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Soybean diseases: what to watch

Seedborne diseases may affect next year's crop.

November 12, 2007  By Albert Tenuta

6aWhen it comes to soybean seed, there is no substitution for 'good quality seed'.
There are many factors that can impact seed quality and are often referred to
as the 'Three Ps' (physical, physiological and pathological). Physical factors
include mechanical damage (cracks) to the seed coat and environmental stresses
that result in considerably smaller seed size. When it comes to physiological
factors these include storage conditions which, if improperly done, could significantly
lower seed quality (germination and vigour). And finally, pathological refers
to the many organisms that can infect the seed and lead to disease problems.

Soybean seed disease problems are most noticeable after an unusual growing
season or when harvest is delayed and the diseases are allowed to develop. These
diseases will lower seed quality which will impact germination rates and seed
vigour. The planting of low quality seed, due to seed infection, will result
in stand establishment problems such as poor and uneven emergence.

One way to avoid infected seed or reduce the likelihood of introducing a disease
into a field is to use certified seed. Although producers can have their own
seed tested for diseases such as phomopsis seed mould, purple seed stain, viruses
(soybean mosaic and been pod seed mottle) and white mould, few actually do so.
When purchasing certified seed, these tests will have been undertaken. Here
are some of the most common seed diseases that occur in Ontario and what you
should be looking for:


Phomopsis seed mould
Phomopsis seed mould has traditionally been Ontario's most destructive seed
disease. This disease has two distinct phases. The first is witnessed on the
stems and pods and this phase of the disease is referred to as 'pod and stem
blight'. Pod and stem blight symptoms appear as rows of small, black spots (specks),
which occur near stem nodes or on the pods. This phase of the disease can also
appear as 'islands of black specks' which are surrounded by a thin black border.
The black specks or 'pycnidia' are the fruiting bodies or overwintering structures
of this fungus. Infected seed is light weight, shrunken with numerous fine cracks.
The fungus grows completely over the seed coat resulting in a white, crusty
and chaulky appearance. The majority of the seed infection occurs during or
after the yellow pod (R7) soybean development stage and therefore, a delay in
harvest that allows the crop to stay in the field, especially under cool, wet
conditions, can result in substantial infection.

6bThe fungus can survive in soil, within crop residues and on infected seed.
Severely infected seed should not be kept since numerous studies have shown
infected seed can substantially lower germination and emergence rates in both
field and laboratory experiments. Low to moderate levels of seed infection can
be reduced with the use of fungicide seed treatments.

Purple seed stain
Infected seed has a distinctive purple discolouration varying from violet to
pale purple to dark purple over part or all of the seed coat. This discolourization
is often confined to the upper two layers of the seed coat. The embryo is not
discoloured or affected. In most cases, a seven to 13 percent reduction in emergence
can occur in the field. In laboratory studies, germination has been reduced
by as much as 30 percent.

Viruses: soybean mosaic virus and bean pod mottle

Soybean mosaic virus is spread from plant-to-plant by aphids while bean pod
mottle virus is spread by the bean leaf beetle. Both can be seed-transmitted
and can lower germination and vigour of the seed. A common symptom of viral
infection is uneven crop maturity or 'green stem' in which stems and leaves
remain green even though pods have matured. Leaves of infected plants are distorted,
wrinkled, puckered and have a typical mosaic pattern (yellow mottling) that
is most evident on younger leaves. Virus symptoms are often confused with hormonal
herbicide injury. Plants infected with soybean mosaic virus are scattered in
the field and generally, the area affected is smaller than if the cause was
herbicide injury. In addition, there is no pattern to the injury. Infected plants
may be stunted.

Seeds infected with soybean mosaic virus are smaller and have a brown/black
discolourization or fanning pattern from the hilum. Bean pod mottle virus symptoms
on the seed are similar and have brown or black streaks that also extend from
the hilum. Both viral diseases are often mistaken for physiological stress which
can also cause seed coat discoloration. There are no chemicals to treat virus-infected
seed and no rescue treatments exist within season to reduce your risk to viral
diseases. The best protection is to plant seed that is virus-free.

White mould
When we think of white mould, we rarely consider the affect of the disease on
soybean seed quality. In most years, this is true. However, with the amount
of white mould in the 2004 crop the possibility does exist and it could affect
seed quality. Pods infected with white mould can result in seed infection. Infected
seed has a loose, white fungal growth on the seed. Sclerotia (hard, black bodies)
may be found in the seed sample.

*Albert Tenuta is an extension plant pathologist for the Ontario Ministry
of Agriculture at Ridgetown.



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