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When your plants try to deliver a message

By George Mbakaya | July 21st 2018 at 00:00:00 GMT +0300

Did you know plant disease control measures depend on proper identification of diseases and of the causal agents?

Without proper identification of the disease and the disease-causing agent, disease control measures can be a waste of time and money and can lead to further plant losses. Proper disease diagnosis is therefore vital.

Often, farmers rely on symptoms for the identification of a disease problem but, use of symptoms is an inadequate method because similar symptoms can be produced in response to different causal agents.

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Today’s article presents the various steps associated with accurate plant disease diagnosis. The process may vary with different diseases and conditions but the overall process is relatively consistent. The steps require careful observations and questions.

Determine what is normal

Identification of affected plants is one of the first steps in diagnosing a plant disease. Knowing the identity of the plant species affected allows the farmer to utilise various resources that contain lists of plant diseases associated with specific plants.

Recognise healthy plant appearance. Before you take an in-depth look at your ailing plant, step back and look around. How healthy are its neighbours?

It is important to know the normal appearance of the plant species you are investigating. Each plant species has special growth habits, colours and growth rates. If you do not know what to expect of the plant, you cannot recognise when something is wrong.

For instance, does the plant normally have new foliage that is yellow or red and becomes darker green as the foliage ages? It is important to know what the normal appearance of a plant is before you decide there is a problem.

Once the “normal” appearance of the specific plant is determined, several comparisons can be made between the problem plants and healthy plants.

Compare characteristics such as overall size, shape, and colouration; leaf shape, size, colouration, and distribution; root distribution and colouration; and bark, stem or trunk texture and colouration. It is also important to note normal events, such as leaf drop, that may occur in a healthy plant.

The affected parts of the plants should also be noted. Are there symptoms on the roots, leaves, stems, flowers, or fruit? Is the entire plant involved? Is only one limb or side of a plant involved? Answers to these questions can assist in the identification of the problem. Since diseases are plant-specific, this narrows the field of likely illnesses. Now you can check your plant for clues that match up with these diseases.

Examine the entire plant, including some roots. Root damage can show up as leaf symptoms. Some experts advise focusing your examination downward from where you see the problem. Make notes on exactly what you see in the way of symptoms: oddly coloured tissues, shrunken plant parts, swelling or galls, whatever you notice.

Insect damage

Now consider for a moment some key differences between the symptoms, above, that you see with disease damage, versus those associated with insect damage. There’s still a possibility that what you have is caused by a living organism that chews, rasps, sucks, bores holes or ties leaves up in rolls to make housing. If you see those, start suspecting insect damage.

It is also important to assess the activities that have been conducted around the affected plants. The problem may not be due to anything that the grower has done; the problem could be related to what his/her neighbour has done.

Information pertaining to the growing environment to which the affected plant has to be exposed. It is especially important to document changes in the environment.

Environmental factors to consider include: extreme temperatures, rainfall, hail, lightning, prolonged drought, temperature inversions and prevailing winds. Site factors such as soil type, possible drainage problems, and soil pH should also be evaluated.


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