Agriculture Reference
In-Depth Information
CHAPTER 3
SURVEYS OF VARIATION IN VIRULENCE AND FUNGICIDE
RESISTANCE AND THEIR APPLICATION TO DISEASE
CONTROL
JAMES K.M. BROWN
3.1 INTRODUCTION
Genetic variation has been discovered in almost all plant pathogens which have been
studied, both in characters which affect their ability to infect host plants and in traits
which are not related to pathogenicity. Knowledge of variation in pathogenicity
characters, gained through surveys of pathogen populations, is valuable in planning
effective measures for disease control, such as the use of resistance genes in crop
varieties or the application of fungicides. Conversely, knowledge about such
epidemiological factors as the dispersal of inoculum helps in the planning of pathogen
surveys and in predicting how pathogens may adapt to control measures. This chapter
reviews methods of surveying variation in traits related to pathogenicity in fungi and
ways of applying the results of surveys to disease control. In particular, it focuses on
traits important in agriculture, especially virulences and responses to fungicides. It also
reviews two sets of recent developments: the application of molecular genetics to
surveys of pathogenic variation and the dissemination of the results of surveys and
recommendations arising from them
via
the internet.
3.2 CHARACTERISING INDIVIDUAL PATHOGENS
The two kinds of phenotypic variation that are most often studied in pathogen
surveys are adaptation to different host cultivars and responses to fungicides.
3.2.1 Host cultivar range
The most intensively studied type of adaptation of pathogens to different host
cultivars is that which follows the gene-for-gene relationship, first defined for flax
rust by Flor (1956). Many aspects of the genetics and molecular biology of gene-
for-gene interactions were reviewed by Crute
et al.
(1997) and Dangl and Jones
(2001). A gene-for-gene interaction operates between a specific avirulence gene
in a pathogen and a specific resistance gene in a plant. The resistance
protein detects the presence of a pathogen with the corresponding functional
avirulence gene, most likely by detecting the interaction of the avirulence protein with a
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