Antibiosis Activity Research Articles

Stimulating disease suppression in soils: sulphate fertilizers can increase biodiversity and antibiosis ability of root zone bacteria against Streptomyces scabies

The value of ammonium sulphate (+SO 4) fertilizer as a method of controlling potato common scab varies from field to field and from year to year. We examined the influence of two fertilizer regimes on the biodiversity and antibiosis activity (in vitro) of culturable rhizobacterial populations recovered from fields with a similar history of potato common scab. Root zone soils treated with +SO 4 fertilizers were colonized by significantly ( P=0.05) more species-diverse communities of bacteria than those managed with ammonium nitrate fertilizers (+NO 3). In bioassays conducted on artificial media adjusted to pH 4.9 (acid) or pH 6.8 (base), in vitro antibiosis in bacterial strains against Streptomyces scabies was found to be dependent on fertilizer regime ( P=0.023), with more isolates with antibiosis ability being recovered from +SO 4 compared to +NO 3 treated soils. Overall, bacterial species from the genera Bacillus (18–35%) and Pseudomonas (15–18%) were major components of those rhizoflora communities antagonistic to S. scabies. Under our field conditions, soil acidification with +SO 4 treatments stimulated the development of rhizobacterial communities that generated secondary metabolites with (in vitro) antibiosis ability, against S. scabies. This occurred both in the liquid and vapour phase (volatile gases); the latter most notably by Bacillus cereus, Bacillus pumilus and Pseudomonas chlororaphis. We postulate that in hot, dry seasons, antibiosis against S. scabies mediated by +SO 4 treatments, and the competitive bacterial communities they engender, would be less effective in controlling potato common scab, as the active moieties would be rapidly volatilized into the atmosphere. In contrast, biocontrol would be more effective in wetter seasons, the active moieties being retained, in and around the potato root zone for greater biologically significant periods of time.

Developing disease-suppressive soils through crop rotation and tillage management practices

A field laboratory was established in Prince Edward Island, Canada, to determine the effects of 2- and 3-year crop rotations, with conventional and minimum tillage treatments, on the severity of soilborne diseases of potato. The 2-year rotation consisted of spring barley and potato (cv. ‘Russet Burbank’), and the 3-year rotation was barley (undersown with red clover), red clover and potato. Examination of potato stem, stolon, and tuber tissues revealed significantly ( P=0.05) lower levels of canker and black scurf caused by Rhizoctonia solani, in plants grown in 3-year vs. 2-year rotations. The severity of dry rot ( Fusarium spp.) and silver scurf ( Helminthosporium solani) was significantly ( P=0.05) lower in tubers from plots managed with 3-year rotations and minimum tillage practices. Potato tubers harvested from 3-year rotational soils were significantly ( P=0.05) less diseased than those from 2-year rotational soils following inoculation with Phytophthora erythroseptica, causal agent of pink rot. In greenhouse experiments using field soils from 2- and 3-year rotations, we found that potato plants growing in 3-year rotational soils were significantly ( P=0.05) less diseased than those growing in 2-year rotational soils following inoculation with P. erythroseptica. Analysis of root zone bacteria recovered from the rhizosphere (exoroot) and potato root tissues (endoroot) showed that the greatest antibiosis activity inhibiting the growth of soilborne pathogens in vitro occurred in bacterial isolates recovered from the endoroot tissues of 3-year rotation crops under minimum tillage management. Our evidence supports the view that soil agroecosystems can be modified through rotation and conservation tillage practices to improve disease suppression by enhancing the antibiosis abilities of endophytic and root zone bacteria (endo- and exoroot).

Deterrent and antibiosis activity in Thespesia populnea Cav. against the spotted bollworm, Earias vittella (Fabricius)

Earias vittella (Fabricius) feeds on some malvaceous plant species, but has never been reported to attack Thespesia populnea Cav., even though this plant belongs to the same family. The factors responsible for the failure of E. vittella to establish on T. populnea were, therefore, studied. When whole fruits or fruit pericarps of this plant were provided as larval food, larvae failed to develop and none of them pupated. However, on developing seeds of these fruits, larval growth and development were normal and comparable to those of larvae reared on developing seeds of the most favoured host plant, Abelmoschus esculentus Moench. Ether extracts from fruit pericarps exhibited antifeedant and antibiosis activity against this insect. Ether as well as ethanol extracts affected oviposition to some extent. In a choice situation, significantly less eggs were laid on treated than on untreated okra fruits. However, in a no-choice situation most of the eggs were laid on treated okra fruits.

Pseudomonas cepacia and Pseudomonas putida as winter wheat inoculants for biocontrol of Rhizoctonia solani

Pseudomonas cepacia R55 and R85 and Pseudomonas putida R104, antagonistic towards plant pathogenic fungi in vitro, were assessed as seed inoculants for winter wheat (cv. Norstar) grown in a growth chamber in soil infested with Fusarium solani or Rhizoctonia solani isolate AG-1, AG 2-1, or AG-3. Infestation of soil with R. solani AG-1 or AG 2-1 reduced root dry weight of uninoculated plants by 62 and 78%, respectively, whereas R. solani AG-3 or F. solani had no effect on plant biomass. Pseudomonad inoculants increased (relative to plants subjected to disease) the winter wheat root dry weight by 92–128% and shoot dry weight by 28–48% in the soil infested with R. solani AG-1. The shoot material of all plants inoculated with pseudomonads also had significantly (P < 0.05) higher total Fe contents than the uninoculated treatment in the R. solani AG-1 infested soil. Pseudomonas cepacia R55 produced the highest (P < 0.01) total Fe contents in the shoots, but it had no effect on N and P content. Pseudomonas cepacia R85 significantly increased total N (P < 0.05) and total P (P < 0.01) of wheat shoots, and P. putida R104 increased the percentage (P < 0.05) and (or) total P content (P < 0.01) in the soil infested with R. solani AG-1. Pseudomonas cepacia R85 also significantly (P < 0.05) increased wheat shoot biomass in R. solani AG-3 infested soil. All three pseudomonads produced fluorescent siderophores when cultured in a low-iron medium. These results suggest an in situ antibiosis activity of three fluorescent pseudomonad strains towards phytopathogenic fungi and suggest that the plant growth response was probably due to protection against damage caused by R. solani. Key words: biocontrol, pseudomonads, Rhizoctonia solani, winter wheat, siderophores.