K.V.B.R. Tilak

Suppression of maize root diseases caused by Macrophomina phaseolina, Fusarium moniliforme and Fusarium graminearum by plant growth promoting rhizobacteria

A plant growth-promoting isolate of a fluorescent Pseudomonas sp. EM85 and two bacilli isolates MR-11(2) and MRF, isolated from maize rhizosphere, were found strongly antagonistic to Fusarium moniliforme, Fusarium graminearum and Macrophomina phaseolina, causal agents of foot rots and wilting, collar rots/stalk rots and root rots and wilting, and charcoal rots of maize, respectively. Pseudomonas sp. EM85 produced antifungal antibiotics (Afa+), siderophore (Sid+), HCN (HCN+) and fluorescent pigments (Flu+) besides exhibiting plant growth promoting traits like nitrogen fixation, phosphate solubilization, and production of organic acids and IAA. While MR-11(2) produced siderophore (Sid+), antibiotics (Afa+) and antifungal volatiles (Afv+), MRF exhibited the production of antifungal antibiotics (Afa+) and siderophores (Sid+). Bacillus spp. MRF was also found to produce organic acids and IAA, solubilized tri-calcium phosphate and fixed nitrogen from the atmosphere. All three isolates suppressed the diseases caused by Fusarium moniliforme, Fusarium graminearum and Macrophomina phaseolina in vitro. A Tn5:: lacZ induced isogenic mutant of the fluorescent Pseudomonas EM85, M23, along with the two bacilli were evaluated for in situ disease suppression of maize. Results indicated that combined application of the two bacilli significantly (P = 0.05) reduced the Macrophomina-induced charcoal rots of maize by 56.04%. Treatments with the MRF isolate of Bacillus spp. and Tn5:: lacZ mutant (M23) of fluorescent Pseudomonas sp. EM85 significantly reduced collar rots, root and foot rots, and wilting of maize caused by Fusarium moniliforme and F. graminearum (P = 0.05) compared to all other treatments. All these isolates were found very efficient in colonizing the rhizotic zones of maize after inoculation. Evaluation of the population dynamics of the fluorescent Pseudomonas sp. EM85 using the Tn5:: lacZ marker and of the Bacillus spp. MRF and MR-11(2) using an antibiotic resistance marker revealed that all the three isolates could proliferate successfully in the rhizosphere, rhizoplane and endorhizosphere of maize, both at 30 and 60 days after seeding. Four antifungal compounds from fluorescent Pseudomonas sp. EM85, one from Bacillus sp. MR-11(2) and three from Bacillus sp. MRF were isolated, purified and tested in vitro and in thin layer chromatography bioassays. All these compounds inhibited R. solani, M. phaseolina, F. moniliforme, F. graminearum and F. solani strongly. Results indicated that antifungal antibiotics and/or fluorescent pigment of fluorescent Pseudomonas sp. EM85, and antifungal antibiotics of the bacilli along with the successful colonization of all the isolates might be involved in the biological suppression of the maize root diseases.

Antifungal characteristics of a fluorescent Pseudomonas strain involved in the biological control of Rhizoctonia solani.

A plant growth-promoting isolate of a fluorescent Pseudomonas spp. EM85 was found strongly antagonistic to Rhizoctonia solani, a causal agent of damping-off of cotton. The isolate produced HCN (HCN+), siderophore (Sid+), fluorescent pigments (Flu+) and antifungal antibiotics (Afa+). Tn5::lacZ mutagenesis of isolate EM85 resulted in the production of a series of mutants with altered production of HCN, siderophore, fluorescent pigments and antifungal antibiotics. Characterisation of these mutants revealed that the fluorescent pigment produced in PDA and the siderophore produced in CAS agar were not the same. Afa- and Flu- mutants had a smaller inhibition zone when grown with Rhizoctonia solani than the EM85 wild type. Sid- and HCN mutants failed to inhibit the pathogen in vitro. In a pot experiment, mutants deficient in HCN and siderophore production could suppress the damping-off disease by 52%. However, mutants deficient in fluorescent pigments and antifungal antibiotics failed to reduce the disease severity. Treatments with mutants that produced enhanced amounts of fluorescent pigments and antibiotics compared with EM85 wild type, exhibited an increase in biocontrol efficiency. Monitoring of the mutants in the rhizosphere using the lacZ marker showed identical proliferation of mutants and wild type. Purified antifungal compounds (fluorescent pigment and antibiotic) also inhibited the fungus appreciably in a TLC bioassay. Thus, the results indicate that fluorescent pigment and antifungal antibiotic of the fluorescent Pseudomonas spp. EM85 might be involved in the biological suppression of Rhizoctonia-induced damping-off of cotton.

