Octobr 2006. Disease diagnostics – the case for plant diseases. Several members raised there concern that there were few pictures of diseases sent to PestNet compared those of insects and weeds.
It was pointed out that there was more to diagnosing a pathogen than taking a photograth in the field. A picture is only a guide to what the problem might be. But frustratingly, there seems to be even a dearth of one good photo of many less common diseases (for example mango scab). A good sets of pictures is needed in which a pathogen has been isolated and pathogenicity demonstrated by production of the same symptoms. Even better is when the pictures are accompanied by information that can aid in diagnosis and control. Anna Snowdon’s Color Atlas of Postharvest Diseases Volume 1 is a great example of this sort of approach.
A good photo of symptoms verified as caused by a particular pathogen, together, preferably, with a photo of the causal agent illustrating key diagnostic features, is much more accessible than a dried herbarium specimen. One problem, however, is that isolation from symptoms caused, say, by Elsinoe mangiferae, the cause of scab may yield Colletotichum and so the disease is misdiagnosed. But a start has to be made somewhere. While it is easy to amass lots of pictures of insects associated with a host it is more difficult to acquire them for the diseases – a photo can then be an aid to providing further diagnostic or control advice.
For those interested in disease diagnosis and pathogen biology there is a small list of lessons covering key plant diseases that are representative of a range of pathogens on the American Phytopathology Website at:
While few are tropical plant disease problems, the pathogen genera covered are a good spectrum of the pathogen genera infecting tropical plants. Frequently, there are pictures of the pathogen as well as plant symptoms.
Another contribution stated that the reason insect/weed enquiries outnumber diseases, from the Pacific at least, is that there are few plant pathologists compared to entomologists. This was noted in an excellent review of Pacific plant pathology by Bob Fullerton in 1991. There are microscopes and in most places they are in good order as SPC has supported servicing and repair, but it takes a certain amount of practice to take good photographs down a microscope, even if you are able to prepare the cultures in the first place. As Richard Davis suggests: it’s a little more than just point and shoot!
PestNet with FAO support has been to many Pacific Island countries, giving out cameras and training people how to use them to take photos of insects/weeds and disease symptoms. Perhaps the next step should be photo microscopy? I would like to hear what others think, especially people who have been helped with identification through the ADAP supported Pacific Islands Distance Diagnostic and Recommendation System.