Lateral transfer of a phytopathogenic symbiont among native and exotic ambrosia beetles

Different ambrosia beetle species can coexist in tree trunks, where their immature stages feed upon symbiotic fungi. Although most ambrosia beetles are not primary pests and their fungal symbionts are not pathogenic to the host tree, exceptional situations exist. Notably, Xyleborus glabratus carries a phytopathogenic symbiont, Raffaelea lauricola, which causes laurel wilt, a lethal disease of some Lauraceae species. Both X. glabratus and R. lauricola are natives of Asia that recently invaded much of the coastal plain of the southeastern USA. This study examined ambrosia beetles that breed in susceptible trees in Florida (USA), including avocado (Persea americana), redbay (P. borbonia) and swampbay (P. palustris). Raffaelea lauricola was recovered from six of eight ambrosia beetle species that emerged from laurel wilt‐affected swampbay trees, in addition to X. glabratus. Controlled infestations with cohorts of the six species other than X. glabratus revealed that each could transmit the pathogen to healthy redbay trees and two could transmit the pathogen to healthy avocado trees; laurel wilt developed in five and one of the respective beetle × host interactions. These results indicate flexibility in the lateral transfer of a non‐native ambrosial fungus to other ambrosia beetles, and for the first time documents the transmission of a laterally transferred phytopathogenic symbiont by new ambrosia beetle species. Additional work is needed to determine whether, or to what extent, the new beetle × R. lauricola combinations play a role in spreading laurel wilt.     

Responses of avocado to laurel wilt,caused by Raffaelea lauricola

Laurel wilt, caused by Raffaelea lauricola, threatens native and non‐native species in the Lauraceae in the south‐eastern USA. Avocado, Persea americana, is the most important agricultural suscept of laurel wilt. Grafted plants (clonal scions on seedling rootstocks) of 24 cultivars were screened against the disease in the field from 2008 to 2010. Disease was induced with either mycelial plugs or conidial suspensions of R. lauricola. There were significant differences in the severity of disease that developed on different cultivars, and West Indian cultivars were most susceptible (P < 0·05). Simmonds, a West Indian cultivar that comprises 35% of the commercial production in Florida, was consistently susceptible and was used as a standard genotype in different studies. Disease severity increased significantly on cv. Simmonds as plant size (stem diameter) increased (P < 0·0042). In greenhouse studies, internal (sapwood) and external disease severities on cv. Simmonds were correlated (P < 0·0001), and a threshold was evident, in that external symptoms developed only after moderately severe symptoms had developed internally. Latent infection was uncommon; R. lauricola was usually isolated on a semiselective medium or detected via qPCR only from discoloured xylem of inoculated cv. Simmonds. As basipetal movement of the pathogen was common, its movement among trees via root grafts is probable. Greater understanding is needed of the movement of R. lauricola in naturally and artificially infected trees, and whether sufficient tolerance exists in avocado to assist in the management of this important new disease.     

Management of laurel wilt of avocado,caused by <Emphasis Type="Italic">Raffaelea lauricola</Emphasis>

Laurel wilt is caused by Raffaelea lauricola, a nutritional symbiont of an Asian ambrosia beetle, Xyleborus glabratus. American members of the Lauraceae plant family are most susceptible and 300 million trees have been killed by the disease in the southeastern USA since 2003. Recently, commercial production of an important crop in the laurel family, avocado (Persea americana), has been affected in southern Florida. We summarize studies in which diverse measures were tested for managing the disease. In all studies, trees were treated with potential laurel wilt control measures and subsequently inoculated with R. lauricola. On potted plants in greenhouse experiments, commercial nutritional products (Greenstim and Keyplex 350) and SAR products (Agri-Fos and Nutri-Phite), when applied as soil drenches or foliar sprays, had either no impact on, or increased laurel wilt symptom development compared to non-treated control treatments. Bark applications of Tilt (a propiconazole fungicide for which emergency registration had been obtained in 2010) in a surfactant (Pentrabark) enabled significant laurel wilt protection in greenhouse studies on small potted plants, but Pentrabark and other surfactants moved little propiconazole into the xylem of fruit-bearing trees in field studies. In efficacy studies in the field, Propiconazole Pro (an injectable formulation of propiconazole), Tilt, and two experimental formulations of another triazole fungicide, tebuconazole, decreased the development of laurel wilt compared to nontreated control trees when applied as undiluted injections into branches and scaffold limbs (microinjection), or injection of dilute fungicide into tree flare roots (macroinfusion). However, symptoms developed in all treated trees by 10–11 months after inoculation with R. lauricola, indicating that trees would need to be re-treated at least on an annual basis. Regardless of how fungicides were applied, insignificant levels of different active ingredients entered fruit. Although fungicide treatment of fruit-bearing avocado trees is not a concern for food safety and several triazole and DMI fungicides can protect avocado trees from laurel wilt, cost-effective measures with which the xylem could be loaded with and protected by these products remains a challenge. Management of laurel wilt in commercial avocado production areas is discussed.     

