BSPP Presidential Meeting 1999

Biotic Interactions in Plant-pathogen Associations

Session VIII - Disease complexes and complex aetiology

Microbial diversity, complex disease interrelationships and root health
Richard A. Sikora.

Professor for Soil-Ecosystem Phytopathology and Nematology, Institut fr Pflanzenkrankheiten, Universitt Bonn, Nussallee 9, D-53115 Bonn, Germany. 

Additive damage impact to the root system caused by the simultaneous activity of multiple species of plant pathogens and parasitic nematodes should consistently lead to severe crop loss. This is especially true if estimated losses reported for each individual organism are valid. However, when calculating additive damage impact the sum seldom reflects that extrapolated from data generated in greenhouse and/or pesticide based field trials.

Furthermore, complex disease interrelationships that lead to synergistic interactions should be devastating and lead to syndromes and ultimately to chaos. This is at times the case, but only in isolated instances and on specific crops. The controversy over the existence of `interactions verses `interrelationships as it relates to root health also has led to heated debate in the past. Complex disease interrelationships and the disease syndromes they ultimately cause have been defined as: a group of signs and symptoms that occur together and characterize a particular abnormality.

In the past a great deal of research was conducted on classical synergistic interactions that were based on statistical verification. A recent review of the literature resulted in only a small number of new reports on this important subject. The vast majority of studies dealt with interrelationships related to biological control which is a logical development of this subject.

The term `chaos has been recently used to described a complex disease and the `edge-of-chaos for the buffering capacity or antagonistic potential of an ecosystem. When the `edge-of-chaos in the soil of an agro-ecosystem is established, the buffering capacity is fully expressed and the impact of individual pathogens and/or parasites alone or in combination is significantly reduced.

However, when the `edge-of-chaos is overstepped, complexity is out-of-control and complex disease syndromes result. The following questions need to be addressed: 1) what factors are responsible for the shift from a healthy root exhibiting symptoms of minor additive disease impact to a disease syndrome 2) does the buffering system or antagonistic potential play a major roll in root health or are only individual antagonists important and 3) what are the causes of the breakdown in root health?

It seems logical that: 1) the development of a complex disease syndrome in the root system is closely associated with the level of the antagonistic potential in the system 2) the level of antagonistic potential varies from non-existent to full disease suppressiveness in all soils and 3) a community of beneficial organisms living in or on the root system is the driving factor responsible for sustainable root health. Furthermore, it can be hypothesized that shifts in both structure and density of rhizosphere microbial communities specific for a plant species are responsible for the presence of or lack of disease in susceptible cultivars. Complex disease syndromes result when the level of distortion within these microbial communities is greatest.

A number of crops that are considered highly sensitive to root pathogens and parasitic nematodes are initially produced in sterile tissue culture or under near sterile conditions in seedling production systems. These seedlings lack `rhizosphere specific microbial communities that form the buffering system that prevents complex disease syndromes from forming. Advanced plant breeding also has led to new problems. New resistant cultivars that have been shown be resistant to nematodes, are often intolerant to early root infection. What importance do `rhizosphere specific microbial communities play in the expression of tolerance in such cultivars?

In principle `chaos begins at the root surface or the `cutting-edge of the plants defence system and expands into the root tissue of plants lacking natural defence mechanisms. Disease can be offset by using inside-out and/or outside-in strategies of biological system management. These two approaches attempt to re-establish parts of the microbial community defence system that is eliminated by these production technologies. Model strategies being followed to offset the impact of complex diseases syndromes in modern banana tissue culture and vegetable transplant production systems will be outlined. The advantages and disadvantages of the approaches will be discussed.

Mutualism and antagonism: Ecological interactions among bark beetles, mites and fungi
Kier D. Klepzig, 1J.C. Moser, 2M.J. Lombardero, 2M.P. Ayres, and 2R.W. Hofstetter

USDA Forest Service, 2500 Shreveport Hwy, Pineville, LA 71360, USA.
Dartmouth College, Hanover, New Hampshire USA.

Symbiosis may be defined as: "the acquisition and maintenance of one or more organisms by another that results in novel structures and (or) metabolism". Relationships among symbiotic organisms may change over time and ranges of resources. Other organisms may indirectly facilitate or interfere with these relationships. Interactions among bark beetles and their associated fungi and mites are complex examples of the manner in which symbioses change and are indirectly affected by other organisms. These complex relationships have been extensively studied in the southern pine beetle (SPB), a bark beetle that kills healthy living trees through mass colonization. The SPB is consistently associated with three main fungi. Two of these fungi (Ceratocystiopsis ranaculosus and Entomocorticium sp. A.) are carried in a specialized structure (mycangium) in female SPB. The third fungus is carried phoretically on the exoskeleton. Both C. ranaculosus and Entomocorticium sp. A are also carried by phoretic mites of SPB. Due to the effects of these fungi on SPB larval development, their competitive interactions have significant implications. The two mycangial fungi provide nutrition to developing larvae, while the phoretic fungus interferes with larval development. These interactions appear to be mediated by phoretic mites which have mutualistically symbiotic relationships with the SPB associated fungi they vector. The multiple interdependencies in this system provide novel opportunities for control of, and further research on, this damaging forest pest complex.

Disease complexes, marram grass degeneration and sand dune succession
Wim H. van der Putten

Netherlands Institute of Ecology NIOO-CTO PO.Box 40 6666 ZH Heteren, the Netherlands.

The role of soil-borne diseases in the spatio-temporal dynamics of natural vegetation is receiving increasing attention of ecologists. In natural vegetation, disease phenomena are often less pronounced than in agro-ecosystems. I will start with a brief overview of examples of soil-borne diseases thus far known. Then, I will continue with an overview of research on coastal sand dunes and soil-borne diseases contributing to succession.

In coastal foredunes, thus far, we assume that soil-borne disease complexes consist of combinations of plant-parasitic nematodes and plant-pathogenic fungi. Both surveys, selective elimination studies and inoculation trials have been carried out and results will be discussed. New results and plans for future developments in our research will be presented concerning possible contributions of plant parasitic nematodes to the functioning of natural soil disease complexes. Finally, we may discuss possible interactions and synergism of phytopathology studies in agro-ecosystems and those in natural vegetation.