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Sclerotinia in soya beans:

Know your enemy

By Nico Barnard, agronomist, Pannar Seed

clerotinia, a disease found in B: When weather conditions are D: Sclerotinia that is growing, releases
crops across the globe, can favourable, the sclerotia germinate and oxalic acid which kills the flower; the
cause a 0,25t/ha loss for every small mushroom-like structures, known sclerotinia then penetrates even deeper
10% of plants infected. It occurs as apothecia, start to grow upwards. In into the flower. This process is repeated
S in 64 different plant families order for them to develop, these apothecia until the leaf axil is reached via the flower
and in over 400 plant species, including need wet soil, humidity higher than 75% stalk. Infection spreads from the leaf axil
some well-known weeds. It can infect and relatively cool soil temperatures of to the stems, which then start to die in
almost all annual broadleaf crops. 12 to 24°C. Sclerotia buried as deep as an upward manner. Leaves that die and
It is essential that anyone wishing 5cm can still produce apothecia, but wither are the first symptom, which is
to minimise damage, knows and most are formed on sclerotia present just usually only observed two to three weeks
understands the disease. As there below the soil surface. Conditions must after the spores have infected the flowers.
is no single management regime remain favourable for several days in The sclerotinia grows actively in the
that can control it, an integrated order for apothecia to reach maturity. maturing soya bean plant and forms
management plan is the only way Apothecia are small, funnel-shaped new sclerotia, which resemble mouse
to limit the impact of the disease. mushrooms that are light- to salmon- droppings. During the harvesting
coloured with a diameter of 2 to process the sclerotia are distributed by
Life cycle of sclerotinia in soya beans 5mm. Once conditions are ideal and the combine harvester, allowing a new
A: Sclerotinia can survive as undefined apothecia have matured, thousands generation to spread. Infection can
black structures, known as sclerotia, of spores are released up to 16cm only spread from one plant to the next
in the soil for seven to ten years. The high into the air, where air currents when leaf stalks or stems touch and
inside is cream- to salmon-coloured. spread it over larger distances. sclerotinia then grow from plant to plant.
The best way to control the disease is by
Figure 1: The life cycle of sclerotinia. C: Sclerotinia preventing the sclerotia from germinating
spores germinate and forming apothecia, or by protecting
on the flowers the flowers and preventing spores from
of soya beans at germinating on the flower. Nothing can be
air temperatures done about plants that are already infected
below 28°C. The as corrective spraying cannot be applied.
flowers must be
wet, either from Sclerotinia control
heavy dew, mist Unfortunately, all yield-enhancing practices
or rain, for at least contribute to the occurrence of sclerotinia.
three consecutive The more leaves a soya bean field can
mornings for spores produce, the better the yield. There are
to germinate (keep two options for managing sclerotinia:
in mind that spores • Option 1 is to open up the leaf canopy
can germinate on to allow the soil to dry out faster and
any dead plant to keep the plants as dry as possible.
material). The This can be achieved by using wider
soft petals are rows (1,1 to 1,5m). Wider rows mean
energy-rich and lower yields if sclerotinia does not
the ideal medium occur. The prevalence of sclerotinia
for the spores to in wider rows is normally lower and
germinate on. yield losses are limited. This practice

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