Plants Invisible Enemies

Plants, through their ability to fix carbon dioxide by photosynthesis, are the primary producers of the food that feeds the world’s human population as well as the many animals and other organisms that are heterotrophic for carbon compounds. It is not surprising, therefore, that among the latter there is a considerable number which, in order to have first call on these rich pickings, have adopted the parasitic mode of life. 

 They range from higher plants themselves, the parasitic angiosperms, to viroids, naked fragments of nucleic acid, in some instances less than 300 nucleotides in length. Between these extremes of size, there are plant pathogenic organisms among the fungi, nematodes, algae, Oomycetes, Plasmodiophoromycetes, trypanosomatids, bacteria, phytoplasmas and viruses.

In almost all of these categories there are organisms that cause catastrophic plant diseases, affecting the lives of millions of people by competing for the plant products on which they depend for food, fibre, fuel and cash. In this chapter all 11 classes of plant pathogenic agent will be introduced and those that are particularly destructive will be highlighted together with the impact that they have had on the people who have been most seriously affected. However, the first imperative of a plant pathologist is to establish unequivocally the cause of disease.

The correct diagnosis of a plant disease and its cause is not always an easy task. In the first instance symptoms may be ill defined which make their association with any organism problematic (Derrick and Timmer, 2000) and, secondly, plants grow in environments which are notably non-sterile. In particular, besides supporting a microflora on their aerial parts, the phylloplane, they are rooted in soil which may contain in excess of 1 million organisms per gram. 

The plant pathologist is therefore faced with trying to determine which, if any, of the organisms associated with the diseased plant is responsible for the symptoms. 

This is normally achieved by the application of the postulates of Robert Koch, a German bacteriologist of the 19th century, which for plant pathogens may be stated as follows:

(1) The suspected causal organism must be constantly associated with symp-toms of the disease.

(2) The suspected causal organism must be isolated and grown in pure culture.

(3) When healthy test plants are inoculated with pure cultures of the suspected causal organism they must reproduce at least some of the symptoms of the disease.

(4) The suspected causal organism must be reisolated from the plant and shown to be identical with the organism originally isolated.

Clearly, these criteria can only be met with organisms that can be cultured, ruling out all obligate pathogens which include a number of important fungi, many phytoplasmas and all viruses and viroids. Establishing these organisms as causal agents of disease usually involves purification of the suspected agent rather than culture and the demonstration that these purified preparations reproduce at least some of the disease symptoms.