Cercospora Leaf Spot - Beet
Cercospora leaf symptoms, Severely affected beet plants (Photo E. Crawford) & heavy disease pressure on leaf
Pathogen
Fungus
Hosts
Sugar beet
Symptoms
The disease appears as small (2 – 3 mm) round grey spots with reddish margins. Under humid conditions and with the use of a hand lens, black spots (spore producing structures) can be seen in the centre of the lesions, which helps to distinguish it from Ramularia.
Development
It usually appears on individual plants and can spread if temperature and humidity are both high. Under severe disease pressure the lesions coalesce to the extent that whole leaves can be lost.
Overwinters as spores on old leaves and plant debris. Can also be seed-borne but this is thought to be a minor source of infection.
Favourable Factors
Although Cercospora develops above 17 °C, under UK conditions it requires daytime temperatures of 25°C or greater and high humidity for rapid spread.
Importance
In warmer climates where there are frequent storms or irrigation is used, Cercospora can cause severe damage resulting in a yield loss of as much as 50 %.
In the UK its future development will depend on summer weather conditions. The rapid spread of Cercospora infections across the Netherlands within 5 years suggests its development in the UK should be closely monitored.
Control
Avoid short rotations of less than 3 years
Remove crop debris
In continents where Cercospora is more economically important growers can select tolerant varieties which can slow disease development helping fungicides to work more effectively. Tolerant varieties are available in the UK but these are not actively promoted as the risk of this disease is not currently considered to be of major concern
Foliar fungicides should be applied at disease onset (usually late July – Early August) and if lifting beet beyond October a second application should be made 3 – 4 weeks later.
Heavy disease pressure on leaf
Severely affected beet plants. Suffolk, Oct. 2016. (Photo E. Crawford)
Symptoms on a leaf seen in Suffolk Oct. 2016 (Photo E. Crawford)
Cercospora on fodder beet leaves, Cornwall, Oct 2016 (Photo T. Nicholson)
