Effect of seed and root exudates on the growth ofXanthomonas phaseoli var.fuscans

In a medium containing bean, barley and wheat seed exudates,Xanthomonas phaseoli var.fuscans (Burk.) Starr et Burk. grew substantially better than in that containing root exudates of these plants. When the bacteria were cultivated in a medium containing root exudates of bean plants deprived of cotyledons after eleven days of growth, growth was slower than in the presence of root exudates of control plants. On the other hand, the growth was stimulated in a medium containing root exudates of bean plants deprived of leaves. It was found that seed exudates of these plants contained biologically active peptides stimulating the growth of the microorganism. These peptides were not found in root exudates. These findings suggest a relationship between the survival ofXanthomonas phaseoli var.fuscans in the rhizosphere of bean and the exudation of biologically active peptides originating from the stock substances of seeds and cotyledons.     

Evidence Suggesting Protozoan Predation on Rhizobium Associated with Germinating Seeds and in the Rhizosphere of Beans (Phaseolus vulgaris L.)

Changes in populations of microorganisms around germinating bean (Phaseolus vulgaris L.) seeds, in the rhizosphere of bean, and in a model rhizosphere were studied. Strains of Rhizobium phaseoli that were resistant to streptomycin and thiram were used, and as few as 300 R. phaseoli cells per g of soil could be enumerated with a selective medium that was devised. A direct role was not evident for bacterial competitors, lytic bacteria, antibiotic-producing microorganisms, bacteriophages, and Bdellovibrio in the suppression of R. phaseoli around germinating seeds and in the rhizosphere. Protozoa increased in numbers in the soil upon planting of the seeds. The extent of colonization of soil by R. phaseoli was inversely related to the presence of large numbers of bacteria and protozoa. Colonization of R. phaseoli was improved upon suppression of protozoa with thiram and also when the soil was amended with other protozoan inhibitors and mannitol to simulate seed and root exudation. The data support the view that the decrease in numbers of R. phaseoli is caused by an increase in protozoan predation, the protozoa increasing in number because they prey on bacteria that proliferate by using seed and root exudates as nutrients.     

The Inhibition of Rust Fungus Growth in Allopurinol-Treated Bean Plants: Indirect Evidence for the Involvement of Xanthine Oxidoreductase in the Biotrophism of Uromyces phaseoli

Allopurinol [4-hydroxypyrazolo(3,4-d)pyrimidine], a specific, potent inhibitor of xanthine oxidoreductase, effective in vitro and in vivo, was applied to bean plants as soil drench at a 400 μM concentration 8–10 days before inoculation and strongly reduced the development of Uromyces phaseoli in bean leaves. Allopurinol was ineffective on uredospore germination, presumably due to the absence of any xanthine oxidoreductase activity in the extract of germinated uredospores. The concentration of allopurinol used for the treatment did not significantly influence the level of ureides in leaves mainly because low concentration of these compounds were found in leaves and also probably because allopurinol-insensitive biosynthetic route/s of these compounds are active in bean plants. This paper examines the possibility that host xanthine oxidase is in some way involved in the biotrophic nutritional process leading to the growth of bean rust fungus.     

Fungicide Enhancement of Nitrogen Fixation and Colonization of Phaseolus vulgaris by Rhizobium phaseoli

