Bacterial root rot / Bacterial wilt (Pseudomonas solanacearum)
Introduction
The disease is caused by a bacterium, Pseudomonas (Ralstonia) solanacearum (Smith) Dowson and is more prevalent in China during rainy season (Zing et al., 1988) while the same disease is known to occur sporadically in India (Mathew et al., 1994).It is also known as bacterial wilt. The disease is very destructive and spread rapidly. The disease first appears on few plants that act as infective centers from which the disease spreads to the neighboring plants. If there are many infective centers, the disease spread more rapidly to whole the field causing heavy mortality of the plants.
Symptoms
The plants start wilt but the wilted leaves maintain green colour. The disease symptom is not significant in the underground portion at first, only some brown stripes can be seen in the xylem if the root is cut opened. As the disease progress the rotting will be spread to the stem and branches. The seriously affected portion of the roots will become black, the root bark rots and fall off. A slimy rotten milky juice exudes from the cut infected roots.
Causal organism
Pseudomonas solanacearum
Systematic position
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Betaproteobacteria
Order: Burkholderiales
Family: Burkholderiaceae
Genus:Pseudomonas
Species: solanacearum
Bacillus solanacearum, Burkholderia solanacearum
Burkholderia solanacearum, Pseudomonas solanacearum, Ralstonia solanacearum
Description of pathogen
Pseudomonas solanacearum is aerobic non-spore producingGram-negative motile rod shaped bacterium. It is soil-borne pathogen, motile with a polar flagella tuft. The organism grows aerobically and does not form endospores. Cells are 0.5-0.7 x 1.5-2.0 µm and are non-encapsulated. P. solanacearum is catalase positive, oxidase positive, and reduces nitrates. The pathogen does not hydrolyze starch and does not readily degrade gelatin. In broth culture, the organism is inhibited by concentrations of NaCl greater than 2%. The bacteria have 1 - 3 polar flagella at each end measuring 1.90 ´ 0.80 µm.Predisposing factors
Although the optimum temperature ranges from 28-36°C, the pathogen can live within the temperature ranges 10-40°C. Its growth rate will be very much below 18°Cor above 38°C. If the bacterium is exposed to a temperature 53°C it will die. The pH range for its growth is 5-9 and best at 7-8. The disease will spear very quickly in the water logged lands. Similarly the newly planted mulberries and saplings are easily affected and die quickly.
Disease cycle
The pathogen is soil born and can remain in the soil for several years. Soil and diseased mulberry plants and other host plants in the contaminated areas are the primary source of inoculation. Affected saplings and cuttings are the main sources of primary inoculums in non contaminated areas. Once the disease is established in a locality, the pathogen will be disseminated by rain, irrigation water, cultivation etc.
Resistant varieties
|
Resistant reaction |
Mulberry varieties |
|
Highly resistant |
Nil |
|
Resistant |
Qing No. 10, No. 4 and Dong – 9 |
Control
The affected plants should be uprooted and burned. For management of the disease, water logging condition should be avoided. Contaminated soil should be disinfected with formalin solution after diluting with water (1:100) or o.2% bleaching powder to be applied in the spots as disinfectants at the rate of 10 - 20 kg/m2
Related literature
Sharma DD, Nishitha Naik V, Chowdary NB, Mala VR, (2003) Soilborne diseases of mulberry and their management - a review. Int. J. Indust. Entomol., 7(4): 93-106.
Teotia RS, Sen SK, (1994) Mulberry diseases in India and their control. Sericologia, 34(1)1-18.