Biological Control of Crown Gall: Seed Inoculation

S ummary : Peach seeds were inoculated with the nonpathogenic isolate 84 of Agrobacterium radiobacter var. radiobacter biotype 2 before sowing in natural soil heavily inoculated with the tumour inducing biotype 2 isolate 27 ( A. radiobacter var. tumefaciens ). Nonpathogens (presumably isolate 84) became predominant in the biotype 2 population on roots, on underground stems and in the soil round plant crowns. Significant ( P < 0·001) biological control of crown gall was achieved. Total gall incidence on plants grown from inoculated seed was 31% and from uninoculated seed 79%; the corresponding gall incidence on plant crowns was 12% and 76%. Dusting seed with Thiram (3·1 g/kg seed) did not significantly reduce disease incidence. Infection appeared to occur through undamaged lenticels indicating that wounding is not a necessary prerequisite for crown gall induction in peach.     

Population Heterogeneity of Agrobacterium tumefaciens in Galls of Populus L. from a Single Nursery

This study focused on the natural crown gall infections occurring in a Leuce poplar nursery. Soil effects on crown gall frequency were detected, indicating that contamination was due to a resident Agrobacterium tumefaciens population, which was present before seedling plantation. The crown gall frequency on poplar progenies varied from 3 to 67%, indicating the feasibility of improvement in crown gall resistance. Of 129 tumor isolates, 128 were pathogenic. These isolates were of biotype 1 or 2. Biochemical, serological, and antibiotic resistance typing results concurred, indicating the presence of four biotype 1 and two biotype 2 resident subpopulations. No significant change was noticed in the relative proportions of subpopulations from one year to another. Pathogenic subpopulations both in vitro and in planta were susceptible to Kerr K84 (P. B. New and A. Kerr, J. Appl. Bacteriol. 90:172-179, 1972). In addition, no serological cross-reactions were found to occur between K84 and the pathogenic subpopulations.     

New Antagonistic Strains of Non‐Pathogenic Agrobacterium vitis to Control Grapevine Crown Gall

Graft unions of nursery stock of grapevine (Vitis vinifera L.) collected in Japan yielded non‐pathogenic strains of Agrobacterium. On the basis of classic diagnostic tests, a sequence analysis and a previously reported multiplex PCR method, the non‐pathogenic strains ARK‐1, ARK‐2 and ARK‐3 were identified as Agrobacterium vitis. Stems of grapevine seedlings were inoculated with both a cell suspension of seven mixed strains of A. vitis (Ti) as a pathogen and one of a new strain or A. vitis strain VAR03‐1, one of the biological control agents against crown gall previously reported, as competitors to assay the suppression of tumour formation caused by the pathogen. In a test with a 1:1 cell ratio of pathogen/nonpathogen, strains ARK‐1, ARK‐2 and ARK‐3 reduced the tumour incidence.. In particular, strain ARK‐1 was strongest at inhibiting tumour formation in this study. Strain ARK‐1 established populations on roots of grapevine tree rootstock and persisted on roots for a year. ARK‐1, ARK‐2 and ARK‐3 did not produce a halo of inhibition against A. vitis (Ti) strain on YMA medium. Moreover, strain ARK‐1 did not reduce tumour incidence on the stems of grapevine when ARK‐1 was dead or only culture filtrate was used. This result indicates the possibility that these new strains inhibit grapevine crown gall in planta by a different mechanism other than VAR03‐1. In particular, one of the new strains, named ARK‐1, was most effective in inhibiting tumour formation on grapevine and appears to be a promising new agent to control grapevine crown gall.     