Differential effect of various endomycorrhizal fungi on nodulating ability of green gram by Bradyrhizobium sp. (Vigna) strain S24

Bradyrhizobium sp. (Vigna) strain S24 interacted differentially with eight vesicular-arbuscular mycorrhizal (VAM) fungi and caused significant variations in nodulation and growth parameters of green gram. Coinoculation with Scutellospora calospora resulted in the highest nitrogenase activity and dry biomass. The nodulation competitiveness of strain S24 was significantly higher (60–63%) in the presence of Glomus mosseae, G. fasciculatum and Scutellospora calospora when compared to treatment with single inoculation of S24 (51%). Percentage VAM colonization was higher in treatments having higher nocule occupancy of introduced strain (S24).

Can Azospirillum strains capable of growing at a sub-optimal temperature perform better in field-grown-wheat rhizosphere

Abstract. Two Azospirillum brasilense strains, CDJA and A40, capable of growing and producing plant growth-promoting (PGP) substances at the sub-optimal temperature (SOT) of 22°C, were tested for their ability to survive, colonize and enhance wheat growth and yield under field conditions upon inoculation. The response was compared with that of A. brasilense strain, A9, impaired in growth and PGP activities at SOT (22°C) but otherwise comparable to CDJA and A40 at 37°C. A field experiment was carried out in a split-plot design with four levels of N as main plots and three strains and an uninoculated control as subplots. A differential response in the establishment of the strains and in plant growth and yield was obtained, due to the categories of strains, particularly at lower levels of N (0xa0kg and 40xa0kg N ha–1). The results clearly demonstrated that strains capable of growing and producing PGP substances at SOT are better inocula for wheat.

Bacterial Biocontrol Agents and their Role in Plant Disease Management

The newer innovations in the field of agriculture has led to enhanced destructive potential of diseases. These Innovations include practice of continuous monoculture and use of high doses of nitrogenous fertilizers that enhance disease susceptibility. To combat high incidence of fungal, bacterial and viral diseases, modern agriculture is now largely dependent on use of different formulations of agrochemicals. Increased environmental pollution by agrochemicals has led to considerable changes in attitude towards the use of pesticides in agriculture. It has led to more stringent regulations on their use and, in some cases, removal from the market. Consequently, interest has been focused on alternatives to chemicals for controlling pests and diseases. The use of biological control measure is one of the best strategies available to combat the pests and diseases in an ecofriendly manner and much experimental work is being carried out all over world to assess its commercial acceptability and applicability.

Influence of soluble phosphorus fertilizer on the interaction between a vesicular-arbuscular mycorrhizal fungus and Azospirillum brasilense in barley (Herdeum vulgare L.)

SummaryPot-culture studies were carried out to examine the response of barley (Hordeum vulgare L.) to inoculation with Azospirillum brasilense and Glomus versiforme, singly and/or in combination, under varying levels of nitrogenous [(15NH4)2SO4] and soluble phosphatic (single superphosphate) fertilizers. The interaction between both the endophytes led to increased growth and nutrition of the barley plants. Roots from plants inoculated with Azospirillum brasilense and Glomus versiforme exhibited very low acetylene reduction activity. N2 fixation in the plants increased with the increase in plant growth but the mycorrhiza alone gave a low level of N2 fixation in the plants compared to combined inoculation with both the endophytes.

