In vitro and in vivo screening of Bacillus spp. for biological control of Rhizoctonia solani.
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Date
2003
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Abstract
The increasing concerns about chemical pesticides that are environmentally hazardous
and the continuous development of resistance by palhogens to chemical pesticides have
led to this study. Many studies have shown that some Gram-negative bacteria, such as
Pseudomonas flouresens, control plant diseases and promote plant growth. In this study
Gram positive bacteria, Bacillus sp., were chosen because of their ability to produce
endospores. Endospores can be used in stable, dry formulations. The advantage of using
endospores is their ability to survive harsh conditions such as droughts and high
temperatures, which give a long shelf life to the biological control agent.
Bacillus isolates were recovered from the rhizosphere of 12 different crops, and were
subsequently screened in vitro for their antimicrobial activity. Of 130 isolates, 87
exhibited antimicrobial activity against the test organisms: Rhizoctonia solani, Pythium
sp., Phytophthora cinnamoni, Fusarium sp., and single representatives of Gram negative
and Gram positive bacteria, namely, Erwinia carotovora and Staphylococcus aureus
respectively. The Bacillus isolates B77, B81 and B69 inhibited all the test organisms
investigated, which suggests that they produced broad spectrum antimicrobial compounds
or more than one antimicrobial compound. Of the isolates that showed antimicrobial
activity, 78 of them did not inhibit Trichoderma harzianum K D, which is a registered
biological control agent; indicating their potential for combined application.
Selected Bacillus isolates were tested for the biological control of R. solani under
greenhouse conditions in wheat, cabbage, tomato, maize, and cucumber seedlings.
Bacillus isolates were applied as seed treatments, and the inoculated seeds were planted
in R. solani infested speedling trays. Shoot dry weight measurement of seedlings
indicated that 12 out of 19 Bacillus isolates showed significantly different shoot dry
weight in wheat whereas all the isolates tested in tomato and cucumber gave significantly
different shoot dry weight. No significantly different shoot dry weight was obtained for
maize or cabbage. Seed emergence findings indicated that none of the Bacillus isolates
gave significantly different emergence percentage on wheat, cabbage, tomato, and maize
but all of them showed significantly different emergence percentage on cucumber. The
results indicate that both the pathogen and the biological control agents exhibited varying
levels of specificity on each crop tested.
The biological control potential of the best Bacillus isolates was tested on bean and maize
crops in the field. Green bean and maize seeds were coated with the selected Bacillus
isolates and then sown under field conditions. For each isolate, four replicate treatment
plots were established, with and without a R. solani inoculum. Percentage emergence,
plant survival levels to harvesting and yield of maize cobs and green beans pods were
measured. For all parameters measured the positive and negative controls were not
significantly different thereby rendering the results for the entire field study inconclusive.
However, Bacillus isolates B77, BII, R5 and R7 improved green bean pod yield and
Bacillus Isolate B8I increased maize yield, indicating their potentials as plant growth
promoting rhizobacteria (PGPR).
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.
Keywords
Bacillus (Bacteria), Soilborne plant pathogens--Biological control., Seedlings--Diseases and pests., Rhizoctonia solani--Biological control, Theses--Plant pathology.