Agriculture Reference
In-Depth Information
re-infection of barley seeds is determined by the quantity of inoculum available in
the crop during flowering and the extent to which the flowers remain open during
pollination. High levels of seed infection are associated with varieties that display an
open flowering habit. The flowering period may be extended in cool, moist
conditions, leading to a potential increase in seed infection. Some cultivars of barley
display a 'closed' flowering habit and so avoid infection. Loose smut was rarely
recorded in the closed flowering cultivar Golden Promise, even during years of
relatively high loose smut infection (Rennie and Seaton, 1975).
Most inoculum for loose smut reinfection probably originates within diseased
crops and, given suitable conditions at flowering, the disease tends to multiply over
successive generations. However, infection can also spread between neighbouring
crops and Rennie (unpublished observations) found that seeds developing in healthy
barley crops could become infected by spores released in diseased crops 200 m
upwind. Infected volunteer barley plants within seed crops can also act as an
important inoculum source for developing seeds.
Because seed infection is deep seated, loose smut was not controlled by chemical
seed treatments until the introduction of the systemic fungicide carboxin. Prior to its
introduction, elite seed stocks were multiplied in isolation or, occasionally, high
value infected stocks intended for further multiplication were subjected to a hot
water treatment at a temperature that killed the fungus but not the seed. Because
loose smut was not controlled by conventional seed treatments, including those
based on organomercury, the European Union required action to be taken against the
disease during seed multiplication and standards for loose smut were included in
Cereal Seeds Regulations in the UK (Anon., 1985).
In practice, the disease is controlled by treating seed of susceptible cultivars
intended for multiplication with an effective systemic seed treatment fungicide
(Reeves
et al
.
,
1994; Soper, 1995). In the early 1980s, the fungus developed
resistance to carboxin and loose smut levels increased in stocks of susceptible
cultivars until effective alternative seed treatment fungicides were used on infected
stocks (Wray and Pickett, 1985). Seed treatments that are effective in controlling
loose smut tend to be more expensive than those that offer more limited control of
seedborne diseases and, for this reason, they are more likely to be used on high
value seed intended for further multiplication. Loose smut is usually effectively
controlled in certified cereal seed but infection can build up to damaging levels in
stocks of farm-saved seed of susceptible barley cultivars, especially where the same
seed stock is multiplied over several years, in the absence of an effective seed
treatment (Rennie, 1987; Cockerell and Rennie, 1996). Surveys of certified and
farm-saved seed have shown that the incidence of loose smut varies between seasons
and cultivars. High seed infection levels tend to be associated with increasing
popularity of susceptible cultivars (Cockerell
et al.,
2005).
13.3.3 Lettuce mosaic virus
Lettuce mosaic virus (LMV) was first reported by Jagger in Florida (Jagger, 1921)
and subsequently shown to be seed transmitted by Newhall (1923). Significant crop
