Fourteen strains of Pseudomonas fluorescens isolated from rhizosphere soil of rice were tested for their antagonistic effect towards Rhizoctonia solani, the rice sheath blight fungus. Among them, PfMDU2 was the most effective in inhibiting mycelial growth of R. solani in vitro. Production of chitinase, beta-1,3-glucanase, siderophores, salicylic acid (SA) and hydrogen cyanide (HCN) by P. fluorescens strains was evaluated. The highest beta-1,3-glucanase activity, siderophore production, SA production and HCN production were recorded with PfMDU2. A significant relationship between the antagonistic potential of P. fluorescens against R. solani and its level of beta-1,3-glucanase, SA and HCN was observed.

The effect of soil application of biocontrol agents (Pseudomonas fluorescens, Trichoderma viride and T. harzianum) in combination with chitin on induction of phenolics and defense enzymes in coconut roots infected with Ganoderma lucidum, the causal agent of Ganoderma disease, was investigated. Soil application of these biocontrol formulations in combination with chitin induced a significant increase in the activities of peroxidase (PO), polyphenol oxidase (PPO), phenylalanine ammonia-lyase (PAL), chitinase and beta-1,3-glucanase in the G. lucidum infected palms. Activities of both PAL and PO reached maximum levels within 3 d while the activity of PPO reached the maximum level 6 d after application of a mixture of P. fluorescens, T. viride and chitin. Isozyme analysis revealed that unique PO3 and PPO2 isozymes were induced in coconut palms treated with P. fluorescens + T. viride + chitin. Accumulation of phenolics was recorded 3 d after treatment and reached maximum levels 9 d after treatment application. Activity of chitinase was significantly increased from the third day after treatment imposition and continued to increase up to 9 to 12 d in all treatments. Chitinase isozyme analysis revealed that a unique Chit3 isoform was induced in coconut roots treated with P. fluorescens + T. viride + chitin. The beta-1,3-glucanase activity was maximum 9 d after treatment application. The mechanisms by which P. fluorescens + T. viride + chitin reduced the incidence of Ganoderma disease in coconut may be related to its ability to induce defense mechanisms in coconut palms.

Rhizoctonia solani isolates varying in their virulence were tested for their ability to produce oxalic acid (OA) in vitro. The results indicated that the virulent isolates produced more OA than the less virulent isolates. In order to isolate OA-detoxifying strains of Pseudomonas fluorescens, rhizosphere soil of rice was drenched with 100 mM OA and fluorescent pseudomonads were isolated from the OA-amended soil by using King's medium B. These isolates were tested for their antagonistic effect towards growth of R. solani in vitro. Among them P. fluorescens PfMDU2 was the most effective in inhibiting the mycelial growth of R. solani. P. fluorescens PfMDU2 was capable of detoxifying OA and several proteins were detected in the culture filtrate of PfMDU2 when it was grown in medium containing OA. To investigate whether the gene(s) involved in OA-detoxification resides on the plasmids in P. fluorescens PfMDU2, a plasmid-deficient strain of P. fluorescens was generated by plasmid curing. The plasmid-deficient strain (PfMDU2P-) failed to grow in medium containing OA and did not inhibit the growth of R. solani. Both PfMDU2 and PfMDU2P- were tested for their efficacy in controlling sheath blight of rice under greenhouse conditions. Seed treatment followed by soil application of rice with P. fluorescens strain, PfMDU2, reduced the severity of sheath blight by 75% compared with the control, whereas PfMDU2P- failed to control sheath blight disease.

The induction of defence compounds and enzymes involved in the phenylpropanoid pathway were studied in the ripe and green chilli fruits inoculated with Colletotrichum capsici and Alternaria alternata. Total phenols and the activity of phenylalanine ammonia lyase (PAL), peroxidase (PO), polyphenol oxidase (PPO) and catalase (CAT) increased in the inoculated ripe and green chilli fruits compared to the corresponding healthy fruits. Total phenols and the activities of the enzymes were at the maximum 2 -3 d after inoculation and thereafter declined sharply in ripe chilli fruits, whereas slowly in green chilli fruits. In comparison with ripe chilli fruits, green chilli fruits were more resistant as they showed higher accumulation of total phenols and also higher activities of enzymes.

