A comparison of the mechanisms of action of vinclozolin,procymidone, iprodione and prochloraz against Botrytis cinerea

The mechanisms of action against Botrytis cinerea of the dicarboximide fungicides vinclozolin ( I ), procyrnidone ( II ), iprodione ( III ) and the less closely related compound prochloraza ( IV ) have been compared. They all inhibited mycelial growth much more than spore germination. None of the compounds affected respiration, membrane permeability or RNA production but III inhibited DNA synthesis and IV inhibited protein synthesis. Although chitin biosynthesis was inhibited by all the fungicides it was barely affected at the ED₅₀ concentrations (the concentration required to reduce the growth or germination of the test species by 50%) and is thus unlikely to be the primary target. The fungicides altered fungal lipid composition. I and II inhibited triglyceride production but III had no effect on it; III and IV reduced sterol biosynthesis. No common primary mode of action was found.     

Cross-Resistance Patterns Among Sterol Biosynthesis Inhibiting Fungicides (SBIs) in Cercospora beticola

Thirty single-spore isolates of Cercospora beticola, collected from several fields in northern Greece, representing a broad spectrum sensitivity to the sterol demethylation-inhibiting (DMIs) fungicide flutriafol, were tested for sensitivity to eleven other sterol biosynthesis-inhibiting (SBI) fungicides and to the guanidine fungicide dodine. Sensitivity was measured as EC₅₀ values for each fungicide and log-transformed EC₅₀ values to each fungicide were pairwise correlated and the correlation coefficient estimated. These pairwise comparisons showed high correlation coefficients between the DMIs suggesting a cross-resistance relationship between these fungicides. However, the degree of cross-resistance between DMIs varied greatly. Conversely, low correlation coefficients were obtained for the pair-wise comparisons with the morpholine fungicide fenpropimorph suggesting a lack of cross-resistance between morpholines and DMIs in C. beticola. Similarly, there was no correlation between the sensitivity (EC₅₀ values) to dodine and all the other fungicides tested, indicating that there was no negative cross-resistance relationship between dodine and SBIs in C. beticola. Based on these results, combinations or alternations of fungicides which show no cross-resistance relationship should be used to control the disease in areas where reduced sensitivity to DMIs has been already observed.     

Effects of sterol biosynthesis-inhibiting (SBI) fungicides on cytochrome P-450 oxygenations in fungi

The fungicides miconazole, fenarimol, and etaconazole block ergosterol biosynthesis in fungi by inhibiting sterol 14α-demethylation, which is mediated by a cytochrome P-450 enzyme. The sensitivity of cytochrome P-450-dependent hydroxylation or demethylation of several substrates to these fungicides and similar compounds was compared to that of fungal growth and sterol 14α-demethylation. Demethylation of p-chloro-N-methylaniline (PCMA) by sporidia of Ustilago maydis and 11α-hydroxylation of progesterone by Aspergillus nidulans were relatively insensitive to these compounds and to metyrapone. The ability of a sterol 14α-demethylation-deficient mutant to demethylate PCMA indicates that this substrate is not demethylated by the sterol 14α-demethylation system of U. maydis. The 14α-hydroxylation of progesterone by cells of Curvularia lunata was quite sensitive to the three fungicides, and also to metyrapone and isopropylphenylimidazole. This system was less sensitive to the three fungicides than sterol 14α-demethylation, but was appreciably more sensitive than PCMA demethylation. A study of progesterone 14α-hydroxylation in cell-free preparations of C. lunata showed the reaction to be inhibited by CO, and to be competitively inhibited by low concentrations of miconazole. These data suggest that the primary action of sterol biosynthesis-inhibiting (SBI) fungicides is competitive inhibition of sterol/steroid-type cytochrome P-450 enzymes rather than interference with the function of sterol carrier proteins or enzyme-modulating phospholipids.     

Synergistic antifungal interactions of zinc or copper with anilazine

Two isolates of Botrytis cinerea and one of Colletotrichum coccodes were used to test for synergistic or antagonistic interactions (a) between pairs of 11 common fungicides and (b) between mixtures of individual fungicides with components of foliar fertilisers. Two screening assays monitored the effects with time of inhibitor combinations on spore germination and subsequent mycelial growth. The combinations that gave positive results for interaction in the screening assays were further investigated by a third quantitative assay, which compared the observed inhibition of mycelial growth with the net inhibition predicted by a non-interaction model. This model assumed that the components of a combination of substances each inhibited the test fungus independently of one another. Only mixtures of anilazine with zinc or copper resulted in appreciable deviation from the predicted behaviour based on a non-interaction model. The inhibition of mycelial growth by combinations of anilazine with Zn²⁺ (inorganic salt or formulated with urea) was consistently greater than predicted for each of three fungi on either of the two test media. Similar effects were observed with Cu²⁺, although the synergism was less marked.     

