|
| Kingdom |
Fungi |
| Phylum |
Basidiomycota |
| Class |
Basidiomycetes |
| Order |
Ceratobasidiales |
| Family |
Ceratobasidiaceae |
Rhizoctonia stem canker is an economically important disease in many
crops. However, very little is known about its economic importance in
sweetpotato. The fungus causes more damage in plant beds than in the field.
Generally, only a few plants are affected and sometimes the infection heals over
after infected sprouts are planted in the field.
When the fungus is associated with other root fungi the damage can be
serious. This is really what causes most important losses.
R. solani is extremely widespread, occurring throughout the
world in all arable soils. It has been reported in so many other crops
from Africa, Asia, Caribbean, Europe, Pacific Islands, North America and South
America. However, effect on sweetpotato had only been recorded in the United
states.
The mycelia of R. solani are
made up of hyaline hyphae
when young, becoming brown as they mature. Mature hyphae are 4-6 µm thick and
the length of their cells is between 60-200 µm. Hyphae arise at a distinctive
straight angle and are constricted at the point of branching with a septum
formed in the branch near the constriction (see figure). As hyphae mature they
become rigid with branches at right angles. There are also monilioid hyphae made
up of short barrel shaped cells. These cells have thick walls and are mostly
branched. The sclerotia are
compact masses of cells generally formed in affected areas. The cells on the
surface of sclerotia are dark brown due to the presence of pigments but the ones
in the centre are colourless. Sclerotia are dark in colour, almost black, of
various shapes, but mostly flat and of different sizes.
Sometimes sclerotia can be as thin as a film.
The disease is more common in seedbeds. Healthy plants in the field
rarely develop symptoms of the disease.
The most obvious symptom is the presence of water-
soaked sunken lesions, in the stems of seed sprouts, near the soil line.
The stem cortex decays causing stunting and yellowing. Occasionally, white
powdery growth is observed on the sol surface and on stems of sprouts.
When the root system is affected, black cankers can be observed in the
taproot and lateral small roots, the whole plant becomes affected and dies.
On fleshy roots the fungus causes brown rotted areas or cracked sunken
cankers often covered with fungal mycelium.
The fungus survives as actively growing mycelium
in plant debris, as resting mycelium in dry organic matter, such as plant
refuse or in its sclerotial stage,
depending on the temperature and moisture of the soil.
The incidence and severity of the disease depends on temperature and
moisture. The fungus grows well across a wide range of temperatures 20-28 °C.
Moisture approaching saturation results in high disease severity. Poorly drained
soils have the same effect on severity.
Damage could still be moderate even in cases where there is a high soil
inoculum level because of good soil composition and aeration. The disease
can also be very mild when there are natural enemies of R. solani, such
as protozoa, nematodes and earthworms, or when there are antagonistic fungi and
bacteria such asTrichoderma spp., Gliocladium spp., Pythium spp.,
Verticillium sp., Fusarium sp., Pseudomonas sp., Bacillus
subtilis and Streptomyces rimosus.
R.solani is considered to be a polyphagous
fungus and includes many physiological strains grouped as anastomosis groups
(AG).
The fungus produces pectic, cellulolytic, and other enzymes that hydrolyse
cell walls and other cell components.
R. solani is world wide in distribution producing root rot, stem rot, and
foliage diseases in numerous host crops.
Sweetpotato is considered a secondary host, but probably the disease is
present wherever sweetpotato is grown.
Cultural control
Use of healthy and clean material for transplanting.
Solarization for 6 weeks.
Use of antagonistic microorganisms (this is also considered biological
control).
Host-plant resistance
No information has been found in this respect.
Chemical control
Several fungicides, effective against R. solani, can be used for soil
treatment as well as for dipping material to be transplanted. Fungicides
recommended are metasodium for soil fumigation, benomyl and tryazole group
fungicides as dips before planting.
Clark, C.A. and Moyer, J.W. 1988. Compendium of sweet potato diseases. APS
Press. 74 p.
Parmeter, J.R. Jr. (Ed.) 1970. Rhizoctonia solani, Biology and
Pathology. University of California Press. 255 p.
Mordue, J.E.M. 1974. Thanatephorus cucumeris. CMI Descriptions of
pathogenic fungi and bacteria No. 406. 2 p.
Contributed
by: Teresa Ames |
