Crop: Potato. It has also been reported on tomato
Why is it a problem? Early blight is a fungus that, looks like late blight, can defoliate and even destroy unprotected potato crops. However, treatment is not often required.
Where and when is it a problem? Early blight co-occurs with late blight, but it is more prevalent in warmer conditions, so also extends down to lower altitude areas such as Tsirang and Sarpang. Although ubiquitous it rarely causes widespread defoliation in the open air. It can cause severe defoliation of seed potato production in greenhouses.
Brown, angular necrotic spots develop on leaves. The spots are surrounded by concentric rings of leathery tissue. These are called “bullseye” spots. Necrotic (dead) spots are most common on the lower shaded leaves and stems. Tuber lesions are dry, dark and pressed into the tuber surface, with the underlying flesh turning dry and brown.
Confusion with other pests: Late blight also cause lesions, but early blight lesions are very characteristic owing to their defined margins and concentric rings.
Lifecycle: Early blight overwinters in plant debris, soil and infected potato tubers.
Dispersal: Initial infection can be through infected seed potatoes. Spores are readily moved between plants and fields by wind and water.
When can damage be expected? Generally damage requires higher temperatures and less humidity compared to late blight. Early blight develops more slowly than late blight so generally gets dominated when they co-occur. It rarely causes economic problems in Bhutan.
Hosts: It is mainly a pest of potato, but has also been reported on tomato and chilli in Bhutan.
Management is rarely needed in Bhutan. Prevention is best.
Look for lesions on the lower, shaded stems. Consider taking management action when 2-3 leaves are affected per 20 plants.
Effect of variety
Resistance is not specifically bred for, but considerable differences in susceptibility has been seen among varieties in other countries.
- Remove all affected leaves, and destroy them (burning or deep burial). Use polythene bags to transport them out of the field, as you otherwise spread the disease.
- Remove plant debris after harvest as it harbours the disease.
- Plant seed potatoes at recommended spacings. For table potatoes rows should be 70 cm apart and plants within a row 20 cm apart. For seed potatoes the recommendation is 60 cm and 15 cm.
- Avoid overhead irrigation as this can splash the disease from plant to plant.
- The use of fungicide is not recommended as it is not economically justified in Bhutan. If severe symptoms are found then contact NPPC for advice.
Image acknowledgements: NPPC & CT,IMP(University of Connecticut)