Agriculture Reference
In-Depth Information
CHAPTER 13
SEEDBORNE DISEASES
W.J. RENNIE AND VALERIE COCKERELL
13.1 INTRODUCTION
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Almost 90% of all the world s food crops are grown from seeds (Schwinn, 1994) and
seeds are widely distributed in national and international trade. Germplasm is also
distributed and exchanged in the form of seeds in breeding programmes. However,
many plant pathogens can be seed transmitted and seed distribution is a very
efficient means of introducing plant pathogens into new areas as well as a means of
survival of the pathogen between growing seasons.
Disease-causing organisms (in plants: usually fungi, bacteria, viruses and
nematodes) may be carried with, on or in seeds and, in suitable environmental
conditions, may be transmitted to cause disease in developing seedlings or plants.
With some diseases, the pathogen attacks the germinating seedling and affects
seedling establishment and hence plant populations; with others, disease symptoms
are not seen until a later stage of growth.
Studies on the epidemiology of seedborne pathogens are important to understand
how the diseases they cause can be controlled most effectively. With seedborne
infection, optimum control is often achieved most cost-effectively, and usually with
least disruption to trade, by the application of seed treatment chemicals, or through
avoiding infection by producing seed in areas or environments that are unfavourable
for disease development. Maude (1996) gives a comprehensive review of how
seedborne pathogens may be controlled through exclusion and elimination. In some
special circumstances, it may be appropriate to test seed before marketing or sowing
and infected seed may be rejected or treated with an appropriate chemical. Effective
control through seed testing requires not only sensitive and accurate test methods but
also appropriate and adequate sampling of the seed lots to be examined (Cockerell
et al
.
,
2003; Kruse, 2004).
A point sometimes not appreciated is that seed which is traded nationally or
internationally is not necessarily disease-free. Many importing countries have
legislation that aims to prevent or limit the introduction of non-indigenous
seedborne pathogens (Ebbels, 2003) and national seed certification schemes may
have standards for the most damaging seedborne diseases, especially those that
cannot easily be controlled by seed treatment chemicals (Rennie, 1993). Frequently
a distinction has to be made between diseases that are already widely distributed but
which can have an effect on crop performance and those that are of limited
distribution but have the potential to cause serious crop losses if introduced into new
areas. The former are usually controlled through tolerances or standards in domestic
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