Global Theme on Agroecosystems
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Global Theme on Agroecosystems
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Biological Control of Late Leaf Spot of Groundnut Background Groundnut ( Arachis hypogea L.) is very important crop in more than 100 countries in the world. Its yields in developing countries are very low ranging from 0.3 to 0.9 t ha -1 compared to very high yields of 2.8 t ha -1 in USA. The occurrence of diseases and pests are the main causes for poor yields in groundnut. The crop is prone to many fungal diseases where ever it is grown. The most important fungal foliar diseases with attributed crop losses of more than 50% are late leaf spot (LLS) caused by Phaeoisariopsis personata and rust caused by Puccinia arachidis. Chemical control of these diseases is being widely practiced to achieve higher yields all over the world. But the undesirable effects of chemical fungicides on ecosystem, development of fungicide tolerant strains, increasing cost of fungicides necessitated the search for the non-chemical/biological methods to control of these fungal diseases. Biological control offers promising alternative for chemical fungicides/pesticides Several fungi and bacteria were reported to have broad spectrum antifungal activity and offered promising disease control in several crops under greenhouse and field environments. A few of these biocontrol agents (bacteria and fungi) are commercially exploited in several parts of the world. Thus, biological control appears to be a promising, eco-friendly and cost effective alternative disease management option for control of fungal diseases in crop plants. Search for rhizobacteria as potent antagonists to late leaf spot pathogen, Phaeoisariopsis personata. One hundred and fifty bacterial strains were isolated from the rhizosphere of groundnut plants from different areas of Andhra Pradesh and Karnataka? These strains were numbered as GRS 1 to GRS 150 and were tested for in vitro antagonism against P. personata. Of the 150 bacterial strains tested, only 10 strains inhibited >90% conidial germination of P. personata (Figure 1).
The length of the germ tube was also greatly reduced by these bacterial strains.
The ten bacterial strains which inhibited > 90% conidial germination of P. personata were tested for control of late leaf spot on susceptible cultivar TMV 2 under greenhouse environment. When the plants reached 35-days-old, the bacterial strains were applied as a foliar spray at three different intervals i.e. at 24h before, at 0 h (same time) and at 24h after the pathogen inoculation. The concentration of bacterial strains was log 9 cfu/ml and pathogen was 20000 conidia/ml. The severity of late leaf spot was measured as percent leaf area damaged, percentage of defoliation and disease score on 1-9 rating scale up to 30 days after pathogen inoculation. Percent leaf area damaged, percent defoliation and disease score were significantly less in the pots sprayed with all the ten bacterial strains than control (Table 1 and figure 2). Two bacterial strains GRS 94 and GRS 100 were found effective even when applied 24 h before the pathogen inoculation. These two cultures were submitted to Indian Type Culture Collection, IMTECH and Chandigarh for strain identification.
Table 1. Late leaf spot score at 30 days after inoculation in simultaneous treatment of bacterial strains
Scope for further research Determining the ability of the promising rhizobacterial strains to establish on the phylloplane of groundnut using genetically marked antibiotic resistant mutants will help to design an appropriate spray schedule of these bacterial strains. Development of formulations for long-term storage of these bacteria and suitable method for field application is essential. Also, determining the fungicide tolerance of the bacterial strains makes it feasible to integrate them with fungicides for effective control of late leaf spot.
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