Evaluation of the effectiveness of a consortium of three plant-growth promoting rhizobacteria for biocontrol of cotton Verticillium wilt

We investigated the biocontrol efficacy of a consortium of three rhizobacteria (hereafter BBS) against cotton Verticillium wilt, along with the effect on plant growth, in a series of greenhouse and field experiments. BBS treatment inhibited germination of Verticillium dahliae spores by 88.4%. Under greenhouse conditions, BBS reduced Verticillium wilt by 86.1% compared to the untreated control, and promoted plant growth by 49.9%. In field experiments, nine L/acre of BBS suspension reduced Verticillium wilt by 76.0% and increased cotton yields by 13.7%. Soil inoculation with BBS also improved the breaking tenacity and uniformity index of harvested cotton. Soil properties improved with BBS treatment in field experiments, including an increase in organic matter and the availability of nitrogen (N), phosphorus (P) and potassium (K). There was no significant difference in the biocontrol efficacy of BBS among eight tested cotton cultivars.     

Characterization of Two Fungal Isolates from Cotton and Evaluation of their Potential for Biocontrol of Verticillium Wilt of Cotton

Two isolates (CVd‐WHw and CVn‐WHg) recovered from Verticillium‐wilt‐symptomatic cotton grown in Hubei Province of China were identified based on their morphology, growth characteristics in culture, specific amplification and identification of internal transcribed spacer (ITS) rDNA sequence. According to the morphological characteristics, specific PCR amplification and ITS sequences, CVd‐WHw was identified as V. dahliae and CVn‐WHg as Gibellulopsis nigrescens. In bioassays, the two isolates had significantly lower pathogenicity to cotton plant than V. dahliae isolate CVd‐AYb. Cotton pre‐inoculated with isolate CVn‐WHg or CVd‐WHw exhibited reduced disease indices of Verticillium wilt compared with those inoculated with CVd‐AYb alone. Cotton co‐inoculated with CVn‐WHg or CVd‐WHw and CVd‐AYb provided increased protection from subsequent CVd‐AYb inoculation. These results suggest that the two isolates have the potential to be developed as biocontrol agents for the control of Verticillium wilt in cotton. To our knowledge, this is the first report of a cross‐protection phenomenon using Gibellulopsis nigrescens against Verticillium wilt caused by V. dahliae on cotton.     

Endophytic and ectophytic potato-associated bacterial communities differ in structure and antagonistic function against plant pathogenic fungi

Differences between endophytic and ectophytic bacterial communities with stress on antagonistic bacteria, were studied by comparing the composition of communities isolated from the rhizosphere, phyllosphere, endorhiza and endosphere of field-grown potato plants using a multiphasic approach. Terminal restriction fragment length polymorphism analysis of 16S rDNA of the bacterial communities revealed discrete microenvironment-specific patterns. To measure the antagonistic potential of potato-associated bacteria, a total of 2648 bacteria were screened by dual testing of antagonism to the soilborne pathogens Verticillium dahliae and Rhizoctonia solani. Composition and diversity of bacterial antagonists were mainly specific for each microenvironment. The rhizosphere and endorhiza were the main reservoirs for antagonistic bacteria and showed the highest similarity in their colonisation by antagonists. The most prominent species of all microenvironments was Pseudomonas putida, and rep-PCR with BOX primers showed that these isolates showed microenvironment-specific DNA fingerprints. P. putida isolates from the rhizosphere and endorhiza gave nearly identical fingerprints confirming the high similarity of bacterial populations. The phlD gene, involved in the production of the antibiotic 2,4-diacetyl-phloroglucinol, was found only among Pseudomonas isolates from the rhizosphere and endorhiza. Evaluation of the bacterial isolates for biocontrol potential based on fungal antagonism and physiological characteristics resulted in the selection of five promising isolates from each microenvironment. The most effective isolate was Serratia plymuthica 3Re4-18 isolated from the endorhiza.     

