are ubiquitous in soil and aquatic environments and they have a wide host range. Pythium root rot (Figure 20) is usually restricted to juvenile or succulent tissues or, on older plants, to root tips or lateral roots. Mature, lignified tissues are seldom affected. Infection results in poorly developed root systems, shoot stunting, and chlorosis. Severely diseased seedlings (Figure 21) rarely reach minimal grading standards (by size). Depending upon numerous factors, including soil or growing medium, moisture level, infection often occurs at or slightly below the root collar. In containers (Figure 22), the bottom of the growing cavity often remains wet for prolonged periods, providing ideal conditions for infection. The pathogen enters through root tips, proliferating in young cells and causing rapid collapse and rootlet death. Spread into older roots is limited to the cortex.

Life history (Figure 23)

Pythium species can survive for long periods as thick-walled resting spores (oospores) or as saprophytes on dead organic matter. They are weak competitors, and thus survival as mycelium is uncommon. Survival and infection are primarily through zoospores and sporangia for short and intermediate periods, and oospores for longer periods. Oospores are resistant to extreme temperatures and other adverse conditions, and remain viable in soil for several years. Germination is stimulated by increased moisture or the presence of the host root. From 10 to 18°C, infection is through zoospores produced by both oospores (sexual) and sporangia (asexual). Zoospores travel short distances through the soil water, attracted to the host's root exudates. Infection at higher temperatures usually results from direct germination of oospores and sporangia.


Pythium root rot is seldom a problem when either the bareroot soil or container-growing medium are well drained and slightly acid. Selecting a bareroot site with such soil helps alleviate Pythium-caused problems. Proper water management is a key component of control. Sometimes soil fumigation or fungicide drenches in bareroot nurseries, or just the latter in containers, is justified. The problem may carry over from one crop to the next, intensifying with time (e.g., on root pieces in bareroot nurseries and on used styroblocks in container nurseries). Pythium-free peat should be used in both bareroot and container nurseries. When diseased stock is lifted, care should be taken not to leave diseased roots, as they serve as refugia for the pathogen. Styroblocks should be thoroughly sanitized between crops. Irrigation water, especially from ponds or reservoirs that receive run-off from Pythium-infected areas, can be a source of inoculum. Chlorination of contaminated water may be justified.

Selected References 

Agrios, G.N. 1988. Plant pathology. 3rd ed. Academic Press, Inc., New York, N.Y.

Gill, D.L. 1970. Pathogenic Pythium from irrigation ponds. Plant Disease Rep. 54: 1077-1079.

Hendrix, F.F. Jr. and W.A. Campbell. 1973. Pythiums as plant pathogens. Ann. Rev. Phytopathol. 11: 77-98.

James, R.L. 1982. Pythium root disease of Douglas-fir and grand fir seedlings at the Coeur d'Alene Nursery, Idaho. U.S. Dep. Agric. For. Serv., North. Reg. Rep. 82-10.

Look Alikes

Other Fungi



Corky root disease
Cylindrocarpon root rot

Fungus gnats

Saturated Soil
High Salinity
Nutrient Deficiency


Pythium root rot

Principal, locally grown hosts

Host age and season when damage appears


Nursery type and location















All spruces, Douglas-fir, sometimes other species


Usually late summer through following spring






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click on this image to see a larger version     Figure 20. pythium root rot on bareroot Douglas-fir.






click on this image to see a larger version     Figure 21. Pythium root rot on Douglas-fir.






click on this image to see a larger version     Figure 22. Pythium root rot on container-grown spruce. Note that damage is most severe on roots lower in the container.







click on this image to see a larger version     Figure 23. Life history of Pythium root rot.