Regulation of Pectate Lyase Synthesis in Pseudomonas fluorescens and Erwinia carotovora

Inducible synthesis of extracellular pectate lyase occurs in Erwinia carotovora, a bacterial soft-rot pathogen of plants, and, to a lesser extent, in a nonpathogenic isolate of Pseudomonas fluorescens. A combination of pectin and a heat-labile factor in fresh potato tissue or acetone powders of the tissue provided the best carbon source for induction. Yields of inducible pectate lyase were much greater than those usually reported. The pathogen, but not the saprophyte, produced a small amount of constitutive enzyme when grown on glucose. The relatively low level or absence of constitutive synthesis in these bacteria did not result from catabolite repression. Attempts were made to relieve any existing catabolite repression by restricting growth through slow feeding of glucose or by growing the organisms on glycerol. These conditions did not significantly alter the differential rate of lyase synthesis compared with changes observed in the presence of inducers. Previous growth history did not affect induction in the pathogen. However, P. fluorescens previously cultured on glucose required 10 to 20 generations of growth on inducing medium before appreciable lyase synthesis occurred. Differences between the pathogen and nonpathogen suggest that regulation of pectate lyase synthesis is related to pathogenicity of soft-rot bacteria.     

Modeling competition for infection sites on roots by nonpathogenic strains of Fusarium oxysporum

By use of plane and solid geometry and probability models, efficiencies of infection and competition for nutrients and infection sites by a nonpathogenic strain of Fusarium oxysporum (C14) with F. oxysporum f. sp. cucumerinum on the rhizoplane of cucumber were calculated. The model is derived from previously published data. Efficiencies for successful infection were 0.04 chlamydospores per infection site for both pathogen and nonpathogen. Observed successful infections by the pathogen in competition with the nonpathogen were close in values to the competition ratio (CR) calculated as the number of chlamydospores on the infection court of the pathogen divided by the total number of both pathogen and nonpathogen at relatively low densities. When total chlamydospores were, on average, closer than 175 μm apart, however, competition for nutrients/mutual inhibition occurred. At such densities there was an overestimation of the effect of competition for infection sites. These relationships were modeled at inoculum densities of pathogen and/or nonpathogen of 5000 chlamydospores per g soil and above, however, in the field, maximum densities of 1000 colony forming units/g (cfu) were observed. Most likely models of competition for infection sites at this density of the pathogen revealed that infection efficiency was only approximately halved, even when 0.98 of the possible 30 infection sites were occupied by the nonpathogen. It is conclude that competition for nutrients and/or infection sites is an insignificant factor in biocontrol of Fusarium wilt diseases by nonpathogenic fusaria.     

Seed Transmission of Pea Seed-borne Mosaic Virus in Peas: Early and Late Expression of the Virus in the Progeny

Abstract Field trials at Summerland and Creston in 1984 and 1985 on loam soils naturally intested with Agrobadonum nonfactures showed that A radiobacter Strain 84 K-S4, did not control crown gall on Antonovka apple seedlingy when applied as a root-dip. K-84 controiled crown gall at Sidney on sandy loam soil m 1984 and 1985, but tailed in 1986. At Creston, higher than standard concentrations of K-84 did not control crown gall of seedlings, whether the seedlings were grown in unsterililzed soil 1986) or in sterile soil, 1987 before transplanting them into A nonfactures naturally intested soil. It a field trial in 1986 at Summerland, root-dip treatment with K-84 alone or in combination with broadeast treatments of melam-sodium of formalin tailed to control crown gall on Antonovka seedlings. However, crown gall intection was significantly reduced with metam-sodium or K-84 treatment alone when seedlings were raised in sterile soil before transplanting in naturally instead safe at Summerland in 1987.     

Cocolonization of the Rhizosphere by Pathogenic Agrobacterium Strains and Nonpathogenic Strains K84 and K1026, Used for Crown Gall Biocontrol

The crown gall biocontrol agent strain K84 and three mutants derived from it, K1026 (Tra⁻ deletion mutant of pAgK84), K84 Agr⁻ (lacking pAgK84), and K1143 (lacking pAgK84 and pNoc), significantly reduced gall formation caused by two pathogenic strains resistant to agrocin 84 in peach × almond seedlings planted in infested soil. Cocolonization of roots by pathogenic and nonpathogenic strains was observed in these biocontrol experiments under field conditions. In spite of the efficient biocontrol observed, average populations consisting of 10² and 10⁶ pathogenic agrobacteria per g of root were found 8 months after planting. The total numbers of pathogenic bacteria on roots were similar for plants treated with the biocontrol strains and for the untreated plants. Strain K84 and the genetically engineered organism K1026 survived at a level of 10⁶ agrocin 84-producing bacteria per g of root. The population size of genetically engineered strain K1026 was not significantly different than the population size of wild-type strain K84 8 months after root inoculation. Strains K84 and K1026 controlled two pathogens resistant to agrocin 84 without reducing the total number of pathogenic bacteria in the root system. In addition, this study shows that some biological control activity of strain K84 against agrocin 84-resistant pathogens is independent of plasmids pAgK84 and pNoc.     

