Thinking with your eyes: masterclasses in plant health

Dr. Eric Boa holding maize lethal necrosis disease in Rwanda.
Dr. Eric Boa teaching field diagnosis in South Africa (Dr. Congo in the picture).
Dr. Eric Boa teaching field diagnosis in South Africa.
Dr. Eric and fellows at table with samples

Thinking with your eyes: masterclasses in plant health

Eric Boa

Modern science has advanced our knowledge greatly. Huge technological leaps now reveal the genetic code of many organisms, a task that takes days rather than years to complete. We can map precisely where to apply fertilizer in fields using GPS. Smartphones give us rapid access to a wealth of information about crop pests and diseases. Who needs experts to give advice on the best way to stop fall armyworm eating your maize. All you need is an app and a few swipes.
If only this was true. Technology and machines will not replace people. We need skilled people who understand crops, recognize symptoms and can interpret them. Or when to send a sample to the laboratory for analysis. Technology can help immeasurably but machines won’t replace people. People have unique abilities in handling a lot of information quickly and fine-tuning decisions about when and if to spray a pesticide, or try improving soil fertility first and seeing what happens. 
One of our greatest skills is observing. This is especially true in diagnosing plant health problems. We think with our eyes, gathering visual information and comparing observed symptoms with a mental list of possible causes. Like many other scientists I used to believe that diagnosis was something you did in the laboratory, but years of working in the field has taught me that diagnosis is first and foremost a practical skill. Like all skills, you need to learn things and gain practical experience.
Traditionally field diagnosis – the recognition and interpretation of symptoms – is something you learn on the job. Over the years you gain more knowledge of symptoms and how to recognize the difference between citrus bacterial canker and citrus scab; when you understand that a lack of phosphorus on maize can look a little like corn stunt phytoplasma. I’ve worked with many different crops, from bamboo and cloves to neem trees, cocoa, maize and potatoes – to name only a few. I’m not an expert in all the pests and diseases of the crops I’ve worked with and I worried about the time it would take to make basic diagnoses in the field.
I soon realised that sending samples to be diagnosed in the laboratory was not the answer. Sending photos has never been easier, but even this method often fails to identify the cause. If I didn’t know the cause then I couldn’t give advice on control measures. I started to think that there had to be a better way to become a field diagnostician and it was from this starting point that I began to develop training material. At first the main audience was foresters, since I originally worked in tree health. There are particular difficulties in diagnosing perennial plants. 
Changes in appearance can be temporary in trees: yellow foliage becomes green and healthy after a drought is broken. Other symptoms are general and tricky to interpret. I realised that I couldn’t teach all the symptoms of all the pests and diseases affecting trees and so a different approach was developed. Over 18 years a short course has evolved, suitable for all levels of knowledge – postgraduate, farmer, forester, extension agent, agronomist and even agrochemical dealers.
Masterclasses in plant health began with field diagnosis but now include giving advice and writing extension material. In each of these two to three courses participants learn basic principles. They learn new ways to look at plants which allow them to start from a broad diagnosis to something more specific. One of the key principles in field diagnosis, the first masterclass because without a likely cause you can’t give a useful recommendation, is to eliminate possibilities.
There’s a fundamental difference in teaching practical field diagnosis skills compared to learning about specific pests and diseases. University courses introduce students to root rots and leaf rusts, stem borers and virus diseases. They don’t teach you how to distinguish mite damage from phytoplasmas. When I first saw soybean rust (or thought I saw soybean rust) I was in the lowlands of Bolivia. I had a hand lens but that didn’t help much since I wasn’t sure what to look for. Soybean rusts don’t have the same powdery appearance of coffee rust – which happens to be bright orange and distinct from other leaf diseases.
Smartphone apps and tablets can of course help a lot, so technology does have a place to play. But it’s no substitute for thinking on the spot. This is particularly true when trying to tell the difference between nutrient deficiency symptoms and a wide range of diseases, particularly those caused by viruses, which induce similar effects. The tea mosquito (Helopeltis spp.) causes a wilt, as does Phytophthora and Ralstonia. The field diagnosis masterclass aims to help you recognize some of the essential differences between similar symptoms. For example, insect-induced wilts tend to occur in patches. Bacterial wilt affects the whole plant, often leading to dramatic drooping of leaves.
Dutch elm disease, a fungal disease, has killed many trees in Europe. Elm is a popular roadside and woodland tree and when leaves start to dry up and branches die it usually signals the end of the tree. So when people see the same symptoms in sycamore tree, another widely planted amenity tree, they often worry that this is another tree disease. But it’s not. Squirrels strip the bark on the trunk in years of limited food supply, and it is this physical damage that causes the wilting. Field diagnosis courses have been taught to many people who are responsible for looking after plants but don’t have expert knowledge of pests and diseases.
