Phytophthora blight of peony

The causative agent of this destructive disease is the oomycete Phytophthora paeoniae. This pathogen is a soil-borne organism that persists in the environment as resilient oospores, which can remain viable in the soil for several years, waiting for favorable conditions to infect host plants.

Peonies (Paeonia spp.) are the primary hosts for this pathogen. The disease affects all vegetative and reproductive parts of the plant, including stems, leaves, flower buds, and the crown. It is most damaging during early spring when young, tender shoots are actively developing and are highly susceptible to penetration by zoospores.

Symptoms typically manifest as dark brown or black lesions starting at the base of the stems, which leads to rapid wilting and collapse of the entire shoot. Infected flower buds often turn black and fail to bloom, frequently becoming covered with a greyish mold. Root rot is also common, leading to a foul-smelling, mushy decay of the underground storage organs.

Environmental conditions play a critical role in disease progression. High soil moisture, poor drainage, and cool to moderate temperatures are major drivers of infection. Water splash from irrigation or rainfall helps spread the pathogen's spores to healthy tissues or neighboring plants, rapidly exacerbating the spread of the blight throughout a garden bed.

The impact of Phytophthora blight is significant, as it can cause complete plant death if left untreated. Management strategies involve stringent sanitation practices, such as removing infected materials and improving soil aeration. Chemical control options include the use of preventive fungicide applications, particularly those labeled for the control of oomycetes, to protect the foliage and crown during periods of high risk.

• Improve soil drainage to prevent waterlogging.
• Increase spacing between plants to improve airflow.
• Sanitize pruning tools after every cut.
• Apply soil drenches during the early spring growth phase.