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Watch out for late blight during this wet weather

By George Mbakaya | Published Sat, May 20th 2017 at 00:06, Updated May 20th 2017 at 00:08 GMT +3

For the last three weeks, tomato and potato farmers have had crazy weather. Heavy rains come with lots of problems. They provide prime conditions for harmful pathogens to propagate and spread. Late blight is one such disease. If not identified and treated early, it can ruin the entire crop. The disease attacks tomatoes and potatoes and, besides wiping the entire crop, it can provide a source of infection for other plants.

Farmers need to understand that late blight is not any other tomato or potato disease. Most tomato diseases affect the leaves and cause limited damage to fruit, and while they may reduce the harvest, the entire crop is not lost. Besides, because most pathogens are not readily dispersed by wind, their effects are localised. Late blight kills plants outright and is highly contagious. Its occurrence in your farm can affect other farms because the spores are easily dispersed by wind.


If it is your first time you are experiencing late blight or you are not sure how to identify it, there are some indicative signs you should look out for. Late blight begins with lesions developing on the stems, leaves or even fruit of the plant (in case of tomatoes). These look like dark water-soaked spots and they will grow larger until they cover the entire leaf. Once this happens, the leaf will die but remain on the stem. Sometimes these lesions are accompanied by a fuzzy white substance growing on the underside of the lesion. Lesions on the potato stems range from dark brown to black in colour while those on tomatoes tend to be shiny and olive-coloured.


Upon confirmation that the disease ravaging your crop is late-blight, you have limited options to save your plants. It is advisable that you uproot the plant immediately so that it does not spread to other plants. Do not compost blight-infected plants. The plants you pull out should be placed in large, sealed plastic bags and left in the sun for several days. This will kill the pathogen and prevent it from spreading back into your garden or a neighbour’s farm.

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Tomatoes and potatoes affected by late blight are still safe to eat, but potatoes should not be stored or used as seed potatoes. If your tomato fruit has been affected, you may still save the seeds, as the disease does not transfer to seeds. However, if you allow the tomato to rot before collecting them, the seeds will die.

Spray preventively, if there is need. Once plants are infected with late blight, it’s too late to save them. Before using any products, read the label and use them accordingly. In most cases, effective protection requires that plants be sprayed as often as weekly throughout the growing season. Remember that while these sprays can help reduce the likelihood of infection, you still need to monitor plants closely.

It is impossible to control the disease on a plant if five to 10 per cent of the foliage is infected. For protection, products which contain the contact fungicide “chlorothalonil” are most effective at preventing late blight. Other products listed as fair at controlling late blight include those with copper or the chemical formulations of “mancozeb”.

Given the ongoing rains, some farmers are already counting their losses. If you are one of the farmers who have been affected by late blight this season, there is not much you can do other than salvage any good fruit you still have. But for those planning to grow tomatoes or potatoes in future, there are a few things that you can do to help prevent this disease from attacking your farm.

If possible, avoid planting tomatoes and potatoes where you had them last year. Be sure to give plants plenty of space, as this will assist in airflow and light around the plants, which will help them resist the disease. When plants are crowded together, it will take longer for the leaves to dry off, making them more vulnerable to the pathogen. Use trellises and supports that will keep the vines off the ground. Weed control is also important. Some weeds are from the same family as tomatoes and potatoes and can carry and spread the infection.

Knowing the right disease symptoms and signs to look for while incorporating an integrated disease management plan, can prevent or limit the severity of plant disease outbreaks.

The writer is an expert on sustainable agriculture and agricultural solutions. Send your crop-related questions to [email protected]