Black Root Rot (Thielaviopsis)

This fungus generally favors soil temperatures between 55 and 65°F, particularly if the soil is wet. The fungus can be found in field soil but has also been found associated with commercial peat moss.

Symptoms

• stunting of foliage and root systems
• blackened areas on roots
• yellowing of leaves between the veins or along the margins
• branch dieback

Signs

• Dark brown, multicellular spores form in the infected roots. The individual cells appear to snap apart.
• Light colored spores are formed in a long, tapering cell and extruded in chains.

Management

• Steam field soil and peat moss. Heat it to 180°F and hold it at that temperature for 30 minutes.
• There are some very effective fungicides for use in managing this plant pathogen. Check with Penn State Extension or the Pennsylvania Department of Agriculture to determine what is currently available for use.

Holly: Ilex crenata, 'Helleri', 'Hoogendorn', 'Nigra', 'Green Cushion', 'Mobjack Supreme' and 'Hetzii' are very susceptible. Ilex vomitoria and Ilex opaca are moderately resistant. English holly (Ilex aquifolium) and Chinese holly (Ilex cornuta) are highly resistant.

References

• Daughtrey, M. L., Wick, R. L., and Peterson, J. L. 1995. Compendium of Flowering Potted Plants. APS Press. St. Paul, MN. 90 pp.
• Lambe, R. C. and McRitchie, J. J. 1986. Holly diseases. Diseases of Woody Ornamental Plants and Their Control in Nurseries. North Carolina Agricultural Extension Service, Raleigh, NC. 130 pp.