Agriculture Reference
In-Depth Information
maximum infection rate is usually low (Hewett, 1978). The infection rate (
r
), the
rate at which the population of the pathogen increases (Van der Plank, 1963), is
expressed as the unit increase of infected plants per day and is influenced by
climatic factors and the susceptibility of the host plant.
Didymella fabae
was
estimated to spread at a rate of 0.07 units per day; over the course of a season, the
disease could spread up to 5-10 m from a single focus of infection (Hewett, 1973).
The relatively low levels of seed infection in many seed-producing areas
(Hewett, 1973; Wallen and Galway, 1977) and the modest rates of disease
development in arable areas suggested that disease control could be relatively easily
achieved by limiting seed production to areas of low rainfall and setting a standard
of around 1% for commercial seed (Hewett, 1973). For further seed production,
lower initial seed infection was necessary, especially in early generations and in the
absence of effective seed treatments. Control of disease through standards in seed
certification schemes is likely to be most effective where field beans are not widely
grown and where there is a rotational interval between crops.
Levels of seed infection vary between years and between varieties grown in the
same year and are strongly influenced by weather conditions during seed production
(Hewett, 1973; Biddle, 1994). Hewett (1966) found that of 180 seed samples from
commercial crops grown in England in 1964-65 only a small proportion were
infected whereas the following year over a third of samples tested were infected at
levels of 3 to 10%. In 1992 a survey of 451 seed lots of winter beans produced in the
UK showed that 31% contained levels of seedborne infection which exceeded 1%
and 3% had more than 10% infection. Seed treatment of infected seed stocks often
gives only partial control and, in practice, a combination of low seed infection and
appropriate chemical treatment is most effective (Knott
et al.,
1994).
Bond and Pope (1980) found that infected volunteer plants from old field bean
crops could act as an important source of inoculum for crops in neighbouring fields.
They reported spread up to 120 m into neighbouring fields. They also highlighted the
risks from minimum cultivation techniques and drew attention to the risk of spread
between crops where spring and winter cultivars were grown in the same area. These
additional influences on disease development resulted in a recommendation that seed
crops should be isolated by at least 50 m, not only from other bean crops but also
from fields in which a bean crop had been grown during the previous year. These
actions were in addition to a requirement that certified seed of the first generation
should have no infected seeds in a sample of 600. The discovery of the teliomorph
D. fabae,
produced over winter on infected bean straw in stubble fields, suggests
that the pathogen can also be spread through the dispersal of airborne sexual spores
(Jellis and Punithalingam, 1991).
13.3.5 Pea bacterial blight
(
causal agent
Pseudomonas syringae
pv.
pisi)
Bacterial blight is a widely distributed and potentially damaging disease of peas.
The disease is seedborne and primary disease foci develop principally on plants
grown from infected, or infested, seed. Water-soaked lesions develop on stems,
leaflets and stipules of infected plants. Infection often runs along the veins, causing