Particular bacterial strains in certain natural environments prevent infectious diseases of plant roots. How these bacteria achieve this protection from pathogenic fungi has been analysed in detail in biocontrol strains of fluorescent pseudomonads. During root colonization, these bacteria produce antifungal antibiotics, elicit induced systemic resistance in the host plant or interfere specifically with fungal pathogenicity factors. Before engaging in these activities, biocontrol bacteria go through several regulatory processes at the transcriptional and post-transcriptional levels.

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Biological control of soil-borne pathogens by fluorescent pseudomonads

Plant growth-promoting rhizobacteria (PGPR) are the rhizosphere bacteria that can enhance plant growth by a wide variety of mechanisms like phosphate solubilization, siderophore production, biological nitrogen fixation, rhizosphere engineering, production of 1-Aminocyclopropane-1-carboxylate deaminase (ACC), quorum sensing (QS) signal interference and inhibition of biofilm formation, phytohormone production, exhibiting antifungal activity, production of volatile organic compounds (VOCs), induction of systemic resistance, promoting beneficial plant-microbe symbioses, interference with pathogen toxin production etc. The potentiality of PGPR in agriculture is steadily increased as it offers an attractive way to replace the use of chemical fertilizers, pesticides and other supplements. Growth promoting substances are likely to be produced in large quantities by these rhizosphere microorganisms that influence indirectly on the overall morphology of the plants. Recent progress in our understanding on the diversity of PGPR in the rhizosphere along with their colonization ability and mechanism of action should facilitate their application as a reliable component in the management of sustainable agricultural system. The progress to date in using the rhizosphere bacteria in a variety of applications related to agricultural improvement along with their mechanism of action with special reference to plant growth-promoting traits are summarized and discussed in this review.

Plant growth-promoting rhizobacteria (PGPR): emergence in agriculture

Biological control of soilborne pathogens by introduced microorganisms has been studied for over 65 years (9, 49), but during most of that time it has not been considered commercially feasible. Since about 1 965, however, interest and research in this area have increased steadily (9), as reflected by the number of books (10, 47,49, 152) and reviews about it (11,26,30, 106, 143, 153, 173, 174, 183) that have appeared . Concurrently, there has been a shift to the opinion that biological control can have an important role in agriculture in the future, and it is encouraging that several companies now have programs to develop biocontrol agents as commercial products. This renewed interest in biocontrol is in part a response to public concern about hazards associated with chemical pesticides. Microorganisms that can grow in the rhizosphere are ideal for use as biocontrol agents, since the rhizosphere provides the front-line defense for roots against attack by pathogens. Pathogens encounter antagonism from rhizosphere microorganisms before and during primary infection and also during secondary spread on the root. In some soils described as microbiologi­ cally suppressive to pathogens (172), microbial antagonism of the pathogen is especially great, leading to substantial disease control. Although pathogen­ suppressive soils are rare, those identified are excellent examples of the full potential of biological control of soilborne pathogens.

Biological control of soilborne plant pathogens in the rhizosphere with bacteria

▪ Abstract Agricultural soils suppressive to soilborne plant pathogens occur worldwide, and for several of these soils the biological basis of suppressiveness has been described. Two classical types of suppressiveness are known. General suppression owes its activity to the total microbial biomass in soil and is not transferable between soils. Specific suppression owes its activity to the effects of individual or select groups of microorganisms and is transferable. The microbial basis of specific suppression to four diseases, Fusarium wilts, potato scab, apple replant disease, and take-all, is discussed. One of the best-described examples occurs in take-all decline soils. In Washington State, take-all decline results from the buildup of fluorescent Pseudomonas spp. that produce the antifungal metabolite 2,4-diacetylphloroglucinol. Producers of this metabolite may have a broader role in disease-suppressive soils worldwide. By coupling molecular technologies with traditional approaches used in plant patholog...

Microbial populations responsible for specific soil suppressiveness to plant pathogens.

The rhizosphere is a hot spot of microbial interactions as exudates released by plant roots are a main food source for microorganisms and a driving force of their population density and activities. The rhizosphere harbors many organisms that have a neutral effect on the plant, but also attracts organisms that exert deleterious or beneficial effects on the plant. Microorganisms that adversely affect plant growth and health are the pathogenic fungi, oomycetes, bacteria and nematodes. Most of the soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the playground and infection court where the pathogen establishes a parasitic relationship with the plant. The rhizosphere is also a battlefield where the complex rhizosphere community, both microflora and microfauna, interact with pathogens and influence the outcome of pathogen infection. A wide range of microorganisms are beneficial to the plant and include nitrogen-fixing bacteria, endo- and ectomycorrhizal fungi, and plant growth-promoting bacteria and fungi. This review focuses on the population dynamics and activity of soilborne pathogens and beneficial microorganisms. Specific attention is given to mechanisms involved in the tripartite interactions between beneficial microorganisms, pathogens and the plant. We also discuss how agricultural practices affect pathogen and antagonist populations and how these practices can be adopted to promote plant growth and health.

