Agriculture Reference
In-Depth Information
FUMIGANTS Pathogen inoculum can be destroyed by soil fumigants like methyl
bromide, chloropicrin or dichloropropene. Although expensive, this is effective
against a wide range of root diseases (Sumner et al. , 1997). Sealing the soil
surface with polythene sheeting after application retains the fumigants within
the soil for longer and increases their effectiveness (Entwistle, 1990). Fumi-
gation has been used to eliminate small foci of white rot infection or to produce
seedbeds free of Fusarium basal rot for raising transplants. However, the use of
fumigants is becoming increasingly unacceptable on environmental grounds; in
particular, methyl bromide has been identified as an ozone-depleting substance
and is being phased out (Clarkson and Whipps, 2002).
SOIL HEATING In hot countries, solarization (see Nematode Pests, above) acts
similarly and destroys pathogens in the top 25-30 cm of soil by raising tempera-
tures by 10-15°C (Clarkson and Whipps, 2002). For example, in southern
Spain covering wet soil with a thin film of polythene for 8 weeks starting in July
reduced the number of white rot sclerotia from 100 to 150/kg of soil to
undetectable levels and doubled garlic yields in white rot-infested plots (Prados-
Ligero et al. , 2002). Solarization is effective only in regions with high insolation;
for example, in a cool summer in Victoria, Australia, solarization did not
suppress pink root disease but it did increase yields and improve the preventative
effect of the fumigant dazomet on the disease (Porter et al. , 1989). In cool
climates steam can be used to heat-sterilize soil and destroy root-infecting
pathogens, but it is expensive and feasible only on a small scale for eliminating
infection 'hot spots' or for producing disease-free beds for transplant raising
(White et al. , 2000). Land treated with solarization or fumigants can be fairly
rapidly reinfected from nearby soil or by pathogens which have survived deep in
the soil (Porter et al. , 1989; Prados-Ligero et al. , 2002).
PATHOGEN GERMINATION STIMULANTS One interesting, biologically based tech-
nique is the application of chemicals that stimulate propagule germination in the
absence of host plants, and this approach has been tested with some success
against white rot. The sclerotia of Sclerotium cepivorum are stimulated to germi-
nate near allium roots by the characteristic 'flavour volatiles' (see Chapter 8)
resulting from the exudation of precursors into the rhizosphere. If host plants are
absent after it germinates, the pathogen dies. Onion oil and the flavour volatile
diallyl disulfide (DADS), which can be produced chemically, stimulate the germi-
nation of white rot sclerotia. In field trials on organic soils in Ontario, Canada,
injection of 85.5% DADS at a rate of 10 l/ha in 500 l/ha water reduced white rot
incidence in bulbs at harvest to negligible levels in plots which, untreated,
produced 2-8% incidence but, on a site with higher disease incidence, there was
no suppression of the disease (Hovius and McDonald, 2002). Greenhouse trials
showed that application of DADS to the soil resulted in 80% destruction of
sclerotia in 2 months compared with 100% survival in untreated soil. A
similar but less marked effect of DADS on sclerotial survival was observed in
Search WWH ::




Custom Search