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Beware, dreaded El Nino rains will bring floods of plant diseases

By George Michaels Mbakahya | October 3rd 2015
A farmer shows the destruction of his banana plantation by floods at the Tana Delta following days of heavy rains.

NAIROBI: To a farmer, rain is generally a welcome blessing. Wet weather and plants are usually a match made in heaven. However, sometimes there can be too much of a good thing. Problems caused by heavy rains in different regions of the country may soon start to show up in the farms with the much anticipated El Nino rains.

The next few months will present unique challenges to farmers due to overabundance of rain. These include the treatable problems like leaf spot, yellowing leaves due to Nitrogen deficiency and more serious ones such as root rot and stem blight.


Most plant diseases require moisture to infect and spread and are therefore worse during wet weather. This is especially true of the leaf spot diseases, which are easy to recognise by the circular tan, brown or purple spots they cause on the plant foliage. On ornamental plants, leaf spot diseases are typically just a cosmetic problem, though a few are more serious.

On vegetables, leaf spot diseases cause great concern since they often result in reduced yields. Most leaf spot diseases are specific, only capable of infecting a few species of plants. When a plant in your yard develops leaf spot, do not worry about it spreading to everything in your yard, though it will likely spread to plants of the same type (from one tomato plant to another, for example).

Pruning and spacing plants to promote good air circulation will help them dry quickly and can reduce leaf spot problems. Removing spotted leaves from plants will help slow spread of the disease but will not cure it. Remember to sanitise the pruners before moving on to other plants so that you don’t spread the disease.

Fungicides can be used to control leaf spot diseases, but not always needed. There are numerous types of fungicides, including several organic, but the right type to use will depend on the disease you are trying to manage. For instance, applying copper based fungicides weekly at first sign of disease will prevent its spread. This organic fungicide will not kill leaf spot, but prevent fungus from germinating.

If a plant in your yard develops leaf spot, send a sample to your local extension officer for recommendations on how to control it. Some farmers will experience symptoms such as yellowing of leaves at the bottom of the plants that eventually fall off. This brings me to the second problem in the garden: nitrogen deficiency.


If the older leaves of your plants are simply turning yellow without spots, they are most likely showing symptoms of nitrogen deficiency. Nitrogen leaches out of the soil rapidly with heavy rainfall, leaving fast growing plants like flowers and vegetables hungry. If the plants in your garden are losing lots of yellow leaves, you should fertilise with slow release or organic fertiliser to replace the nitrogen lost through the rains.

In the long term, mulching with organic matter (such as well rotted garden compost or manure) provides a steady trickle of nitrogen to stabilise levels. In the short term, applying high nitrogen fertilisers such as Sulphate of ammonia or poultry manure pellets will remedy the problem. In addition, spraying plants with liquid fertilizer will help plants recover quickly.

Excessively wet conditions will negatively affect crop production in many ways. Abnormally high amounts of rain will leach nutrients, especially nitrogen, from the soil. Nitrogen added to the soil in the form of granular fertiliser is especially vulnerable to leaching.


Plant roots need oxygen to survive, especially the tender root hairs that are responsible for absorbing water and nutrients from the soil. Oxygen exists in the small air spaces between soil particles. When these spaces fill up with water, delicate root hairs drown. As a result, plants will wilt, particularly during the heat of the day. This is often a temporary problem, and plant roots are able to recover as long as the soil dries out within a day or two.

When the soil stays wet for an extended time, root rot disease is likely to set in. With these diseases, once a plant wilts it does not recover but gets progressively worse.

There is no way to save infected plants or eradicate these diseases which live in the soil. In wet areas, only varieties that are moisture tolerant and known to be resistant to root rot should be planted.

In addition to wilting, another symptom of root rot can be discoloration, which means that root rot can be easily mistaken for a fertility problem. When die back sets in, it indicates that the problem is not with fertility, but with rot. Good drainage is key to avoiding and reducing root rot.


Stem blights attacks many vegetables and flowers. Tomatoes, pepper and beans are especially susceptible. If plants in your farm begin to wilt, be sure to inspect lower part of the stem at soil level.

If you can see a white cotton film covering the stem and surrounding soil, your plant has stem blight. There is no way to treat or eradicate this disease, which persists in the soil from year to year. Do not add infected plants to your compost pile as this will only spread the disease.

While some rain is good to keep the garden watered, heavy rains almost every day can lead to some serious garden issues. The first thing you should be on the lookout for is crop diseases. With all the rain we are expecting in the coming weeks, conditions will be ripe for the spread of diseases.

The writer is a consultant on sustainable agriculture and agricultural innovations.

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