Sterol content and other characteristics of pimaricin-resistant mutants of Aspergillus nidulans

Pimaricin-resistant mutants of Aspergillus nidulans were selected on a medium containing the polyene-antibiotic. Some resistant mutants contained markedly reduced amounts of ergosterol, but others contained almost normal levels of this sterol. Most resistant mutants which lacked ergosterol had a biochemical lesion in sterol C-22 desaturation. Analysis of sterols in one of these isolates showed the presence of 5,7-ergostadienol, 5,7,24(28)-ergostatrienol, and 5,8-ergostadienol. The sterol C-14 demethylation inhibitor, fenarimol, was more toxic to this mutant than to the wild type. On the other hand, mutants inactive in sterol C-22 desaturation were resistant to oligomycin but showed wild type sensitivity to carboxin. Attempts to select sterol C-14-demethylation-deficient mutants of Aspergillus nidulans, Monilinia fructicola, and Pyricularia oryzae on polyene-containing media were unsuccessful. Apparently C-14-methyl sterols do not support growth of these filamentous fungi.     

Effects of sterol biosynthesis-inhibiting fungicides and plant growth regulators on the sterol composition of barley plants

A number of sterol biosynthesis-inhibiting (SBI) fungicides and plant growth regulator analogs were applied as root drenches to barley seedlings and their effect on the total sterol composition of the roots and shoots was measured by gas-liquid chromatography. Prochloraz was found to be inactive in this system, probably because of poor uptake, while the other compounds could be divided into two groups according to their mode of action as assessed by sterol profiling. The morpholines tridemorph and fenpropimorph inhibited the enzyme cycloeucalenol—obtusifoliol isomerase whereas triadimenol, nuarimol, paclobutrazol, and triapenthenol (RSW 0411) inhibited the enzyme responsible for the removal of the C-14 methyl group. Effects of individual diastereo-isomers and enantiomers of some compounds on sterol profiles were compared with their known fungicidal and anti-gibberellin properties. Shoot growth was reduced by all the compounds tested, paclobutrazol, nuarimol, and triapenthenol being the most effective. As well as inducing accumulation of abnormal sterols, SBI fungicide treatment changed the ratio of campesterol to stigmasterol and sitosterol. It is hypothesized that this may reflect changes in membrane architecture and may offer an explanation for the increased frost hardiness sometimes observed with SBI fungicide-treated plants.     

Effects of triazole fungicides on sterol biosynthesis during spore germination of Botrytis cinerea,Venturia inaequalis and Puccinia graminis f. sp. tritici

With three plant pathogens,Botrytis cinerea, Venturia inaequalis and Puccinia graminis f. sp.tritici, the time course of sterol biosynthesis during spore germination was examined by labeling experiments along with the question whether this pathway could be inhibited by triazole fungicides. Conidia ofB. cinerea andV. inaequalis are able to synthesize sterols immediately after the beginning of the germination process when the germ tubes have not yet emerged. On the contrary uredospores ofP. graminis start sterol biosynthesis after 6 to 8 h germination time almost at the end of the germ tube phase, indicating that sterol reserves of the spores are likely to be used for the germ tube growth.The sterol C-14 demethylation appeared to be the rate limiting step within the sterol biosynthetic pathway: the half life of 24-methylenedihydrolanosterol was less than 1 h forB. cinerea. It was more than 1 h forV. inaequalis and 3 h forP. graminis. Independent of these differences in the time course of sterol biosynthesis and in the C-14 demethylation rate, the synthesis of sterols in germinating spores was strongly inhibited by triazole fungicides in all three pathogens examined. In contrast toP. graminis, this inhibition could be demonstrated withB. cinerea andV. inaequalis even in ungerminated conidia, indicating that the fungicides were rapidly taken up and reached their target within 1 or 2 h. These results are discussed along with the question whether spore germination can be used as a bioassay for the estimation of sensitivities of triazole fungicides.     