Evaluation of potential biocontrol rhizobacteria from different host plants of Verticillium dahliae Kleb

AIMS: A screening approach was developed to assess the potential of rhizobacterial strains to control Verticillium wilt caused by Verticillium dahliae Kleb. METHODS AND RESULTS: Sixty randomly chosen antagonistic bacterial strains originally isolated from rhizosphere of three different host plants of V. dahliae--strawberry, potato and oilseed rape--were evaluated for biocontrol and plant growth promotion by analysing in vitro antagonism towards V. dahliae and other plant pathogenic fungi, production of fungal cell wall-degrading enzymes and plant growth-promoting effects on strawberry seedlings. To test the plant growth-promoting effect, a microplate assay with strawberry seedlings was developed. Although the rhizobacterial strains were isolated from different plants they showed effects on the growth of strawberry seedlings. According to the in vitro biocontrol and plant growth-promoting activity, the three best candidates Pseudomonas putida B E2 (strawberry rhizosphere), Ps. chlororaphis K15 (potato rhizosphere) and Serratia plymuthica R12 (oilseed rape rhizosphere) were selected for greenhouse experiments to verify the in vitro screening results. Under greenhouse conditions the isolates selected according to this strategy were as effective, or more effective than commercial biocontrol agents and may therefore possibly be valuable as antagonists of V. dahliae. CONCLUSIONS: In this study, the screening strategy resulted in a selection of three interesting biocontrol candidates against Verticillium: Ps. putida B E2 (strawberry rhizosphere), Ps. chlororaphis K15 (potato rhizosphere) and Ser. plymuthica R12 (oilseed rape rhizosphere). SIGNIFICANCE AND IMPACT OF THE STUDY: A new combination of in vitro screening methods including a microplate assay with strawberry seedlings to test the plant growth promoting effect which allow to more efficiently select potential biological control agents was developed successfully.     

Seedling vaccination by stem injecting a conidial suspension of F2, a non-pathogenic Fusarium oxysporum strain, suppresses Verticillium wilt of eggplant

Several bacterial and fungal strains have been evaluated as biocontrol agents (BCAs) against Verticillium dahliae. In these studies, the BCAs were applied as a root drenching inoculum; however, this application method may have an adverse effect on the native, beneficial for the plants, microbial community. In the present study, it was evaluated whether endophytic application by stem injecting a conidial suspension of the non pathogenic Fusarium oxysporum F2 strain, isolated from a V. dahliae suppressive compost amendment, could reduce significantly Verticillium wilt symptom development in eggplants. It was revealed that stem injection of F2 seven days before transplanting the seedlings to soil infested by V. dahliae microsclerotia resulted in reduced disease severity compared to the control treatment. To visualise F2 ramification into the plant vascular system eggplant stems were injected with an EGFP transformed F2 strain. It was shown that F2 colonises effectively the plant vascular tissues over a long period of time as it was assessed by F2 DNA levels. In parallel, qPCR analysis showed that the application of F2 reduced significantly the amount of V. dahliae DNA in the stem tissues compared to the control treatment.     

Suppression of plant wilt diseases by nonpathogenic Fusarium oxysporum Fo47 combined with actinomycete strains

In order to build integrated strains with superior growth-promoting and disease-suppression effects, the biological control efficacy of Fo47 solid agents combined with actinomycetes strains toward Fusarium oxysporum and Verticillium dahliae were investigated in experiments on watermelon, cotton and eggplant. Five actinomycetes strains were prepared by solid fermentation. The count of viable solid agents, initially with propagules at 10⁷–10¹¹ CFU/g, slowly decreased after being stored one year at room temperature. After being inoculated into sterile soil for 50 days, the viable count of strain Fo47 remained at a stable level. The suppressive effects of Fo47 combined with strain QLP12 on Fusarium wilt on watermelon and cotton, and Verticillium wilt on eggplant, reaching 58.47%, 50.73% and 58.82%, respectively. This was significantly better than the single strain Fo47 alone, and growth of these treated plants and the colonisation rate of Fo47 were increased substantially as well. These results indicate that solid integrated agents with a high viability count and superior stability in soil could increase disease suppression and promote plant growth by synergy with different strains. The increased suppression obtained by Fo47 combined with actinomycete strains was not due to a simple addition of different mechanisms of biocontrol agents. By being intelligently integrated, these combinations increase disease suppression and provide the best biocontrol effect.     

