Diseases of Spinach Caused by Lower Fungi-Downy Mildew, White Rust, and Damping Off

Spinach is affected by three diseases caused by lower fungi: Downy Mildew (Peronospora farinose f. sp. spinaciae), White Rust (Albugo occidentalis), and Damping –off (Pythium species). Downy Mildew is the most destructive and widespread disease of spinach. White Rust, although a very damaging disease of spinach, occurs only in the United States east of the Rocky Mountains. Spinach is very susceptible to damping-off and root rots, vulnerable to overwatering, and can suffer serious losses of stand due to these diseases.

Downy Mildew is characterized by irregular, yellow lesions on cotyledons and true leaves which later turn bright yellow. Lesions enlarge, become tan and dry, and a purplish growth occurs on leaf undersides. Severe infections result in curling and distortion of leaves and a general blight. The pathogen is composed of many distinct pathotypes (races) and new strains are being detected. Like other Downy Mildews, this pathogen is favored by cool, moist weather, dense plant canopies, and can progress rapidly under favorable conditions. The best means of controlling Downy Mildew is the use of resistant cultivars; the ability of the pathogen to produce new strains can thwart this effort. Seed treatments with mefenoxam can protect young seedlings and foliar fungicide sprays can be applied to emerged crops.

Symptoms of White Rust are distinctive as raised white pustules or blisters on both leaf surfaces. Pustules may be surrounded by a chlorotic halo or occur in concentric rings. Severe infections can cause lesion coalescence and leaf necrosis. A. occidentalis survives as Oospores in infected tissues and disease development is favored by cool, wet weather. Management consists of crop rotation, resistant cultivars, and preplant and foliar fungicides (mefenoxam, phosphorous acid fungicides).

Damping-off can occur before seeds emerge or post emergence causing stunting, yellowing, wilting, and eventual collapse of seedlings. This disease is favored by excess soil moisture and spinach is particularly sensitive to this situation. Pythium species are natural inhabitants of the soil and will persist in infested soils as Oospores. Severity of damping-off is influenced by cultivar, soil properties, irrigation management, and pathogen populations. Damping-off occurs most frequently in heavy, poorly drained soils, low areas in fields, and in fields with history of repeated spinach production. Prevent Damping-off by avoiding saturated soil conditions and careful seedbed preparation, field selection, planting techniques, and irrigation.  Seed can be treated with mefenoxam or fungicides can be applied as a drench after planting.