© P. Beales 1997
|their own right. From
June each year, there is infection of young leaves by oak powdery mildew, Microspheara
alphitoides. However, Q. robur is more susceptible than Q. petraea and
hybrids show intermediate susceptibility.
This disease illustrates well some of the characteristics
of plant diseases in natural communities where man has little influence. It gives a hint
of the roles that plant pathogens play in ecosystems, and why there needs to be a
Biodiversity Action Plan to conserve even these disease causing organisms.
1) One species is more susceptible than
2) Whilst many individual leaves on a
tree may be infected and die, M. alphitoides alone is rarely a cause of tree death.
3) The hybrids show a complete range of
4) When hybrid acorns germinate they
often form a monoculture of seedling oaks near the parent tree. Some become heavily
diseased and die, whilst
others (presumably with more resistance characteristics) live and grow to maturity.
5) Oak powdery mildew and its hosts are a
good example of how hosts and pathogens can co-evolve over many millenia and remain in
balance provided that there is an absence of interference from man.
Conservation of host and pathogen are necessary to
preserve the total ecosystem. To eradicate the pathogen would lead to a series of
unpredictable changes in the evolution of oaks and their ecosystem.