Agronomist notebook: Rains are here, watch out for grey mould in tomatoes

Philomon Endong'a on his tomato farm in Mosop, Nakuru County. Botrytis affects all the above-ground parts of tomatoes in both greenhouse and open field. PHOTO | FILE | NATION MEDIA GROUP

What you need to know:

  • Botrytis affects all the above-ground parts of tomatoes in both greenhouse and open field. This fungus occurs as both a pathogen and a saprophyte.
  • Gray mould is favoured by cool and humid conditions that stress the plants, making them susceptible to the disease.
  • To minimise incidents of transmission of the disease from an open field to the greenhouse, farmers should avoid planting tomatoes or susceptible crops outside the structure.
  • Cut infected stems of dead plants to prevent the build-up of spores and the spread of infection. Remove all plant debris from the previous crop as this acts as a host to the grey mould and hence infecting the next crop.

The ongoing rains in various parts of the country have increased disease infestations in crops, raising production costs for farmers. The worst affected are tomato growers, who have seen the humid conditions severely affect the crops.

John, a farmer in Baringo, called me the other day, noting his tomato fruits had turned whitish and soft upon touch and some had ruptured skin near the centre of the decayed area.

The anomaly had affected plants in both greenhouse and open field. I listened to him and was able to identify Botrytis disease, commonly referred to as grey mould, as what had attacked his crops. It is caused by the asexual fungus Botrytis cinerea (B. cinerea).

Looking at the fruits, one can easily think that the disease is late blight. But unlike late blight, which makes the entire fruits turn black to brown, in Botrytis, the tomato fruits turn partially black to brown near the fruit stalk.

Some lesions then expand to concentric rings girdling the entire stem and causing wilting above the infection site. The disease is mostly characterised by tan to brown soft spots or blotches and affects many vegetable crops, including pepper, potato, and onions.

Botrytis affects all the above-ground parts of tomatoes in both greenhouse and open field. This fungus occurs as both a pathogen and a saprophyte. It can also cause diseases such as damping off, stem girdling, foliar blighting and post-harvest rots.

Gray mould is favoured by cool and humid conditions that stress the plants, making them susceptible to the disease. The pathogen spores are easily transported by people, wind, rain and greenhouse air currents. Planting other crops nearby may serve as pathogen sources.

WELL VENTILATED

To minimise incidents of transmission of the disease from an open field to the greenhouse, farmers should avoid planting tomatoes or susceptible crops outside the structure.

Also, avoid carrying out management practices in the open field then moving into the greenhouse as this increases the chances of introducing the pathogen in the structure.

Don’t work on wet plants. If pruning, it is advisable to do the exercise in early afternoon as this allows the wounds to dry up quickly. Prune regularly to remove old laterals and dense leaves to improve air circulation on the crop. Ensure the pruned leaves are well disposed of to avoid further spread of the disease.

The greenhouse should be well ventilated to reduce the relative humidity and prevent grey mould. Consider installing fans to move air within the greenhouse. Also, check on the crop spacing to allow good air circulation between the plants. Practice greenhouse sanitation by ensuring all the infected plants are removed.

If using a shade net, it is ideal to have a polythene sheet on the roof as this would prevent the raindrops from affecting the tomatoes. During dry conditions, avoid overhead irrigation as this encourages too much leaf wetness.

Cut infected stems of dead plants to prevent the build-up of spores and the spread of infection. Remove all plant debris from the previous crop as this acts as a host to the grey mould and hence infecting the next crop.

Mulching is recommended in the greenhouse as the fungus can easily survive in the soil, becoming a source of infection to the next crop. Before planting in the greenhouse, it’s advisable to wash down the walls and the floor to reduce infection cases.

Use certified seeds that are resistant or tolerant to such diseases and practice crop rotation. Chemicals with copper as an active ingredient help prevent the occurrence of the disease.

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