The number and weight of pods and the weight and nitrogen content of the tops of beans (Phaseolus vulgaris) derived from seeds inoculated with a thiram-resistant strain of Rhizobium phaseoli were increased if the seeds were treated with thiram before sowing in soil. A greater percentage of the nodules on 21-day-old plants were derived from the resistant strain, more nodules were formed, and these nodules were more effective in the presence of the fungicide than in its absence. These differences in nodule numbers were no longer present in 56-day-old plants, and only a small percentage of the nodules contained the resistant strain. The abundance of the fungicide-tolerant R. phaseoli increased rapidly soon after planting the seed and subsequently fell markedly, but the rate of decline was less if the seeds had been treated with the chemical. Protozoa also proliferated if thiram had not been applied to the seed, but their numbers were deleteriously influenced by thiram. Bdellovibrio, bacteriophages, and lytic micro-organisms acting on R. phaseoli were rare under these conditions. Ciliates and flagellated protozoa were initially suppressed by planting thiram-coated bean seeds in nonsterile soil, but the former were inhibited longer than the latter and the ciliate numbers never fully recovered if the seeds were treated with the fungicide. The resistant strain grew well in sterile soil also inoculated with a protozoa-free mixture of soil microorganisms whether thiram was added or not, but after an initial rise in numbers, its abundance fell if the mixture contained protozoa; the rate of this fall was delayed by the fungicide. The numbers of R. phaseoli were consistently less in sterile soil inoculated with the rhizobium plus a mixture of soil microorganisms containing ciliates and other protozoa than if the inoculum contained other protozoa but no ciliates. These results suggest that a suppression of protozoa, and possibly especially the ciliates, accounts for the enhanced growth of beans and the greater initial frequency of nodules formed by the thiram-resistant R. phaseoli in the presence of this fungicide. Thiram applied to uninoculated seed enhanced bean growth if thiram-resistant R. phaseoli were present in soil.     

Diversity of Rhizobium-Phaseolus vulgaris symbiosis: overview and perspectives

Common bean (Phaseolus vulgaris) has become a cosmopolitan crop, but was originally domesticated in the Americas and has been grown in Latin America for several thousand years. Consequently an enormous diversity of bean nodulating bacteria have developed and in the centers of origin the predominant species in bean nodules is R. etli. In some areas of Latin America, inoculation, which normally promotes nodulation and nitrogen fixation is hampered by the prevalence of native strains. Many other species in addition to R. etli have been found in bean nodules in regions where bean has been introduced. Some of these species such as R. leguminosarum bv. phaseoli, R. gallicum bv. phaseoli and R. giardinii bv. phaseoli might have arisen by acquiring the phaseoli plasmid from R. etli. Others, like R. tropici, are well adapted to acid soils and high temperatures and are good inoculants for bean under these conditions. The large number of rhizobia species capable of nodulating bean supports that bean is a promiscuous host and a diversity of bean-rhizobia interactions exists. Large ranges of dinitrogen fixing capabilities have been documented among bean cultivars and commercial beans have the lowest values among legume crops. Knowledge on bean symbiosis is still incipient but could help to improve bean biological nitrogen fixation.     

Contest and scramble competitions in two bruchid species,Callosobruchus analis andC. Phaseoli (Coleoptera: Bruchidae)

We performed multiple-generation competition experiments between Callosobruchus analis and C. phaseoli with different bean sizes. In each system, we supplied 5 g of mung beans (Vigna radiata) every 10 days. We examined three types of bean conditions: 5 g of large beans, 5 g of small beans, and a mixture of 2.5 g of large and small beans. In small bean condition, C. analis dominated C. phaseoli in all three replicates and C. phaseoli was extinct by the 260th day. On the contrary, C. phaseoli overcame C. analis within 250 days in large beans in all three replicates. In mixed beans condition the two bruchid species coexisted more than 500 days in two out of the three replicates. Even in the exceptional case, both species coexisted for 460 days. These results were examined in the light of the predictions from short-term larval competition experiments and a game theoretical model by Smith and Lessells (1985). The density and frequency dependent results during larval competition inside a bean was concluded to be a main factor to produce the above long-term competition results.     

EXPERIMENTS IN THE SUDAN GEZIRA ON CONTROL OF WILT OF DOLICHOS BEAN (DOLICHOS LABLAB) ASSOCIATED WITH ATTACK BY COCKCHAFER GRUBS (SCHIZONYCHA SP.)

