Agriculture Reference
In-Depth Information
eliminating potential weed hosts in and near lettuce field borders, avoiding the
planting of new lettuce fields adjacent to old fields and ploughing in lettuce fields
immediately after harvest. The seed certification programme in Australia includes
growing seed crops of lettuce in geographic isolation from commercial crops, in
areas of low aphid activity, and the careful inspection of plants to remove any with
disease symptoms. Outbreaks of disease in commercial lettuce crops in North
America have been associated with breaches in seed certification rules (Raid
et al.,
1996).
In northern Europe, where aphid pressure may be lower than in California and
where commercial lettuce production is probably less intensive, a standard of no
LMV in 2200 seeds has been shown to give effective practical control (Van Vuurde
and Maat, 1981; Dinant and Lot, 1992). These high standards for seed health have
required the introduction of seed testing procedures, usually based on ELISA
techniques, that can accommodate a high throughput of samples of 2000-30000
seeds (Zerbini
et al.,
1995).
13.3.4 Ascochyta leaf and pod spot of field beans
(
causal agent
Didymella fabae)
Field bean (
Vicia faba
) seed may become infected by
Didymella fabae
from
diseased pods in infected crops. Seed infection is in the form of mycelium, largely in
the seed testa rather than the cotyledons. Heavily infected seeds of susceptible
cultivars may show purple-brown discolouration and, on imbibition, dark pycnidia
of the pathogen may be seen on infected seeds. Lesions may develop on the primary
foliage leaves of seedlings grown from infected seeds. Seed infection appears not to
affect germination or seedling establishment (Hewett, 1973) and the proportion of
infected seeds that produce seedlings with primary lesions varies from nil to 50%
depending on growing conditions (Hewett, 1973; Wallen and Galway, 1977; Gaunt
and Liew, 1981). Transmission is often higher in winter-sown crops.
Short-range disease spread occurs by rain splash transmitting the spores, which
ooze from pycnidia in leaf lesions in wet weather. Disease development depends
very much on environmental conditions and high seed infection is associated with
wet seasons. During dry periods, disease development and spread is halted (Hewett,
1973; Biddle, 1994).
In conditions suitable for disease development, Hewett (1973) found that lesions
occurred at a height of 30 cm towards the end of May in south-east England and by
the end of June leaves 60 cm above ground bore lesions. His observations suggested
a rate of spread through the crop of approximately 1 m per month. For successful
seed re-infection, there must be rainfall early in the development of a crop so that
the pathogen is carried sufficiently high in the leaf canopy to re-infect the pods and
seeds as they develop. Hewett (1973) considered that above-average rainfall in May
and June was required in south-east England for successful re-infection of
developing seeds. In his trials, the maximum increase in seed infection from that on
the seed sown to that on the harvested seed was eight times.
Ascochyta leaf and pod spot, like many polycyclic seedborne diseases, develops
relatively slowly (Hewett, 1978). Disease increase with time is logarithmic but the
