Plant Pathology Thrust Into The News

Plant disease has had some coverage in the mass media of late in recent months, with the outbreak of the ash dieback disease in the UK. This plant ailment is caused by a fungus called Chlara fraxinea. The spores of this fungus are spread by the wind and when they settle upon the leaves of an ash tree, will germinate and penetrate the leaf surface. It will grow within the leaves and spread further into branches, killing the tree as it goes. This causes wilting and die back of leaves and over time, as the fungus spreads, it causes the death of whole branches and eventually leads to the death of the entire tree, which can take a number of years. 

Although the recent coverage of this has heightened the public awareness of the threat of plant diseases, there are still a number of diseases of trees and agricultural crops that have been relatively under publicised in terms of their threat. For instance, a disease known as Sudden Oak Death, caused by a fungal-like pathogen known as Phytopthora ramourum, was mainly associated with high mortality in a number of tree and plant species in the USA. However, as of 2009 this disease was found to be infecting Japanese larch trees (Larix kaempferi) in South-West England. Since then it has spread to Wales and Northern Ireland. Japanese larch is an important forestry species and the losses to this disease could have a high impact in an industry where recovery would be slow. This example represents a sinister and unpredictable side to plant diseases. The ability of P. ramourum to infect a Japanese larch was the first recorded instance of this disease being able to infect a commercially important conifer tree species. It is an example of an increase in the host range of this plant pathogen and can be considered to be evolution in action as the organism has been able to adapt to a new environment. 

Evolution and adaptability of plant pathogens is one of the main reasons why the problems caused by many well known diseases such as Late Blight of Potato, that have been studied for over a hundred years, have not yet been eradicated. The usual way that plant pathogens are tackled agriculturally are to either produce pesticides that will kill the pathogen or to breed resistant plant varieties that do not allow the pathogen to infect. However, this imposes selective pressures upon the disease causing organism and drives the adaptation of new pathogen strains that can be resistant to the pesticides or able to overcome resistant plant varieties. This is the same process that is driving the appearance of antibiotic resistant strains of bacteria worldwide. 

            Not only does this process apply to organisms that are already pathogenic, but can also apply to organisms that grow harmlessly alongside plants. Most plants have a whole ecosystem of microbes that grow within them. These are called endopyhtes and can include fungi, bacteria and other microbes. Most grow harmlessly and will feed on excess nutrients within plant tissues (saprotrophy) and some may even provide a nutritional service to the plant, such as nitrogen fixing bacteria in leguminous plant species. Sometimes however, these organisms may come under a selective pressure where it could become more advantageous for them to directly steal the nutrients from the plant, instead of just feeding on nutrients the plant releases as part of its development. This could entail directly killing parts of the plant (necrotrophy) or directly extracting the nutrients while it is still living (biotrophy). The endophyte would then be considered a pathogen as it is inflicting a disease upon its host plant. 

The case of Ash Dieback disease is an example of this process happening in nature.  Originally, Chlara fraxinea would likely have been a saprotroph that would have lived passively within the tissues of tree species. At some stage, there may well have been a change in selective pressure upon this organism; that has driven the evolution of pathogenicity within this fungal species on Ash trees. In 1992 in Poland, swathes of Ash trees were observed to be dying and the disease was identified as Chlara fraxinea. Since then it has spread across Europe and now this year has been identified in the UK leaving 80 million ash trees at risk.