What every potato farmer should know about late blight and early blight

Potato diseases results into high yield losses both in the field and during storage. This condition has forced farmers to resort to excessive use of agrochemicals, which is posing danger to the environment, to humans and beneficial organisms. Cost of inputs like agro-chemicals and labour is also increasing at an alarming rate. The potato diseases are caused by fungi, bacteria or viruses. The diseases are therefore described as fungal, bacterial or viral diseases.

Management of potato diseases require a multi-faceted approach to eradication of menace facing the farmers. This requires research and efficacy trials by research institutions and information flow to the farmers through the available communication channels such as input providers and Viazi Soko digital platform, just to mention but a few. Pest Control and Product Board (PCPB) and agrochemical companies provide a list approved agrochemicals list, at least semi-annually, a contribution that keeps the sector up-to-date with suitable products.

Some of the most serious fungal diseases are late blight and early blight. Most farmers tend to think that these diseases, especially late blight, are caused by cold weather conditions which is in fact, not true. The truth is that the pathogen spreads faster in wet conditions.

Late blight is the most important potato disease because it is caused by an oomycete (water mould) Phytophthora infestans and the disease damages leaves, stems and tubers. This disease damages leaves, stems and tubers. There is soft rot of tubers often observed in the store when the field is infected with late blight.  Some of the symptoms includes; small pale to dark green spots appearing on underside of the leaves. Later spread to stem and tubers. Round, dark brown watery looking blotches appear on the underside of the potato leaves. The blotches appear wet and irregular at the margins; the blotches appear brown when dry and black when wet.

Symptoms on the stem are dry, dark brown elongated blotches. Infected stem wilts and dries. On the tuber, light brown blotches that are slightly sunken may appear on the surface. The key indicator of late blight infection is the white downy growth on the underside of the leaf. The lesions are confined to areas between the leaf veins.

Transmission of the disease depends on the life cycle of the Phytophthora infestans which can last between three and fifteen days depending on the prevailing weather conditions and the level of plant resistance. Sources of this fungus can be: infected plant from neighbouring field, wild plant, volunteer crops or infected crops in the field which releases spores which reaches healthy plant through being carried away by wind. The favourable conditions for the spread of the fungal pathogen include; high humidity (>85%) and warm temperatures (>15 ̊ C).

To control this disease, farmers are advised to; use of healthy potato tuber at planting, use varieties with high late blight resistance, cover the tubers always during hilling to prevent tuber infection, destroy leaves that are infected to prevent tubers coming into contact with the spores before harvesting, harvest the tubers when they are fully mature to avoid incidences of skin damages and spores entry during harvesting and storage. Lastly, use approved chemicals with fungicides after emergence and repeat regularly based on the prevailing weather conditions.

A potato plant affected by late blight disease.

Source: NPCK potato production handbook, www.npck.org

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