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Production of perithecia, asci, and ascospores by Indian isolates of Magnaporthe grisea is rare and has not been found among the Southern Indian isolates of the blast pathogen. From among 138 monoconidial isolates that infect rice and other hosts, we now report the distribution of mating types (MAT1-1 and MAT1-2) of M. grisea in finger millet and paragrass (Brachiaria mutica)-infecting isolates. Twenty-eight of the 96 finger millet isolates, 5 of the 16 paragrass isolates, and none of the 26 rice isolates produced fertile perithecia in laboratory matings with fertile testers. Backcrosses of ascospore progenies to the parental M. grisea isolate but not to the tester strain resulted in fertile perithecial formation, and a further backcrossing scheme indicated definite fertility patterns of Mendelian inheritance in M. grisea.

About 200 isolates of the bacterial blight pathogen Xanthomonas oryzae pv. oryzae were obtained from infected rice (Oryza sativa L.) leaf samples from the Palghat District of Kerala State in southern India during January and February, 1998, when there was a severe epidemic of bacterial blight in the farmers' fields. One hundred and forty of these isolates were assayed for pathogenicity on rice cv. IR24 (no R gene), IRBB21 (Xa21), and NH56 (Xa4+xa5+xa13+Xa21) at their maximum tillering stage by the standard clip-inoculation technique (1) at the Regional Agricultural Research Station (Kerala Agricultural University), Pattambi. In two separate experiments, all the isolates were found to be pathogenic to IR24, producing lesions greater than 10 cm in length. None of the isolates induced susceptible symptoms on cv. NH56 and the average lesion length was less than 2 cm. Twenty of the 140 isolates, however, showed virulence to IRBB21 (seeds supplied by S. R. McCouch, Cornell University, and by P. Ronald, University of California-Davis) and induced lesions of 10.0 to 18.0 cm. We report here the presence of Indian races of X. oryzae pv. oryzae that are virulent to IRBB21 (Xa21), contradicting an earlier report of avirulence (2). These results suggest that the deployment of Xa21 alone will not be a sound strategy for the management of bacterial blight in southern India. References: (1) S. S. Gnanamanickam et al. Plant Dis. 78:173, 1994. (2) G.-L.Wang et al. Mol. Plant-Microbe Interact. 9:850, 1996.

Bacterial antagonists of both fluorescent and nonfluorescent groups were screened for in-vitro inhibition of the rice sheath blight (ShB) fungus Rhizoctonai solani Kuhn and chitinase production in the laboratory. Twelve percent of the total 1,757 strains screened inhibited R. solani while 31% of the total 1,366 strains tested were positive for chitinase activity. The efficient strains were then evaluated in the field for ShB suppression. Two strains from each of the three seed-bed experiments were chosen for the field test in a hot-spot location. Additional treatments were a Bacillus and validamycin, the fungicide. There was no correlation between chitinase activity in the antagonists and ShB suppression in the seed-bed or field plots. Two most efficient Pseudomonas putida and P. fluorescens strains afforded 68 and 52% ShB suppression while validamycin afforded 27% disease control.

Fluorescent and nonfluorescent strains of bacteria isolated from rice rhizospheres on the International Rice Research Institute (IRRI) farm were evaluated for in vitro antibiosis towards the sheath blight (ShB) pathogen Rhizoctonia solani, and for suppression of ShB in detached rice leaves. Efficient strains were located on the basis of consistent performance in two laboratory tests. Among nine efficient strains, 3 strains were identified as Pseudomonas fluorescens, 5 strains were tentatively identified as Bacillus spp. and one strain was identified as Enterobacter. In three greenhouse tests lowland rice soils with optimum pH for rice growth (pH 5.5–6.5), acidic pH (pH 5.0) and boron toxicity were found more suitable for biological control of ShB and, less frequently, also yield increases than were alkaline (pH 6.9) and zinc-deficient soils. Bacterial treatments afforded significant ShB reductions in 3 field experiments, but no significant yield increases resulted. In direct-seeded rice best performances by bacterial treatments in terms of ShB suppression were 66 and 98% during DS 1988 and WS 1988, respectively, which were comparable to or better than the performance of validamycin (a fungicide routinely used for ShB control) which afforded 42 and 63% disease suppression, respectively, in the same experiments. Although bacterial treatments caused ShB reductions both in direct-seeded and transplanted rice crops, disease control was more pronounced in direct-seeded than in transplanted crops. These results indicate that carefully selected strains of bacterial antagonists have the potential for ShB suppression in rice at least in areas where direct-seeding is practised.

In laboratory tests Sarocladium oryzae, the sheath rot pathogen of rice was found to inhibit the mycelial growth of other stem-attacking rice pathogens. Among those inhibited, Sclerotium oryzae and Gaeumannomyces graminis var. graminis were most sensitive while Pyricularia oryzae and Rhizoctonia solani were less sensitive. Tissue-based tests made with rice culm segments established that Sarocladium oryzae inhibits mycelial growth and delays sclerotium formation in R. solani. Cerulenin, the toxin produced by Sarocladium oryzae showed a toxicity pattern towards rice pathogens similar to that of Sarocladium oryzae. The stem rot pathogen, Sclerotium oryzae was most sensitive to cerulenin. In two greenhouse experiments, IR58 rice plants inoculated with Sarocladium oryzae alone or together with Sclerotium oryzae, G. graminis var. graminis or R. solani were found to have reduced plant height and increased tiller number. Sheath rot severity increased when Sarocladium oryzae was inoculated as a single pathogen or together with others. Sheath rot inoculation reduced stem rot in rice plants by 76 and 58%, respectively, in Experiment 1 and 2. By its known antagonistic interaction towards stem rot and crown sheath rot pathogens which are sensitive to it and by other unknown interactions, sheath rot emerges as the dominant disease.

