S. Shahid Shaukat

Nematicidal activity of some strains of Pseudomonas spp.

Culture filtrates of strains of Pseudomonas spp. were able to produce juvenile mortality of Meloidogyne javanica, the root-knot nematode in vitro. Seed treatment or soil drench with Pseudomonas sp. (strain 82 and 51) and Pseudomonas aeruginosa (strain-78) significantly reduced nematode population densities in soil and subsequent gall formation due to M. javanica in mungbean, under glasshouse conditions. P. aeruginosa strain-78 multiplied on talc powder and applied in soil reduced root-knot development and nematode population density under field conditions in mungbean used as first crop or succeeding crop. Strain-78 used with either Paecilomyces lilacinus or Bradyrhizobium japonicum suppressed root-knot disease compared with either antagonist used alone. Strain-78 applied in the soil amended with a brown seaweed (Sargassum tenerrimum) also inhibited root-knot development. The toxic principles of the strain-78 were partially characterized. The active factors were heat labile, sensitive to extreme pH values, polar in nature and having a molecular weight of less than 8000 MW.

Mixtures of plant disease suppressive bacteria enhance biological control of multiple tomato pathogens

Abstract. Plant growth-promoting rhizobacteria (PGPR) strains CHA0 (Pseudomonas fluorescens), IE-6 S+ (Pseudomonas aeruginosa) and 569Smr (Bradyrhizobium japonicum) were tested singly and in combinations for biological control against multiple tomato pathogens (root-infecting fungi and root-knot nematodes). Strains CHA0 and IE-6S+ inhibited in vitro growth of 569Smr while IE-6S+ suppressed CHA0. The bacterial species not only inhibited the radial growth of three root-infecting fungi, Macrophomina phaseolina, Fusarium solani and Rhizoctonia solani (AG 8), but also caused substantial mortality of Meloidogyne javanica juveniles. Used as a soil drench the three bacteria not only suppressed root-infecting fungi and root-knot nematodes but also enhanced growth of tomato plants both under glasshouse and field conditions. The suppressive effect was generally more pronounced when the bacteria were employed together. Strain IE-6S+ exhibited better rhizosphere colonization than CHA0 and 569Smr. Populations of CHA0 in the rhizosphere declined when the bacterium was used with either IE-6S+ and/or 569Smr, while populations of IE-6S+ in the rhizosphere were enhanced when used in combination with CHA0 and/or 569Smr. IE-6S+ was the only bacterium that colonized inner root tissues of tomato plants. When using an iron chelator to create iron deficiency in the soil, the biocontrol efficacy of the bacteria against F. solani and R. solani was enhanced while against M.phaseolina and M. javanica this activity remained unchanged. Only strain 569Smr gave significant suppression of M. phaseolina in both iron-deficient and iron-sufficient soils.

Nematicidal and allelopathic potential of Argemone mexicana, a tropical weed.

Argemone mexicana L. (Papaveraceae), a tropical annual weed, is phytotoxic to many crop species. This study was designed to examine the allelochemical and nematicidal potential of A. mexicana and to better understand the role of this weed in the ecosystem. A methanol-soluble extract of the leaf material caused greater juvenile mortality of Meloidogyne javanica than did ethyl acetate or hexane extracts indicating the polar nature of the toxins. Decomposing tissues of A. mexicana in soil at 50 g kg−1 were highly deleterious causing 80% mortality of tomato plants. At 10 g kg−1 plant growth was enhanced, while at 30 g kg−1 plant growth was substantially retarded. M. javanica population densities in the rhizosphere and in roots, and gall formation were significantly suppressed when 10, 30 or 50 g kg−1A. mexicana was allowed to decompose in the soil. To establish whether decomposition was necessary to produce phytotoxic symptoms, or whether the shoot extract alone could interfere with plant growth, an aqueous shoot extract was applied to soil. Whereas a 50% extract promoted plant growth, a 100% (100 g/500 mL distilled water) concentration significantly reduced plant height, and fresh weights of shoot and root. In general, decomposing plant material caused greater phytotoxicity compared to the aqueous extract. Addition of N as NH4NO3 partially alleviated the phytotoxic action of A. mexicana,and also reduced severity of root-knot disease. Adding Pseudomonas aeruginosa to soil amended with A. mexicana resulted in decreased density of M. javanicain the rhizosphere and in tomato roots, suppressed galling rates and enhanced plant growth.

Trichoderma harzianum enhances the production of nematicidal compounds in vitro and improves biocontrol of Meloidogyne javanica by Pseudomonas fluorescens in tomato

Aims:  To determine the influence of soil‐borne fungus Trichoderma harzianum on the biocontrol performance of Pseudomonas fluorescens strain CHA0 and its 2,4‐diacetylphloroglucinol (DAPG) overproducing derivative CHA0/pME3424 against Meloidogyne javanica.

