How to prevent and treat oleander black spot disease? Here are the methods
Symptoms
The spots occur on the edges or middle of the leaves, semicircular or circular. When several spots join together, they form a wavy pattern, present on both the front and back sides, with the front darker than the back; the spots are grayish-white or grayish-brown; in the later stage, a black powdery mold layer develops on the spots, generally occurring on overwintering leaves.
Pathogen
The pathogen is a fungus, Alternaria sp. The conidia are olive-colored, oval, with both longitudinal and transverse septa, in chains, borne on relatively long conidiophores.
Disease pattern
Generally, old leaves, lower leaves, and suckers sprouting at the root zone are most affected; spores spread through airflow and rainwater; if there is a lot of rain during the disease period, the condition will worsen.
Prevention and treatment methods
(1) Horticultural control: Strengthen management and improve the plant's ventilation and light penetration. Apply more phosphorus and potassium fertilizer to boost tree vigor.
(2) Chemical control: When needed, spray once with 75% chlorothalonil 700 times dilution.