Agriculture Reference
In-Depth Information
CHAPTER 6
DISPERSAL OF FOLIAR PLANT PATHOGENS: MECHANISMS,
GRADIENTS AND SPATIAL PATTERNS
H.A. McCARTNEY, B.D.L. FITT AND J.S. WEST
6.1 INTRODUCTION
Dispersal has long been recognised as fundamental to the development of plant
disease epidemics, for without dispersal many epidemics would fail to progress. In
recent years, agriculture, especially in the developed world, has come under
increasing pressure to produce crops in sustainable and environmentally friendly
ways. Consequently, there is an urgent need for more efficient disease management
systems. Understanding the temporal and spatial dynamics of disease epidemics is
crucial to the development of such systems. For example, when developments in
precision agriculture lead to spatially targeted crop spraying, there will be a need to
understand and predict disease patch dynamics. The role of dispersal in gene-flow
within plant pathogen populations is little understood, but may be crucial to
understanding fungicide resistance breakdown or the distribution of alleles
conferring virulence within populations. Knowledge of dispersal processes is also
needed to understand movement of new pathogens into a landscape, for example the
introduction of exotic pathogens into a country or the movement of pathogens due to
climate change. Thus, knowledge of dispersal will increasingly be needed by policy
makers devising plant health protection strategies. It is evident that now, perhaps
more than ever, there is a need to understand the nature and scope of the dispersal of
plant pathogen propagules.
Plant pathogen propagules include fungal spores, virus particles and bacteria
(cells and spores) and there are many different mechanisms by which each can be
dispersed from infected host plants. For example, many plant viruses are dispersed
by insect vectors; insects, birds and farming activities can spread both bacteria and
fungal spores. Many soil-borne pathogens can be spread in ground water or by
agricultural operations (see also Chapter 14). However, many economically
important crop diseases are caused by foliar fungal pathogens, for which the main
routes of dispersal are wind-borne (Chapter 15) or splash-borne (Chapter 16) spores.
The scale of dispersal by these processes ranges from a few centimetres for some
spores spread by rain-splash up to hundreds of kilometres for some spores carried by
the wind. This chapter will concentrate on the spread of such pathogens.
For foliar pathogens, disease spread is the direct consequence of spore dispersal,
although spatial patterns of disease may be quite different from the spore dispersal
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