Wilt of dolichos bean ( Dolichos lablab ) in the Sudan Gezira appeared to be due primarily to cockchafer grubs ( Schizonycha sp.) in the soil attacking the hypocotyls or roots of plants up to about 6 weeks after sowing. Many wilted plants also showed symptoms of ashy stem blight ( Macrophomina phaseoli ), but in these the fungus was probably a secondary invader rotting roots weakened or damaged by unfavourable soil conditions or insects. Wilt was often severe in dolichos sown on land cropped to sorghum ( Sorghum vulgare ) in the preceding season, sorghum roots in the soil harbouring the grubs. Under moderate grub attack seed dressings containing organomercurial and γ-benzene hexachloride (BHC) gave satisfactory-protection at 0.089%γ-BHC/seed but were inadequate when wilt was severe. Dieldrin and aldrin at 0.044 or 0.089%/seed gave excellent protection in all experiments, but their performance under exceptionally severe wilt conditions has yet to be tested, as also the relative efficiencies of seed treatment and soil treatment under such conditions. Fungicide-insecticide seed treatment also reduced preemergence rotting of germinating seeds due to grub attack.
Wilt rarely occurred in dolichos planted on land fallow in the preceding season, but, even in the apparent absence of wilt, seed treatment often appreciably improved emergence, plant populations, growth and yields. These effects possibly resulted from control of root damage by soil insects, such damage reducing growth and yields but not sufficiently severely to cause wilting. Pending further investigation a powder seed dressing containing organomercurial plus 20% dieldrin, and applied at 1:450 by weight to seed (about 5–6 g./acre of dieldrin), is recommended for dolichos bean in the Gezira.     

Multiplication of Xanthomonas campestris pv. Phaseoli and Intercellular Enzyme Activities in Resistant and Susceptible Beans

In order to assess resistance to common bean blight, populations of two isolates of Xanthomonas campestris pv. phaseoli were monitored in leaves of two Phaseolus vulgaris breeding lines, BLT87-2 (susceptible) and OAC88-1 (partially resistant) and a resistant tepary bean accession, P. acutifolius P. I. 440795. The breeding line OAC88-1 possesses resistance to common bacterial blight which was incorporated from P. acutifolius by an interspecific cross. In susceptible, leaves, bacterial populations increased to 10⁸ CFU/g leaf at 3 wk after inoculation whereas, in resistant leaves, bacterial populations declined to 10¹ - 10³ CFU/g leaf. In partially resistant leaves the population first declined similar to that in resistant P. acutifolius but later increased, and typical bacterial blight symptoms appeared. Cellulase, protease and amylase activities were monitored in culture and intercellular leaf spaces. Only cellulase activity was, clearly related to bacterial growth in the susceptible host; other enzyme activities were variable in their relationship to host resistance and bacterial growth. Differences between strains in cellulase activity inside partially resistant leaves corresponded to their ability to secrete cellulase in culture. Measuring cellulase activity in intercellular wash fluids may be a simple and sensitive method for determining X. c. phaseoli populations in leaves.     

Identification and survival of the causal organism of leaf smut disease of cowpea in Nigeria

Protomycopsis phaseoli (Ramak and Subram) is the causal agent of the cowpea leaf smut disease in Nigeria and not Entyloma vignae as claimed by some authors. This pathogen formed dark ash-grey to sooty-black lesions of 3–10 mm in diameter, while young lesions had yellow haloes. P. phaseoli produced dark reddish-brown chlamydospores that are globose to oval measured 23.8 μm, thick-walled and rugose. The chlamydospores germinated and produced globose vesicles. The pathogen grew on potato dextrose agar only when the leaf tissue was dipped in acidified water (1% H₂SO₄). The organism was slowly growing at 24–28 °C with snow white colour. Chlamydospores of P. phaseoli in infected cowpea leaves survived longer when buried in the soil for five months than when they were left on the soil surface for the same period at temperatures (26–27 °C) and humidity (70–82%) prevailing in Ibadan. Destruction of leaf debris before crop emergence, long period of rotation and no tillage cropping are suggested to prevent the onset and spread of leaf smut disease of cowpea. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Translocation of 14C-Labelled Assimilates in Dwarf Bean Plants Infected with Xanthomonas phaseoli (E.F.Sm.) Dowson

The distribution of ¹⁴C-labelled assimilates in dwarf beansinfected on one unifoliate with Xanthomonas phaseoli or systemicallyinfected with the pathogen was compared with the distributionin plants subjected to water stress. Unifoliates on systemically-infectedand water-stressed plants retained a much greater proportionof assimilated ¹⁴CO₂ than equivalent leaves on healthy plants,but unifoliates directly inoculated with X. phaseoli (localinfection) retained only a slightly larger proportion than controlplants. Local infection had little influence on the distribution ofexported radiocarbon but systemic infection produced markedeffects and there were similar changes in water-stressed plants.These changes seem to be related mainly to changes in growthpatterns, particularly to the reduced development of trifoliateleaves. The main causes of the altered assimilate distributionappear to be modified source-sink relationships, but the natureof the vascular system of dwarf bean and interference with vascularfunction by X. phaseoli seem to be more significant factorsthan in other diseases where assimilate distribution has beenstudied.     