Intraspecific Variation within Populations of Fusarium oxysporum Based on RFLP Analysis of the Intergenic Spacer Region of the rDNA

Appel, D. J., and Gordon, T. R. 1995. Intraspecific variation within populations of Fusarium oxysporum based on RFLP analysis of the intergenic spacer region of the rDNA. Experimental Mycology 19, 120-128. Fifty-six isolates of Fusarium oxysporum, including F. oxysporum f. sp. melonis and nonpathogenic strains, were chosen from a larger collection to represent diversity in vegetative compatibility groups (VCGs), mitochondrial DNA (mtDNA) haplotype, geographic distribution, and virulence. Using PCR, a 2.6-kb fragment including the intergenic spacer (IGS) region of the ribosomal DNA was amplified from each isolate. The enzymes EcoRI, Sau 3A, Cfo1, and Ava1I, cut this fragment differentially, revealing 5, 6, 6, and 7 patterns, respectively. Among the 56 isolates, a total of 13 unique IGS haplotypes was identified. Among most F. o. melonis isolates. IGS haplotype correlated with VCG and mtDNA haplotype, but did not differentiate among races. However, a race 1 isolate found in VCG 0131 shared virulence, mtDNA, and IGS haplotypes characteristic of VCG 0134; this isolate may represent a conversion in VCG from 0134 to 0131. Four nonpathogens shared the pathogen vegetative compatibility phenotypes. One race 1,2 isolate associated with VCG 0134 shared both IGS haplotype and VCG with a nonpathogen, but these isolates did not share the same mtDNA haplotype. Another nonpathogenic isolate shared mtDNA and IGS haplotypes with pathogen group 0131 and may simply be an avirulent mutant of a pathogenic strain. For the other two nonpathogenic isolates, vegetative compatibility indicated a close relationship to the pathogen, but differences in both mtDNA and IGS haplotype suggest otherwise. Overall, the IGS haplotype was more variable among the nonpathogenic F. oxysporum VCGs among which 12 of the 13 IGS haplotypes were found. Nonpathogenic isolates that shared a common mtDNA haplotype, but were associated with different VCGs, often had different IGS haplotypes.     

Induction of pathogen resistance and pathogenesis-related genes in tobacco by a heat-stable Trichoderma mycelial extract and plant signal messengers

Heat-stable mycelial extracts of the nonpathogenic fungus Trichoderma longibrachiatum induced resistance in tobacco seedlings ( Nicotiana tabacum L. cv. Wisconsin 38) to the pathogen Phytophthora parasitica var. nicotianae (race 0), which did not involve a hypersensitive response. Resistance could not be induced with mycelial extract prepared in the same manner from P. parasitica . The nonpathogenic mycelial extract induced expression of PR-1b and osmotin (PR-5) genes to a higher level than did mycelial extract from the pathogenic fungus. The tissue-specific pattern of PR gene induction by the nonpathogenic mycelial extract was different from that of the pathogenic mycelial extract and was consistent with the ability of the former to cause disease resistance. The expression patterns of these two PR genes and the accumulations of their encoded proteins also were affected by salicylic acid (SA), methyl jasmonate (MeJA), ethylene (E) and combinations of these plant signal messengers. However, only combined SA and MeJA treatment mimicked the pattern of PR gene mRNA and protein accumulation induced by the nonpathogenic mycelial extract. E inhibitors blocked both mycelial extract-induced and SA/MeJA-induced PR gene expression, and the cis pattern of responsiveness on the osmotin promoter was the same for the mycelial extract, SA, E, or E/MeJA. Seedlings treated with P. parasitica spores in the presence of SA/MeJA were protected from pathogen colonization. However, these seedlings exhibited symptoms of cell death (disease symptoms) both in the absence and presence of P. parasitica spores, in contrast to seedlings treated with nonpathogenic mycelial extract, which remained healthy. These results suggest that the signal transduction pathways for elicitation of defense responses by exogenously applied heat-stable nonpathogenic mycelial extract and SA/MeJA overlap at the point of PR protein induction but are not identical.     