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The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms

Naturally occurring fluorescent pseudomonads involved in suppression of black root rot of tobacco

Five Fusarium solani mulants, able to grow on a medium containing 800 p.p.m. tomatine, were obtained after treatment of the wild-type strain (to which 100 p.p.m. tomatine was fungistatic) with either N-methyl-N'-nitro-N-nitrosoguanidine or u.v. light. The mutants produced a severe rot in green tomato fruits (containing tomatine) in contrast to the wild-type strain which did not rot green tomatoes. The mutants were neither more nor less aggressive on red tomatoes (without tomatine) and on pea plants than the wild-type strain. We suggest that tomatine is liberated in the very early stages of infection and contributes effectively to the inhibition of further growth of the wild-type strain of F. solani. Furthermore, the mutants contained less sterol per gram of mycelium and this may explain their relative insensitivity to tomatine.

Induction of Fusarium solani mutants insensitive to tomatine, their pathogenicity and aggressiveness to tomato fruits and pea plants

Two Fusarium solani mutants of mating type (+), insensitive to tomatine and digitonin, pathogenic for green tomato fruits (containing tomatine) and with low sterol content, were crossed with a mating type (−) strain of Nectria haematococca (the perfect state of F. solani), sensitive to tomatine and digitonin, non-pathogenic for green tomato fruits and with high sterol content. Most asci produced had a reduced spore number. Tomatine insensitivity and mating type were inherited independently in 421 randomly selected F1 ascospores (P = 0·80). Linkage analysis showed that the mutations for tomatine insensitivity, induced in one mutant after N-methyl-N′-nitro-N-nitrosoguanidine and in the other after u.v. treatment, belonged to the same locus or were closely linked (n = 400, P = 0·096). Pathogenicity for green tomatoes, low sterol content and insensitivity to digitonin were always inherited together with tomatine insensitivity. We suggest that tomatine is a resistance factor of green tomatoes against F. solani and that low sterol content is a mechanism of insensitivity to tomatine and digitonin.

Genetic analysis of tomatine insensitivity, sterol content and pathogenicity for green tomato fruits in mutants of Fusarium solani

Verticillium lecanii and Aphanocladium album infected in 5 days 90–95 % of the teliospores of Puccinia horiana, the major pathogen of Chrysanthemum. This suggests the possibility to expand the use of V. lecanii, a commercial biocontrol agent against aphids, in order to control P. horiana, V. lecanii and A. album parasitized other microcyclic rusts but with a lesser frequency: P. dianthi (82 and 88 %), P. malvacearum (72 and 60 %) and P. glomerata (57 and 61 %). Cladosporium sphaerospermum and C. uredinicola were less effective than V. lecanii and A. album against all rust species tested.

Zusammenfassung

Hyperparasitismus von Puccinia horiana und anderen mikrozyklischen Rostpilzen



Verticillium lecanii und Aphanocladium album infizierten in 5 Tagen 90—95% der Teleutosporen von Puccinia horiana, dem wichtigsten Krankheitserreger von Chrysanthemen. Es erscheint deshalb moglich, V. lecanii, das gegen Blattlause eingesetzt wird, gegen P. horiana anzuwenden. V. lecanii und A. album befielen auch andere mikrozyklische Rostpilze, jedoch in geringerem Ausmas: Puccinia dianthi (82 und 88%), P. malvacearum (71 und 60%) und P. glomerata (57 und 61%). Cladosporium sphaerospermum und C uredinicola waren gegen alle gepriiften Rostpilze weniger wirksam als V. lecanii und A. album.

Hyperparasitism of Puccinia horiana and other microcyclic rusts.

In a location with cypress spurge (Eupborbia cyparissias L. #3 EPHCY) and naturally occurring Uromyces scutellatus (Pers.) Lev. s.str., the percent of shoots deformed by this systemic rust increased from 6% in 1981 to 48% in 1982, followed by a decrease in both deformed and normal shoots in 1983 and 1984. After inoculation in 1981 of two cypress spurge crowns at a location near Zurich by teliospores of Uromyces alpestris Tranzsch., more than 80% deformed shoots were obtained in 1983 and 1984. Inoculations of cypress spurge root segnents by U. scutellatus s.str. or U. alpestris in experimental plots in 1982 gave 50 to 100% infected plants, and in most cases more than 50% deformed shoots in 1984. The collection of U. alpestris studied (E-52) could be a promising candidate for mycocontrol of cypress spurge because of its pathogenicity and specificity. Additional index words. Weed mycocontrol, rust fungi, Uromyces alpestris, EPHCY.

H. Kern

Papers

Naturally occurring fluorescent pseudomonads involved in suppression of black root rot of tobacco

Induction of Fusarium solani mutants insensitive to tomatine, their pathogenicity and aggressiveness to tomato fruits and pea plants

Genetic analysis of tomatine insensitivity, sterol content and pathogenicity for green tomato fruits in mutants of Fusarium solani

Hyperparasitism of Puccinia horiana and other microcyclic rusts.

Potential control of weedy spurges by the rust Uromyces scutellatus