Epidemiological studies on jute diseases

Summary Hyphae of M. phaseoli failed to grow on unsterilized natural soil and were completely lysed within 4 days of exposure. Germination of sclerotia in natural soil was inhibited indicating soil fungistasis. Lysis of mycelium and inhibition of germination of sclerotia could be annulled by addition of various organic nutrients and fertilizers to natural soil or by autoclaving the soil. Germination of dormant sclerotia in natural soil was stimulated by root-exudates of host and non-host plants. Population of sclerotia buried in unsterilized natural soil gradually declined and after 15 months only 35 per cent of the initial number could be recovered; more than 80 per cent of these germinated when nutrients were added. Data suggest poor saprophytic ability of M. phaseoli in mycelial form and the involvement of dormant sclerotia in the survival of the organism in soil.     

Occurrence of <Emphasis Type="Italic">Xanthomonas axonopodis</Emphasis> pv. <Emphasis Type="Italic">phaseoli</Emphasis>, the causal agent of common bacterial blight disease,on seeds of common bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.) in upper Egypt

Common bean seed lots collected from different seed dealers and Malawii agriculture station were screened for the presence of Xanthomonas axonopodis pv. phaseoli. In the laboratory the pathogen was isolated following the routine laboratory assay method, i.e. direct plating method using yeast extract-dextrose-calcium carbonate agar medium (YDC). Yellow, convex, mucoid colonies of Xanthomonas were consistently isolated on YDC from seed samples. The presumptive pathogen was confirmed by isolation on semiselective medium, such as mTBM and MD5A. Further, the pathogen was confirmed by biochemical, physiological and, finally, the pathogenicity tests. Five samples out of seven were positive for Xanthomonas. The isolates were found to cause common blight of 3-week-old common bean plants by 7 d after inoculation. Bacteria with the same characteristics as those inoculated were re-isolated from the infected plants.     

Genotypic Characterization of the Common Bean Bacterial Blight Pathogens, Xanthomonas axonopodis pv. phaseoli and Xanthomonas axonopodis pv. phaseoli var. fuscans by rep-PCR and PCR–RFLP of the Ribosomal Genes

Common bacterial blight (CBB), caused by Xanthomonas axonopodis pv. phaseoli and X. axonopodis pv. phaseoli var. fuscans is one of the most destructive diseases of common bean worldwide. The interrelatedness, genetic diversity and geographical distribution of the CBB pathogens was assessed using restriction fragment length polymorphism (RFLP) analysis of polymerase chain reaction amplified 16S ribosomal gene, including the 16S–23S intergenic spacer region and repetitive element PCR (rep‐PCR). RFLP profiles generated by the restriction endonucleases MboI, RsaI and HaeIII differentiated X. axonopodis pv. phaseoli from X. axonopodis pv. phaseoli var. fuscans and non‐pathogenic Xanthomonas species associated with common bean. Cluster analysis of rep‐PCR profiles revealed a high level of genetic differentiation (G_ST = 0.56) between the two CBB pathogens, showing that they are genetically distinct. Significant levels of genetic diversity were observed within each strain, indicating that the two bacteria are not clonal. More genetic diversity was observed in X. axonopodis pv. phaseoli (H = 0.134; I = 0.223) than X. axonopodis pv. phaseoli var. fuscans (H = 0.108; I = 0.184). However, no geographical differentiation was evident for either X. axonopodis pv. phaseoli var. fuscans (GST = 0.013) or X. axonopodis pv. phaseoli (G_ST = 0.017). This lack of geographical differentiation has important practical implications, as available host resistance genes are likely to be effective in controlling the disease in diverse geographical areas.     

