|
|
Kingdom |
Eubacteria |
|
Class |
Proteobacteria |
|
Order |
Gram-negative aerobic rods and cocci |
|
Family |
Pseudomonadaceae |
Bacterial wilt can
cause severe yield reduction, in the range 30 - 80%. However, it is
geographically restricted.
The causal organism, Ralstonia solanacearum, is globally
distributed as a serious pathogen causing bacterial wilt in a wide range of crops.
However, the strain which causes bacterial wilt of sweetpotato has only been recorded in some parts of China.
It is the subject of quarantine regulations restricting sweetpotato and stock movement
from the affected regions.
The sweetpotato strain of R. solanacearum also infects other crops
such as potato, tomato, capsicum and peanut. It is possible that this
disease may spread by movement of other crop species. For example, seed
potatoes are commonly exported from China to Vietnam, and often carry bacterial
wilt. Any suspected case of bacterial wilt in areas exposed to Chinese
seed materials should be reported to plant protection authorities.
Sprouts from infected mother roots become wilty and the base of the stem
shows progressive degeneration: initially the base of the stem becomes
watersoaked before turning yellowish and brown. The vascular bundles turn brown,
and this symptom extends upwards. Infected sprouts usually fail to develop
roots after transplanting.
Healthy sprouts or stem cuttings may become infected in the field. As
for sprouts, the base of the stem becomes watersoaked, then yellowish brown.
Brown streaks in the stem (in vascular tissue) may develop resembling Fusarium Wilt. The plants may
appear wilted, but may recover as their root system develops. They may
appear stunted and hungry, with older leaves turning yellow.
Fibrous roots may have brown or watersoaked patches. Storage roots may
show no symptoms, or may develop yellowish brown longitudinal streaks. In
more severe infection, greyish watersoaked lesions may develop on the surface,
and the storage root proceeds to decay with a distinctive odour.
The Pseudomonads are gram-negative, flagellated, aerobic bacteria.
Within Ralstonia solonacearum, the sweetpotato-infecting strain has been
placed in Race 1, Group 2 on the basis of host range, being pathogenic on peanut
and pepper but not on tobacco. It is grouped in Biotype IV on the basis of
physiological and biochemical tests. Strains in this group are unable to
metabolize lactose, maltose or cellobiose, and were sensitive to the antibiotic
oleandomycin (see He et al, 1983).
The bacterium may be transferred either in soil or in plant material. A
field may become infested through infested planting material, or via irrigation
water or composts containing infested plants.
Development of the disease is favoured in warm humid weather. It is
also more severe on poorly drained clay loam soils than on sandy soils.
However, it does not survive well in flooded conditions, such as in rice paddy.
It prefers soils that are slightly acidic.
The sweetpotato strain of Ralstonia solanacearum has been found to infect a
number of other solanaceous crops, icluding tomato, potato, eggplant and
capsicum, as well as peanut. Cereals are non-hosts and appropriate break crops in rotation
with sweetpotato.
Rotation with non-host crops is the most important measure to reduce
infestation on affected fields. The pathogen may survive for over 3 years
in upland fields, but is greatly reduced after 1 year in flooded paddy.
Planting material should be sourced from disease-free crops.
Cultivars differ in their susceptibility, although no immunity has been
found.
Soil amendments (lime) to reduce soil acidity may help reduce the severity of
disease.
Establishing the crop in the cooler months may reduce infection, which mosty
occurs through wounded tissue immediately after transplanting.
In China, quarantine regulations restrict the movement of sweetpotato and
livestock from infested areas.
Clark, C.A. and Moyer, J.W. 1988. Compendium of sweet potato diseases. APS
Press. 74 p.
He, L.Y., Sequeira, L. and Kelman, A. 1983. Characteristics of strains of
Pseudomonas solanacearum from China. Plant Dis. 67:1357-1361.
Kersters, K., Ludwig, W., Vancanneyt, M., DeVos, P., Gillis, M. and Schleifer,
K-H. 1996. Recent changes in the classification of the pseudomonads: an overview. Systematic and Applied Microbiology 19,
465-477.
Yabuuchi, E., Kosako, Y., Yano, I., Hotta, H. and Nishiuchi, Y. 1995.
Transfer of two Burkholdaria and an Alcaligenes species to
Ralstonia gen. nov.: proposal of Ralstonia pickettii (Ralston,
Palleroni and Doudoroff 1973) comb. nov., Ralstonia solanacearum (Smith
1896) comb. nov. and Ralstonia eutropha (Davis 1969) comb. nov.
Microbiol. Immunol. 39, 897-904.
Zhen, G.B. and Fan, H.Z. 1962. Identification of the pathogen causing
bacterial wilt of sweet potato. J. Plant Prot. 1:243-253.
Contributed
by:
Jane O'Sullivan |
