Sudden Death Syndrome (SDS)
SDS
is a fungal disease caused by Fusarium solani f. sp. glycines. Although
the symptoms of SDS develop after mid-July in Iowa, infection by the
fungus occurs as early as the seedling stage when soils are cold and
wet. Research has shown that low soil temperatures favor fungal infection
and that fields planted early have a higher SDS risk than later-planted
soybeans.
SDS symptoms
are fairly easy to identify
Although the fungus colonizes the roots early in the season, leaf symptoms
rarely appear until mid July. Leaves of infected plants initially
show scattered yellow spots between leaf veins (Figure 1). The irregular
shape of the spots and the green areas around the leaf veins give SDS
its characteristic pattern. The tissue in these spots starts to die
and enlarges to form brown streaks between the veins, called interveinal
necrosis (Figure 2). Only the midvein and major lateral veins remain
green. Leaflets drop eventually but the petioles remain on the stem.
Another
symptom of SDS is rotted roots. Diseased plants have deteriorated taproots
and lateral roots. The root cortex is light gray to brown, and the discoloration
may extend up into the stem 2 inches above ground. Sometimes bluish
fungal colonies can be seen on the root if soil moisture is high (Figure
3).
Foliar
symptoms of SDS look similar to some brown stem rot (BSR) strains and sometimes the two
diseases are misdiagnosed. A good way to distinguish between the two
is to check roots of diseased plants. Plants with SDS will have root
rot, while BSR does not cause root rot.
Management of
Sudden Death Syndrome
If
you find light SDS in your fields, the disease may or may not develop
into a severe problem in the future. If the disease is light (no hot
spots with severe defoliation), scout for the disease in the next soybean
crop.
If the disease is already causing damage in your fields, avoid planting
susceptible varieties. Many
SDS-tolerant varieties are available in Iowa.
The fungus can
overwinter in cysts of the soybean cyst nematode (SCN), increasing its
ability to survive from season to season. Thus, the presence of SCN
may favor development of SDS. Management practices to reduce soybean
cyst nematode populations are thought to also delay onset of SDS.
Two years between
soybean crops and maintaining good crop nutrition have been shown to
reduce incidence and severity of SDS.
SDS severity has
been associated with compacted soil. Therefore, deep tillage may also
reduce severity of SDS on headlands and other parts of the field where
the soil is compacted.
|