Biological control of common blight of bean (Phaseolus vulgaris) causedby Xanthomonas axonopodis pv. phaseoli by using the bacteriumRahnella aquatilis

The aim of this study was to evaluate the bacterium Rahnella aquatilis (Ra) for protection of bean plants against common blight disease caused by Xanthomonas axonopodis pv. phaseoli (Xap). Xap isolates were isolated from a naturally blighted leaves of bean plants grown in Assiut governorate. The blight symptoms were produced by all three isolates, but the isolates differed in their degree of the pathogenicity. Xap1 was the most virulence one against bean plants. The effect of Ra against common blight of bean plant was tested. In vitro studies, Ra exhibited inhibitor effect against the pathogen. Under greenhouse and field conditions, beanvariety “Giza 6” treated by Ra resulted in marked disease suppression. Ahigh decrease of the disease was correlated with a reduction of the bacterial multiplication. In physiological studies, bean plants treated by Ra exhibited higher phenolic compounds contents and higher activity of peroxidase (PO) enzyme than untreated plants. In conclusion, application of Ra was effective and could be recommended for controlling the bean common blight disease.     

The effect of Rhizobium inoculation and nitrogen fertiliser on nitrogen fixation and seed yield of dry beans (Phaseolus vulgaris)

The potential benefit to be derived from seed inoculation of Phaseolus vulgaris beans with effective strains of Rhizobium phaseoli, was investigated in field experiments over three years on a site low in soil nitrogen and lacking indigenous effective strains of R. phaseoli. Inoculation with R. phaseoli (strain RCR 3644) produced significant increases in nodulation, nitrogenase activity and plant growth in all experiments. In trials in 1978 and 1979, with cv. Seafarer, inoculation, in the absence of nitrogen fertiliser doubled seed yields. In 1978, the seed yields from inoculated beans without nitrogen fertiliser (1–6 t/ha) were not significantly different from those obtained with uninoculated beans receiving the optimum nitrogen fertiliser treatment of 120 kg N/ha (1–75 t/ha). In 1979, with lower rainfall favouring more efficient utilisation of nitrogen fertiliser, inoculation gave seed yields (1–88 t/ha) equivalent to those obtained with 60 kg N/ha (1–70 t/ha) but significantly less than with 120 kg N/ha (2–88 t/ha). More precise estimates from nitrogen response curves showed that inoculation supplied the fertiliser equivalent of 105 and 70 kg N/ha in 1978 and 1979 respectively. In both years, significant benefits were also obtained by the combination of inoculation and nitrogen fertiliser. In a separate experiment in 1979, with four R. phaseoli strains inoculated onto eight bean cultivars, three were highly effective nitrogen fixers on all cultivars. Two strains (RCR 3644 and NVRS 963A) each increased mean yields, in the absence of nitrogen fertiliser, from 1–39 t/ha uninoculated to c. 2–5 t/ha inoculated whilst strain RCR 3622 was outstanding with a mean yield of 3-0 t/ha. An analysis of the nitrogen content of seed showed that gains from nitrogen fixation were 37–57 kg N/ha/growing cycle for the combination RCR 3644 with cv. Seafarer. However, 106 kg N/ha/growing cycle was recorded for the combination RCR 3622 and cv. Aurora.     

B. thuringiensis is a Poor Colonist of Leaf Surfaces

The ability of several Bacillus thuringiensis strains to colonize plant surfaces was assessed and compared with that of more common epiphytic bacteria. While all B. thuringiensis strains multiplied to some extent after inoculation on bean plants, their maximum epiphytic population sizes of 10⁶ cfu/g of leaf were always much less than that achieved by other resident epiphytic bacteria or an epiphytically fit Pseudomonas fluorescens strain, which attained population sizes of about 10⁷ cfu/g of leaf. However B. thuringiensis strains exhibited much less decline in culturable populations upon imposition of desiccation stress than did other resident bacteria or an inoculated P. fluorescens strain, and most cells were in a spore form soon after inoculation onto plants. B. thuringiensis strains produced commercially for insect control were not less epiphytically fit than strains recently isolated from leaf surfaces. The growth of B. thuringiensis was not affected by the presence of Pseudomonas syringae when co-inoculated, and vice versa. B. thuringiensis strains harboring a green fluorescent protein marker gene did not form large cell aggregates, were not associated with other epiphytic bacteria, and were not found associated with leaf structures, such as stomata, trichomes, or veins when directly observed on bean leaves by epifluorescent microscopy. Thus, B. thuringiensis appears unable to grow extensively on leaves and its common isolation from plants may reflect immigration from more abundant reservoirs elsewhere.     

