| Aphanomyces root rot is a
sugarbeet disease well known to producers in many growing
regions of the United States. It has caused substantial
losses annually in the Red River Valley and Southern
Minnesota, and in Texas. The disease was first positively
identified from western Nebraska and eastern Wyoming during
the 1999-growing season. The disease was found to be
widespread in Nebraska (5 of 11 sugarbeet-growing counties).
This observation suggests that it has likely been present in
this area for some time and has simply been confused with
other root diseases such as Rhizoctonia root and crown rot.
The disease is caused by a soilborne fungus that can
attack plants at both the seedling stage, and as a later rot
of older sugarbeets. The seedling disease is commonly
referred to as black root. It does not affect initial stand
establishment, but can affect stands 1-3 weeks after
emergence by stunting, reducing vigor, and even causing
death of young plants. Symptoms are first observed on stems
as grayish, water-soaked lesions that soon progress to black
and eventually cause the stem to become thin and
thread-like. The cotyledons generally do not wilt until very
advanced stages. If soil conditions become unfavorable for
further disease development, plants may often survive and go
on to produce a normal crop. However, even if disease stops,
further losses can occur because the thin, delicate stems
become more susceptible to breakage from high winds in the
spring.
A root rot of older beets may occur at any point of the
season, and is highly dependent upon environmental
conditions. Wilting, yellowing, and loss of the shiny green
color to leaves are symptoms associated with the root rot of
older beets. Plants often recover at night, but are more
prone to wilt again in the day. If soils remain moist and
warm, infection can proceed rapidly, resulting in
interveinal yellowing and death of older leaves. Root
symptoms begin as yellowish, water-soaked lesions that
progress into the interior of the root. As disease
progresses, the affected tissues turn dark brown to black.
Infection can occur anywhere on the root, but usually begins
as a tip rot. Disease can progress to the point where the
entire root is rotted. At this stage, above ground symptoms
can be deceiving. Plants affected to this degree are easily
pulled up or dislodged because of severe root deterioration,
yet still often maintain vigorous, healthy-looking tops.
Plants can also recover from the root rot phase, like
that in the seedling stage if environmental conditions
become limiting for further disease progress. This can occur
if temperatures become lower or moist soils dry rapidly.
Roots such as these may retain scabby, superficial lesions
from prior infection, but still may produce relatively
normal yields.
Aphanomyces root rot, like other soilborne diseases like
rhizomania and Rhizoctonia root and crown rot, is going to
be a difficult challenge for sugarbeet producers. Once they
become established, they are unlikely to ever be eradicated
because of their ability to remain viable in soils for many
years. Very few options exist at this point other than sound
agronomic practices that modify the soil environment.
However, there are steps that may be taken to reduce chances
of severe yield losses. Planting early into cool soils will
help seedlings become established quicker. The avoidance of
over-irrigating can also be beneficial. Seed treated with
the fungicide Tachigaren will also protect the seedling from
the black root phase, but will not provide season long
protection.
The key to successful disease management is to avoid
undue stresses on the plants and establish a rapid and
vigorously growing crop. A plant that is unhealthy is more
susceptible to infection. Therefore, the older and more
established a crop is when and if infection does occur, the
better the chances for yields to be adversely affected. This
would include factors such as proper fertility, weed
control, and careful irrigation management during periods of
warm weather. Several cultivars are also available that have
some tolerance to Aphanomyces root rot, which may be useful
to those that have this problem in their fields. |