Effect of the new pyrazole carboxamide fungicide penthiopyrad on late leaf spot and stem rot of peanut

BACKGROUND: Management of early leaf spot (Cercospora arachidicola Hori.), late leaf spot [Cercosporidium personatum (Berk. & MA Curtis) Deighton] and stem rot (Sclerotium rolfsii Sacc.) of peanut (Arachis hypogaea L.) in the southeastern USA is heavily dependent upon sterol biosynthesis inhibitor (SBI) and quinone outside inhibitor (QoI) fungicides. Effective new fungicides with different modes of action could improve overall disease control and extend the utility of the current fungicides. Penthiopryad is a pyrazole carboxamide fungicide being evaluated for use on peanut. Field experiments were conducted from 2004 to 2007 to determine the effect of a range of rates (0–0.36 kg AI ha^?1) of penthiopyrad on leaf spot and stem rot and the relative efficacy of penthiopyrad and current fungicide standards chlorothalonil, tebuconazole and azoxystrobin. RESULTS: Leaf spot control in plots treated with penthiopyrad at 0.20 kg AI ha^?1 or higher was similar to or better than that for the chlorothalonil standard. The incidence of stem rot for all penthiopyrad treatments was usually less than that for the tebuconazole or azoxystrobin standard treatments. Pod yields for all penthiopyrad treatments were similar to or higher than those for the respective standards. CONCLUSION: Penthiopyrad has excellent potential for management of late leaf spot and stem rot of peanut, and may complement current SBI and QoI fungicides. Copyright © 2008 Society of Chemical Industry     

New Cases of Negative Cross-resistance between Fungicides,Including Sterol Biosynthesis Inhibitors

A survey of fungicide resistance in Mycosphaerella graminicola and Tapesia acuformis, two major pathogens of winter wheat in France, respectively responsible for speckled leaf blotch and eyespot, led to the characterization of two types of resistant strains to sterol 14α-demethylation inhibitors (DMIs). Most of the strains of M. graminicola collected in France in 1997–1998 were resistant to all DMIs, and only in a few strains was the resistance to several triazoles associated with increased susceptibility to pyrimidine derivatives (i.e., fenarimol, nuarimol) and triflumizole. On the other hand, in T. acuformis the most prevalent strains were those which exhibited negative-cross resistance between DMIs. In both fungi such a phenomenon could be related to changes in cytochrome P450 sterol 14α-demethylase, the target site of these fungicides. For Botryotinia fuckeliana, the causal agent of grey mould, the extensive monitoring conducted in French vineyards before the marketing of fenhexamid revealed the presence of highly resistant strains to this promising botryticide (only in tests involving mycelial growth measurements). Negative cross-resistance to edifenphos and several sterol biosynthesis inhibitors, such as prochloraz and fenpropimorph, was observed in fenhexamid resistant strains. Synergism of the antifungal action of fenhexamid by cytochrome P450 inhibitors, such as the DMI fungicides, was only recorded in fenhexamid resistant strains. These data and those previously obtained with edifenphos resistant strains of Magnaporthe grisea (rice blast pathogen) suggest that in fenhexamid resistant strains of B. fuckeliana the same cytochrome P450 monooxygenase could be involved in detoxification of fenhexamid and activation of edifenphos. Received 6 September 1999/ Accepted in revised form 13 September 1999     

Relationship between chemical structure and biological activity of triazole fungicides against Botrytis cinerea

The inhibitory activity of commercial and experimental triazole fungicides on the target enzyme, sterol 14α-demethylase (P450_14DM), was studied in a cell-free sterol synthesis assay of Botrytis cinerea Pers. ex Fr. In order to assess structure-activity relationships, the inhibitory activities of the compounds on radial growth of the fungus were tested as well. The EC₅₀ values (concentrations of fungicide inhibiting radial growth of B. cinerea on PDA by 50%) of all triazoles tested ranged between 10^?8 and 10^?5 m. IC₅₀ values (concentrations of fungicide inhibiting incorporation of [2-¹⁴C]mevalonate into C4-desmethyl sterols by 50%) generally ranged between 10^?9 and 10^?7 M and correlated with inhibition of radial mycelial growth. However, differences in IC₅₀ values did not reflect quantitatively the observed differences in EC₅₀ values, since the ratio between EC₅₀ and IC₅₀ increased with decreasing fungitoxicity. For a limited number of compounds the correlation between intrinsic inhibitory activity and fungitoxicity was low. Both in-vitro tests were used to investigate structure-activity relationships for stereoisomers of cyproconazole, SSF-109 and tebucona-zole. Fungitoxicity and the potency to inhibit cell-free C4-desmethyl sterol synthesis correlated for all stereoisomers tested. Mixtures of isomers of tebucona-zole or cyproconazole were slightly less active than the most potent isomer. The high activity of several commercial triazoles in both experiments implies that poor field performance of triazole fungicides against B. cinerea is due neither to insensitivity of the P450_14DM nor to low in-vitro sensitivity of the fungus.     

