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December 2018
SANSOR will only certify seed lots that have been produced on
fields registered with them and produced according to the specifica-
tions and requirements of the SA Seed Certification Scheme. During
the registration process the origin of the seed is verified to ensure
that it is acceptable for certification.
Field inspections, during which several aspects such as varietal
purity and isolation distances are controlled, are conducted by
trained and authorised seed inspectors. After harvesting, processing
and packaging, the seed is sampled and tested by registered seed
testing laboratories to assess the germination potential and physical
purity of the seed lots.
The percentage of sclerotia in seed samples is determined by us-
ing the methodology prescribed by the International Seed Testing
Association (ISTA) Rules for Seed Testing, at the same time as when
general quality testing is conducted by a registered seed testing
laboratory. The presence and level of occurrence of sclerotia (or
seed that has been transformed into visible fungal sclerotia) in a
seed sample is determined by the seed analyst, in the process of
conducting a standard ISTA purity analysis. The virulence of the
sclerotia is not determined during these examinations.
Certified seed lots must comply with the minimum physical
requirements as stipulated in the South African Seed Certification
Scheme. The scheme states that a sample of 1 000 g shall not con-
tain more than 0,2% sclerotia of
Sclerotinia
sclerotiorum
, calculated
on a total weight basis. The sample must be taken by an authorised
sampler in the methodology prescribed by ISTA to ensure that it is
representative of the seed lot.
The results obtained are reported on a Report of Analysis,
issued by the registered seed testing laboratory and submitted to
SANSOR for assessment and approval before a final seed lot certifi-
cate will be issued.
Post control grow-outs are planted by the Department of Agricul-
ture, Forestry and Fisheries (DAFF) to verify the varietal purity and
identity of the production unit.
Mr BP Greyling, a mega producer and Agricultural Writers’ SA
Farmer of the Year in 2011, farms on Langfontein near Wakkerstroom
in Mpumalanga. He shared his experience on the use of certified
seeds, ‘I have planted soybeans for the past 32 years. I won’t say that
I never had
Sclerotinia
on my farm, but I can say that in 32 years, I
never had
Sclerotinia
on any field where I planted certified seeds’.
He is convinced that buying certified seeds is the first and most
important step towards keeping soybean fields
Sclerotinia
-free.
Good management practices
Good management practices used by seed production companies
against
Sclerotinia
infection:
Cultivar selection
A number of varieties has proven to be either more tolerant or more
susceptible to
Sclerotinia
infection than others. This information,
obtained through internal product research, can usually be obtained
from the seed companies marketing those varieties.
There is currently no soybean variety known to be completely
resistant to
Sclerotinia
. The use of more tolerant varieties is an effec-
tive tool in managing the disease.
Avoid planting highly susceptible varieties in fields with a history
of
Sclerotinia
infection.
Plant population and
general planting direction
Unfortunately, producers follow production practices that can of-
ten create the ideal micro climate and environment for diseases to
develop. Some of the following practices are associated with high
infection rates: Narrow row spacing, high seeding rates, late planting
dates, and the over application of nitrogen.
Cultivation practices used should discourage the formation of a
cool and humid micro climate wherein
Sclerotinia
can thrive.
Wider rows, and planting from north to south, will allow sunlight
and the movement of air into the rows, especially after irrigation or
spells of rain.
Soybean plants are very adaptable and known to compensate in
cases where lower seeding rates are observed.
Crop rotation
Sclerotinia
has a wide host range including soybean, sunflower,
dry bean, potato, pea, cucumber and some common weeds, to name
a few. Crop rotation towards non-susceptible crops such as maize
and sorghum can help to reduce the level of sclerotia occurrence
in the soil. Rotation crops need to be planted for at least three con-
secutive planting seasons before soybeans can be reconsidered
on the same field.
Weed control
Several broad-leaved weeds are known to be
Sclerotinia
hosts and
aids in the spread of the disease. A well-planned spray programme
should be followed as recommended by chemical sales agents.
Fertility and plant nutrition
High soil fertility, especially the use of nitrogen-rich manures
and fertilisers, favours
Sclerotinia
development by promoting lush
plant growth and early canopy closure. Having soil fertility tests con-
ducted on a regular basis will help avoid over-fertilising fields that
are prone to
Sclerotinia
infection.
Irrigation management
Excessive irrigation, above what is needed to maintain yield
potential, should be avoided during flowering to minimise moisture
at the soil surface and below the crop canopy. Low moisture levels
within the soybean canopy are critical for reducing the potential for
infection.
Occasional, heavy watering is better than frequent, light watering.
Avoiding excessive irrigation is especially important during the
critical periods of infection from early flowering to early pod devel-
opment stages.
Chemical control
Chemical applications can be a component of an integrated man-
agement system for
Sclerotinia
. Some foliar-applied fungicides and
herbicides have efficacy against
Sclerotinia
, although none of them
offers complete control.
Harvesting
As previously mentioned, sclerotia often get distributed by combine
harvesters. It is therefore imperative to harvest the fields where the
fungus was observed, last.
Going forward
Further research is needed to effectively manage this dreaded dis-
ease, with a special focus on the breeding of new, more resistant
cultivars, biological control agents and integrated disease manage-
ment systems.