Agriculture Reference
In-Depth Information
The epidemiological significance of very low levels of seedborne infection is not
well known. Roberts (1992) found that soil water content had a significant effect on
transmission of
P. syringae
pv.
pisi
from seed to seedling and seed transmission
increased with increasing soil moisture. Hollaway
et al
. (1996) estimated that
approximately 10% of infected seeds would give rise to infected seedlings, even in
relatively dry seed beds, and that a seed lot with 0.1% infection would, therefore,
result in a primary infection of 0.01%, which would be sufficient to cause a field
epidemic (Taylor and Dye, 1976).
In a series of glasshouse and field trials, Roberts (1993) and Roberts
et al
. (1995)
showed that pea blight could result in significant yield loss. Yield reductions of up
to 71% were recorded in glasshouse trials, following inoculation with the pathogen,
and a model constructed from field trials data suggested that a yield loss of 0.98
tonnes ha
-1
would be expected where 5% of the leaf area was diseased at growth
stage 208. A feature of the field trials was the considerable spread of infection into
plots sown with healthy seed and the interaction between disease and bird damage.
In retrospect, the seed sampling and testing procedures introduced in 1986 did not
significantly reduce the proportion of seed samples infected with pea blight, though
they probably limited the incidence of infection in certified seed. However, despite
initial concerns, pea blight did not appear to develop to epidemic levels in spring-
sown crops of combining peas and there were few field reports of serious yield loss.
The statutory controls on certified seed have now been removed and, while a
voluntary Code of Practice aimed at minimizing the occurrence of blight in pea
crops was introduced in 1994 (Anon., 1994), there is currently relatively little testing
of spring-sown seed. In contrast,
P. syringae
pv.
pisi
has been shown to have a
damaging effect on autumn-sown combining peas and a combination of frost
damage in the spring and pea blight can lead to significant plant loss. For this
reason, growers of autumn-sown combining peas are advised to have seed tested for
pea blight and to avoid sowing seed in which infection is found (A.J. Biddle,
personal communication).
The pathogen can survive on or within seeds between seasons and seed infection
remains the most important source of disease outbreaks. However, diseased pea
debris and infected volunteer plants can also play a role in overwintering and crop
rotations are usually considered as part of a strategy in controlling pea blight
(Hollaway and Bretag, 1997).
13.4 FUTURE DEVELOPMENTS
The seed industry is becoming increasingly international and there are pressures to
relax, or remove, trade barriers between countries. One of the challenges for the
future, therefore, is to encourage and facilitate trade in seeds while, at the same time,
ensuring acceptable control of damaging seedborne pathogens. Increasingly, import
restrictions will focus on those pathogens that, following scientifically sound risk
assessments, are shown to present the greatest threat (Ebbels, 2003). Seed
production, especially of high value seed, such as hybrid and genetically modified
varieties, will be concentrated in countries, or regions, in which specific seedborne
