Sclerotium Root Rot is a serious disease of sugarbeets. During the last few seasons it
has caused a large amount of crop loss in the Sacramento Valley of California. The
pathogen, Sclerotium rolfsii, is a soilborne fungus widely
distributed in the southern U.S., as well as the warmer parts of the world. The disease is
also known by the name "southern blight" because of its prevalence in the
southern regions of the country. The fungus has a very broad host range, causing disease
in over a thousand species of dicotyledonous plants, and millions of dollars of loss, on a
variety of crops, each year.
Symptoms of this disease can be severe. The initial symptom of infection is wilting of
the leaves (see image #1). Observation of the root reveals the presence of a white cottony
growth on its surface (see image #2).
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#1 Image #2
This white growth is called mycelium which is the vegetative growth form of the fungus. As the infection proceeds, the
mycelium will eventually cover the entire surface of the root. The tissues of the root
become soft and have a water-soaked appearance that is evident when cut open (see Image
#3). Many spherical, dark brown, structures may be observed on or in the infected tissue,
or in the surrounding soil (see image #4).
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These structures resemble mustard seeds and are calledsclerotia, the survival form of the fungus. These structures will remain in the soil
over the winter and be available to cause infection during following years.
In the fall of the year symptoms are confusing. Beets may have symptoms of Sclerotium
root rot, but without signs of the fungus. This is due to changing environmental
parameters. As temperatures cool, other soil microorganisms become more competitive and
attack the pathogen, the mycelium may be destroyed and the sclerotia may not develop.
Sclerotia may persist for several years depending on the soil conditions. Many factors
affect the survival of the sclerotia, such as temperature and moisture. The sclerotia will
survive for 5 years or more in dry conditions, but survival under cool and moist
conditions is much shorter. Sclerotia will persist for a long time in soils low in organic
matter and available nitrogen.
Initial infection occurs by the germination of the sclerotia. The optimal soil
temperatures for germination are from 70° to 86° F. After the sclerotia germinate, the
fungus grows to the sugarbeet root. Chemicals produced by the fungus attack the root
tissue in advance of the fungus. These chemicals cause severe damage and even death of the
tissue, making the root more easily invaded by the fungus. Once the fungus has infected a
root, it is then able to grow through the soil and infect adjacent roots in the row.
Because of this ability to spread down the row, a few initial infections can result in a
large amount of disease. Operations that move soil have the potential to infest new fields
with the disease. Soil adhering to implements, vehicles, or shoes has the potential to
Certain conditions may increase the amount of disease caused by Sclerotium
Disease severity may increase during periods of temperature and soil moisture
fluctuations, which apparently stimulate the sclerotia to germinate. The disease usually
appears when the canopy is fully developed and soil temperatures are high. Free soil water
is not required for infection, as it is for some other soilborne
diseases, but moist soils favor disease development. Plant to plant spread of the
fungus is more likely in moist soil. Cultivation practices may throw soil into the crowns
of the sugarbeets which may increase infection.
Control of this disease is difficult. Certain crop rotations may reduce the inoculum
levels of the fungus in the soil. Growing nonsusceptible crops such as corn, wheat or
barley or growing winter crops such as peas, in infested fields, results in rapid decline
in the population of sclerotia. Rotations with susceptible crops such as beans or
sunflowers are likely to maintain high levels of sclerotia and result in severe disease
during the next sugarbeet crop.
Other cultural practices may also be effective control measures. Deep plowing will bury
the sclerotia deep in the soil so they will not be able to infect the crop. Sclerotia more
than 5" deep in the soil will not germinate and will be destroyed by other soil
Incorporation of organic matter into soil has been shown to reduce the disease severity
due to Sclerotium rolfsii. The addition of compost or oat straw or corn stalks has been
effective. The disease control observed with these amendments may be due to the release of
ammonia which is toxic to the sclerotia. The amendments also increase the populations of
soil microorganisms. Some of these microorganisms will attack and kill the
Reduction of Sclerotium root rot has been accomplished with certain nitrogen
fertilizers. Anhydrous ammonia, urea, ammonia sulfate and other ammonium forms of
fertilizers reduce the population of viable sclerotia in soil. This reduction is
apparently due to the stimulation of antagonistic soil microorganisms that are attacking
and killing sclerotia. The use of large amounts of nitrogen on sugarbeets would result in
low sugar concentrations in the root, so nitrogen application alone is probably not a
viable control measure. The timing of the nitrogen application may be more important that
the quantity. Side dressing the nitrogen may stimulate the beneficial microorganisms to
become established on the root surface and protect the sugarbeet from infection.
Additional studies are being planned to determine the optimum rates and application
methods for these fertilizers to achieve the maximum disease control and sugar yield.
Although no sugarbeet varieties are resistant to this pathogen, preliminary work
conducted by the USDA indicates that resistance may be possible. The development of
disease resistance screening techniques is underway to identify resistant sugarbeet
If adequate land is available, growers should avoid fields with high populations of the
pathogens, and choose non-infested fields to grow sugarbeets. The Holly Sugar Company's
plant pathology laboratory is able to determine the population of this pathogen in soil
samples from grower's fields. Contact your Holly agriculturist for more information.
Although several fungicides are active against the fungus, none are available that will
economically control the disease in sugarbeets. Large amounts of these chemicals are
required, and they are difficult to apply in amounts large enough to be effective.
Soil fumigants, such as metham-sodium, may be possible control tools. Since the
sclerotia are usually found near the soil surface, they should be readily accessible to
attack by these types of chemicals. Researchers in India have shown the sclerotia to be
killed by metham-sodium. More work is needed to determine rates and application methods
for this fumigant.
Other chemicals, not developed as soil fungicides, are reported to be active against
Sclerotium rolfsii. In a study conducted in India, aldicard and phorate were shown to
reduce disease incidence in greenhouse and field trials. Additional experiments are needed
to determine the usefulness of these chemicals.
Biological agents have shown promise for the control of this disease. Gliocladium
virens is a soil fungus that is pathogenic to Sclerotium rolfsii. It has been tested for
disease control in tomato with encouraging results. The biological control agent is added
to the soil in alginat pellets containing wheat bran. The fungus is able to become
established and parasitize the sclerotia of Sclerotium rolfsii, killing them.
Management of this disease will be due to an integrated approach on the part of the
growers. Proper rotations with nonsusceptible crops are a major aspect of control. Use of
some chemicals will aid in limiting the disease pressure. Optimal use of nitrogen
fertilizers will help reduce inoculum. Proper sanitation will keep the fungus out of
non-infested fields. The development of biological agents and resistant germplasm will
contribute to disease control. A total management scheme to maintain a healthy crop and a
healthy soil is needed to overcome this malady.