Armillaria Root Rot

Affected trees show thin, chlorotic foliage, distressed cone crops, and abundant resin flow or brown liquid leaching at the tree base. Trees slowly decline and often die standing, though some may blow over in wind. Beneath the bark at the root collar, you'll find the diagnostic white, stringy mycelial fans adhering to the wood and inner bark. Black shoe-string-like structures called rhizomorphs may extend into surrounding soil. Bark beetles often hasten decline in stressed trees.

Armillaria root rot is difficult to manage once established, so prevention through stress reduction is critical. Plant only high-quality, well-acclimated stock and ensure proper planting depth. Maintain good drainage, avoid waterlogging, and irrigate deeply rather than with frequent light watering that wets the crown. In landscape hedges, remove healthy-appearing trees next to diseased ones because infection spreads through root contact. On sites with known disease centers, clear all stumps and roots greater than 1 inch in diameter and leave the ground fallow 1 to 3 years before replanting.