Effects of cotton–maize rotation on soil microbiome structure

Verticillium wilt is a disastrous disease in cotton-growing regions in China. As a common management method, cotton rotation with cereal crops is used to minimize the loss caused by Verticillium dahliae. However, the correlation between soil microbiome and the control of Verticillium wilt under a crop rotation system is unclear. Therefore, three cropping systems (fallow, cotton continuous cropping, and cotton–maize rotation) were designed and applied for three generations under greenhouse conditions to investigate the different responses of the soil microbial community. The soil used in this study was taken from a long-term cotton continuous cropping field and inoculated with V. dahliae before use. Our results showed that the diversity of the soil bacterial community was increased under cotton–maize rotation, while the diversity of the fungal community was obviously decreased. Meanwhile, the structure and composition of the bacterial communities were similar even under the different cropping systems, but they differed in the soil fungal communities. Through microbial network interaction analysis, we found that Verticillium interacted with 17 bacterial genera, among which Terrabacter had the highest correlation with Verticillium. Furthermore, eight fungal and eight bacterial species were significantly correlated with V. dahliae. Collectively, this work aimed to study the interactions among V. dahliae, the soil microbiome, and plant hosts, and elucidate the relationship between crop rotation and soil microbiome, providing a new theoretical basis to screen the biological agents that may contribute to Verticillium wilt control.     

Island Cotton Gbve1 Gene Encoding A Receptor-Like Protein Confers Resistance to Both Defoliating and Non-Defoliating Isolates of Verticillium dahliae

Verticillium wilt caused by soilborne fungus Verticillium dahliae could significantly reduce cotton yield. Here, we cloned a tomato Ve homologous gene, Gbve1, from an island cotton cultivar that is resistant to Verticillium wilt. We found that the Gbve1 gene was induced by V. dahliae and by phytohormones salicylic acid, jasmonic acid, and ethylene, but not by abscisic acid. The induction of Gbve1 in resistant cotton was quicker and stronger than in Verticillium-susceptible upland cotton following V. dahliae inoculation. Gbve1 promoter-driving GUS activity was found exclusively in the vascular bundles of roots and stems of transgenic Arabidopsis. Virus-induced silencing of endogenous genes in resistant cotton via targeting a fragment of the Gbve1 gene compromised cotton resistance to V. dahliae. Furthermore, we transformed the Gbve1 gene into Arabidopsis and upland cotton through Agrobacterium-mediated transformation. Overexpression of the Gbve1 gene endowed transgenic Arabidopsis and upland cotton with resistance to high aggressive defoliating and non-defoliating isolates of V. dahliae. And HR-mimic cell death was observed in the transgenic Arabidopsis. Our results demonstrate that the Gbve1 gene is responsible for resistance to V. dahliae in island cotton and can be used for breeding cotton varieties that are resistant to Verticillium wilt.     

Potential of soil-derived fungal biocontrol agents applied as a soil amendment and a seed coating to control Verticillium wilt of sugar beet

