Leaf Spots and Blights of Beet and Swiss Chard

Beet (Beta vulgaris) and Swiss chard (Beta vulgaris subsp. cicla) are subject to numerous leaf spots including, Downy Mildew, Powdery Mildew, Rust, and Cercospora and Ramularia leaf spots. Cercospora is one of the most important diseases on these crops and significant damage can occur in warm, temperate regions. Powdery and Downy Mildew can reduce yield when the diseases are severe. Ramularia decreases the quality of the edible foliage and is especially important in seed crops. Rust is generally a minor problem, but it has been increasing in prevalence.

Cercospora leaf spot caused by Cercospora beticola is characterized by numerous, small, off white to brown lesions with a reddish margin. When spots increase in size, they can cause extensive loss of foliage. The pathogen may be seedborne and survives in infected weed hosts and in infected crop debris and soil as sclerotia for up to two years. The disease is favored by warm, wet weather and is spread in wind and water splash. Start with certified, disease-free seed; destroy volunteer plants and infected weed hosts, and plow under infected crop debris after harvest. Fungicides must be applied before infection and symptom development.

Powdery Mildew caused by Erysiphe betae appears like other powdery mildews as colonies of superficial, white fungal growth, on both leaf surfaces, and starting on older leaves first. Leaves with severe infections can senesce and die. Epidemics are associated with alternating dry weather and periods of high relative humidity with moderate temperatures. Damage is increased on stressed plants. Powdery Mildew overwinters on infected weed hosts, in buried roots, and as cleistothecia in infected crop debris. Control is reliant upon preventive fungicide sprays (sulfur, copper, sterol-inhibitors).

Downy Mildew is caused by Peronospora farinose f.sp. betae and can infect plants at any stage. Initial symptoms consist of yellowing and distortion of youngest leaves, where the pathogen may become systemic and invade the growing point. A dark heart rot of the crown may develop.  Severe infection results in a rosette of brittle, deformed, chlorotic leaves which curl downwards.  Leaves may also develop individual Downy Mildew lesions which are yellow to brown and irregular. With abundant moisture, a dense, purple-gray sporulation appears on both leaf surfaces of infected plants. The pathogen is seedborne and persists in the soil as Oospores or as overwintering mycelia in weed hosts and volunteer plants. Start with certified, disease-free seed, destroy weed hosts and volunteer plants, and bury infected crop residue after harvest. Fungicides such as mefenoxam or mancozeb may give some control.

Ramularia leaf spot is caused by Ramularia beticola and occurs throughout Europe and North America. Leaf spots occur as large, pale brown spots without the distinctive red margin of Cercospora. Leaf spots appear on lower leaves first and multiple lesions can lead to early death of the leaf. Disease development is favored by cool conditions with high relative humidity. The pathogen produces survival structures (pseudosclerotia) which can be found in crop debris and soil and may also be seedborne. Plant resistant cultivars, remove volunteer and weed hosts, and bury crop residues after harvest. Control by fungicides is generally only used in seed crops.

Rust caused by Uromyces betae produces characteristic orange, raised pustules on both sides of the leaf. Rust lesions may occur in concentric circles or be surrounded by necrotic tissue. The disease, when severe, can cause yellowing, early senescence, and leaf death. Inoculum consists of windblown spores from weed hosts or volunteer plants and may be seedborne. Rust development is favored by warm, moist conditions and is inhibited by dry periods. Remove diseased volunteer and weed hosts and avoid planting production fields next to seed fields. Fungicides can be applied, where needed.