Agriculture Reference
In-Depth Information
seeds regulations while the latter are often the subject of strict import or quarantine
regulations.
Before considering control strategies for specific seedborne diseases, the
following epidemiological information must be available.
13.2 EPIDEMIOLOGY
(a) What quantity of inoculum must be present on seeds, and in what form, to
transmit the pathogen to developing seedlings and cause disease?
The large majority of seedborne pathogens are carried passively on seed surfaces or
are established as hyphae or spores in or under the seed coat. Successful
transmission of infection to developing seedlings often depends on the location of
infection on the seed and on the amount of inoculum present. Theoretically, one
spore of
Tilletia tritici
on a single wheat seed can result in a bunted plant but several
workers have shown that, in practice, bunt does not occur in the growing crop until
the level of contamination is much higher (40-100 spores per seed); seed
contamination needs to be even higher before infection occurs in some less
susceptible cultivars (Heald, 1921; Oxley and Cockerell, 1996). The relationship
between the amount of inoculum on the seed and disease transmission is usually
strongly influenced by environmental conditions and cultivar susceptibility.
(b) How is seed-to-seedling transmission influenced by environmental conditions
during germination and subsequent plant development?
Where inoculum is superficial on the seed surface, or is established only in the tissues of
the seed coat, successful transmission of the pathogen to the germinating seedling
depends not only on the amount and location of inoculum on the seed but also on
environmental conditions during germination and seedling establishment.
Microdochium
nivale
, which can cause seedling blight in cereals, is often present on seed in the form of
mycelium in the pericarp. If germination is rapid, in conditions where moisture is
adequate and the temperature is above 10°C, there may be relatively little transfer of
infection to developing seedlings and the pathogen has little or no effect on germination
or seedling establishment (Hewett, 1983). However, where the soil temperature is
between 5 and 10°C and germination proceeds slowly, the fungus readily establishes
within the tissues of young seedlings and significant seedling losses can occur
(Hewett, 1983; Richards, 1990). Disease transmission is frequently reduced at low soil
water contents. Infection of young pea seedlings by
Pseudomonas syringae
pv.
pisi
is
greatly reduced in dry seed beds (Holloway
et al
.
,
1996).
On the other hand, where inoculum is deep-seated and the embryonic axis of the
seed is infected, environmental factors may have relatively little effect on pathogen
transmission. Barley seed infected with
Ustilago nuda
, where inoculum becomes
established in the scutellum tissues of the seed embryo, is likely to give rise to plants
showing symptoms of loose smut over a very wide range of environmental conditions.
