TEN YEARS OF BIOCONTROL OF BACTERIAL WILT OF POTATO IN CHILE.
Ciampi L., Fuentes R., Schöbitz R., Bernal G.
Instituto de Producción y Sanidad Vegetal, Facultad de Ciencias Agrarias. Universidad Austral de Chile. Casilla 567, Valdivia, Chile.
The potato plant (Solanum tuberosum L) is one of the most affected crops in the world by insects, weeds and diseases which cause severe losses. Among pathogens, viruses, bacteria, fungi and nematodes can affect the plant or tubers, in the field, storage and produce distribution. The classical manner of control through exclusion, eradication, protection and host resistance has failed for many diseases. Among them, bacterial wilt caused by Ralstonia solanacearum E.F. Smith arises as one of the greatest challenges. Important work done at the University of Wisconsin in the late 70's under the supervision of Dr. Luis Sequeira established the particular way by which this bacterium can be distributed throughout the world inside infected but healthy appearing potato tubers: the latent infection. Once a field infection was reported it will be an almost impossible task to eradicate the pathogen.
In Chile bacterial wilt was found in the Metropolitan Region, far away from the main area in which certified tubers and produced. Major efforts have been made by sanitary authorities to keep main potato production areas free of the disease: severe sanitary controls, quarantines in infected farms and prohibition of potato introduction to Southern Chile. However, the spread of the pathogen is real, and slowly year after year expands its geographical distribution. This caused another concern: how to biologically control bacterial wilt and to stop its spreading.
The first step was to look for antagonistic microorganisms, easy to manage and harmless to man and plants. Many were found, among them isolates BC8 and A47. The first was a Pseudomonas fluorescens and the second one was a Bacillus subtilis. Isolation and purification of the metabolites responsible for the antagonistic activity was carried out. For isolate A47 we revealed the presence of an extracellular, thermostable and hydrolysis-resistant metabolite of the iturin group. For isolate BC8, the experiments suggested that it would correspond to a siderophore-like compound; the fluorescent pigment was purified by reverse-phase chromatography. Furthermore, spectral feature and amino acid composition revealed similarity with the properties described for other pseudobactins.
Isolate BC8 was studied under field conditions using organic soil as carrier. Interesting results were obtained: isolate BC8 originally obtained from the inside of healthy tubers could get into the plant and to protect it from wilt infection. However, the amendment process used to field test the isolate is only good for small experiments. Also, the selection of overproducer mutants of inhibitory substances was investigated. Strain A47 was cultured in presence of acridine orange. After exposing A47 cells to different concentrations of the mutagenic agent, it was possible to select one strain that in vitro produced an increased biocontrol activity against Gram negative bacteria.
Finally the work in construction of an alginate matrix to store, to carry and to inoculate antagonistic bacterial cells into the soil system has become our main field of research. Studies in the formulation are well developed and will be presented in details in another abstract in this same symposium by the same authors.
Research funded by FONDECYT 1951105 grant.