From among 125 strains of fluorescent and 52 strains of nonfluorescent bacteria initially screened in the laboratory for their antibiosis towards the bacterial wilt pathogen, Pseudomonas solanacearum, strain Pfcp of Pseudomonas fluorescens and strains B33 and B36 of Bacillus spp., were chosen and evaluated further in greenhouse and field tests. Pfcp treated banana (Musa balbisiana), eggplant and tomato plants were protected from wilt upto 50, 61 and 95% in greenhouse and upto 50, 49 and 36% respectively in field. Protection afforded by the Bacillus strains was lower. In bacteria-treated plants which were subsequently inoculated with P. solanacearum plant height and biomass values increased and were close to those of nontreated and noninoculated control plants.

Four semiselective media that varied with respect to carbon source, amino acids, and antibiotics were evaluated for the growth of various strains of the rice bacterial blight pathogen, Xanthomonas oryzae pv. oryzae in pure and mixed culture. Plating efficiencies in pure culture ranged from 40 to 90% on these media, and colonies developed in 3-7 days, depending on the strain of X. o. oryzae. In mixtures with Erwinia herbicola and Pseudomonas putida(1:1:1), X. o. oryzae was recovered only on XOS medium. Recovery of a faster growing X. o. oryzae strain (X1-5) was improved by adding 50-100 mg/L of FeEDTA to XOS medium, but recovery of other strains was reduced or unaffected

Native strains ofPseudomonas fluorescens exhibitedin vitro antibiosis towards isolates of races 1 and 4 ofFusarium oxysporum f.sp.cubense, the Panama wilt pathogen of banana. The seedlings ofMusa balbisiana seedlings treated withP. fluorescens showed less severe wilting and internal discolouration due toF. oxysporum f.sp.cubense infection in greenhouse experiments. In addition to suppressing Panama wilt, bacterized seedlings ofM. balbisiana also showed better root growth and enhanced plant height.

Coronatine was detected in culture filtrates of 12 out of 19 pathogenic strains of Pseudomonas glycinea examined, but not in culture filtrates of strains of P. phaseolicola, P. syringae, or P. tabaci. A reversed-phase high-performance liquid chromatographic procedure was developed for coronatine quantitation. Generally, coronatine production by strains in culture correlated with their ability to induce systemic symptoms (chlorosis and stunting) in inoculated soybean plants. Application of purified preparations of coronatine to unifoliate leaves of soybean plants resulted in localized chlorosis, development of chlorosis in subsequently developing trifoliate leaves, and stunting of plant growth, similar to symptoms induced by infection. Coronatine was demonstrated in soybean leaves infected with P. glycinea but was not detected in healthy leaves. The results indicate that coronatine can play an important role in the development of symptoms of bacterial blight of soybean, but the demonstration that some pathogenic strains do not produce coronatine indicates that it may not be essential for pathogenicity.

Strains of fluorescent and nonfluorescent bacteria that were isolated from rice rhizospheres of Southern India and showed antagonism towardsRhizoctonia solani were evaluated for biological control of rice sheath-blight (ShB). Efficient strains of bacteria inhibited mycelial growth ofR. solani, affected sclerotial viabilityin vitro and protected IR 20 and TKM 9 rice seedlings from infection byR. solani in greenhouse tests. Pretreatment of sclerotia in bacterial suspensions resulted in reductions in ShB lesion sizes up to 31 to 44% in IR20 and 58 to 74% in TKM 9 rice. In field plots, IR 50 and TKM 9 rice plants raised from bacterized seeds had 65 to 72% less ShB than those plants from untreated seeds.

Systemic chlorotic mottling of trifoliate leaves of soybean cultivars developed following inoculation of unifoliate leaves with strains of Pseudomonas syringae pv. glycinea. This symptom could not be reproduced by application of coronatine, a toxin previously isolated from this bacterium, but resembled systemic symptoms that were produced on bean (Phaseolus vulgaris) by inoculation with P. syringae pv. phaseolicola or by application of toxic principles derived from this pathovar. Attempts to isolate a second toxin from pv. glycinea culture fluids using methods employed for the isolation of phaseolotoxin from pv. phaseolicola were not successful. Evidence was obtained for the presence of inhibitory material that interfered with assays for toxin with Escherichia coli K12 as the test organism. Partial separation of the toxin from the inhibitor and other substances was achieved by high-performance liquid chromatography of culture fluids and further purification was effected by ion exchange, partition, and thin-layer chromatography. Biological activity of the pv. glycinea toxin preparation was similar to a toxin prepared from pv. phaseolicola. Both toxins produced similar systemic symptoms on soybean leaves, inhibited E. coli (reversed by arginine, citrulline, and certain tripeptides), inhibited ornithine carbamoyl transferase, and caused the accumulation of ornithine in soybean leaves, as has been reported for phaseolotoxin. There were small differences between the pv. glycinea and pv. phaseolicola toxins in chromatographic behaviour. It is concluded that, in addition to coronatine, pv. glycinea produces a second toxin that is similar to that produced by pv. phaseolicola.

Severity of stem-rot disease of peanut caused byRhizoctonia solani was reduced by 54.9 and 68% in plants of two cultivars treated in the greenhouse with antagonistic strains ofPseudomonas fluorescens. These strains were selected based on theirin vitro toxicity to mycelial growth and sclerotial germination ofR. solani. In field experiments, bacterization of peanuts withP. fluorescens resulted in taller plants (by 25.7%) and increased yields (by 59.0%).