Verticillium dahliae (Vd) is an emerging threat to sugar beet production. Control measures such as fungicides are not available and the utilisation of resistant cultivars is very limited. Hence, we explored the potential of two soil-derived fungal biocontrol agents (BCAs), Fusarium oxysporum F2 (FoF2) and Verticillium tricorpus 1808 (Vt1808), against Verticillium wilt of sugar beet. Pathogenicity tests revealed that Vd caused over 90% disease incidence and severity and led to a significant yield reduction, whereas BCAs neither inhibited nor promoted plant growth. Germination rate was higher in BCA-treated seeds compared to untreated ones. Viability of both BCAs was significantly reduced after six months of storage in liquid methylcellulose formulation (MC), while BCA concentrations remained stable on stored seeds treated with MC. In contrast, Vt1808 produced in sand-rye flour formulation remained 100% viable after storage. Similarly, post-storage analysis of the FoF2 talcum powder formulation revealed improved colony forming units, but increments were not significant. In vitro, both BCAs caused no growth inhibition zones and only insignificant Vd growth reductions were observed. In the greenhouse, soil amendment with a higher dose of FoF2:Vt1808 mixture resulted in substantial reductions of disease severity on the crown (33.3%), disease incidence (55.6%) and disease severity (68.8%) on the beet as well as an improved yield (32.9%). In contrast, seed coating did not reduce symptoms on the crown and on the beet. In the field, both BCAs did not provide significant disease reductions. Nevertheless, a promising result was achieved with the application of FoF2. Despite the need for improvement of the biocontrol activity, the results of this study demonstrate the suitability of the optimised BCA formulations for utilisation in commercial sugar beet production.     

Impact of Plant Species and Site on Rhizosphere-Associated Fungi Antagonistic to Verticillium dahliae Kleb.

Fungi with antagonistic activity toward plant pathogens play an essential role in plant growth and health. To analyze the effects of the plant species and the site on the abundance and composition of fungi with antagonistic activity toward Verticillium dahliae, fungi were isolated from oilseed rape and strawberry rhizosphere and bulk soil from three different locations in Germany over two growing seasons. A total of 4,320 microfungi screened for in vitro antagonism toward Verticillium resulted in 911 active isolates. This high proportion of fungi antagonistic toward the pathogen V. dahliae was found for bulk and rhizosphere soil at all sites. A plant- and site-dependent specificity of the composition of antagonistic morphotypes and their genotypic diversity was found. The strawberry rhizosphere was characterized by preferential occurrence of Penicillium and Paecilomyces isolates and low numbers of morphotypes (n = 31) and species (n = 13), while Monographella isolates were most frequently obtained from the rhizosphere of oilseed rape, for which higher numbers of morphotypes (n = 41) and species (n = 17) were found. Trichoderma strains displayed high diversity in all soils, but a high degree of plant specificity was shown by BOX-PCR fingerprints. The diversity of rhizosphere-associated antagonists was lower than that of antagonists in bulk soil, suggesting that some fungi were specifically enriched in each rhizosphere. A broad spectrum of new Verticillium antagonists was identified, and the implications of the data for biocontrol applications are discussed.     

Root Hairs Play a Key Role in the Endophytic Colonization of Olive Roots by <Emphasis Type="Italic">Pseudomonas</Emphasis> spp. with Biocontrol Activity

The use of indigenous bacterial root endophytes with biocontrol activity against soil-borne phytopathogens is an environmentally-friendly and ecologically-efficient action within an integrated disease management framework. The earliest steps of olive root colonization by Pseudomonas fluorescens PICF7 and Pseudomonas putida PICP2, effective biocontrol agents (BCAs) against Verticillium wilt of olive (Olea europaea L.) caused by the fungus Verticillium dahliae Kleb., are here described. A gnotobiotic study system using in vitro propagated olive plants, differential fluorescent-protein tagging of bacteria, and confocal laser scanning microscopy analysis have been successfully used to examine olive roots–Pseudomonas spp. interactions at the single-cell level. In vivo simultaneous visualization of PICF7 and PICP2 cells on/in root tissues enabled to discard competition between the two bacterial strains during root colonization. Results demonstrated that both BCAs are able to endophytically colonized olive root tissues. Moreover, results suggest a pivotal role of root hairs in root colonization by both biocontrol Pseudomonas spp. However, colonization of root hairs appeared to be a highly specific event, and only a very low number of root hairs were effectively colonized by introduced bacteria. Strains PICF7 and PICP2 can simultaneously colonize the same root hair, demonstrating that early colonization of a given root hair by one strain did not hinder subsequent attachment and penetration by the other. Since many environmental factors can affect the number, anatomy, development, and physiology of root hairs, colonization competence and biocontrol effectiveness of BCAs may be greatly influenced by root hair’s fitness. Finally, the in vitro study system here reported has shown to be a suitable tool to investigate colonization processes of woody plant roots by microorganisms with biocontrol potential.     