Influence of Temperature, Wetness Duration, and Leaf Type on the Quantification of Monocyclic Parameters of Bean Rust

Monocyclic parameters of bean rust (Uromyces phaseoli var. typical) were quantified in growth chambers, on rwo bean cultivars for three temperatures (17, 21, and 25 °C), two types of leaves (unifoliolate and trifoiiolate leaves), and nine leaf wetness periods (0, 4, 7, 10, 13, 16, 19, 22, and 25 hrs). The expression of disease was greatly influenced by past-inoculation temperatures. The incubation and latent periods were shortest at 21 °C for both cultivars and leaf types. For both cultivars, trifoiiolate leaves were more susceptible than unifoliolate leaves. A wetness period of at least four hours was required for disease to occur. The maximum disease efficiency for both cultivars occurred with 22 hrs of leaf wetness at 17 °C. The disease efficiencies for temperatures of 17–29 °C and leaf wetness periods of 0–25 hrs were adequately described by a response-surface model. Because of the great influence of temperature and leaf wetness on infection, bean rust is unlikely to occur at high temperatures (> 25°C) and short leaf wetness periods (< 7 hrs).     

Origin of sterols in uredospores of Uromyces phaseoli

The sterols from healthy bean leaves are β-sitosterol, stigmasterol, campesterol and 28-isofucosterol. An additional sterol observed in bean leaves infected with Uromyces phaseoli was identified as 7,(Z)-24(28)-stigmastadien-3β-ol, which is the major sterol of the uredospores of the fungus. The fungus appears to stimulate sterol synthesis, but most of the increased sterol content of infected leaves can be attributed to the sterol of the uredospores.     

Contest and scramble competitions in two bruchid species,Callosobruchus analis andC. phaseoli (Coleoptera: Bruchidae)

Larval competition between contest and scramble strategists was investigated using the two bruchid species, C. analis (contest species) and C. phaseoli (scramble species) with two different sized mung beans (large and small beans). In both sized beans, the adult emergences of each species dependen on total density of the initial larval densities of the two species and the ratio of the two densities. The emergence of one species was suppressed by the existence of the other species when the initial larval density per bean of the former species was less than that of the latter one. There were many cases in which both C. analis and C. phaseoli emerged from one bean in large beans, but such cases were quite rare in small beans. C. analis performed interference behavior only at late larval stages, whereas C. phaseoli was superior in exploitative competition all through their larval stages. These, combined with the niche segregation inside a bean, are throught to be the major factors of observed density- and frequency-dependent competition results. Based on the above experimental results, long-term competition results between the contest and scramble species were predicted.     

Studies of the infection of the winged bean by Synchytrium psophocarpi in Papua New Guinea

Infection of leaves and stems of Psophocarpus tetragonolobus by Synchytrium psophocarpi only occurred following spray inoculation of motile zoospore suspensions and incubation for a minimum of 12 h in polyethylene bags or a mist chamber. The incubation period was 7 days and generation time 22 days at temperatures of 31 ^oC max, 24 ^oC min and r.h. of 90% max, 70% min. Young, 1–2 day-old leaves were most susceptible; there was no infection on 10 day-old leaves and susceptibility was not increased by the removal of leaf waxes. No infection occurred when plants were grown from seed from infected pods, seed inoculated with zoospores or sporangia and seed sown in soil containing infected leaf debris. Resting spores were not found in infected tissues stored for 12 wk or in plant debris. S. psophocarpi did not infect Arachis hypogaea, Glycine max, Phaseolus aureus, P. coccineus, P. vulgaris, Pisum sativum, Psophocarpus scandens, Vicia faba, Vigna sesquipedalis and V. unguiculata. S. minutum did not infect winged bean. Inoculation confirmed the susceptibility of the winged bean lines UPS 62, UPS 122, UPS 126 and resistance of two Thai winged bean lines 1602/1 and 1611/2.