Agriculture Reference
In-Depth Information
yield. Gladders
et al
. (2001) found that canker incidence at harvest could be related
to the development of phoma leaf spot in the previous autumn/winter.
Light leaf spot has been integrated into a canker warning system (Gladders
et al.,
2004), which is available to farmers on the internet (http//phoma.csl.gov.uk).
9.8.3 Sclerotinia
Sclerotinia sclerotiorum
, the cause of sclerotinia stem rot of winter oilseed rape, has
a complex life cycle. Sclerotinia is a weak pathogen and generally requires a food
base from which to infect undamaged tissue. With oilseed rape this food base is
generally the senescing petals. In order to infect oilseed rape, sclerotia germinate in
the soil to produce apothecia and ascospores are discharged into the atmosphere
where they land on rape petals; when these senesce they are blown off the raceme
and must adhere to the stems. They generally become trapped in the axils of petioles
and racemes. The chance of all these necessary factors being coincident is low.
However, such an occurrence took place in England and Wales in 1991 with the
result that many crops became infected (Turner and Hardwick, 1995). This led to
widespread spraying against the disease. Recent seasons have seen less severe
attacks but spray applications have continued at a high level. It is a high profile
disease, with only a few plants per field showing symptoms necessary for the
disease to be alarmingly obvious. Farmers are not prepared to take the risk of crops
being unprotected as action has to be taken long before symptoms appear in the
crop.
There are basically three phases in the infection cycle:
•
sclerotial germination and ascospore release;
•
petal infection;
•
petal retention on the canopy and infection.
A number of schemes to forecast the occurrence of sclerotinia stem rot have been
devised and have concentrated on the first two elements in the infection cycle.
One of the simplest is the use of depots of buried sclerotia devised by the Danish
extension services (Buchwaldt, 1986), where the carpogenic germination of buried
sclerotia is monitored in established fields of oilseed rape. If germination is between
0 and 25%, there is a risk of a minor attack. If germination is from 26 to 30% a week
before full flower, then the level of sclerotinia stem rot is likely to be above the
economic threshold for spraying. It is also suggested that the system could be
modified by a point system weighted by agronomic and edaphic factors as is the
case in Sweden (Twengström and Sivald, 1995).
The use of depots of buried sclerotia was evaluated as part of a risk forecasting
scheme to predict apothecial emergence dates (Sansford, 1995c). Sclerotial
germination was immediately preceded by a dry/drier period of weather and/or a
mean daily temperature (on a weekly basis) rising above 10°C. Sclerotial
germination declined after temperatures reached a daily mean of >15.3°C.
