Agriculture Reference
In-Depth Information
losses have been reported in the United States (Grogan
et al.,
1952; Raid
et al.,
1996) and in Europe (Broadbent, 1951).
Within growing crops, the virus is spread by aphids in a non-persistent manner,
with
Myzus persicae
and
Macrosiphum euphorbiae
among the most common
vectors. A small but significant proportion of infected plants (usually up to 10%)
produce infected seed and this results in plants that act as a primary source of
inoculum in the following season. Diseased plants that develop from infected seeds
are often dwarfed and may fail to form marketable heads.
The virus may survive in lettuce-growing areas within susceptible weeds, and
Phatak (1974) has reported seed transmission in
Senecio vulgaris
. Effective weed
control, particularly in areas where virus-transmitting aphids are active, is important
in disease control. Insecticide control of aphids, especially using quick-acting
'knock down' insecticides may limit virus spread. Some cultivars show tolerance to
the virus; symptom development in these cases may be limited but diseased plants
remain an important source of inoculum for crops of susceptible cultivars. However,
seed is the primary source of infection in commercial lettuce crops and the principal
means of spread of the virus (Grogan, 1980; Raid
et al.,
1996).
Seed production schemes usually aim to produce lettuce seed that is either free
from LMV or that has a low level of infection, and certified seed will usually meet
standards for maximum permitted infection. Grogan
et al.
(1952) reported
observations on LMV in California which suggested that typical seed infection of
1-3% would result in around one infected plant in each 1 m row before thinning.
Where aphid activity was high and between-plant transmission occurred, up to 30%
of the plants could become infected before thinning took place. The rate of seed
transmission in lettuce varies from 0.1% to 37% (Shukla
et al
.
,
1994) and depends
on the time of infection of the mother plant (Couch, 1955), the virus strain (Dinant
and Lot, 1992), the lettuce cultivar (Falk and Guzman, 1984) and the temperature at
which the infected plants are grown.
Grogan was able to show that virus-free seedlings grown in isolation from other
commercial lettuce crops could produce virus-free seed. In further studies, he
demonstrated the benefits of using virus-free seed to produce commercial lettuce
crops (Grogan, 1980). Plants grown from virus-free seed in a 55 m
2
plot initially
showed no symptoms of LMV, whereas 2% of plants grown from the untested seed
showed mosaic symptoms. Immediately prior to harvest, approximately 2% of plants
within the experimental block showed symptoms whereas infection in the
commercial crop had increased to 12%.
These studies also showed that aphid movement was from plant to plant within
rows rather than across furrows, encouraging the view that virus transmission
between neighbouring crops grown from different seed sources would be lower than
spread within crops. In California, seed infection above 0.1% usually gave
inadequate disease control in commercial crops: where aphid numbers and activity
were high even this low level of seed infection resulted in 23% LMV infected plants
in one trial. It became apparent that to ensure effective, consistent control of LMV,
seed had to have less than 0.022% seedborne infection (Grogan, 1983). This was
achieved by testing 30,000 seeds and accepting only those seed lots that showed
no infection. Additional control measures in support of this programme included