Effects of sterol biosynthesis inhibitor fungicides in the phytopathogenic fungus,Nectria haematococca: ergosterol depletion versus precursor or abnormal sterol accumulation as the mechanism of fungitoxicity

The relative importance of the depletion of ergosterol versus the accumulation of precursor or abnormal sterols in the mechanism of fungal growth inhibition by sterol biosynthesis inhibitor fungicides is incompletely understood. In order to investigate this problem further, the degree of inhibition of the growth of Nectria haematococca by fungicides with different enzymatic targets in the sterol biosynthetic pathway was determined and compared with their effects on the sterol profile. The sensitivity of N. haematococca was highest towards fenpropimorph, followed by tebuconazole, terbinafine, fenpropidin and tridemorph. Terbinafine, a squalene epoxidase inhibitor, induced a very large accumulation of squalene without very significant inhibition of ergosterol biosynthesis and growth. The fungus appeared able to tolerate large amounts of squalene. In the case of tebuconazole, a sterol 14α-demethylase inhibitor, it seemed that the accumulation of C4 mono- and dimethyl sterols was responsible for fungitoxicity. Fenpropimorph and fenpropidin seemed to be good inhibitors of both sterol Δ¹⁴-reductase and Δ⁸→Δ⁷-isomerase, whereas tridemorph was a better inhibitor of Δ⁸→Δ⁷-isomerase than of the Δ¹⁴-reductase. Large accumulations of Δ^8,14- or Δ⁸-sterols and correspondingly large decreases in the ergosterol content are both implicated in the fungitoxicity of these compounds in N. haematococca. © 1998 Society of Chemical Industry     

Possible methionine biosynthesis inhibition by pyrimidinamine fungicides

The broad-spectrum fungicide CGA 219417 inhibits mycelial growth of Botrytis cinerea Pers ex Fr. Pseudocercosporella herpotrichoides (Fron.) Deighton and Helminthosporium oryzae B. de Haan on defined media lacking amino acids. The growth inhibition of B. cinerea is reversed by the addition of a mixture of 19 amino acids at a concentration of 100 μM each or by the addition of methionine or homocysteine in concentrations of 100 μM or 1 mM, respectively. In the case of B. cinerea, the reversal of growth inhibition by methionine could also be shown for pyrimethanil and mepanipyrim. These findings suggest that the pyrimidinamine fungicides inhibit the biosynthesis of methionine in phytopathogenic fungi.     

Similarities between the systemic fungicides triforine and triarimol

The fungicides triforine and triarimol, though structurally unrelated have a similar antifungal spectrum. Neither fungicide inhibited germination of conidia of Cladosporium cucumerinum, but both inhibited subsequent mycelial growth. Triforine, like triarimol, proved to be an effective inhibitor of ergosterol biosynthesis. Two mutants of C. cucumerinum selected for resistance to triarimol also exhibited resistance to triforine and to the triarimol analogues ancymidol and bis(4-chlorophenyl)-3-pyrimidinemethanol. Resistance of these mutants, however, did not extend to the sterol inhibitors triparanol and trans-clomiphene or to sodium o-phenylphenate and 2,6-dichloro-4-nitroaniline, two compounds usually tolerated by fungi exhibiting general hydrocarbon resistance. Toxicity of either triforine or triarimol to C. cucumerinum was annuled by β-carotene, vitamin A, progesterone, testosterone and farnesol. The various similarities between triforine and triarimol suggest a common mode of action for the two compounds.     

纳他霉素对灰葡萄孢不同生育阶段菌体的毒力及其生物学性状的影响

以多菌灵为对照药剂,采用生长速率法、悬滴法和浸渍法分别测定了不同温度下纳他霉素对灰葡萄孢Botrytis cinerea菌丝生长、分生孢子萌发和菌核萌发的毒力,研究了常温下两种药剂对菌株产孢和菌核形成的影响。结果表明:纳他霉素及多菌灵对灰葡萄孢不同生育阶段菌体的抑制作用随温度降低而有不同程度增强;分生孢子对药剂最为敏感;温度对药剂对菌丝毒力的影响最显著;多菌灵对菌株不同发育阶段的抑制活性均高于纳他霉素。供试药剂对菌株产孢时间和菌核产生时间无显著影响,但多菌灵可显著刺激菌株产孢和菌核形成。     