Selection and evaluation of nitrate-tolerant strains of Rhizobium leguminosarum biovar viceae specific to the lentil

Seventeen strains of Rhizobium leguminosarum biovar viceae specific to the lentil (Lens culinaris L.) were screened, using the high-yielding lentil cultivar L 4076, for their tolerance to three levels of NOinf3sup-: 0, 4, and 8 mM NOinf3sup-. Preliminary screening of this symbiosis for nodulation and N fixation in the presence of NOinf3sup-showed significant variations among the strains. The number of nodules decreased and nitrogenase activity was depressed in all strains in the presence of NOinf3sup-. Strains L-1-87, L-27-89, L-33-89, and L-40-89 tolerated 8 mM NOinf3sup-. Four strains, three tolerant of NOinf3sup-(L-1-87, L-27-89, and L-33-89), and one sensitive (L-11-89) to NOinf3sup-, were selected from preliminary screening and used in a pot experiment to assess the symbiosis in the presence of 6 mM NOinf3sup-at three stages of plant growth, viz., 40 days, 60 days, and at the final harvest. In general, the weight of nodules and C2H2 reduction activity was significantly higher after 60 days than after 40 days. Inoculation with strain L-1-87 produced the maximum number of nodules, and root and shoot biomass both in the presence and the absence of NOinf3sup-. Nitrate reductase activity in the tops and nodules was assayed only after 60 days and did not show significant variations among strains and NOinf3sup-treatments. The grain yields for all strains except L-11-89 were significantly higher in the presence of NOinf3sup-than in the absence of NOinf3sup-, indicating that tolerant strains contributed symbiotically fixed N to the plants N pool, resulting in an additive effect on yield. Inoculation with strain L-1-87 produced the maximum grain yield and this strain appears to have potential use as an inoculant in the presence of high levels of soil N.

A note on the effect of non-symbiotic nitrogen fixers and organic wastes on yield and nitrogen uptake of barley

Abstract Pot trials were conducted to study the response of barley (Hordeum vulgare L.) to inoculation with Azotobacter chroococcum and Azospirillum brasilense in soils amended with city compost (CC) and paddy (rice) straw compost (PSC). The total nitrogen content in the rhizosphere soil was greater in amended soils than in the controls, the maximum being in the case of PSC at 30 days after combined inoculation with Azotobacter and Azospirillum. The nitrogen percentages in roots and shoots were maximum with CC inoculated with Azotobacter and Azospirillum at 30 days after sowing. The Azospirillum population was significantly greater in soils amended with CC and PSC than in unamended soils. It was greater in soils inoculated with both the nitrogen fixers than in soils inoculated with the single organism. Azospirillum + PSC produced the highest grain yield. Maximum N uptake in the barley straw was found in unamended soils plus Azotobacter.

Survival of Azospirillum brasilense and Azotobacter chroococcum in Organic-Amended Soil-Based Carriers

Summary Laboratory studies on the survival of Azotobacter chroococcum and Azospirillum brasilense in different carriers showed that soil amendment with organic matter, like bagasse, city compost, and paddy straw compost, supported the multiplication of both nitrogen fixers, as compared to the control series.

Establishment of Azotobacter and Azospirillum in the rhizosphere of barley (Hordeum vulgare L.) in organic-amended soils

Summary Pot trials were conducted to study the survival of Azotobacter and Azospirillum in the rhizosphere of barley crop which was inoculated with Azotobacter chroococcum and Azospirillum brasilense and grown in soils, amended with organic matter, such as city compost (C.C.) and paddy straw compost (P.S.C.). The two asymbiotic nitrogen fixers showed higher population in organic-amended soils than in unamended soils, which served as control. At the same time the proliferation was more pronounced in inoculated series.