The fungus Cercospora beticola affects both leaves and bulbs, with potentially serious yield losses. High temperatures and humidity, typically in late summer, increase damage

 

Identification

Small round leaf spots (3-5 mm diameter) are first found on older leaves then transfer to younger leaves. Tan to light brown centres have reddish-brown borders. In heavy infections, spots coalesce and leaves turn brown but stay attached to the plant.

 

Cercospora leaf spot.
(Photo: M. Braithwaite, Plant Diagnostics Ltd.)

 

Importance

CLS occurs sporadically but can result in significant DM loss. 

 

Spread

CLS survives between crops via residues, soil, seed and infected plants. Spores are produced in spring and transferred by wind, water splash, insects and  equipment. Spread is greatest when leaves are wet at night and days are warm and humid.  

 

Management

Rotate crops every year. Sow seed only from reputable sources. Use treated seed. Keep infected residues 100 m away from other susceptible crops. If irrigating, avoid prolonged leaf wetness; water from mid-day when leaves are dry. Fungicides are available.