Stem Rot
Phytophthora syringae (primary), P
18 host plants
Last updated
This profile synthesizes data from multiple published sources. Expert field review is in progress.
Stem rot caused by Phytophthora creates dark brown to black cankers on stems and crowns of crabapples, peonies, and dianthus, typically appearing on the lower portions of plants. You may notice an alcoholic odor coming from infected bark, and the leading edge of a canker often looks irregularly mottled. In winter and early spring during cool, wet weather, this disease can spread rapidly through nursery stock and young trees.
Prevention is far more effective than trying to treat stem rot once it takes hold. Plant on well-drained sites, never mound soil against tree trunks, and remove trees from wet ground storage quickly. If you're moving nursery stock, keep trees off the ground during rainy weather and never heel them in deeper than the original collar line. Chemical options like phosphonate sprays can help prevent infection if applied before the disease arrives.
Quick Reference
Management
Peak activity during cool, wet winter and early spring (November-April in the Puget Sound region). P. syringae is most aggressive at 50-59°F with moisture and rain splash. Nursery stock is especially vulnerable during fall/winter harvest and storage. Less active during warm, dry summers (June-September). Stem wounds made in late season (September-October) are particularly vulnerable to infection. # Source: PNW Plant Disease Management Handbook
Cool (50-59°F) wet weather; poorly drained sites; soil or mulch piled against stems; rain splash; overhead irrigation; late-season wounds; nursery stock in storage or heeled-in beds; saturated or near-saturated soil. # Source: PNW Plant Disease Management Handbook - Stem Rot (P. syringae)
Cultural Controls
- Prevent mechanical injury to trees, especially late in the season.
- Avoid leaving trees on the ground during rainy winter weather.
- Plant on well-drained sites.
- Avoid mounding soil around the trunks.
- Also, do not \"heel-in\" higher than the collar.
- Harvest highly susceptible nursery trees during dry weather.