根系土壤假丝酵母菌Candida sp. YIN9对大丽轮枝菌和全齿复活线虫的抑制作用

[目的] 研究黄萎病抗性棉(海 7124)根际土壤中酵母菌株对棉花黄萎病病原真菌大丽轮枝菌和全齿复活线虫的拮抗效果,为生物防治棉花黄萎病和全齿复活线虫提供理论依据。[方法] 通过镜检、糖发酵实验、碳源同化实验、26S rRNA测序对菌株的形态、生理生化特征及其系统发育关系进行鉴定,并利用七叶苷筛选、刚果红染色、平皿对峙实验、盆栽实验、平板生测实验测试其产酶活性以及抑制大丽轮枝菌和杀线虫活性。[结果] 从大批黄萎病抗性棉(海 7124)根际土壤中筛选出编号为YIN9的酵母菌菌株,分类鉴定结果表明:YIN9菌株属于假丝酵母属Candida。平皿对峙实验结果表明:菌株YIN9对大丽轮枝菌的抑菌率达59%;将菌株YIN9的无菌发酵滤液与大丽轮枝菌孢子共培养12 h后镜检发现,用菌株YIN9处理的实验组,大部分棉花黄萎病病菌孢子不能正常萌发。盆栽实验结果表明:菌株YIN9对棉花黄萎病的平均防治效果为60.02%,可以显著降低感病棉棉花黄萎病的发病率和病情指数。此外,与从黄萎病抗性棉根际土壤中筛选获得的其他酵母菌株相比,菌株YIN9具备较高的杀线虫活性:菌株作用全齿复活线虫48 和60 h后,线虫死亡率分别为90%和100%。将菌株YIN9发酵液煮沸后,其抑制大丽轮枝菌和杀线虫活性均急剧下降,进一步测试发现,该菌株拥有较高的蛋白酶和纤维素酶活性。[结论] YIN9中的生防因子可能是热不稳定性物质,具备较高的杀线虫活性,可以显著提高感病棉对黄萎病的抗性。     

Microbial communities associated with the root system of wild olives (Olea europaea L. subsp. europaea var. sylvestris) are good reservoirs of bacteria with antagonistic potential against Verticillium dahliae

Wild olive trees, namely oleaster, are considered the ancestor of cultivated olive and a unexplored source of genetic variability that might contain important traits of agronomic and biotechnological interest. The longevity and genetic diversity of oleasters may have favoured selection of specific and well adapted rhizosphere microbial populations that can constitute unique reservoirs of microbial antagonists of Verticillium dahliae, the main soilborne fungal pathogen of olive worldwide. The objective of this present study was to determine the structure and diversity of bacterial communities in the rhizosphere and endosphere of oleaster from 11 havens in Cádiz and Córdoba provinces of Andalusia, southern Spain. To carry out the study we used a multiphasic approach. First, the occurrence and diversity of rhizosphere bacteria was monitored by a cultivation-independent-approach, using fluorescent terminal restriction fragment length polymorphism (FT-RFLP) analyses of amplified 16S rDNA sequences. FT-RFLP patterns revealed a high heterogeneity in the composition of the sampled rhizosphere bacterial communities and suggested the existence of plant genotype-site-specific communities, with each oleaster haven being a unique reservoir of bacterial diversity. Secondly, to investigate the antagonistic potential of these root-associated bacterial populations, a total of 675 bacterial isolates obtained from oleaster rhizosphere and endosphere were screened by dual testing for inhibition of in vitro growth of the highly virulent, olive defoliating pathotype of V. dahliae. Out of 675 tested bacterial isolates, 94 (14%) showed a strong antagonistic activity against a defoliating V. dahliae pathotype. Of the antagonistic bacteria, a slightly lower proportion (12.9% of total bacteria) were inhabitant of the oleaster rhizosphere compared to that in the endosphere (16.5%). The biotechnological potential of those isolates was assessed by in vitro production of different hydrolytic enzymes, indole-1.3-acetic acid (IAA), siderophores, and antimicrobial compounds. Overall, most of bacterial antagonists (58.5 to 78.3%) showed proteolytic, lipolytic, and chitinolytic activity, and produced IAA and siderophores. Finally, analysis of the 16S rDNA gene sequence indicated that most of the 94 bacterial antagonists belong to genera Bacillus (56.4%), Pseudomonas (27.7%), and Paenibacillus (7.4%). Overall, the rhizosphere and endosphere of wild olives were proved as a good reservoir of bacteria antagonists against V. dahliae. Several of those bacteria showing high and broad antagonism potential may therefore be considered for further analyses as promising biocontrol agents against V. dahliae in olive.     