<Emphasis Type="Italic">Fusarium sambucinum</Emphasis> isolate FS-94 induces resistance against fusarium wilt of tomato via activation and priming of a salicylic acid-dependent signaling system

The wheat rhizosphere-inhabiting nonpathogenic Fusarium sambucinum isolate FS-94 protected tomato from Fusarium wilt (F. oxysporum f. sp. lycopersici) in laboratory experiments. Seed soaking or immersion of seedling roots in a FS-94 spore suspension prior to inoculation with the pathogen delayed the appearance of wilt symptoms and significantly reduced disease severity in plants of a susceptible tomato cultivar. Quantification of fungal ergosterol in infected tomato showed that protection against wilt agent was related to limitation of the pathogen growth in plants exposed to FS-94. Incubation of tomato seedlings in a FS-94 spore suspension for 48 or 72 h led to plant protection and increased the salicylic acid (SA) concentration in their roots, suggesting that this isolate was involved in a plant-mediated mode of action and induced resistance. Soaking tomato seeds in the spore suspension did not induce SA accumulation in seedling roots, but nevertheless resulted in a significant reduction in wilt severity when the seedlings were challenged with the pathogen. In response to pathogen attack, the SA content in susceptible seedlings grown from FS-94-treated seeds started to increase within 1 day and remained elevated for 72 h. This suggests that F. sambucinum isolate FS-94 primed a SA-dependent signaling system in tomato.     

Hypersensitive reaction and induced resistance in rice against the Asian rice gall midge Orseolia oryzae

Rice seedlings of the resistant variety Phalguna showed premature tillering, browning of central leaf, and tissue necrosis at the apical meristem following artificial infestation with avirulent biotype 1 of the Asian rice gall midge, Orseolia oryzae (Wood-Mason) (Diptera: Cecidomyiidae). Tissue necrosis representing a typical hypersensitive reaction (HR), accompanied by maggot mortality, was observed within 4 days after infestation. However, reinfestation of secondary tillers subsequent to HR in primary tiller, did not lead to HR in secondary tillers though maggot mortality was seen. Artificial infestation with the weed gall midge O. fluvialis did not result in HR either in gall midge susceptible TN 1 or resistant Phalguna rice varieties. Resistance in Phalguna against the virulent biotype 4 could be induced by either prior, simultaneous, or subsequent infestation with the avirulent biotype 1. The duration of effectiveness of such induced resistance varied with the sequence and time lag between infestations.     

Biological control of crown gall, Agrobacterium tumefaciens

Biological control of crown gall caused by Agrobacteriurn turnefaciens (Smith & Townsend) Conn, pioneered by Dr A. Kerr in South Australia, is effected through the establishment of a high population of the related non-pathogen A. radiobacter (Beijerinck & van Delden) Conn, strain 84 in the rhizosphere of susceptible plants. Strain 84 produces a bacteriocin to which many strains of the pathogen in Australasia, North America and Britain are sensitive. The disease is present in Britain on a variety of hosts including cherry. At East Malling cherry leaf scars, invaluable as an avenue of infection for bacterial canker infectivity titrations, have been used successfully in crown gall studies. Live cells of strain 84, but neither an avirulent strain of the pathogen nor a soil bacterium highly antagonistic to A. tumefaciens in vitro, inhibited gall formation in cherry leaf scars. Heat-killed cells had no effect. In a field experiment at East Malling hardwood cuttings of the new rootstock Colt have been dipped in strain 84 and total inhibition of crown gall is expected to ensue. The results of other experiments where the disease is already established on cherry-rootstock layer-beds and in blackberry plantations are less predictable. In time we hope to solve this problem. Only time will show whether this method of biological control is long lasting or will eventually break down.     

