Abstract: In a laboratory study chickpea root nodulating bacterium Rhizobium sp., strain Thal-8 significantly inhibited the growth of pathogenic fungi (Alternaria alternata, Fusarium sp. Ascochyta rabiei, Drechslera sp. and Curvularia sp.) in potato dextrose agar media. The inhibition rate displayed differences in accordance with different fungi cultures. However, the inhibitory effect of Rhizobium sp., strain Thal-8 occurred most on pure cultures growth of Alternaria alternata and Drechslera sp. (54 and 45%, respectively) and least on Curvularia sp. (3%).
INTRODUCTION
Microorganisms that can grow in the rhizosphere are ideal for use as biocontrol agents since, the rhizosphere provides front line defence for roots against attack by pathogens (Weller, 1998). Of these, the rhizobia are reported as effective biocontrol agents for the inhibition of certain soil-borne plant pathogens (Chakrabotry and Purkayastha, 1984; Ehteshamul-Haque and Ghaffar, 1992). Many species of rhizobia promote plant growth and also inhibit the growth of certain pathogenic fungi. There are reports where Paecilomyces lilacinus, Trichoderma harzianum (Ehteshamul-Haque et al., 1992) Stachybotrys atra (Butt and Ghaffar, 1972) and Memnoniella ehinata (Dawar et al., 1993) have shown promising results for the control of root infecting fungi. Rhizobia are also reported to significantly inhibit the growth of pathogenic fungi i.e Macrophomina phaseolina (Tassi) Gold, Rhizoctonia solani Kuhn and Fusarium sp., in both leguminous and non-leguminous plants (Ehteshamul-Haque and Ghaffar, 1993). An experiment was therefore carried out to study the effect of Rhizobium sp., strain Thal-8 on growth of leguminous, non-leguminous and anaerobic pathogens. Thal-8 is a local isolate that is being used by NARC for producing bioinoculant for chickpea. This inoculum is being widely used by chickpea farmers in Pakistan. It was personally communicated by some farmers that Thal-8 inoculated plants not only gave better yield but they were also more resistance to the disease (Ascochyta blight). The reported study was conducted with the objective to find out the additional benefits of Rhizobium sp. strain Thal-8 so that it may be used as biocontrol agent both for leguminous and non-leguminous plants.
MATERIALS AND METHODS
Three leguminous pathogens (Ascochyta rabiei, Alternaria alternata and Fusarium sp.) were isolated from infected plants of chickpea. A non-leguminous pathogen (Drechslera sp.) was isolated from infected wheat plant while an anaerobic pathogen (Curvularia sp.) was isolated from rice. Rhizobium sp., strain Thal-8 was obtained from Soil Biology and Biochemistry Lab., National Agricultural Research Centre, Islamabad. Potato dextrose agar media was used for culture growth. All the cultures were inoculated against Rhizobium sp. on solidified media in Petri dishes, which were divided into two halves by marking with a permanent marker on under sides. Rhizobium sp. was inoculated in the form of streak while fungi cultures were inoculated in the form of 1mm disc cut from fifteen days previously grown cultures. Both Rhizobium sp. and fungi cultures were inoculated at the same day. After inoculation cultures were incubated at 25°C (±3°C) in an incubator. The colony radii of all fungi cultures were measured after five days of incubation while colony radius of Ascochyta rabiei was measured after 10 days. Percent decrease in colony radii of all fungi cultures due to Rhizobium sp., strain Thal-8 was calculated by using following formula.
Percent decrease in colony radius = B-A/A x 100
where
| B | = | Colony radius away from Rhizobium sp. |
| A | = | Colony radius facing Rhizobium sp. |
RESULTS AND DISCUSSION
In this study, Rhizobium sp., strain Thal-8 significantly inhibited the growth of leguminous as well as non-leguminous pathogens. But the growth of anaerobic pathogen was not much affected. The inhibition rate displayed differences in accordance with different fungal cultures (Table 1). This is due to influence of antifungal compounds of rhizobia on the fungal colonies. Thal-8 is also reported to produce acid which have antifungal effect (Khokhar et al., 2001).
| Table 1: | Percent decrease in colony radii of pathogenic fungi due to Rhizobium (Thal-8) |
Difference in the efficacy of rhizobia on different hosts may also be due to the quality/quantity of root exudates which influence the bacterial colonization (Parke, 1991). Rhizobia are also reported to produce toxic metabolites (Chakrabotry and Purkayastha, 1984) which have inhibitory effect on soil-borne plant pathogens.
The rhizobia present in the rhizosphere of plants presumably prevent the contact of pathogenic fungi by covering hyphal tip of the fungus and by parasitizing it (Tu, 1978). Besides parasitizing the hyphae rhizobia also produce antibiotics (Malajezuk, 1983) which resulted in the lysis of the fungal hyphae (Malajezuk et al., 1984). Due to aerobic nature of Rhizobium the growth of anaerobic pathogen was not much affected. Since, the rhizosphere provide front line defense for roots against attack by pathogens, the rhizobia present in the rhizosphere are ideal for use as biocontrol agents. The growth of Fusarium solani, Macrophomina phaseolina and Rhizoctonia solani was significantly inhibited due to rhizobia (Omar and Abd-Alla, 1998). Beside the fixation of nitrogen rhizobia are reported to produce plant growth regulators such as auxins, cytokinins and gibberallins like substsances that stimulate and enhance plant growth (Triplett et al., 1981; Sheng, 1993). There is therefore need to select potential Rhizobium (biocontrol agents) that could be effective against more than one pathogens on more than one host plants.
Integration of two or more Rhizobium strains could also provide better results than their separate use. Bradyrhizobium sp. and Rhizobium trifolii significantly controlled M. phaseolina, Rhizoctonia solani and Fusarium solani infection on sunflower and chickpea (Siddiqui et al., 1998). There is also need to establish a correlation between the population of pathogens in the soil and inoculum dose of biocontrol agents required for effective suppression of the disease as suggested by Dawar et al. (1993). It is concluded from this study that Rhizobium sp. (Thal-8) could be used as a biocontrol agent both for leguminous and non-leguminous plants.
ACKNOWLEDGMENT
The work has been carried out by the help of Mr. Tariq Sultan, Soil Biology and Biochemistry Lab. NARC who is sincerely acknowledged.