Phytophthora oak and larch blight

Pathogen and disease type. Phytophthora nemorosa is a pathogenic oomycete causing severe plant diseases. Often mistaken for a true fungus, this pathogen is highly specialized and thrives in moist environments. It acts as a systemic parasite that infiltrates the vascular and cortical tissues of host trees, leading to chronic infections.

Affected crops. The disease primarily affects deciduous species, particularly oak trees (Quercus), as well as larch (Larix). It is a significant concern for both natural forest ecosystems and commercial nurseries. Due to its mobility in water, it can easily spread through nursery irrigation systems or forest watersheds.

Symptoms and signs. Early indicators include necrotic leaf spots, which often lead to premature defoliation. In more severe stages, the pathogen causes bleeding cankers on the stems or trunks. These lesions are often dark and ooze sap. Root infection is also common, leading to crown dieback, reduced vitality, and eventual mortality of the host tree.

Conditions for development and severity. The spread of Phytophthora nemorosa is heavily dependent on wet, mild conditions. Rain splash and water runoff serve as the primary transport mechanisms for the pathogen's spores. The severity of the damage is linked to the disruption of the host's hydraulic integrity, eventually causing total physiological collapse of the affected specimen.

Control and prevention measures. Management strategies include strict nursery hygiene and the use of certified, disease-free planting material. Chemical control options involve the application of phosphite-based fungicides, which can suppress the disease when applied proactively. Integrated management requires

• removal and destruction of symptomatic trees
• improvement of site drainage
• avoidance of soil movement from infested areas