ABSTRACT

As much an art as a science Diagnosis of diseases and other problems in potatoes is as much an art as a science. Descriptions in books, even this one, can never fully describe the range of symptoms that may develop in the field. Photographs often show ‘typical’ symptoms, but from this norm variations occur due to resistance of the host, the prevailing environmental conditions or interaction with other pathogens or pests. An example of this variability is manifest in viruses. In one cultivar, a virus may exhibit severe mosaic symptoms, yet in another, the symptoms may be milder or non-existent. However, in this second cultivar, if an additional virus is present symptoms may be catastrophic, leading to severe stunting or death. Confusion in visual diagnosis of symptoms can be overcome by resorting to specific laboratory (or field) diagnostic tests. But even then, where a pathogen, pest or disorder is confirmed and seems to be the obvious cause, it is essential that the diagnostician eliminates other causes or implicatory factors. Where a diagnostician relies solely on visual symptoms, perhaps in the field situation, diagnosis may not be clear-cut. A preliminary diagnosis, before confirmation in the laboratory, may be based on a large proportion of the key diagnostic characteristics of a causal agent or the symptoms being present.