牛至精油及其主要成分香芹酚和百里酚对16种植物病原真菌的抑制活性

为评价牛至精油及其2种主要成分香芹酚和百里酚在植物真菌病害防治方面的应用前景,采用菌丝生长速率法和孢子萌发法测定3种物质对16种植物病原真菌菌丝生长和孢子萌发的抑制活性,并采用黄瓜子叶法测定其对黄瓜灰霉病的防治效果。结果表明,牛至精油在500 mg/L浓度处理下可完全抑制番茄早疫病菌Alternaria solani、黄瓜灰霉病菌Botrytis cinerea和小麦根腐病菌Bipolaris sorokiniana等12种植物病原真菌的菌丝生长,对棉花黄萎病菌Verticillium dahliae的菌丝生长抑制率也达到97.66%。牛至精油对这13种病原真菌的EC₅₀在83.09~236.58 mg/L之间,香芹酚和百里酚对这13种病原真菌的毒力均高于牛至精油,其中香芹酚对黄瓜灰霉病菌、油菜菌核病菌Sclerotinia sclerotiorum、小麦全蚀病菌Gaeumannomyces graminis、棉花黄萎病菌和小麦茎基腐病菌Fusarium pseudograminearum菌丝生长的毒力较强,EC₅₀分别为9.09、22.45、24.28、28.40和28.80 mg/L;百里酚对黄瓜灰霉病菌和小麦全蚀病菌菌丝生长的毒力较强,EC50分别为21.32 mg/L和27.08 mg/L。香芹酚和百里酚对棉花黄萎病菌、玉米大斑病菌Setosphaeria turcica、苦瓜枯萎病菌Fusarium oxysporum f.sp.momordicae、黄瓜灰霉病菌和黄瓜炭疽病菌Colletotrichum orbiculare的孢子萌发亦有较强毒力,其EC₅₀为3.78~289.07 mg/L。1 000 mg/L牛至精油对黄瓜灰霉病的预防效果与400 mg/L嘧霉胺悬浮剂的预防效果无显著差异;500 mg/L百里酚对黄瓜灰霉病的预防效果和治疗效果均与400 mg/L嘧霉胺悬浮剂的效果相当,而香芹酚的治疗效果显著优于400 mg/L嘧霉胺悬浮剂。表明牛至精油及其主要成分香芹酚和百里酚的抑菌谱广且活性强,具有作为植物源杀菌剂的开发应用潜力。     

哈茨木霉菌与5种杀菌剂对番茄灰霉病菌的协同作用

采用对峙法、菌丝生长速率法和离体叶片法分别测定了哈茨木霉菌Trichoderma harzianum与5种不同作用机制杀菌剂联用对番茄灰霉病菌Botrytis cinerea的抑制作用。结果表明,联用可增强哈茨木霉菌及啶酰菌胺、嘧菌酯、咯菌腈、氟啶胺和啶菌噁唑5种供试药剂对番茄灰霉病菌的抑制作用,即具有协同作用,因此可考虑将杀菌剂与木霉菌联合用于番茄灰霉病的防治。     

Nested PCR‐RFLP is a high‐speed method to detect fungicide‐resistant Botrytis cinerea at an early growth stage of grapes

BACKGROUND: Grey mould caused by the fungus Botrytis cinerea Pers. ex Fr. is one of the major diseases in grapes. The use of fungicides is a simple strategy to protect grapes against B. cinerea disease. However, phenotypes exhibiting resistance to fungicides have been detected in B. cinerea populations. The variation of fungicide‐resistant B. cinerea isolates renders B. cinerea disease control difficult in grapevine fields. RESULTS: The authors have developed a nested polymerase chain reaction–restriction fragment length polymorphism (PCR‐RFLP) method to detect fungicide‐resistant B. cinerea isolates at an early growth stage of grapes in grapevine fields. The nested PCR‐RFLP method was carried out to detect benzimidazole‐, phenylcarbamate‐ and/or dicarboximide‐resistant B. cinerea isolates from grape berries and leaves at Eichorn–Lorenz growth stage 25 to 29. This method successfully detected fungicide‐resistant B. cinerea isolates at an early growth stage of grapes. In addition, only 8 h was required from tissue sampling to phenotyping of fungicide resistance of the isolates. CONCLUSION: It is proposed that the early diagnosis of fungicide‐resistant B. cinerea isolates would contribute to further improvement of integrated pest management against B. cinerea in grapevine fields, and that the nested PCR‐RFLP method is a high‐speed, sensitive and reliable tool for this purpose. Copyright © 2008 Society of Chemical Industry     

