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- Biology and Survival of Pseudomonas Solanacearum in Malaysia
Biology and Survival of Pseudomonas Solanacearum in Malaysia
Dissertation Abstract:
Bacterial wilt, caused by Pseudomonas solanacearum (Smith, 1896) Smith 1914, is a disease of many crops. The lack of a truly effective method to control the disease, especially in the tropics, has been attributed to the pathogen's many strains and lack of information on the ecology and variation of the pathogen's population. This study was conducted to explain and understand some aspects of the ecology and variation of the pathogen in Malaysia which will be useful in the formulation of more effective control measures.
A study of the distribution of bacterial wilt caused by P. solanacearum showed the disease to be widespread in all areas (northern, central, and southern parts of Peninsular Malaysia, and East Malaysia) sampled. The disease occurred in many crops, weeds, and ornamentals. Two hundred and forty-three isolates of the pathogen were obtained from 25 plant species belonging to nine families with the pathogenicity confirmed on their respective hosts. The study indicated the needs for strict quarantine measures presenting the introduction of exotic strains to the country and the reconfirmation of the status of the known pathogen hosts.
The environmental conditions in Malaysia were conducive to the growth of the pathogen and the development of the disease, which were observed throughout the year regardless of dry or wet weather conditions. The disease also occurred in many types of mineral (clayey, loamy, and sandy) and organic (peat and muck) soils. The type of hosts and their varietal susceptibility also played sign ificant roles in determining the occurrence of the disease.
Cultural characteristics of P. solanacearum isolate on a tetrazolium medium and a medium containing 0.1 percent L-tryosine showed that isolates differed in colony size, fluidity, and production of formazan and melanin. However, these could not be used to differentiate strains within race 1.
Greenhouse pathogenicity studies revealed the presence of pathovars of race 1, which differed in their pathogenicity to ginger, ground nut, and tobacco. Tomato and Irish potato were the universal hosts of all Malaysian isolates but all isolates were non-pathogenic to Heliconia sp. Biovar determination study indicated that isolates were biovar 2, 3, and 4. The majority of the isolates were biovar 3, followed by biovar 4 and 2. Determination of race based on host range, origin, and biovar showed that isolates were in races 1 and 3. Race I was the predominant race and was widespread, while race 3 was confined to the highlands.
Attempts were made to isolate virulent and temperate bacteriophages of P. solanacearum. One virulent bacteriophage was obtained from diseased brinjal (Solanum melongena L.) plants and their rhizosphere soils. However, temperate bacteriophages were obtained from lysogenic strains of the pathogen. Forty-three percent of the P. solanacearum isolates were lysogenic; 14 temperate bacteriophages were isolated from these. The reaction of 68 P. solanacearum isolates to all the phages showed that none of the phages were specific to race ofbiovar.However, 13 lysotypes were distinguished and all isolates could be typed.
Thirty-eight percent of the isolates tested for bacteriocin production were bacteriocinogenic. Studies of the concentration of bacteriocin produced and the sensitivity of P. solanacearum isolates to the bacteriocin showed that a few avirulent bacteriocinogenic strains produced relatively high concentrations of bacteriocin and were able to inhibit the growth of a large number of the virulent P. solanacearum isolates tested. This bacteriocin and its avirulent producer could therefore be useful in the development of biological control measures for the pathogen. However. a few isolates were not inhibited by any of the bacteriocin produced by the 30 avirulent bacteriocinogenic strains tested.
A study on the population ofthe virulent strain (race 1, biovar 3) in sterilized and nonsterilized soils showed that the pathogen was not capable of long-term survival in the soil. Population density trends of the avirulent strain in the presence of the arivulent bacteriocinogenic strain at ratios of 1:0, 1:1, 1:10. and 1:100 showed similar trends. This indicated that the avirulent strain was ineffective in controlling the population of the virulent strain in the soil at the ratios tested or that the method of soil application was not effective. Greenhouse study on the population dynamics of the virulent strain (race 1, biovar 3) of P. solanacearum showed that the bacterial population in the soil decreased significantly after planting with a non-host crop. This indicated that the non-host crops viz. Ipomoea aquatica Forsk., Brassica rapa L., and Raphanus sativus L. var. hortensis Baker could be used in crop rotation programs.