Post‐gall induction performance of Adelges Abietis (L.) (Homoptera: Adelgidae) is influenced by clone,shoot length,and density of colonising gallicolae

1. We evaluated the effect of clone (one susceptible and one resistant clone), shoot length, crown level, and gallicola density on post‐gall induction performance of Adelges abietis. Galls that had been successfully induced by one fundatrix on a range of shoot sizes were selected, and the number of gallicolae that could colonise the gall was manipulated. 2. Post‐induction gall development success was inversely related to shoot length and was higher on the susceptible clone than on the resistant clone. As gallicola density did not influence the proportion of galls that successfully completed development, reduced post‐induction gall development on large shoots was not likely to be result of an insufficient stimulus from gallicolae. 3. Clone was the only factor that significantly influenced gall volume and galls were larger on the susceptible clone than on the resistant clone. As gall volume did not increase when more gallicolae attempted to colonise a gall, competition within a gall increased. Gallicola survival was inversely related to the number of colonising gallicolae. Our results suggest that gall size may be limiting at natural densities. 4. Previous studies report positive relationships between gall induction success and fundatrix density, and between gall size and fundatrix density. As each fundatrix produces one egg mass of gallicolae, this study suggests that there may be a trade‐off between the successful induction of a large gall and subsequent survival of gallicolae. 5. In the present study, clone influenced all measures of post‐gall induction performance. Performance was always higher on the susceptible than on the resistant clone.     

Biological Control of Crown Gall in Grapevine and Raspberry by Two Pseudomonas spp. with a Wide Spectrum of Antagonistic Activity

Pseudomonas aureofaciens B-4117 and P. fluorescens CR330D inhibited the growth of a wide range of plant pathogens, including Agrobacterium tumefaciens , when tested on agar media. In a series of nursery-based trials with natural pathogen inoculum, application of either B-4117 or CR330D significantly reduced the incidence and severity of crown gall caused by A. tumefaciens on grapevine and raspberry. The extent of disease control depended upon the variety tested. Both bacteria reduced disease during seedling root production and grafting. The disease incidence on root cuttings of three grapevine varieties was reduced by 56-80% and the disease severity index (DSI) was decreased by 75-86%. Depending on the scion variety, the number of healthy rooted grafts increased by 2-3.5-fold, while the DSI was reduced by 1.5-3-fold. The results suggest that there is potential in using these antagonists to diminish the influence of latent rootstock infection on graft sensitivity to crown gall. Pretreatment of rooted raspberry seedlings with P. aureofaciens B-4117 prevented the development of crown galls caused by A. tumefaciens strain K24 or by a mixture of A. tumefaciens pathogenic strains previously isolated from raspberry. Both Pseudomonas spp. persisted on the root surfaces of inoculated vine cuttings and in non-sterile soil. The advantages of using the antagonistic bacteria as biocontrol agents of crown gall are discussed.     

Nutritional similarity between leaf-associated nonpathogenic bacteria and the pathogen is not predictive of efficacy in biological control of bacterial spot of tomato