Reduced sensitivity of Botrytis cinerea to two sterol biosynthesis-inhibiting fungicides: fenetrazole and fenethanil

Isolates of Botrytis cinerea having reduced sensitivity to the sterol biosynthesis-inhibiting (SBI) fungicides fenetrazole and fenethanil were obtained from one out of four sites from which isolates were tested. Reduced sensitivity was associated with poor disease control by fenetrazole, which had been applied with dichlofluanid. Conidial germination and hyphal growth of B. cinerea from the four sites were tested in vitro on media amended with the fungicides. Following fenetrazole or fenethanil treatment, at the site where control had failed, populations of B. cinerea were detected with higher EC₅₀ and EC₉₀ values than at the three other sites. Germination of conidia of B. cinerea was markedly inhibited by 1.0 μg/ml fenetrazole and 0.5–1.0 μg/ml fenethanil. The frequency of isolates insensitive to 1.0 μg/ml fenetrazole or to 0.5 μg/ml fenethanil was 3.4 and 1.8 times higher, respectively, at the site where control had failed, compared with another site where SBI fungicides had never been applied to control grey mould. Grey mould caused by selected isolates of B. cinerea which expressed the phenotype of low sensitivity to SBI fungicides in leaves of tomato, pepper and Senecio cineraria was not controlled by either fenetrazole or fenethanil (1 .5–3.0 μg/ml). However, up to 100% disease reduction was obtained when leaves infected by sensitive isolates were treated with the fungicides.     

Effects of certain herbicides on the fate of sporangiospores of Mucor piriformis and conidia of Botrytis cinerea and Penicillium expansum

The effects of several herbicides used in pome fruit orchards on the germination of spores and growth of mycelia of Botrytis cinerea, Mucor piriformis and Penicillium expansum in vitro and the survival of propagules of these fungi in soil were studied. Diuron in agar at 4–128 μg ml^?1 reduced germination of spores of B. cinerea and M. piriformis, and 2,4-D and paraquat at 32 μg ml^?1 similarly affected B. cinerea and P. expansum. Several herbicides at 128 μg ml^?1 in agar reduced growth of B. cinerea and M. piriformis but were ineffective against P. expansum. Propagule survival levels of the three fungi generally were lower in both autoclaved and non-autoclaved soil amended with herbicides than in non-amended soil. This effect was greatest in non-autoclaved soil, suggesting involvement of microbial antagonists. The most effective herbicides for reduction of fungal propagules in soil were 2,4-D, diuron, and paraquat.     

Inhibition of Botrytis cinerea growth and suppression of botrytis bunch rot in grapes using chitosan

Chitosan inhibited growth of Botrytis cinerea in liquid culture and suppressed grey mould on detached grapevine leaves and bunch rot in commercial winegrapes. Germination of B. cinerea was completely inhibited in malt extract broth containing chitosan at concentrations greater than 0·125 g L^?1. However, treated conidia were able to infect detached Chardonnay leaves and pathogenicity was not affected, even after incubation for 24 h in chitosan at 10 g L^?1. When added after conidial germination, chitosan inhibited B. cinerea growth and induced morphological changes suggestive of possible curative activity. The effective concentration of chitosan that reduced mycelial growth by 50% (EC₅₀) was 0·06 g L^?1. As a foliar treatment, chitosan protected detached Chardonnay leaves against B. cinerea and reduced lesion diameter by up to 85% compared with untreated controls. Peroxidase and phenylalanine ammonia‐lyase activities were also induced in treated leaves. In vineyard studies, Chardonnay winegrapes exhibited 7·4% botrytis bunch rot severity at harvest in 2007 after treatment with an integrated programme that included chitosan sprays from bunch closure until 2 weeks preharvest, compared with 15·5% in untreated controls and 5·9% with fungicide treatment. In the following season, botrytis bunch rot severity was 44% in untreated Chardonnay at harvest and the integrated programme (21%) was less effective than fungicides (13·8%). However, in Sauvignon blanc winegrapes, the integrated and the fungicide programme each reduced botrytis bunch rot severity to 4% and were significantly different from the untreated control (11·5%). This study provides evidence that suppression of botrytis in winegrapes by chitosan involves direct and indirect modes of action.