Involvement of a vascular hypersensitive response in quantitative resistance to Ralstonia solanacearum on tomato rootstock cultivar LS‐89

Ralstonia solanacearum causes bacterial wilt disease in Solanaceae spp. Expression of the Phytophthora inhibitor protease 1 (PIP1) gene, which encodes a papain‐like extracellular cysteine protease, is induced in R. solanacearum‐inoculated stem tissues of quantitatively resistant tomato cultivar LS‐89, but not in susceptible cultivar Ponderosa. Phytophthora inhibitor protease 1 is closely related to Rcr3, which is required for the Cf‐2‐mediated hypersensitive response (HR) to the leaf mould fungus Cladosporium fulvum and manifestation of HR cell death. However, up‐regulation of PIP1 in R. solanacearum‐inoculated LS‐89 stems was not accompanied by visible HR cell death. Nevertheless, upon electron microscopic examination of inoculated stem tissues of resistant cultivar LS‐89, several aggregated materials associated with HR cell death were observed in xylem parenchyma and pith cells surrounding xylem vessels. In addition, the accumulation of electron‐dense substances was observed within the xylem vessel lumen of inoculated stems. Moreover, when the leaves of LS‐89 or Ponderosa were infiltrated with 10⁶ cells mL^?1 R. solanacearum, cell death appeared in LS‐89 at 18 and 24 h after infiltration. The proliferation of bacteria in the infiltrated leaf tissues of LS‐89 was suppressed to approximately 10–30% of that in Ponderosa, and expression of the defence‐related gene PR‐2 and HR marker gene hsr203J was induced in the infiltrated tissues. These results indicated that the response of LS‐89 is a true HR, and induction of vascular HR in xylem parenchyma and pith cells surrounding xylem vessels seems to be associated with quantitative resistance of LS‐89 to R. solanacearum.     

Magnesium oxide nanoparticles induce systemic resistance in tomato against bacterial wilt disease

Bacterial wilt is a serious problem affecting many important food crops. Recent studies have indicated that treatment with biotic or abiotic stress factors may increase the resistance of plants to bacterial infection. This study investigated the effects of magnesium oxide nanoparticles (MgO NP) on disease resistance in tomato plants against Ralstonia solanacearum, as well as its antibacterial activity. The roots of tomato seedlings were inoculated with R. solanacearum and then immediately treated with MgO NP; the treated plants showed very little inhibition of bacterial wilt. In contrast, when roots were drenched with a MgO NP suspension prior to inoculation with the pathogen, the incidence of disease was significantly reduced. Rapid generation of reactive oxygen species such as O₂^?˙ radicals was observed in tomato roots treated with MgO NP. Further O₂^?˙ was rapidly generated when tomato plant extracts or polyphenols were added to the MgO NP suspension, suggesting that the generation of O₂^?˙ in tomato roots might be due to a reaction between MgO NP and polyphenols present in the roots. Salicylic acid‐inducible PR1, jasmonic acid‐inducible LoxA, ethylene‐inducible Osm, and systemic resistance‐related GluA were up‐regulated in both the roots and hypocotyls of tomato plants after treatment of the plant roots with MgO NP. Histochemical analyses showed that β‐1,3‐glucanase and tyloses accumulated in the xylem and apoplast of pith tissues of the hypocotyls after MgO NP treatment. These results indicate that MgO NP induces systemic resistance in tomato plants against R. solanacearum.     

Evaluation of organic amendments from agro‐industry waste for the control of verticillium wilt of olive

Biological control of plant diseases using soil amendments such as animal manure and composted materials can minimize organic waste and has been proposed as an effective strategy in crop protection. In this study, 35 organic amendments (OAs) and 16 compost mixtures were evaluated against Verticillium dahliae by assessing both the antagonistic effect on the mycelial growth of two representative isolates of V. dahliae and the effect on the reduction of microsclerotia viability of the pathogen in naturally infested soil. Eleven OAs and five compost mixtures showed a consistent inhibition effect in in vitro sensitivity tests, with solid olive‐oil waste compost one of the most effective. Therefore, a bioassay with olive plants was conducted to evaluate the suppressive effect against V. dahliae of these selected OAs and compost mixtures. Significant reduction in the severity of the symptoms of V. dahliae indicates the potential use of grape marc compost (100% disease severity reduction) and solid olive‐oil waste, combined with other OAs. Microorganism mixtures and dairy waste OAs had a potential suppressive effect when they were combined with compost, showing a 73% and 63% disease severity reduction, respectively. A mixture of agro‐industrial waste with other biological control agents is a promising strategy against verticillium wilt of olive. To the authors' knowledge, this is the first report on the effectiveness of compost extracts (compost teas) on the inhibition of natural microsclerotia of V. dahliae, and also on verticillium wilt suppression in olive with solid olive‐oil waste.     

Role of co‐infection by Pectobacterium and Verticillium dahliae in the development of early dying and aerial stem rot of Russet Burbank potato

Potato early dying (PED) is a disease complex primarily caused by the fungus Verticillium dahliae. Pectolytic bacteria in the genus Pectobacterium can also cause PED symptoms as well as aerial stem rot (ASR) of potato. Both pathogens can be present in potato production settings, but it is not entirely clear if additive or synergistic interactions occur during co‐infection of potato. The objective of this study was to determine if co‐infection by V. dahliae and Pectobacterium results in greater PED or ASR severity using a greenhouse assay and quantitative real‐time PCR to quantify pathogen levels in planta. PED symptoms caused by Pectobacterium carotovorum subsp. carotovorum isolate Ec101 or V. dahliae isolate 653 alone included wilt, chlorosis and senescence and were nearly indistinguishable. Pectobacterium wasabiae isolate PwO405 caused ASR symptoms including water‐soaked lesions and necrosis. Greater Pectobacterium levels were detected in plants inoculated with PwO405 compared to Ec101, suggesting that ASR can result in high Pectobacterium populations in potato stems. Significant additive or synergistic effects were not observed following co‐inoculation with these strains of V. dahliae and Pectobacterium. However, infection coefficients of V. dahliae and Ec101 were higher and premature senescence was greater in plants co‐inoculated with both pathogens compared to either pathogen alone in both trials, and V. dahliae levels were greater in basal stems of plants co‐inoculated with either Pectobacterium isolate. Overall, these results indicate that although co‐infection by Pectobacterium and V. dahliae does not always result in significant additive or synergistic interactions in potato, co‐infection can increase PED severity.     

Colonization of lettuce cultivars and rotation crops by Fusarium oxysporum f. sp. lactucae,the cause of fusarium wilt of lettuce

The severity of fusarium wilt is affected by inoculum density in soil, which is expected to decline during intervals when a non‐susceptible crop is grown. However, the anticipated benefits of crop rotation may not be realized if the pathogen can colonize and produce inoculum on a resistant cultivar or rotation crop. The present study documented colonization of roots of broccoli, cauliflower and spinach by Fusarium oxysporum f. sp. lactucae, the cause of fusarium wilt of lettuce. The frequency of infection was significantly lower on all three rotation crops than on a susceptible lettuce cultivar, and the pathogen was restricted to the cortex of roots of broccoli. However, F. oxysporum f. sp. lactucae was isolated from the root vascular stele of 7·4% of cauliflower plants and 50% of spinach plants that were sampled, indicating a greater potential for colonization and production of inoculum on these crops. The pathogen was also recovered from the root vascular stele of five fusarium wilt‐resistant lettuce cultivars. Thus, disease‐resistant plants may support growth of the pathogen and thereby contribute to an increase in soil inoculum density. Cultivars that were indistinguishable based on above‐ground symptoms, differed significantly in the extent to which they were colonized by F. oxysporum f. sp. lactucae. Less extensively colonized cultivars may prove to be superior sources of resistance to fusarium wilt for use in breeding programmes.     

Heritability of Ulmus minor resistance to Dutch elm disease and its relationship to vessel size,but not to xylem vulnerability to drought

Dutch elm disease (DED) is a vascular wilt disease that causes the occlusion and cavitation of xylem vessels. Therefore, it is hypothesized that those elms that are less vulnerable to cavitation by drought might be more resistant to DED. To test this hypothesis, the relationship between xylem vulnerability to cavitation and susceptibility to DED was examined in progenies of crosses between susceptible and resistant individuals of Ulmus minor. Hydraulic conductivity and xylem vulnerability curves were evaluated and anatomical features such as vessel size, length and grouping were measured. Next, elms were inoculated with Ophiostoma novo‐ulmi, the cause of DED, and pre‐dawn and midday water potentials, stomatal conductance and wilting percentages were assessed. Progenies of R × R crosses showed significantly lower mean wilting percentages (30–50%) than the progeny of S × S crosses (75%). Fifty percent conductivity loss was reached at c. ?1 MPa, pointing out a high vulnerability of this species to drought‐induced cavitation. Crown wilting percentage as a result of inoculation and xylem vulnerability to cavitation by water stress did not show any significant correlation. Nevertheless, significant differences in theoretical hydraulic conductivity and vessel size parameters (diameter, length and size distributions) were found among the tested progenies. Susceptible trees had significantly wider and longer vessels. Xylem structure of resistant elms seems to restrict pathogen spread rather than prevent cavitation.     

Ralstonia solanacearum virulence in tomato seedlings inoculated by leaf clipping

Ralstonia solanacearum is a phytopathogenic bacterium that colonizes the xylem vessels of host plants leading to a lethal wilt disease. Although several studies have investigated the virulence of R. solanacearum on adult host plants, infection studies of this pathogen on the seedling stages of hosts are less common. In a preliminary observation, inoculation of R. solanacearum F1C1 on 6‐ to 7‐day‐old tomato seedlings by a simple leaf‐clip strategy resulted in a lethal pathogenic condition in seedlings that eventually killed these seedlings within a week post‐inoculation. This prompted testing of the effect of this inoculation technique in seedlings from different cultivars of tomato and similar results were obtained. Colonization and spread of the bacteria throughout the infected seedlings was demonstrated using gus‐tagged R. solanacearum F1C1. The same method of inoculating tomato seedlings was used with R. solanacearum GMI1000 and independent mutants of R. solanacearum GMI1000, deficient in the virulence genes hrpB, hrpG, phcA and gspD. Wildtype R. solanacearum GMI1000 was found to be virulent on tomato seedlings, whereas the mutants were found to be non‐virulent. This leaf‐clip technique, for inoculation of tomato seedlings, has the potential to be a valuable approach, saving time, space, labour and costs.     

Wheat leaf resistance to Pyrenophora tritici‐repentis induced by silicon activation of phenylpropanoid metabolism

Tan spot caused by Pyrenophora tritici‐repentis is a disease present in all wheat‐producing countries and silicon (Si) treatment of wheat plants has been shown to increase plant resistance to tan spot. In this study, the effect of phenylpropanoid metabolism on resistance to tan spot was evaluated and some phenolic compounds that accumulated in response to P. tritici‐repentis attack were identified. Furthermore, the effect of Si on phenylalanine ammonia‐lyase (PAL) activity and phenolic compound accumulation were determined in situ. Antifungal activity of differentially accumulated phenolic compounds was also evaluated in in vitro tests. Results showed that the increase in concentration of phenolic compounds was greatest at the onset of infection, and that some compounds showed fungitoxic effects including fungal tip swelling, granulation of germ tube and hyphae, and hyphal hyperbranching. Silicon‐induced reduction in both lesion size and tan spot disease progression were associated with activation of phenylpropanoid metabolism. PAL activity and accumulation of antifungal phenolic compounds were greater in pathogen‐inoculated plants supplied with Si. In these plants, fluorescence indicative of accumulation of phenolic compounds occurred early in epidermal cells and its intensity increased during the evaluation period, showing higher numbers of fluorescent cells around infected cells. Thus, the combined responses of cell fluorescence at sites of infection, increased PAL activity and accumulation of phenols indicate that Si strengthened wheat defence responses to infection by P. tritici‐repentis, reducing the severity of tan spot.     

Increased bacterial endophyte populations in Nicotiana benthamiana with a branched‐chain alkane mixture

The aim of this study was to determine if treatment of soil with a branched‐chain alkane mixture known to induce resistance against Colletotrichum orbiculare also changes populations of bacterial endophytes from Nicotiana benthamiana. Eight culturable bacterial endophyte types matching six species of Bacillus and two species of Pseudomonas were found in roots, stem + petioles and/or leaves. Application of the branched‐chain alkane mixture resulted in significantly higher endophyte populations compared to the water or emulsifier controls for the Pseudomonas sp. LW3, Bacillus simplex LW4 and Bacillus pumilis LW5 colony types in roots and the B. simplex LW4 colony type in stem + petioles. The Pseudomonas sp. LW3 and B. simplex LW4 colony types also had higher populations in pure cultures under in vitro conditions with the branched‐chain alkane mixture compared to the controls. Inoculation with each of the eight colony types increased their population in the plant and induced resistance against C. orbiculare, with the most effective being Pseudomonas sp. LW3 and Pseudomonas alcaligenes SW1. Most of the endophytes could inhibit C. orbiculare growth in vitro, but the level of resistance in planta was not correlated to the ability of the colony type to inhibit C. orbiculare in culture. Thus, a branched‐chain alkane mixture can selectively affect the biomass of a subset of bacterial endophytes, demonstrating that it is a novel in planta endophyte growth promoter.     

Temperature at the early stages of Clavibacter michiganensis subsp. michiganensis infection affects bacterial canker development and virulence gene expression

Clavibacter michiganensis subsp. michiganensis (Cmm), the causal agent of bacterial canker and wilt, causes severe economic losses in tomato net‐houses and greenhouses worldwide. In this study, seedlings which were transplanted and inoculated monthly over 2 years wilted and died earlier in the spring (21–24°C) and autumn (18–23°C) than in the winter (15–18°C) and summer (28–31°C): T₅₀ (the time taken for 50% of the plants to wilt or die) was 2 and 3–4 months after inoculation, respectively. A highly significant correlation was found between the average temperatures during the first month after inoculation and T₅₀; the shortest T₅₀ mortality (70 days) was observed for an average temperature of 26°C. Expression of virulence genes (pat‐1, celA, chpC and ppaA) by Cmm was higher in plants inoculated in the spring than in those inoculated in the summer. In another set of experiments, seedlings were inoculated and maintained in controlled‐environment growth chambers for 2 weeks. Subsequently, they were transplanted and maintained in commercial‐type greenhouses for 4–5 months. The temperatures prevailing in the first 48 h after inoculation were found to affect Cmm population size and virulence gene expression and to have season‐long effects on bacterial canker development.     

Effects of plant age on disease development and virulence of Clavibacter michiganensis subsp. michiganensis on tomato

The effect of plant age at the time of inoculation on the severity of bacterial wilt and canker disease caused by Clavibacter michiganensis subsp. michiganensis (Cmm) was examined in six greenhouse experiments. The period during which inoculations led to wilt and death of tomato plants was defined. This period, designated ‘window of vulnerability’, ranged from transplanting to the 17‐ to 18‐leaf stage. Plants inoculated after this period expressed disease symptoms but did not wilt or die. No significant changes in disease incidence were observed when leaves of different ages were inoculated. Yield accumulation was significantly reduced in plants inoculated within the window of vulnerability compared with those inoculated after this period. Expression of virulence genes, viz. celA, encoding a secreted cellulase, and the serine protease‐encoding pat‐1, chpC and ppaA, was induced during the early stages after inoculation in plants inoculated within the window of vulnerability. Differences in Cmm population between plants inoculated within and outside of this period were insignificant after the first week post‐inoculation, indicating that differences in disease severity, yield loss and expression of virulence determinants are not correlated with Cmm population level. Results suggest that implementation of precautionary measures during the window of vulnerability to avoid secondary spread of Cmm will have a season‐long effect on plant mortality and may minimize, or even prevent, yield losses.     

Comparison of root and foliar applications of potassium silicate in potentiating post‐infection defences of melon against powdery mildew

The application of silicon to the roots or leaves reduces the severity of powdery mildew (Podosphaera xanthii) in melon but the latter treatment is less effective. This study compared key biochemical defence responses of melon triggered by P. xanthii after root or foliar treatment with potassium silicate (PS). Treatments consisted of pathogen‐inoculated or mock‐inoculated plants supplied with PS via roots or foliarly, as well as a non‐treated control. The activity of defence enzymes and the concentration of phenolic compounds, lignin and malondialdehyde were determined from leaf samples at different time points after inoculation. Pathogen‐inoculated plants irrigated with PS showed both an accumulation of silicon and primed defence responses in leaves that were not observed in pathogen‐inoculated plants either sprayed with PS or not treated. These responses included the anticipated activity of peroxidase and accumulation of soluble phenols, the activation of chitinase and repression of catalase, and the stronger activation of superoxide dismutase, peroxidase and β‐1,3‐glucanase. Moreover, the lignin concentration increased in response to inoculation, whereas the malondialdehyde concentration decreased. For the foliar treatment, however, only an increase in lignin deposition was observed compared with the control plants. The results show that silicon strongly plays an active role in modulating the defence responses of melon against P. xanthii when supplied to the roots as opposed to the foliage.     

Factors affecting the virulence of Ralstonia solanacearum and its colonization on tobacco roots

Tobacco bacterial wilt caused by Ralstonia solanacearum is a serious disease affecting tobacco cultivation in southwest China. The response surface methodology was employed to evaluate the optimal conditions of tobacco bacterial wilt, and green fluorescent protein gene (gfp) labelling was applied to monitor the location and survival dynamics of R. solanacearum (Rs::gfp) on tobacco roots and in soil under these optimal conditions. The results showed that the highest wilt incidence was 91.13%, which occurred when the population reached 6.6 × 10⁶ CFU/g soil, the temperature was 30.55 °C, and the humidity was >81.42%. The Rs::gfp densely colonized the root tips and root hairs, and cells of Rs::gfp were observed intermittently in the elongation zone or at the point of the emerging lateral roots. The Rs::gfp number in the rhizosphere soil was 10.75‐, 73.13‐ and 74.86‐times higher than that in the bulk soil at 10, 15 and 20 days after transplantation, respectively. Increased colonization by Rs::gfp was related to the population of the pathogen, the environmental temperature and the humidity in the soil. These three conditions determined whether R. solanacearum would induce tobacco wilt. This is the first study to investigate factors affecting the virulence of a tobacco wilt bacterial pathogen, which is important for conducting field diagnosis and biocontrol of tobacco bacterial wilt.     

Development of an in vitro protocol to screen Clavibacter michiganensis subsp. michiganensis pathogenicity in different Solanum species

Clavibacter michiganensis subsp. michiganensis (Cmm) is a quarantine organism in Europe and in many other countries. It is one of the most severe bacterial pathogens affecting tomato. Screening tomato plants for their resistance level to Cmm requires a large amount of space under quarantine conditions and is therefore costly. This project developed a new inoculation protocol on in vitro tomato plants to facilitate a more economic and higher throughput disease screening. A new method using the PathoScreen system was tested to localize green fluorescent protein-tagged Cmm in planta and to quantify the pathogen based on the percentage of corrected GFP (cGFP%). The system was sensitive in detecting the GFP-tagged Cmm in the shoots, but in the roots a high autofluorescence masked detection and thus sensitivity of the assay. The in vitro protocol was tested on several wild relatives of tomato, which were previously screened in a greenhouse assay. The correlation between wilt symptoms in vitro and wilt symptoms in the greenhouse was overall moderate (r = 0.6462). The protocol worked well in differentiating the two parents that were used in the mapping studies. This study shows that the in vitro protocol can be efficiently used for resistance breeding in many tomato genotypes.     

Fungal endophyte protects Atractylodes lancea from root rot caused by Fusarium oxysporum

Endophytic fungi, which stimulate a variety of defence reactions in host plants without causing visible disease symptoms, have been isolated from almost every plant. However, beneficial interactions between fungal endophytes and pathogens from the same habitat remain largely unknown. An inoculation of Atractylodes lancea plantlets with Gilmaniella sp. AL12 (AL12) prior to infection with Fusarium oxysporum prevented the necrotization of root tissues and plant growth retardation commonly associated with fusarium root rot. Quantification of F. oxysporum infections using real‐time PCR revealed a correlation between root rot symptoms and the relative amount of fungal DNA. Pretreatment with AL12 reduced the accumulation of reactive oxygen species stimulated by F. oxysporum. An in vitro analysis of their interactions under axenic culture conditions showed AL12 could inhibit F. oxysporum growth. Additionally, F. oxysporum infections were shown to decrease salicylic acid (SA) production compared with control plantlets. SA biosynthesis inhibitors, 2‐aminoindan‐2‐phosphonic acid and paclobutrazol, abolished the inhibition of F. oxysporum growth in A. lancea even after inoculation with AL12. The results indicated that the fungal endophyte protected A. lancea not only by direct antibiosis, but also by reversing the F. oxysporum‐mediated suppression of SA production.