It has been demonstrated that for a nonpathogenic, leaf-associated bacterium, effectiveness in the control of bacterial speck of tomato is correlated with the similarity in the nutritional needs of the nonpathogenic bacterium and the pathogen Pseudomonas syringae pv. tomato. This relationship was investigated further in this study by using the pathogen Xanthomonas campestris pv. vesicatoria, the causal agent of bacterial spot of tomato, and a collection of nonpathogenic bacteria isolated from tomato foliage. The effects of inoculation of tomato plants with one of 34 nonpathogenic bacteria prior to inoculation with the pathogen X. campestris pv. vesicatoria were quantified by determining (i) the reduction in disease severity (number of lesions per square centimeter) in greenhouse assays and (ii) the reduction in leaf surface pathogen population size (log(10) of the number of CFU per leaflet) in growth chamber assays. Nutritional similarity between the nonpathogenic bacteria and X. campestris pv. vesicatoria was quantified by using either niche overlap indices (NOI) or relatedness in cluster analyses based upon in vitro utilization of carbon or nitrogen sources reported to be present in tomato tissues or in Biolog GN plates. In contrast to studies with P. syringae pv. tomato, nutritional similarity between the nonpathogenic bacteria and the pathogen X. campestris pv. vesicatoria was not correlated with reductions in disease severity. Nutritional similarity was also not correlated with reductions in pathogen population size. Further, the percentage of reduction in leaf surface pathogen population size was not correlated with the percentage of reduction in disease severity, suggesting that the epiphytic population size of X. campestris pv. vesicatoria is not related to disease severity and that X. campestris pv. vesicatoria exhibits behavior in the phyllosphere prior to lesion formation that is different from that of P. syringae pv. tomato.     

根癌农杆菌转化紫草的研究

紫草 (ＬｉｔｈｏｓｐｅｒｍｕｍｅｒｙｔｈｒｏｒｈｉｚｏｎＳｉｅｂ .ｅｔＺｕｃｃ)是传统中药。其根部含有萘醌类化合物—紫草素及其衍生物 ,具有显著的抗菌、抗炎、抗癌以及促进伤口愈合等生理活性。紫草素同时也是一种名贵化妆品染料。科学家对紫草的研究兴趣是基于其资源的缺乏及紫草植物本身所具有的一些特点 ;如 :紫草素及其衍生物的颜色特性可凭借肉眼观察 ,紫草素及其衍生物只在紫草的根部积累 ,紫草素合成的次生代谢途径受多种酶和外界条件 (光照 ,营养等 )的调节等。紫草细胞培养 (Ｆｕｊｉｔａ等 ,1983;叶和春等 ,1991)可以产…     

Magnaporthe Grisea Genes for Pathogenicity and Virulence Identified through a Series of Backcrosses

We have identified genes for pathogenicity toward rice (Oryza sativa) and genes for virulence toward specific rice cultivars in the plant pathogenic fungus Magnaporthe grisea. A genetic cross was conducted between the weeping lovegrass (Eragrostis curvula) pathogen 4091-5-8, a highly fertile, hermaphroditic laboratory strain, and the rice pathogen O-135, a poorly fertile, female-sterile field isolate that infects weeping lovegrass as well as rice. A six-generation backcrossing scheme was then undertaken with the rice pathogen as the recurrent parent. One goal of these crosses was to generate rice pathogenic progeny with the high fertility characteristic of strain 4091-5-8, which would permit rigorous genetic analysis of rice pathogens. Therefore, progeny strains to be used as parents for backcross generations were chosen only on the basis of fertility. The ratios of pathogenic to nonpathogenic (and virulent to avirulent) progeny through the backcross generations suggested that the starting parent strains differ in two types of genes that control the ability to infect rice. First, they differ by polygenic factors that determine the extent of lesion development achieved by those progeny that infect rice. These genes do not appear to play a role in infection of weeping lovegrass because both parents and all progeny infect weeping lovegrass. Second, the parents differ by simple Mendelian determinants, ``avirulence genes,' that govern virulence toward specific rice cultivars in all-or-none fashion. Several crosses confirm the segregation of three unlinked avirulence genes, Avr1-CO39, Avr1-M201 and Avr1-YAMO, alleles of which determine avirulence on rice cultivars CO39, M201, and Yashiro-mochi, respectively. Interestingly, avirulence alleles of Avr1-CO39, Avr1-M201 and Avr1-YAMO were inherited from the parent strain 4091-5-8, which is a nonpathogen of rice. Middle repetitive DNA sequences (``MGR sequences'), present in approximately 40-50 copies in the genome of the rice pathogen parent, and in very low copy number in the genome of the nonpathogen of rice, were used as physical markers to monitor restoration of the rice pathogen genetic background during introgression of fertility. The introgression of highest levels of fertility into the most successful rice pathogen progeny was incomplete by the sixth generation, perhaps a consequence of genetic linkage between genes for fertility and genes for rice pathogenicity. One chromosomal DNA segment with MGR sequence homology appeared to be linked to the gene Avr1-CO39. Finally, many of the crosses described in this paper exhibited a characteristic common to many crosses involving M. grisea rice pathogen field isolates. That is, pigment-defective mutants frequently appeared among the progeny.     

