A total of 13 microorganisms viz., Aspergillus niger, A. flavus, A. fumigatus, Chaetomium globosum, Corynespora cassicola, Curvularia lunata, Curvularia sp., Eurotium chavelari, Fusarium oxysporum, F. moniliforme, Monilia sp. Rhizopus stolonifer and Trichoderma harzianum was isolated from the Parthenium infested soils of Tamil Nadu with various degree of distribution. Among them, A. niger and R. stolonifer had 100% distribution followed by A. flavus, T. harzianum, C. lunata and F. moniliforme.

Blackgram (Vigna mungo (L.) Hepper) is one of the important pulse crops grown all over the world. Among the virus diseases of blackgram, leaf crinkle caused by Urdbean Leaf Crinkle Virus (Ungrouped/ULCV) is an important disease which leads to considerable losses depending on the season and type of cultivar cultivated. Pre-inoculation spraying of blackgram plants with leaf extracts of Mirabilis jalapa or Bougainvillea spectabilis was found to reduce the leaf crinkle disease to about 90%. The suppression of the disease by the extract was not only due to direct inhibition of the virus. Pre-treatment of blackgram leaves with 10% leaf extracts of M. jalapa and B. spectabilis as foliar spray induced the accumulation of phenolics and enhanced the activities of peroxidase, phenylalanine ammonialyase and polyphenol oxidase. New isoforms of peroxidase and polyphenol oxidase were also induced by the treatments. The induced defense mechanism might have played a role in reducing the disease.

Studies were conducted to evaluate two specific strains of the novel plant growth promoting rhizobacterium (PGPR), Pseudomonas fluorescens for induced systemic resistance (ISR) against urdbean leaf crinkle virus (ULCV) in blackgram (Vigna mungo) under controlled conditions. Results of the studies revealed that the pre-inoculation application of blackgram plants with the strains of P. fluorescens viz., Pf1 and CHAO were found to reduce ULCV infection significantly. Soil and foliar application of P. fluorescens (Pf1) induced the accumulation of phenolics and enhanced the activities of peroxidase, phenylalanine ammonia lyase and polyphenol oxidase. New isoforms of peroxidase and polyphenol oxidase were also induced by the treatment. The induced defense mechanism might have played a role in reducing the disease.

Treatment of acibenzolar-S-methyl (bionTM), salicylic acid and the saprophytic bacterium Pseudomonas fluorescens exhibited induced systemic resistance in Sorghum bicolor (cv. Rio) to Sugarcane mosaic virus (SCMV) isolates from sugarcane. The treatments significantly slowed down the SCMV titre in plants during the initial growth phase. The enhanced induction of total phenols, phenylalanine ammonia lyase (PAL), peroxidase (PO) and polyphenol oxidase (PPO) might have contributed for the induced systemic resistance triggered by various biotic and abiotic inducers. More induction of PO and PPO isozymes were noticed upon application of these inducers. In the present studies, there was a significant decrease of SCMV titre as evidenced by ELISA in these treatments. Among the treatment methods, foliar application was highly effective in case of the abiotic elicitors bion and salicylic acid whereas with P. fluorescens seed treatment was effective.

Total RNA was extracted from the mature leaves of Bougainvillea spectabilis Willd. and messenger RNA (mRNA) was separated out. Using mRNA as template, complementary DNA (cDNA) was synthesized and amplified by Reverse Transcription coupled PCR using gene specific primer. A product of 750 base pair plus was selected based on the size of Bougainvillea antiviral protein (BAP). After elution the product was purified and cloned into pGEM-T Easy vector and mobilized into E. coli strain, JM 109 and sequenced. The partial cDNA sequence of 893 base pairs contained three open reading frames. The sequence of a most possible ORF (297 bp) was translated and its homology with already reported sequences of antiviral proteins was analyzed.