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1 De part ment of Plant Pa thol ogy, Ag ri cul tural Col lege and Re search In sti tute, Tamil Nadu Ag ri cul tural Uni ver sity, Madurai-625 104, Tamil Nadu, In dia 2 De part ment of Phys i cal Sci ences, Ag ri cul tural Col lege and Re search In sti tute, Tamil Nadu Ag ri cul tural Uni ver sity, Madurai-625 104, Tamil Nadu, In dia * Au thor for cor re spon dence Ab stractTreat ment of to mato leaves with aque ous ex tract (0.5%) of the galls of Quercus infectoria sig nif i cantly re duced in fec tion from sub se quent in oc u la tion with Alternaria solani, the tomato early blight patho gen. When the leaves were chal lenge-in oc u lated with A. solani 3 d af ter ap pli ca tion of Q. infectoria gall ex tract (QIGE), the per cent de fo li a tion de creased from 33.6 to 7.3. Two to three day pre-treat ment with QIGE reduced the per cent de fo li a tion by 77 per cent. The bio chem i cal re sponses of to mato plants to QIGE were also stud ied. In tomato plants treated with QIGE, phe no lic con tent in creased rap idly, reached the max i mum at 2 d af ter treat ment. Phenylalanine am mo nia-lyase (PAL) ac tiv ity in creased sig nif i cantly from 1 d af ter treat ment and the max i mum en zyme ac tiv ity was re corded 2 d af ter treat ment at which pe riod a 3-fold in crease in PAL ac tiv ity was ob served when com pared to the con trol. Peroxidase (PO) ac tiv ity was also sig nif i cantly in creased 1 d af ter treat ment and the max i mum ac tiv ity was reached 2 d af ter treat ment. Peroxidase isozyme anal y sis in di cated that PO-1 was in creased dra mat i cally in to mato leaves 1 d af ter treat ment and main tained at the same level through out the ex per i men tal pe riod of 6 d. When to mato leaves were treated with QIGE, a two-fold in crease in chitinase and β-1,3-glucanase ac tiv i ties was re corded 2 and 3 d re spec tively, af ter treat ment. The enhanced ac tiv i ties of de fense-re lated en zymes and el e vated levels of phenolics in QIGE-treated to mato plants be tween 1 and 3 d af ter treat ment sug gest that these in duced bio chem i cal defenses may be in volved in the sup pres sion of early blight by QIGE.List of ab bre vi a tions: GlcNAc-N-acetyl glu cosamine; QIGE-Quercus infectoria gall ex tract; PAL -phenylalanine am mo nia-lyase; PO -perox idase; PR pro teins-pathogenesis-related pro teins In tro duc tionPlants have evolved a wide ar ray of bio chem i cal de fenses to com bat the in vad ing patho gens. Some of the bio chem i cal de fense mech a nisms are con stitu tive and ex ist ing in nor mally de vel oped healthy plants. These in clude phenolics, phe no lic glycosides, un sat u rated lac tones, sapo nins, cyanogenic

Basal stem rot (BSR) caused by the species of Ganoderma is one of the most devastating diseases of numerous perennial, coniferous and palmaceous hosts. In forest systems, Ganoderma has an ecological role in the breakdown or delignification of woody plants. Symptoms of BSR disease can take several years to develop, and the presence of the pathogen (such as indicated by fruiting bodies) is often only visible when the fungus is well established and more than half of the bole tissue has been decayed, leaving no chance for the grower to cure the infected palms. Soils with poor drainage and water stagnation during rainy seasons were found to favour the disease. A limiting factor in controlling the BSR disease is the lack of reliable diagnostic method(s) for early diagnosis. Amendment of calcium nitrate in soil is one of the methods for the management of Ganoderma disease in palms. A combination of biological and systemic fungicides along with good cultural practices will effectively control the BSR disease in palms.

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