Evaluation of biocontrol agents and potassium silicate for the management of powdery mildew of zucchini

Investigations were conducted under greenhouse and field conditions to evaluate the effects of potential biocontrol agents (BCAs) and soluble silicon (Si) on powdery mildew of zucchini caused by Podosphaera xanthii. Five BCAs were applied as foliar sprays to zucchini leaves and Si was drenched weekly into the rhizosphere of these plants.In the greenhouse, all BCAs provided significant control of powdery mildew with fungal isolates, reducing disease levels by up to 90%. Si alone reduced powdery mildew by as much as 35% and improved the efficacy of most of the biocontrol agents. Higher disease pressure reduced the efficacy of Si on powdery mildew but did not affect the performance of the BCAs. In the field, a disease reduction of 10–70% was achieved by BCAs and Si. Lower temperatures and high humidity ranges were suitable for optimal performances. The efficacy of the bacterial BCA, Serratia marcescens – B15 and silicon diminished at temperatures above 25 °C. The fungal BCAs (Clonostachys rosea – EH and Trichothecium roseum – H20) were better suited to higher temperatures (25–30 °C) and were tolerant of low RH values. Application of K₂SiO₂ to zucchini roots increased the level of Si in the leaves, which was responsible for suppression of the disease.     

CGTase,a novel antimicrobial protein from Bacillus cereus YUPP-10, suppresses Verticillium dahliae and mediates plant defence responses

Verticillium wilt is a plant vascular disease caused by the soilborne fungus Verticillium dahliae that severely limits cotton production. In a previous study, we screened Bacillus cereus YUPP-10, an efficient antagonistic bacterium, to uncover mechanisms for controlling verticillium wilt. Here, we report a novel antimicrobial cyclodextrin glycosyltransferase (CGTase) from YUPP-10. Compared to other CGTases, six different conserved domains were identified, and six mutants were constructed by gene splicing with overlap extension PCR. Functional analysis showed that domain D was important for hydrolysis activity and domains A1 and C were important for inducing disease resistance. Direct effects of recombinant CGTase on V. dahliae included reduced mycelial growth, spore germination, spore production, and microsclerotia germination. In addition, CGTase also elicited cotton's innate defence reactions. Transgenic Arabidopsis thaliana lines that overexpress CGTase showed higher resistance to verticillium wilt. Transgenic CGTase A. thaliana plants grew faster and resisted disease better. CGTase overexpression enabled a burst of reactive oxygen species production and activated pathogenesis-related gene expression, indicating that the transgenic cotton was better prepared to protect itself from infection. Our work revealed that CGTase could inhibit the growth of V. dahliae, activate innate immunity, and play a major role in the biocontrol of fungal pathogens.     