Use of biocontrol agents for seed priming against Macrophomina phaseolina and their effect on seed quality were evaluated. Seed transmission was found to be high in discoloured seeds (7.6%). It was 5.5% under unsterilized condition as compared to 2.5% in sterilized seeds. In all the localities, greater infection was noticed in TMV-l than TMV-2 and TMV-7. M. phaseolina infection was observed on seed coat and cotyledon. All the Trichoderma spp. including T viride (Tv-4 & Tv-6); T harzianum (Th-1 & Th-5), T hamatum; T longibrachiatum; T koningii and T pseudokoningii inhibited the pathogen growth significantly and the maximum antagonistic activity was shown by T. harzianum-5. Seed pelleting with Trichoderma spp. protected seeds from M. phaseolina infection and maximum reduction of infection (79.6%) was also shown by T harzianum-5. It also improved seedling vigour, dry matter production and prevented loss of oil content up to six months of storage.

Different antibody-based diagnostic techniques such as direct antigen coating enzyme linked immunosorbent assay (DAC-ELISA), electroblot immunoassay, immunosorbent electron microscopy and dot blot immunoassay were compared with a genome-based technique, viz., reverse transcriptase polymerase chain reaction (RT-PCR) against Sugarcane mosaic virus (SCMV) in different sugarcane cultivars expressing predominant and less predominant symptoms as well as in asymptomatic ones. Polyclonal antiserum raised against SCMV antigen purified from sugarcane cv. CoC 671 was used. All of the antibody-based tests reacted positively with plants showing predominant foliar symptoms; however, the sensitivity was not uniform, with samples from the plants showing less predominant symptoms and in asymptomatic plants. The RT-PCR assay method was more sensitive in detecting the virus from plants of less predominant symptoms also. Hence, a combined diagnostic system utilizing antibody-based techniques can be employed for mass screening and in quarantine operations against the virus, but in doubtful cases RT-PCR can be employed. Among the different sugarcane tissues, SCMV was diagnosed in the leaf lamina, bud, leaf sheath and midrib tissues, but root and pith tissues were negative to the virus in RT-PCR assay. In RTPCR, 870 bp size cDNA was amplified from infected sugarcane with SCMV specific primer.

Serial thermotherapy with hot water involving different treatment series was followed in two sugarcane cultivars namely Co 740 and CoC 671 for assessing its effect on sugarcane mosaic virus (SCMV) titre in the regenerated plants. Observations on virus titre and sett regeneration were recorded at different intervals after planting the treated setts. The direct antigen coating (DAC) ELISA method was followed with the polyclonal antisera raised against the widespread local SCMV strain. All the thermo treatments affected the sett regeneration in both the cultivars. Although none of the treatment series produced virus free plants, the SCMV titre was comparatively reduced in the regenerated plants at an early phase (4 weeks after planting) in severe hot water treatment series but drastic reduction in sett germination was observed. However, the SCMV titre picked up later (8 weeks after planting) in all the treatment series. The early reduction in SCMV titre could be utilized for obtaining more virus free plants in meristem culture.

Studies were carried out on the influence of temperature on growth, survival and antagonistic performance of various strains of Trichoderma spp. against groundnut dry root-rot pathogen Macrophomina phaseolina (Tassi.) Goid. Different isolates of Trichoderma viride, T. harzianum, T. longibrachiatum, T. Hamatum, T. koningii and T. pseudokoningii were employed at various temperatures, viz., 15, 20, 25, 30, 35, 40 and 45°C for studying its cultural behaviour and antagonistic ability. Increased growth of the pathogen ( M. phaseolina ), and growth, sporulation and biomass production of the fungal antagonist ( Trichoderma spp.) were observed between 25 and 35°C. Antagonistic activity of Trichoderma spp. against M. phaseolina was decreased with increase in temperature except for T. pseudokoningii , which showed maximum inhibition at 35°C. Survival of Trichoderma spp. on seed coat was maximum at lower temperature (15°C). At all the temperature regimes T. harzianum strain Th-5 has shown higher suppression of the root-rot pathogen, better growth and survival than strains of other species.