Nutritional Similarity between Leaf-Associated Nonpathogenic Bacteria and the Pathogen Is Not Predictive of Efficacy in Biological Control of Bacterial Spot of Tomato

It has been demonstrated that for a nonpathogenic, leaf-associated bacterium, effectiveness in the control of bacterial speck of tomato is correlated with the similarity in the nutritional needs of the nonpathogenic bacterium and the pathogen Pseudomonas syringae pv. tomato. This relationship was investigated further in this study by using the pathogen Xanthomonas campestris pv. vesicatoria, the causal agent of bacterial spot of tomato, and a collection of nonpathogenic bacteria isolated from tomato foliage. The effects of inoculation of tomato plants with one of 34 nonpathogenic bacteria prior to inoculation with the pathogen X. campestris pv. vesicatoria were quantified by determining (i) the reduction in disease severity (number of lesions per square centimeter) in greenhouse assays and (ii) the reduction in leaf surface pathogen population size (log₁₀ of the number of CFU per leaflet) in growth chamber assays. Nutritional similarity between the nonpathogenic bacteria and X. campestris pv. vesicatoria was quantified by using either niche overlap indices (NOI) or relatedness in cluster analyses based upon in vitro utilization of carbon or nitrogen sources reported to be present in tomato tissues or in Biolog GN plates. In contrast to studies with P. syringae pv. tomato, nutritional similarity between the nonpathogenic bacteria and the pathogen X. campestris pv. vesicatoria was not correlated with reductions in disease severity. Nutritional similarity was also not correlated with reductions in pathogen population size. Further, the percentage of reduction in leaf surface pathogen population size was not correlated with the percentage of reduction in disease severity, suggesting that the epiphytic population size of X. campestris pv. vesicatoria is not related to disease severity and that X. campestris pv. vesicatoria exhibits behavior in the phyllosphere prior to lesion formation that is different from that of P. syringae pv. tomato.     

Selection of brown spot-resistant rice plants from Helminthosporium oryzae toxin-resistant calluses

Helminthosporium oryzae toxin induced electrolyte leakage from rice callus tissues and caused their browning and death. A virulent isolate of the pathogen invaded and colonised callus tissues rapidly, while a less virulent and a nonpathogenic isolate colonised calluses only weakly if at all. Addition of the toxin to calluses permitted colonisation of calluses by the nonpathogenic isolate. Four toxin-resistant calluses were selected and plants regenerated from two of these resistant calluses showed resistance to the pathogen. This resistance was heritable and stability of resistance was observed in the R₁, R₂ and R₃ generations.     

Effect of potential biocontrol agents selected among grapevine endophytes and commercial products on crown gall disease

The current strategies for the control of Agrobacterium vitis crown gall in grape are generally unsuccessful once the pathogen has established in vineyards. Experimental trials were conducted to evaluate the effectiveness of treatments based on non-pathogenic endophytes isolated from asymptomatic grapevines growing in vineyards with high incidence of crown gall and on microorganisms isolated from commercial products. Two-year in planta trials conducted on rootstocks treated with endophytic isolates showed the effectiveness of two bacterial endophytes, both in the genus Curtobacterium, and one fungal isolate in the genus Acremonium in reducing crown gall development. For the commercial biological control agents, Bacillus subtilis SR63 and Trichoderma asperellum T1 were the most effective strains against A. vitis, indicating commercial products could be reserves to draw upon to identify useful biocontrol agents. Based on the combination of data in this work, microorganisms, both endophytes and those formulated in commercial products, were identified that can potentially be exploited for the control of grapevine crown gall disease.