黄萎病不同发生程度棉田中土壤微生物多样性

作物根际土壤微生物群落对土壤生态及作物健康至关重要。以棉花黄萎病不同发生程度棉田的土壤为研究对象,采用理化分析、微生物纯培养及高通量测序技术对土壤理化性质及微生物数量、细菌丰度多样性进行综合分析。结果表明:在纯培养条件下,大丽轮枝菌无菌发酵滤液对细菌生长有明显的抑制作用;棉田接种大丽轮枝菌对土壤中细菌、真菌、放线菌的数量及细菌菌群丰度多样性未产生明显影响,不同采样时间的土壤中细菌菌群结构差异更大。土壤中大丽轮枝菌微菌核数量与棉花黄萎病的发生程度呈显著正相关。土壤肥力对土壤中微生物数量起主导作用,而水稻-棉花轮作能够使棉田有效降盐、减病、改善土壤肥力。通过生物防治、作物轮作、深翻等调控措施增加土壤中有益菌群数量、改善土壤生态、降低棉田土壤中大丽轮枝菌菌源数量是减轻棉花黄萎病危害的基础。     

Biological control of Verticillium wilt of olive by Paenibacillus alvei,strain K165

In the present study, the efficiency of the biocontrol agent Paenibacillus alvei (strain K165) to suppress Verticillium wilt of olive tree was evaluated in greenhouse and field experiments. In planta bioassays were conducted under greenhouse conditions and revealed that K165 significantly decreased symptoms on the susceptible cultivar ‘Amfissis’ by 44.5 and 51.6 % of the final disease severity index and relative area under disease progress curve (AUDPC), respectively. Thereafter, the suppressive effect of K165 against Verticillium dahliae was studied for two consecutive years (2007 and 2008) in a newly established olive orchard of the susceptible cv Amfissis and the resistant cv Kalamon, naturally infested with V. dahliae. The evaluation of K165 was carried out by recording symptoms, isolations and qPCR quantification of the pathogen in olive tissues. In both years, ‘Amfissis’ trees treated with K165 showed significantly lower final disease severity and relative AUDPC values compared to the non treated controls, whereas, in 2008 decreased symptom severity was associated with significantly lower V. dahliae DNA levels in plant tissues, indicating the suppressive effect of the biocontrol agent. However, no significant suppression was observed in ‘Kalamon’. Pathogen isolations along with qPCR quantification revealed a seasonal fluctuation of V. dahliae biomass in olive tissues with higher amounts occurring in May, and lower amounts in February, August and November. This is the first report of biological control of Verticillium wilt of olive tree under field conditions, associated with reduced pathogen levels inside the xylem tissues.     

Dynamic infection of Verticillium dahliae in upland cotton

• Verticillium wilt, an infection caused by the soilborne fungus Verticillium dahliae, is one of the most serious diseases in cotton. No effective control method against V. dahliae has been established, and the infection mechanism of V. dahliae in upland cotton remains unknown.
• GFP‐tagged V. dahliae isolates with different pathogenic abilities were used to analyse the colonisation and infection of V. dahliae in the roots and leaves of different upland cotton cultivars, the relationships among infection processes, the immune responses and the resistance ability of different cultivars against V. dahliae.
• Here, we report a new infection model for V. dahliae in upland cotton plants. V. dahliae can colonise and infect any organ of upland cotton plants and then spread to the entire plant from the infected organ through the surface and interior of the organ.
• Vascular tissue was found to not be the sole transmission route of V. dahliae in cotton plants. In addition, the rate of infection of a V. dahliae isolate with strong pathogenicity was notably faster than that of an isolate with weak pathogenicity. The resistance of upland cotton to Verticillium wilt was related to the degree of the immune response induced in plants infected with V. dahliae. These results provide a theoretical basis for studying the mechanism underlying the interaction between V. dahliae and upland cotton. These results provide a theoretical basis for studying the mechanism underlying the interaction between V. dahliae and upland cotton.