|
| Kingdom |
Fungi |
| Phylum |
Basidiomycota |
| Class |
Basidiomycetes |
| Order |
Auriculariales |
| Family |
Auriculariaceae |
Violet root rot caused by Helicobasidium mompa is a root disease that
develops in the field during the growing season. Affected plants die toward the
end of the growing season. No reports about the effect on yield have been found,
but since the disease causes plants to die, there should be a certain reduction
in yield. The direct effect of the disease on the fleshy roots can cause
further losses.
China, India, Japan, Korea Republic and Taiwan.
The teleomorph stage of the fungus consists of an apically coiled, hyaline,
cylindrical basidia measuring 6-7 x
25-40 µm, with three septae and four
sterigmata that bear kidney shaped to ovoid binucleate basidiospores
that measure 6.0-6.4 x 16-19 µm. The anamorph
stage forms a thick mycelium branched in
straight angle, with a little narrowing on the branching point. The mycelium is
initially white, becoming purplish and purplish brown later on. Mature mycelium
forms strands and cushions on the soil surface, as well as, sclerotia. Sclerotia
are flat to irregular in shape and when transversally cut, show a purple colour
outside and white colour inside .
The disease is called violet root rot due to the colour of the mycelial
mats or cushions of the fungus that cover the affected parts of the plant,
especially at the soil line.
H. mompa is present as mycelium on the soil surface of infected fields
and it is parasitic on the below-ground parts of the plant. Once the plant is
infected and the root system is invaded, the foliage becomes chlorotic
and the leaves at the base of the plant abscise prematurely. The fibrous roots
rot and are packed together by a purplish brown to violet mycelial mat. The
fleshy roots also rot and are covered by bundles of packed mycelia that creep on
the root surface, giving a web-like appearance.
The most conspicuous characteristic of an infested soil is the presence of
mycelial cushions and bundles on the soil surface under the plant.
Initially white, they become pink to brown then purple brown or violet with age.
Symptoms start developing toward the middle of the growing season.
The fungus lives in the soil and spreads from plant to plant through the
mycelium that creeps on the soil surface. It can survive in the soil for at
least 4 years, mainly as sclerotia but also as mycelial strands. Sclerotia are
formed at the end of the growing season, when there are no nutrients available.
As soon as the host and enough moisture are present, the sclerotia start
developing and invade the host. This occurs during the early part of the growing
season.
Dispersion is by rain and irrigation water through the movement of infested
soil especially if the fields are on a slope (erosion associated with water
movement).
When the soil is irrigated for a new crop the fungus grows outside the plant
on the soil surface during the early part of the growing season, forming
infection cushions from which infecting hyphae penetrate the host and invade the
middle lamellae of the tissue in the root system.
Disease severity depends on environmental factors such as temperature. Even
though, the fungus develops in a wide range of temperatures (8-35°C), its best
performance is around 27°C. Other favourable conditions are high moisture, such
as that present during the rainy season, poor drainage and acid soils,
such as those prevailing in forest soils due to the presence of partially
decomposed organic matter.
No information has been found about the role of the basidiospores in the
disease cycle.
H. mompa produces a pigment - helicobasidin - that has toxic effect on
some higher plant species and microorganisms.
The fungus has a wide host range (plurivorous) and also grows well in
decomposing organic matter. It has been reported attacking: Morus (mulberry
tree), Malus domestica (apple), sugar beet, soybean, potato, cotton,
peanuts, tea, plum and grape but probably also infects other hosts.
A few weeks after planting and when the plant is developing a thick canopy,
it is possible to observe the disease under the plant canopy and on the soil
surface as a whitish mycelial growth. Soon after this mycelium becomes pinkish,
then purple and finally purple brown.
Towards the middle of the growing season, the mycelium starts invading the
plant, and the affected parts (the base of the plant at the soil line) show the
mycelial mat. But the most obvious sign of the disease can be observed toward
the end of the growing season; when the affected plants are dug and all the
symptoms described above can be observed, such as the mycelial web growing on
rootlets and fleshy roots and the rotting of such organs.
Cultural control
Sanitation. Crop residues should be destroyed either by burning or deeply
burying infected plants.
Lime amendments to reduce the acidity of the soil.
Rotation for more than 3 years with cereals can prevent the disease.
Early harvesting before the disease becomes severe.
Host-plant resistance
Early maturing varieties can escape the disease.
Chemical control
Nothing has been written about the use of chemicals to control the disease.
Clark, C.A., and Moyer, J.W. 1988. Compendium of sweet potato diseases. APS
Press 74 p.
Holliday, P. 1995. Fungus diseases of tropical crops. Dover Publications,
Inc., New York 607 p.
Hua, Y.X., and Zhou, X.D.(eds).
1984 . Sweet potato cultivation in China. Jiangsu Academy of Agricultural
Sciences//Shandong Academy of Agric.Sciences. Shangai, Shankai Scientific
Technology Press. 378 p.
Ito, K. 1949. Studies on “Murasaki-mompa” disease caused by Helicobasidium mompa
Tanaka. Rev. Appl. Mycol. 30: 377-38 (Abs.).
Contributed
by: Teresa
Ames
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