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The plant detectives: a clinical approach to plant disease

The bamboo plant definitely looks unhealthy. The leaves are brown and yellow and it looks like it is dying. Puzzled looks and frowns show that the extension officer trainees are speculating as to the likely cause. Might it be biotic, perhaps an insect pest or a virus, or an abiotic stress such as drought or nutrient deficiency? Learning to recognize patterns of symptoms has been a major part of the trainees' morning. First they collected and grouped unhealthy plant samples from the surrounding fields, then started to share their knowledge of plant health problems and finding agreement on diagnoses.

Trainee plant doctors examine specimens on a course held in Cuba (Eric Boa)
Trainee plant doctors examine specimens on a course held in Cuba
Eric Boa

Recognising and interpreting plant disease symptoms is something that extension officers like these rarely have much training in. And while a two-day course like this one cannot produce a team of plant pathologists, it can, according to trainer Eric Boa, instil the basic skills needed to be a plant nurse, doctor or detective. In the case of the 'dying' bamboo, there is in fact no crime to solve; the photograph simply shows a healthy plant exhibiting normal albeit twisted leaves.

Diagnosis is critical

Since 1997 the Global Plant Clinic, of which Dr Boa is head, has organised plant disease training courses for over 300 trainees, mostly extension officers, but also researchers, farmers and teachers, in eight countries. That experience has revealed fundamental gaps in knowledge, even for the most common and serious disease threats; in Uganda for example, the symptoms of cassava mosaic disease have been widely publicised, but are still often confused with mite damage. Failure to detect a disease problem, perhaps through ignorance of certain symptoms, risks a damaging spread. A faulty diagnosis, as when a bacterial or nutrient-related disease is blamed on an insect pest, not only means that the true cause remains untreated, but may also lead to wastage of scant resources, unnecessary use of pesticides and environmental damage. Scientists may have reservations about plant disease being diagnosed solely on the basis of field observations, but given the limited availability of more advanced diagnostic facilities in countries like Uganda, teaching basic skills to extension workers is the only realistic option. Boa compares the trainees to Community Animal Health Workers, able to perform a valuable service albeit at a lower level than a fully qualified vet could offer. And in post-conflict situations, such as in the Congo, training plant doctors and establishing local plant health clinics can be a vital element in restoring agriculture quickly.

Letters to the laboratory

Knowing when to turn for help, and how, is of course an important skill in itself. The Global Plant Clinic receives hundreds of samples of diseased plants every year, but many arrive in a sorry state, and with inadequate supporting information. That's not a helpful start to a diagnosis. Whether sending samples to a laboratory in their own country or further afield, extension workers need to know which part of the plant to take a sample from, and how to ensure it reaches laboratories in a useful condition. The trainees write reports based on case studies then read them out to the whole group. They tick how many key pieces of information from a 15-point checklist are present, a useful method for highlighting things left out.

Training for emergency response

Courses are sometimes requested in the context of a particular disease outbreak; the spread of banana bacterial wilt in Uganda was one such example while in Cuba, the national centre for animal and plant health (CENSA) was eager to help staff cope with the greater diversity of crops being adopted by smaller and more independent farmer groups. The practical challenge is how to cope with a greater diversity of plant health problems and assist farmers and extension workers directly. Sending samples is a luxury for many and costly too. Specific courses have also been devised for tree health. In the 1990s, aid agencies and national governments reported a dramatic decline of neem trees in Niger. Previously successful investments in agroforestry were threatened. Subsequently, neem fell out of favour though the belief that it was under attack by some mystery disease was never supported by scientific investigations. If the response at the time to the 'symptoms' of the decline had been informed by a wider awareness of their possible significance perhaps neem would not have suffered a dent in its reputation.

A recent pan-African tree health course in Kenya, led by Boa with support from the World Agroforestry Centre (ICRAF), showed foresters not only how to recognize and interpret symptoms but how to monitor health using the density of the tree crown. Laboratories provide scientific certainty but simple field monitoring provides practical insights for managing problems. That crucially is what can help local communities cope in difficult circumstances.

Date published: September 2005

 

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