Stem rot in oilseed rape
is caused by the fungus Sclerotinia sclerotiorum. It survives in soil
by means of sclerotia,
small black irregularly shaped resting bodies, 1-10mm, which can survive for
more than 10 years. Sclerotia near the soil surface (especially those within
5 cm of the soil surface) germinate in spring to produce pale yellow brown
saucer-shaped fruiting bodies known as apothecia
(2-10mm diameter).
The apothecia generally appear from March onwards as soil temperatures rise
(>10oC) when soil surface is moist. Air-borne spores (ascospores)
are discharged by the apothecia into the air and contaminate petals and other
plant surfaces.
Weather conditions which allow petals to stick
to leaves and stems of oilseed rape and enable sclerotinia to infect the leaf
or stem are required. Petals are an essential part of the infection process,
providing nutrients that enable sclerotinia spores to infect the plant.
Early symptons are pale brown spots on leaves or stems and these appear within
a few days of infection.
Later in the season, the large white lesions occur on the stems or branches
causing premature ripening of the pods.
Black sclerotia are produced within the stem cavity or within the root and
occasionally on the outside of the stem. These sclerotia can be harvested
with seed and hence allow introduction of sclerotinia into new fields. Other
sclerotia fall to the soil at harvest and allow for perpetuation of the disease.
It is difficult to forecast sclerotinia attack as the disease varies from
field to field and year to year. The Decision Guide takes account of the major
factors which influence sclerotinia risk. Two components are particularly
important:
