Stimulatory Effects of Flusilazole on Virulence of Sclerotinia sclerotiorum

Beneficial or harmful: Time-dependent hormesis induced by typical disinfectants and their mixtures with toxicological interaction

Disinfectants and their mixtures can induce hormesis. However, how the mixture hormesis is related to those of components and the interactions in disinfectant mixtures remain unclear. In this paper, the luminescence inhibition toxicities of chlorinated sodium phosphate (CSP), dodecyl dimethyl benzyl ammonium bromide (DOB), dodecyl dimethyl benzyl ammonium chloride (DOC), ethanol (EtOH), glutaraldehyde (GLA), hydrogen peroxide (H2O2), isopropyl alcohol (IPA), n-propanol (NPA), and 20 mixture rays in four mixture systems (EtOH-H2O2, DOB-H2O2, DOC-EtOH, and EtOH-IPA-NPA) containing at least one component showing hormesis to Vibrio qinghaiensis sp.-Q67 (Q67) were determined at 0.25, 3, 6, 9, and 12 h. The synergism-antagonism heatmap based on independent action model (noted as SAHmapIA) was developed to systematically evaluate the interactions in various mixtures. It was shown that five disinfectants (CSP, EtOH, H2O2, NPA, and IPA) and 17 mixture rays exhibited time-dependent hormesis. The hormetic component was responsible for the hormesis of the mixture rays. Most mixture rays showed low- concentration/dose additive action and high-concentration/dose synergism at different time. This study further exemplified the interrelationship between the hormesis in the mixtures and their components and implied the need to pay attention to the time-dependent hormesis and interactions induced by the disinfectants.

Biobased, Macro-, and Nanoscale Fungicide Delivery Approaches for Plant Fungi Control

In this report, two polymeric matrix systems at macro and nanoscales were prepared for efficacious fungicide delivery. The macroscale delivery systems used millimeter-scale, spherical beads composed of cellulose nanocrystals and poly(lactic acid). The nanoscale delivery system involved micelle-type nanoparticles, composed of methoxylated sucrose soyate polyols. Sclerotinia sclerotiorum (Lib.), a destructive fungus affecting high-value industrial crops, was used as a model pathogen against which the efficacy of these polymeric formulations was demonstrated. Commercial fungicides are applied on plants frequently to overcome the transmission of fungal infection. However, fungicides alone do not persist on the plants for a prolonged period due to environmental factors such as rain and airflow. There is a need to apply fungicides multiple times. As such, standard application practices generate a significant environmental footprint due to fungicide accumulation in soil and runoff in surface water. Thus, approaches are needed that can either increase the efficacy of commercially active fungicides or prolong their residence time on plants for sustained antifungal coverage. Using azoxystrobin (AZ) as a model fungicide and canola as a model crop host, we hypothesized that the AZ-loaded macroscale beads, when placed in contact with plants, will act as a depot to release the fungicide at a controlled rate to protect plants against fungal infection. The nanoparticle-based fungicide delivery approach, on the other hand, can be realized via spray or foliar applications. The release rate of AZ from macro- and nanoscale systems was evaluated and analyzed using different kinetic models to understand the mechanism of AZ delivery. We observed that, for macroscopic beads, porosity, tortuosity, and surface roughness governed the efficiency of AZ delivery, and for nanoparticles, contact angle and surface adhesion energy were directing the efficacy of the encapsulated fungicide. The technology reported here can also be translated to a wide variety of industrial crops for fungal protection. The strength of this study is the possibility of using completely plant-derived, biodegradable/compostable additive materials for controlled agrochemical delivery formulations, which will contribute to reducing the frequency of fungicide applications and the potential accumulation of formulation components in soil and water.

Hormetic Effects of Carbendazim on Mycelial Growth and Aggressiveness of Magnaporthe oryzae

Rice blast caused by Magnaporthe oryzae is one of the most destructive fungal diseases of rice worldwide. Stimulatory effects of low doses of fungicides on pathogens are closely relevant to disease management. In the present study, in potato dextrose agar (PDA) amended with carbendazim at a dose range from 0.003 to 0.3 μg/mL, stimulatory effects on the mycelial growth of three isolates sensitive to carbendazim were tested. Carbendazim at concentrations from 0.003 to 0.1 µg/mL showed stimulatory effects on mycelial growth of isolates Guy11 and H08-1a, while carbendazim at concentrations from 0.003 to 0.03 µg/mL stimulated the growth of isolate P131. The maximum stimulation magnitudes were 11.84% for the three isolates tested. Mycelial colonies grown on PDA amended with different concentrations of carbendazim were incubated at 28 °C in darkness for 7 days as the pretreatment. Pretreatment mycelia were inoculated on fresh fungicide-free PDA and subsequent mycelia growth stimulations were still observed, and the maximum stimulation magnitudes were 9.15% for the three isolates tested. Pretreatment mycelia did not significantly change the tolerance to H2O2 and NaCl, except that the tolerance to H2O2 was increased significantly (p < 0.05) when the carbendazim was at 0.3 µg/mL. After five generations of mycelial transference on fungicide-free PDA, the transgenerational hormesis of mycelial were exhibited when transferred onto PDA supplemented with carbendazim at 0.3 µg/mL, and the maximum percent stimulation was 51.28%. The time course of infection indicated that the visible initial necrotic symptoms could be detected at 2 DPI on leaves treated with carbendazim at 0.03 µg/mL, whereas no necrotic symptom could be discerned for the control. Statistical results of lesion area and lesion type at 7 DPI showed that there was a significant stimulation (p < 0.05) on aggressiveness of M. oryzae isolate Guy11 on detached rice leaves at 0.03 µg/mL carbendazim. These results will advance our understanding of hormetic effects of fungicides and provide valuable information for judicious application of fungicides.

Hormetic Effects of Dimethachlone on Mycelial Growth and Virulence of Sclerotinia sclerotiorum

Fungicide hormesis has implications for the application of fungicides to control plant diseases. We investigated the hormetic effects of the dicarboximide fungicide dimethachlone on mycelial growth and virulence of the necrotrophic plant pathogen Sclerotinia sclerotiorum. Dimethachlone at sublethal doses in potato dextrose agar (PDA) increased the mycelial growth of S. sclerotiorum. After the growth-stimulated mycelia were subcultured on fresh PDA and inoculated on rapeseed leaves, increased mycelial growth and virulence were observed, indicating that hormetic traits were passed down to the next generation. Dimethachlone applied to leaves at 0.002 to 500 μg/ml stimulated virulence, with a maximum stimulation amplitude (MSA) of 31.4% for the isolate HLJ4, which occurred at 2 μg/ml. Dimethachlone-resistant isolates and transformants had a mean virulence MSA of 30.4%, which was significantly higher (P = 0.008) than the MSA for sensitive isolates (16.2%). Negative correlations were detected between MSA and virulence in the absence of any fungicide (r = -0.872, P < 0.001) and between MSA and mycelial growth on PDA (r = -0.794, P = 0.002). Studies on hormetic mechanisms indicated that dimethachlone had no significant effects on expression levels of three virulence-associated genes, that is, a cutinase-encoding gene SsCut, a polygalacturonase gene SsPG1, or an oxaloacetate acetylhydrolase gene SsOah1. The results will contribute to understanding hormesis and have implications for the judicious application of fungicides to control plant diseases.

Baseline sensitivity and action mechanism of the sterol demethylation inhibitor flusilazole to Valsa mali

The apple Valsa canker caused by Valsa mali is a devastating branch disease that has seriously threatened the development of the apple industry worldwide. In current study, a total of 115 V. mali strains collected from different apple orchards in Shaanxi Province of China during 2016 and 2017 were tested for their sensitivity to flusilazole. The average EC₅₀ (effective concentrations causing 50% mycelial growth inhibition) value of all tested strains for flusilazole was 0.0892 (±0.0036) μg/mL and the frequency distribution of the EC50 values was unimodal. Flusilazole exhibited both excellent protective and curative activity on detached apple branches, which was significantly better than the commonly used fungicide thiophanate-methyl. After flusilazole treatment, mycelia twisted with offshoot of top increased, the V. mali strains lost the ability of fruiting body production, and cell membrane permeability of the mycelia increased while ergosterol content and pectinase activity decreased. The expression of pectinase genes involved in virulence down-regulated after flusilazole treatment. This study is the first report on the baseline sensitivity of V. mali to flusilazole. These results indicated that flusilazole has a great potential to play an important role in the management of Valsa canker.

Toxic mechanism of three azole fungicides and their mixture to green alga Chlorella pyrenoidosa

Currently, few studies have investigated the joint toxicity mechanism of azole fungicides at different exposure times and mixed at the relevant environmental concentrations. In this study, three common azole fungicides, namely, myclobutanil (MYC), propiconazole (PRO), and tebuconazole (TCZ), were used in studying the toxic mechanisms of a single substance and its ternary mixture exposed to ambient concentrations of Chlorella pyrenoidosa. Superoxide dismutase (SOD), catalase (CAT), chlorophyll a (Chla), and total protein (TP), were used as physiological indexes. Results showed that three azole fungicides and ternary mixture presented obvious time-dependent toxicities at high concentrations. MYC induced a hormetic effect on algal growth, whereas PRO and TCZ inhibit algal growth in the entire range of the tested concentrations. The toxicities of the three azole fungicides at 7 days followed the order PRO > TCZ > MYC. Three azole fungicides and their ternary mixture induced different levels of SOD and CAT activities in algae at high concentrations. The ternary mixture showed additive effects after 4 and 7 days exposure, but no effect was observed at actual environmental concentrations. The toxic mechanisms may be related to the continuous accumulation of reactive oxygen species, which not only affected protein structures and compositions but also damaged thylakoid membranes, hindered the synthesis of proteins and chlorophyll a, and eventually inhibited algal growth. These findings increase the understanding of the ecotoxicity of azole fungicides and use of azole fungicides in agricultural production.

Baseline Sensitivity and Toxic Action of the Sterol Demethylation Inhibitor Flusilazole Against Botrytis cinerea

In the present study, a total of 95 Botrytis cinerea single-spore strains collected from different hosts in Shaanxi Province of China were characterized for their sensitivity to the sterol demethylation inhibitor fungicide flusilazole. The effective concentration for 50% inhibition of mycelial growth (EC₅₀) of flusilazole ranged from 0.021 to 0.372 µg/ml, with an average value of 0.093 µg/ml. Cross-resistance between flusilazole and commonly used fungicides was not detected, and no flusilazole-resistant mutants were induced. Both on detached strawberry leaves and in greenhouse experiments, flusilazole was more effective than the commonly used fungicide carbendazim at reducing gray mold. After culture on PDA plates or detached strawberry leaves, no difference in sclerotia production or pathogenicity was detected between two strains, WG12 (most sensitive to flusilazole) and MX18 (least sensitive to flusilazole). After treatment with flusilazole, however, the two strains lost the ability to produce sclerotia, and oxalic acid and ergosterol contents in mycelium decreased. Interestingly, the inhibition rate of ergosterol content in MX18 was significantly lower than that in WG12. Expression of Cyp51, BcatrD, and Bcmfs1 genes all increased after treatment with flusilazole, especially the Cyp51 and BcatrD genes. However, the expression of Cyp51 gene or BcatrD gene in WG12 and MX18 were significantly different from each other after treatment with flusilazole. In addition, no point mutations in Cyp51 gene were found in MX18. These data suggest flusilazole is a promising fungicide for resistance management of gray mold and also provided novel insights into understanding the resistance mechanism of flusilazole against plant pathogens.

Stimulatory Effects of Boscalid on Virulence of Sclerotinia sclerotiorum Indicate Hormesis May Be Masked by Inhibitions

Hormetic effects of fungicides on phytopathogens are of great importance for proper application of fungicides. The aim of the present study was to investigate the stimulatory effects of the fungicide boscalid on mycelial growth and virulence of the devastating plant pathogen Sclerotinia sclerotiorum. Boscalid in potato dextrose agar (PDA) at a dosage range from 0.0005 to 0.002 μg/ml exerted statistically significant (P ≤ 0.015) stimulations on mycelial growth of S. sclerotiorum, and the maximum stimulation magnitudes were 5.55 ± 0.73% (mean ± SD) for the four isolates tested. Boscalid in PDA at 0.02 μg/ml inhibited mycelial growth of isolates HLJ3H and HLJ4H by 15.0 and 8.9%, respectively. However, after the growth-inhibited mycelia were inoculated on rapeseed leaves, isolates HLJ3H and HLJ4H exhibited virulence stimulations of 8.7 and 17.8%, respectively, indicating that hormesis may be masked by inhibitions. Boscalid sprayed at 0.0001 to 0.1 μg/ml on detached rapeseed leaves had significant (P ≤ 0.041) stimulations on virulence of S. sclerotiorum, and the maximum stimulation magnitudes were 17.90 ± 5.94% (mean ± SD) for the four isolates tested. Experiments on 12 isolates with different levels of virulence showed there was a negative correlation (R = -0.663, P = 0.019) between the maximum virulence stimulation magnitude and virulence of S. sclerotiorum in the absence of fungicide. Boscalid at stimulatory concentrations had no significant effect on the expression levels of three virulence-associated genes that encode cutinase (SsCut), polygalacturonase (SsPG1), and oxaloacetate acetylhydrolase (SsOah1). The molecular mechanisms for hormetic effects of boscalid on S. sclerotiorum remain to be studied in the future.

Stimulatory Effects of Sublethal Doses of Carbendazim on the Virulence and Sclerotial Production of Botrytis cinerea

Stimulatory effects of low doses of fungicides on the virulence of phytopathogens have profound implications for applications of fungicides. The present study demonstrated that carbendazim sprayed at 0.001 to 0.03 μg/ml had stimulatory effects on the virulence of mycelia of Botrytis cinerea, and the maximum percent stimulations were 15.5 and 21.4% for isolates HB459 and HB536, respectively. Potato dextrose agar (PDA) amended with carbendazim at 0.01, 0.02, and 0.05 μg/ml inhibited mycelial growth of isolate HB536 by 0.8, 10.0, and 30.6%, respectively. However, after the inhibited mycelia were inoculated on cucumber leaves, virulence increased by 10.1, 12.9, and 10.8%, respectively. With respect to sclerotial production, carbendazim at 0.005 and 0.02 μg/ml in PDA significantly (P < 0.05) increased, while at 0.1 μg/ml significantly (P < 0.05) reduced the sclerotial number and weight of both isolates compared with nontreated controls. Conidia germination percentages slightly yet statistically significantly (P < 0.05) increased after being inoculated on PDA amended with carbendazim at 0.001 and 0.005 μg/ml. Carbendazim at 0.001∼0.02 μg/ml, either sprayed on cucumber leaves or cosuspended with conidia, exerted significantly (P < 0.05) stimulatory effects on the virulence of B. cinerea conidia. Mechanism studies showed that sublethal doses of carbendazim did not increase the expression levels of pathogenicity-related pectin methylesterase gene Bcpme1, endopolygalacturonase gene Bcpg2, cutinase gene CutA, xylanase gene Xyn11A, or NADPH oxidase gene BcnoxA.

Dissolved organic matter derived from rape straw pretreated with selenium in soil improves the inhibition of Sclerotinia sclerotiorum growth

Sclerotinia sclerotiorum (S. sclerotiorum) is a soil-borne pathogen with broad host range. Dissolved organic matter (DOM) plays a vital role in regulating microbial activity in soil. Exogenous selenium (Se) inhibits plant pathogen growth and enhances the capacity of plants to resist disease. DOM from rape straw with Se treated in soil (RSDOM_Se) was extracted, and the inhibitory effect on S. sclerotiorum growth was investigated. RSDOM_Se inhibited S. sclerotiorum growth, which not only caused severe damage to S. sclerotiorum hyphae but also enhanced soluble protein leakage, thereby improving the growth inhibition ratio by 20.9%. As the action in intercellular, RSDOM_Se led to a significant increase in oxalic acid and decrease in CWDE (cell wall-degrading enzyme, which helps pathogens to invade plants) activities, downregulation of Bi1 (BAX inhibitor-1, required for S. sclerotiorum virulence), Ggt1 (γ-glutamyl transpeptidase, regulates the ROS antioxidant system), CWDE2 and CWDE10 gene expression levels, compared with non-Se treated RSDOM (RSDOM_N). Eight metabolites upregulated in RSDOM_Se were identified by GC-TOF-MS, and among these metabolites, fumaric acid, maleic acid, malonic acid, mucic acid, saccharic acid, succunic acid and phenylacetic acid showed significant inhibition on S. sclerotiorum growth. These findings provide valuable insight into a new approach for developing eco-friendly fungicides.

Hormetic Effects of Mixtures of Dimethachlone and Prochloraz on Sclerotinia sclerotiorum

Previous studies showed that dimethachlone has significant hormetic effects on phytopathogenic fungus Sclerotinia sclerotiorum. The present study investigated hormetic effects of mixtures of dimethachlone and prochloraz on mycelial growth and virulence of two dimethachlone-resistant isolates of S. sclerotiorum. The stimulatory dimethachlone dosage range was around 1 to 100 μg/ml in potato dextrose agar (PDA) medium for mycelial growth of the two isolates, and dimethachlone at 10 and 50 μg/ml had the maximum percent stimulations of 80.6 and 19.3% for isolates JMS14 and HLJ4, respectively. Prochloraz at 0.0003 and 0.002 μg/ml had the maximum percent stimulations of 9.3 and 11.1% for isolates JMS14 and HLJ4, respectively. However, dimethachlone and prochloraz mixed at their respective stimulatory concentrations had the maximum percent stimulations of 48.1 and 9.3% for isolates JMS14 and HLJ4, respectively. After the mycelia with increased and inhibited growth on fungicide-amended PDA were subcultured on PDA without fungicide, mycelial growth for the second generation increased compared with the nontreated control. After the mycelia grown on fungicide-amended PDA were inoculated on rapeseed leaves, the amplitude of virulence stimulation was much greater than that of mycelial growth on PDA amended with fungicide, and the inhibited mycelia also showed substantially increased virulence on leaves. The mixture of dimethachlone at 100 μg/ml and prochloraz at 0.03 μg/ml in PDA inhibited mycelial growth of isolate JMS14 by 59.4%; however, after the inhibited mycelia were inoculated on rapeseed leaves, virulence was stimulated by 69.0%. Spraying sublethal doses of dimethachlone and prochloraz on rapeseed leaves also exhibited significant stimulatory effects on virulence. For isolate JMS14, the stimulatory concentration ranges for dimethachlone and prochloraz were around 1 to 600 μg/ml and 0.0003 to 0.18 μg/ml, respectively. The fitted curve of virulence stimulation for the mixture of dimethachlone and prochloraz shifted to the left on the x axis, denoting dose-additive interactions between the two fungicides with regard to virulence stimulation. Spraying dimethachlone alone at 10 to 50 μg/ml had significant stimulations on virulence, whereas prochloraz alone at 10 to 50 μg/ml had significant inhibitory effects on virulence, and the mixture of dimethachlone and prochloraz at the concentration ratio of 1:1 had greater inhibitory effects than prochloraz alone, indicating dose-additive interactions for the inhibitory effects. Dimethachlone and prochloraz and their mixtures increased tolerance of mycelia to hydrogen peroxide. Dimethachlone significantly increased, whereas prochloraz reduced mycelial cell membrane permeability, and the mixture of the two fungicides had effect-additive interactions with respect to effects on cell membrane permeability. These studies will advance our understanding of hormesis of fungicide mixtures.

Hormetic Effects of Mixtures of Carbendazim and Iprodione on the Virulence of Botrytis cinerea

Hormetic effects of fungicides on mycelial growth and virulence of plant pathogenic fungi have been reported, but the effects of fungicide mixtures on virulence hormesis of plant pathogens remain to be investigated. In this study, hormetic effects of mixtures of carbendazim and iprodione on the virulence of two carbendazim-resistant isolates of Botrytis cinerea were determined. Spraying carbendazim alone at 3 to 800 μg/ml exhibited hormetic effects on virulence to cucumber leaves, and carbendazim at 10 μg/ml had the maximum stimulation of 16.7% for isolate HBtom451. Spraying iprodione alone at 0.0001 to 0.0625 μg/ml exhibited hormetic effects on virulence, and iprodione at 0.025 μg/ml had the maximum stimulation of 18.7% for isolate HBtom451. However, spraying simultaneously carbendazim at 800 μg/ml and iprodione at 0.0625 μg/ml showed inhibitory effects on virulence to cucumber leaves. The mixture of carbendazim at 3 μg/ml and iprodione at 0.0001 μg/ml had much higher virulence stimulations than either fungicide at the same concentration alone. The maximum stimulation for the mixtures occurred at 10 and 0.0005 μg/ml for carbendazim and iprodione, respectively, and these concentrations were much lower than the concentration of their respective fungicide alone eliciting the maximum stimulations. The maximum stimulation amplitude for the mixture was slightly higher than that of each fungicide alone. These results demonstrated that carbendazim and iprodione mainly had dose-additive rather than amplitude-additive interactions when sprayed simultaneously with regard to virulence stimulations. Studies on virulence stimulations for mycelia treated with fungicide in potato dextrose agar showed that the maximum stimulation for the mixtures occurred at concentrations much lower than the concentration of carbendazim alone, indicating a dose-additive interaction when compared with carbendazim hormesis. Studies on potential physiological mechanisms of hormesis showed that increased tolerance to H₂O₂ may be one of the mechanisms for virulence hormesis for the mixtures of iprodione and carbendazim. These studies will advance our understanding of hormesis of fungicide mixtures.

Hormetic Effects of Flusilazole Preconditioning on Mycelial Growth and Virulence of Sclerotinia sclerotiorum

Hormetic effects of fungicides are highly relevant to fungicide applications and management of plant-pathogenic fungi. Preconditioning (i.e., early exposure to relatively low doses of a toxicant) is a special form of hormesis, and fungicide preconditioning of phytopathogenic fungi is inevitable in the field. The present study showed that spraying the demethylation inhibitor (DMI) fungicide flusilazole at 0.1 µg/ml had stimulatory effects on the virulence of Sclerotinia sclerotiorum inoculated at 1 and 24 h after spraying. Flusilazole sprayed at 10 µg/ml showed inhibitory effects on the virulence of S. sclerotiorum inoculated during the first 3 days after spraying. Inoculations on the 5th, 7th, and 10th day after spraying did not show any significant inhibitory or stimulatory effects on the virulence. After growing for 2 days on potato dextrose agar (PDA) amended with flusilazole at a dose range from 0.0005 to 0.25 µg/ml as preconditioning treatments, mycelia were transferred onto PDA without fungicide and subsequent mycelial growth was slower than the nonpreconditioned control. However, after the preconditioned colonies were transferred onto PDA supplemented with flusilazole at 0.2 µg/ml, percent stimulations of mycelia growth compared with the control had a parabolic shape across the preconditioning flusilazole concentration range. Similarly, the mycelial growth of the preconditioned mycelial plugs on PDA amended with other DMI fungicides (prochloraz or tebuconazole) also showed a typical hormetic response, whereas mycelial growth on PDA amended with carbendazim or dimethachlone was inhibited in a dose-dependent manner. Preconditioning S. sclerotiorum with flusilazole on rapeseed plants elicited virulence stimulations in a dose-dependent manner similar to those on mycelial growth on PDA. After disease lesions developed on rapeseed leaves sprayed with flusilazole as the preconditioning treatment were inoculated onto rapeseed plants, virulence was inhibited on leaves without fungicide or sprayed with carbendazim or dimethachlone compared with the nonpreconditioned control, whereas virulence was stimulated on leaves sprayed with flusilazole, prochloraz, or tebuconazole, and the maximum percent stimulation was 10.2%. These results will advance our understanding of hormetic effects of fungicides and of preconditioning hormesis in particular.

Hormetic Effects of Carbendazim on the Virulence of Botrytis cinerea

The ascomycete plant-pathogenic fungus Botrytis cinerea infects more than 1,400 plant species worldwide. Stimulatory effects of sublethal doses of fungicides on plant pathogens are of close relevance to disease management. In the present study, stimulatory effects of carbendazim on the virulence of B. cinerea to cucumber plants were investigated. Spraying carbendazim on cucumber plants at 3 to 200 μg/ml had stimulatory effects on the virulence of carbendazim-resistant isolates of B. cinerea and the maximum percent stimulations were 16.7 and 13.5% for isolates HBtom451 and HBstr491, respectively. Preconditioned mycelia (i.e., mycelia grown on potato dextrose agar [PDA] amended with carbendazim at concentrations of 10, 50, or 200 μg/ml) also showed increased virulence, and the maximum percent stimulations for isolates HBtom451 and HBstr491 were 7.9 and 9.5%, respectively. Compared with mycelia grown on PDA without carbendazim, virulence stimulation magnitudes of spraying carbendazim on leaves increased moderately but the concentrations of carbendazim that elicited the maximum stimulation increased 20- and 8-fold for preconditioned isolates HBtom451 and HBstr491, respectively. The time course of infection indicated that virulence stimulation was mediated by a direct stimulation mechanism. Studies of the physiological mechanism for stimulation demonstrated that carbendazim had no significant effects on tolerance to hydrogen peroxide, or on oxalic acid production in B. cinerea. These studies will deepen our understanding of quantitative features of hormetic effects of sublethal doses of fungicides on plant pathogens.