Black Root Rot

Black root rot thrives in container nurseries and field situations where soil conditions aren't carefully managed, particularly affecting boxwood, daphne, and several landscape shrubs in Western Washington. Your first visible clue is stunted growth, chlorosis, or sudden wilting in otherwise well-watered plants; when you examine the roots, they appear dark or blackened rather than pale and fibrous. The fungus produces thick-walled survival structures (chlamydospores) that persist in soil for years and activate when root exudates signal a host plant is nearby. Neutral to alkaline soils and temperatures between 55 and 70 degrees Fahrenheit create the perfect storm for this disease.

What matters about black root rot is that it persists in soil long after an infected plant is removed, making prevention infinitely easier than dealing with contamination. If you're propagating or growing plants in containers, use only soilless media rather than field soil or recycled potting mix, and keep tools and hands clean to avoid introducing spores to your growing area. Commercial peat moss can harbor the fungus, so source materials carefully and dispose of affected plants and soil promptly rather than composting or reusing it. Once a propagation area or field is contaminated, thorough sanitation and a switch to soilless media become essential.