Why Are There Round Spots with Purple-Red Borders on My Beetroot Leaves?

Circular to oval spots on beetroot leaves, 3–10 mm in diameter, with pale tan, grey, or white centres and a distinct purple-red or reddish-brown border, are the characteristic signs of Cercospora leaf spot (Cercospora beticola) — one of the most widespread and economically significant fungal diseases of beets worldwide. On mature spots, the centre may appear slightly sunken and show tiny black dots (the fungal fruiting bodies). Severely affected leaves turn yellow and collapse. The disease spreads through infected leaf debris, rain splash, and warm, humid weather, and can cause extensive leaf loss in susceptible plants during warm, wet summers.

When and why Cercospora strikes

Cercospora beticola requires warm temperatures (optimum 25–30°C), high relative humidity, and long periods of leaf wetness to produce spores and infect new tissue. In cool, dry climates, it causes little damage. In warm, humid summers — particularly where plants are closely spaced or irrigation wets the leaves — it can cause rapid, severe defoliation. The fungus overwinters in infected crop debris left in the soil, and spores produced on this debris infect new growth in the following season. Crops grown in the same soil repeatedly without rotation have higher baseline inoculum and suffer more severe infections.

Managing Cercospora leaf spot

Remove and bin (do not compost) affected leaves as soon as spots appear — this reduces the spore load available to infect new leaves and slows spread significantly. Improve air circulation by thinning to proper spacing (at least 10 cm between plants) and removing any plants between rows that crowd the bed. Water at soil level rather than overhead to keep leaves dry. Rotate beetroot with unrelated crops (at least a three-year rotation) to reduce soil-borne inoculum from the previous season. In commercial settings, copper-based fungicides are used; these are also available to organic and home growers as registered copper sprays.

Does Cercospora affect the roots?

The primary damage from Cercospora leaf spot is to the leaves, not the roots. However, repeated and severe defoliation substantially reduces the plant's photosynthetic capacity, resulting in slower root development and smaller, less well-filled roots at harvest. An early infection that causes leaf loss in June or July has more impact on root size than a late infection in September when root development is largely complete. Protect the foliage during the period of most active root swelling for the best yield.