The impact of age of sugarcane on SCMV infection was assessed in two sugarcane cultivars viz., CoC 671 and Co 740. The sugarcane plants of one, two, three, four, five and six months aged were inoculated with sugarcane mosaic virus (SCMV) and infection progress was assessed at 15, 30 and 60 days after inoculation. The infection progress was assessed by external symptoms and also by direct antigen coating (DAC)-ELISA with polyclonal antiserum raised against SCMV. The one and two months aged sugarcane plants were highly susceptible and infection progress was very fast whereas five and six months aged plants showed very slow infection progress. All the plants of one and two months aged were assayed positive after 30days. Whereas five and six months aged plants recorded only 22 and 1 per cent infection, respectively even after 60 days of inoculation. Among the two cultivars the progress of infection is relatively lesser in Co 740.The results revealed that age of the plants does have a definite impact on SCMV infection and the progress of infection declined with ageing. It varied with cultivars also.

The meristem tip culture in combination with antiviral chemotherapy was found significant to enhance the sugarcane mosaic virus (SCMV) elimination from the infected sugarcane meristem. Two antiviral chemotherapeutants, viz. ribavirin and 8-azaguanine were applied with MS medium in different concentrations (5, 10, 25 and 50 ppm) and meristem culture was done. The SCMV elimination was assessed in direct antigen coating (DAC)-ELISA with polyclonal antiserum raised against SCMV. Amending the antiviral chemotherapeutant, ribavirin at 50 ppm with MS medium was found promising in SCMV elimination up to 95 per cent when compared to 62 per cent in meristem culture alone and ribavirin did not exhibit any phytotoxicity on sugarcane. However, the other chemotherapeutant, 8-azaguanine exhibited phytotoxicity and reduced regeneration of plantlets. The results indicated that the combined method of antiviral chemotherapy with ribavirin (50 ppm) and meristem culture was effective in SCMV elimination when compared to meristem culture alone.

Biological and chemical control of Pythium aphanidermatum (Edson) Fitz.-Meloidogyne incognita (Chitwood, 1949) disease complex in chilli (Capsicum annum L.) and brinjal (Solanum melongena L.) were evaluated in field. Seed treatement with Diafuran 3G+ Ridomil MZ-72, significantly reduced the incidence of damping-off followed by seed treatment with Ridomil MZ-72, Trichoderma viride, T. harzianum and Paecilomyces lilacinus treatments. There was increase in dry shoot and root weight of both crops, when T. viride was used as seed treatment followed by P. lilacinus and T. harzianum. The nematode gall-index was highly reduced in Diafuran 3G+ treatment followed by P. lilacinus and Diafuran 3G-Ridomil MZ-72 treaments.

The interaction effect of Pythium aphanidermatum (Edson) Fitz. and Meloidogyne incognita (Chitwood. 1949) was tested in field. The concomitant inoculation of P.aphanidermatum and M.incognita caused maximum pre-and post-emergence damping- off in both chilli ev.Co2 and brinjal cv.Co2. The dry shoot and root weight of the plants infected with Pythium and Meloidogyne were found to be reduced significantly.

Biological control of Pythium aphanidermatum (Edson) Fitz. Meloidogyne incognita (Chitwood. 1949) disease complex in chilli with organic amendments viz., Farm yard manure and neem cake were field evaluated. The antagonistic organisms used were Trichoderma viride, T. harzianum (antagonists against Pythium aphanidermatum) and Paecilomyces lilacinus (antagonist against Meloidogyne incognita). The incidence of damping-off of chilli was significantly reduced in all the treatments, when the seeds were treated with antagonistic organisms and the soil was amended with the organic amendments 15 days before sowing viz., Farm yard manure or neem cake at the rate of 20 t/ha. The dry shoot and root weights were increased in T. viride + neem cake treatment followed by T. viride + farm yard manure treatment and the root-knot index was verymuch reduced in P.lilacinus + neem cake treatment followed by T. viride + farm yard manure treatment and the root-knot index was very much reduced in P.lilacinus + neem cake treatment and in P. lilacinus + Farm yard manure treatment.

The infection of bitter gourd plant at the early stage by bitter gourd mosaic virus resulted in significant reduction of number of leaves, leaf area, intermodal length, thickness of vines, length of vines, number of branches, number of flowers, number of fruits and other fruit characters viz, length. of fruits, girth of fruits, weight of fruits and total yield of fruits