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Plantwise Factsheets for Farmers

Late blight disease management in tomatoes

Phytophthora infestans
Tanzania

Recognize the problem

Late blight (“Bakajani chelewa" in Swahili) is a fungus-like mould disease. First, it causes small, water-soaked, greyish-green, irregular areas on leaves. Later, the areas become dark grey and grow over entire leaves. On the undersides of leaves, a fuzzy white fungal growth might be seen early in the morning or after rain. Stem infections can girdle the plant, causing death. Large brown leathery areas may occur on fruits.

Background

The disease affects tomatoes but can also live in Irish potato, peppers and eggplants. The disease likes the cool, wet weather in Tanzania. Late blight is devastating in the wet season and early blight, another fungus, affects crops in the dryer season. Blight can be blown by wind from far away, and can infect your field.

Management

To prevent the problem:
  • Use resistant varieties e.g. Meru, Tengeru 97, Duluti & Tengeru 2010, Shengena
  • Plant away from Irish potato, pepper, eggplants, night shade and infected fields
  • Avoid planting during wet and cool periods
  • Keep wide plant spacing of more than ½ metre to improve air flow
  • Support plants with sticks, and prune plants for better air flow
  • If possible, disinfect knives with bleach before working on the next plant. Wear gloves or wash hands with plenty of water afterwards because bleach is toxic.
  • Remove tomato residues and volunteer Irish potato, pepper and eggplants.
  • Preventatively spray a mixture of 1/2 kg ash, mixed with 1 litre fresh milk and 10 litres water. This can be used in dry weather.
Regularly observe plants. When dry weather is expected, some infection symptoms might be tolerated and infected plant parts can be removed. When wet, cool weather is expected, 2 plants with symptoms are enough to consider immediate action because the disease will spread quickly:
  • Remove and destroy diseased plants but avoid openly carrying materials through your field as much as possible because this will spread the disease
  • Finely chop 20 garlic cloves, soak for 1 day in 10 plastic-bottle-lids of mineral oil, then add 1 plastic-bottle-lid of soap, stir, add 2 litres of water, stir. Spray over plants max. twice with min. 1 week interval
  • Prevent disease spread by spraying products with chlorothalonil or mancozeb (WHO class U). Do not enter the sprayed field for 3 days. Do not spray two weeks or less before harvest. Spray maximum twice per season.
  • Prevent disease spread with mancozeb products combined with copper oxychloride, copper hydroxide or metalaxyl. These are toxic to farmers and consumers (class II moderately hazardous). Do not enter the sprayed field for 3 days. Do not spray two weeks or less before harvest. Spray max twice per season.

When using a pesticide, always wear protective clothing and follow the instructions on the product label, such as dosage, timing of application, and pre-harvest interval.

The recommendations in this factsheet are relevant to: Tanzania

Authors: Mwangi Jubilant, Mansuet Tilya, Hosea D. Mtui, Richard Musebe, Adeltruda Massawe, Martin Kimani
Plantwise
©CAB International. Published under a CC-BY-SA 4.0 licence.

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