Improvement of Pseudomonas fluorescens CHA0 biocontrol activity against root-knot nematode by the addition of ammonium molybdate.

Aims: To improve the efficacy of Pseudomonas fluorescens CHA0 and its genetically modified (GM) derivatives by adding ammonium molybdate to control Meloidogyne javanica, the root‐knot nematode in mungbean.

The influence of mineral and carbon sources on biological control of charcoal rot fungus, Macrophomina phaseolina by fluorescent pseudomonads in tomato

Aims: To determine the influence of various trace minerals and carbon source on the biocontrol performance of Pseudomonas aeruginosa strain IE‐6S+ and P. fluorescens strain CHA0 against Macrophomina phaseolina.

Zinc and glycerol enhance the production of nematicidal compounds in vitro and improve the biocontrol of Meloidogyne javanica in tomato by fluorescent pseudomonads

Aims: To assess the effects of various carbon and mineral sources on the nematicidal potential of biocontrol inoculants of Pseudomonas aeruginosa IE‐6S+ and Ps. fluorescens CHA0 under laboratory and glasshouse conditions. 
Methods and Results: Culture filtrates of strains IE‐6S+ and CHA0, cultured in nutrient yeast extract broth, caused substantial mortality of the juveniles of Meloidogyne javanica. The nematicidal activities of the culture filtrates were altered after amendment with various carbon and mineral sources. Soil amendment with zinc alone or in combination with glycerol improved the biocontrol efficacy against root‐knot nematode, promoted tomato plant growth and enhanced bacterial rhizosphere and endophytic colonization. 
Conclusions: Appropriate quantities of glycerol and zinc alone or in combination enhance the nematicidal activity of Ps. aeruginosa and Ps. fluorescens. Glucose reduces the activity of these bacteria against nematodes. 
Significance and Impact of the Study: Minerals and carbon sources are appealing because they are easy and economical to provide during liquid fermentation of inoculants or as fertilizer amendments to improve the biocontrol activity of indigenous and introduced bacteria.

Differential impact of some Aspergillus species on Meloidogyne javanica biocontrol by Pseudomonas fluorescens strain CHA0

Aims:  The aim was to determine the influence of some Aspergillus species on the production of nematicidal agent(s) in vitro and biocontrol of Meloidogyne javanica in tomato by Pseudomonas fluorescens strains CHA0 and CHA0/pME3424.

Liquid culture carbon, nitrogen and inorganic phosphate source regulate nematicidal activity by fluorescent pseudomonads in vitro

Aims:  The aim of the present investigation was to determine the influence of nutrients on the nematicidal activity by Pseudomonas aeruginosa strain IE‐6S+ and Ps. fluorescens strain CHA0 in vitro.

Effects of Pseudomonas aeruginosa on the diversity of culturable microfungi and nematodes associated with tomato: impact on root-knot disease and plant growth

The available information on Pseudomonas biocontrol inoculants on the non-target fungal and nematode community is scant. The current paper addresses this issue and investigates the effects of biocontrol agents Pseudomonas aeruginosa IE-6 and IE-6S+ (previously shown to suppress several soil-borne plant pathogens) on soil microfungi and plant–parasitic nematodes as well as on the root-knot development and growth of tomato (Lycopersicon esculentum). Furadan, a granular nematicide was included as a treatment for comparative purposes. Treatments were applied to soil at the start of each 52-day-long tomato growth cycle, and their effects on the composition and diversity of rhizosphere and endophytic microfungi and plant–parasitic nematodes were examined at the end of first and fourth growth cycle. Several diversity indices were employed to assess community diversity. A total of 16 genera comprising 23 microfungal species were isolated from the tomato rhizosphere. The most abundant fungal species belonged to the genera Aspergillus, Fusarium, and Penicillium. With a few exceptions, fungi were neither exclusively inhibited nor specifically promoted by the application of treatments at any of the growth cycles studied. However, Paecilomyces lilacinus, an egg and female parasite of root-knot nematode, though exclusively absent in the controls was isolated from the treatments. Both general diversity and equitability of rhizosphere microfungi were greater at first compared to the fourth growth cycle while species richness remained uninfluenced across the growth cycles and treatments. However, Furadan and IE-6S+ treatments considerably abated general diversity and equitability. Of the microfungal species isolated from the rhizosphere seven were also recovered from surface-sterilized root tissue of tomato suggesting that all the endophytes are primarily rhizosphere organisms. Diversity of endophytic fungi was consistently lower compared with that of the rhizosphere. Both general diversity and equitability declined in all three treatments relative to controls in the root tissue but species richness remained unaltered. Diversity and equitability of plant–parasitic nematodes in soil were reduced by all three treatments over the controls at fourth growth cycle whilst species richness did not change at either growth cycle. The biocontrol agents significantly reduced root-knot development and enhanced shoot growth of tomato over the controls. The possible implications of fungal composition and abundance because of biocontrol by Pseudomonas application are discussed.