Agriculture Reference
In-Depth Information
(e) Will reducing seedborne inoculum (for example by selecting relatively healthy
seed, or through seed treatment) lead to reduced levels of disease in the growing
crop?
Reducing seedborne inoculum, through chemical seed treatment or by selecting
healthy seed for sowing, will be an effective disease control measure in situations
where inoculum from other sources is not significant. For example, control of
monocyclic diseases such as loose smut of barley or wheat by seed treatment is
usually very effective. On the other hand, barley net blotch ( P . teres ) or septoria
glume blotch on wheat (caused by Stagonospora nodorum ) cannot usually be
controlled by seed treatment or by sowing healthy seed, since susceptible crops very
quickly become infected from inoculum originating from old stubble and diseased
volunteer plants.
Some polycyclic diseases may be kept in check by reducing the amount of
seedborne inoculum. This usually requires any residual seed infection to be very low
and is probably best achieved by producing seed in areas where the pathogen does
not occur or where environmental conditions are unfavourable for disease
development, so limiting or avoiding seed infection. Hewett (1973) showed that if
seedborne inoculum of Didymella fabae was kept below one infected seed in 200,
ascochyta leaf blight did not develop in plots of field beans in England. Grogan
(1980) established that seedborne inoculum of the aphid-transmitted lettuce mosaic
virus had to be below 0.022% to ensure that disease epidemics did not develop in
commercial fields of lettuce in California.
(f ) What conditions favour the establishment of infection in developing seeds?
In seed multiplication systems, developing seeds usually become infected from
inoculum present within the growing crop. For some diseases, infection may be
systemic and seeds developing on diseased plants may become infected as they are
formed. In these cases, environmental conditions will have little or no influence on
the rate of seed infection.
For many diseases, the pathogen is often carried from infected plants to
developing seeds in wind eddies or by rain splash. Fungal conidia and bacteria often
develop very readily on diseased plants in humid or wet conditions and seed
infection usually depends on the availability of adequate moisture. For many
diseases, therefore, seed infection levels are highest when high humidity or rainfall
occurs during seed development. Hewett (1965) has shown how Microdochium
nivale levels on wheat seed tend to be highest in the wetter western and northern
parts of the United Kingdom. Commercial cabbage seed production in the United
States is concentrated in western Washington state because the dry conditions there
result in a low incidence of seedborne inoculum of Leptosphaeria maculans (cause
of blackleg) and Xanthononas campestris pv. campestris (cause of black rot)
(Gabrielson, 1983). Seed production in areas of low disease pressure, or in environ-
mental conditions unfavourable for seed infection, is a very effective means of
producing healthy seeds and is likely to be most cost-effective when seeds are small
and transport costs low, or when the seed is of sufficiently high value to justify high
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