It’s important for breeders and agronomists, as well as farmers, to understand the differences between different causes, and not just pests and diseases. A field diagnosis can be immensely helpful in deciding to do nothing. Why spray a pesticide when the field diagnosis has eliminated the possibility of pest or disease attack? Of course we all want to know a more precise cause, so that we can identify a precise action to mitigate the symptoms, but in the short term the best action is often to wait and see. Conversely, sometimes you need to take decisive action even though symptoms appear minor. Tuta absoluta, the tomato leaf miner, has now reached Kenya. Small black spots on the fruit, with some rotting below, may not appear important but surely require attention because they don’t look like other rots.
Field diagnosis masterclasses show symptoms of plant health problems on many crops, exposing people to a wide range of pests and diseases. I’ve run 39 courses in 27 countries and learnt a lot about the huge variety of fungi, insects, mites, mammals, bacteria, viruses and other abiotic causes, for example, which cause damage to crops. I always emphasise crops which are relevant to each country but always maintain a broad diversity. Growing herbs and fruit trees may be less important than field crops, such as wheat, but gaining skills in field diagnosis depends on seeing as wide a range of symptoms as possible. It’s surprising how often people from a different country recognize something from elsewhere. All the maize agronomists in South Africa instantly recognized hail damage even though the photograph came from Nepal.
So how do you teach field diagnosis? The varied exercises range from describing symptoms to making a basic diagnosis then proceeding to something more precise. A basic diagnosis for me is deciding whether the cause is biotic or abiotic. Something more precise could be ‘fungus disease’ rather than ‘virus disease’. The aim is to move slowly, building up confidence and convincing people that they know more than they think. The responses have been consistently positive. The greatest frustration is that the training is so short. The temptation to run a week long course is strong, yet it’s important not to attempt too much at the same time.
I’m surprised that field diagnosis skills are not taught more widely or systematically. Why? Because the vast majority of diagnoses are done visually – thinking with your eyes is more important than I first realised. Doctors and veterinarians spend a lot of time developing their ‘field diagnostic skills’. They have to treat patients quickly because the risk of waiting for test results often means that the person or animal becomes more ill.
The masterclass in field diagnosis has exercises on taking a case history, combining patient information with observations made by the doctor. The patient in agriculture is mute – crops don’t talk! But farmers can provide useful details about how a crop is grown, disease development, weather conditions and so on. Extracting useful information from farmers is a skilled job. Plant doctors have to listen carefully, not ask too many questions and probe for more information.
One of the key exercises in the course – and a source of amusement when first explained – is the ABC test. Participants are given a range of photographs showing different symptoms on a key crop. They are given three options for making a basic diagnosis: do symptoms suggest an abiotic (A) or biotic cause (B). They don’t have to say which type, only decide on the broad category. If they didn’t know or aren’t sure they have a third option. The ‘C’ in ABC stands for ‘confused’.
The symptom matrix exercise approaches field diagnosis from a different angle. A table gives a series of symptoms in the first column. The first row has major types of pests and diseases. Participants work in small groups, as they do throughout the course, deciding which pest groups can cause different types of symptoms. Fungi and bacteria both cause cankers, but not viruses. Sometimes it’s not so clear. There are virus ‘wilts’ but these are uncommon and less pronounced than those caused by fungi and bacteria.
Agriculture is the backbone of the Brazilian economy. Plant health problems cause huge losses. Early diagnosis is an effective way to minimize losses. By improving your field diagnosis skills you could have a major impact on saving money, increasing productivity and removing major threats. Field diagnosis is fun and practical. I hope that Brazilian agriculturists agree with me as the opportunity to attend courses becomes available for the first time through Agronomica!


Sobre Eric Boa

A carreira profissional do escocês Ph.D. em Fitopatologia Eric Boa se estende por 23 anos e tem raízes na pesquisa agrícola. Tem experiência em treinamento em diagnóstico fitossanitário para engenheiros agrônomos, inspetores fitossanitários, equipes de assistência técnica, agricultores, comerciantes agrícolas e muitos outros na América Latina, África, Ásia e Europa, por mais de 20 anos. Trabalhou durante 15 anos como pesquisador no CABI, organização filantrópica internacional focada em melhorar a vida da população levando conhecimento científico para resolver problemas da agricultura e meio ambiente. É expert em saúde de plantas, sistemas agroflorestais (bambu) e florestais não madeireiros (selvagem útil fungos); e prestação de serviços nas áreas comuns ("GOING PUBLIC").