Sensitivity distribution of Venturia inaequalis to fenarimol in Québec apple orchards

Cette étude a été faite dans le but d'établir le seuil de sensibilité du Venturia inaequalis au fenarimol, un inhibiteur de la biosynthèse de l'ergostérol. Au cours de la saison 1988, des isolats du V. inaequalis ont été prélevés dans 26 vergers commerciaux du Québec pour un total de 576 isolats monoconidiens. La sensibilité au fenarimol a été établie par des tests d'inhibition de la croissance radiale du mycélium. Les valeurs de ED50 obtenues pour les 26 vergers ont varié de 0,024 à 5,212 μg mL-1 pour une moyenne de 0,156 μ g mL-1. Une réduction de sensibilité, exprimée en valeur de ED50, a été observée dans trois vergers, soit pour 4,51% des isolats testés. Les isolats sensibles avaient un ED50 moyen de 0,079 μg mL-1, et les isolats de sensibilité réduite démontraient un ED50 de 1,714 (μg mL-1, ce qui représente un facteur de résistance de 22. Quatre populations ont été identifiées en fonction de la distribution de fréquence des valeurs de ED50.This study was initiated to quantify the baseline sensitivity of apple scab (Venturia inaequalis) to fenarimol, an ergosterol synthesis-inhibiting fungicide. In 1988, 576 monoconidial isolates of Venturia inaequalis were collected from 26 commercial orchards throughout Quebec. Sensitivity to fenarimol was assessed by radial growth inhibition assay. The ED50 values for the 26 orchards ranged from 0.024 to 5.212 (μ g mL-1 with a mean ED50 of 0.156 μg ml-1. Reduced sensitivity, expressed as ED50, was found in three orchards for an overall frequency of 4.51% of isolates. Sensitive isolates had a mean ED50 of 0.079 μg ml-1, whereas isolates with reduced sensitivity had a mean ED50 of 1.714 μ g mL-1, yielding a resistance factor of about 22. Four populations were identified based on the frequency distribution of ED50 values

A review of non-fungicidal approaches for the control of apple scab

Au Canada, la tavelure est la plus importante des maladies du pommier et la plus coûteuse à réprimer. Un échec dans le contrôle de la tavelure se traduira par une baisse de rendement importante et une diminution de la valeur marchande des fruits. La lutte contre la tavelure est essentiellement basée sur l'application de fongicides. Ces applications répétées de fongicides contribuent à l'augmentation des coûts de production et peuvent avoir un impact indirect sur l'environnement. La dépendance envers les fongicides pourrait être réduite par l'intégration de mesures alternatives aux fongicides, incluant des méthodes génétiques, physiques et biologiques. Les recommandations pour la lutte chimique sont souvent basées sur des études faites avec le cultivar McIntosh qui est très sensible à la tavelure. Ces recommandations étaient justifiées par la présence de 60 à 70 % de ce cultivar dans le nord-est de l'Amérique du Nord. Toutefois, cette situation change dans la mesure où les producteurs plantent de plus en plus de cultivars moins sensibles. L'adaptation de ces recommandations en tenant compte de la sensibilité moindre de certains cultivars permettrait de réduire le nombre d'applications de fongicides. Malheureusement, la recherche sur le développement de cultivars résistants n'a pas donné les résultats escomptés. Toutefois, les nouveaux outils moléculaires peuvent servir à identifier et localiser les gènes de résistance et ainsi permettre une percée importante dans le développement de cultivars résistants. De plus, les applications de fongicides faites au printemps pour réprimer les infections primaires, peuvent être retardées à la suite de traitements d'automne incluant le broyage des feuilles et l'application d'agents de lutte biologique ou d'urée. Ces mesures sont préventives et peuvent s'intégrer dans les programmes de lutte actuels. Toutefois, l'usage de ces mesures est plus complexe que de simples applications de fongicides. Par contre, la lutte intégrée contre la tavelure est plus durable dans la mesure où elle ne dépend pas d'une seule méthode de contrôle et parce qu'elle permet de réduire les risques de développement de résistance aux fongicides dans la population de l'agent pathogène.Apple scab is the single most important disease of apple in Canada and the most costly to control. Failure to control apple scab results in severe yield losses and a reduction in market value of harvested fruits. Currently, the strategy to control apple scab relies on multiple applications of fungicides. These sprays are a significant cost to growers and the indirect environmental impact may be substantial. Reliance on fungicides can be reduced by the integration of non-fungicidal control measures that include genetic, physical, and biological approaches. Recommendations for chemical control of apple scab are often based on the highly susceptible cultivar McIntosh. This was justified because up to 60 to 70% of orchards in northeastern North America were planted with this cultivar. Today, the situation is changing as more and more growers are planting new cultivars, some of which are less susceptible to apple scab. A change in the recommendations to account for cultivar susceptibility could result in a reduction in the number of fungicide applications required. In addition, there has been long-standing research on resistant cultivars, but none of the scab-resistant cultivars have been widely accepted. However, with new molecular techniques to identify and locate the resistance genes, there is potential for progress on this front. Furthermore, fungicide applications for primary infections can be delayed if the number of ascospores is reduced after fall treatments that include leaf shredding, the application of biological control agents, or urea. These methods are preventive and can be integrated into existing management programs. However, the integration of all or some of these methods is more complex than simply the use of fungicides. Nevertheless, integrated management of apple scab may prove more sustainable on a long-term basis, mainly because it does not depend on the use of a single method. Hence the risk of the development of fungicide resistance in the pathogen population is reduced

Effect of plant age, leaf age and leaf position on infection of carrot leaves by Cercospora carotae

On a évalué en serre l'influence de l'âge des plants, des feuilles et de la position des feuilles de carotte (Daucus carota var. sativa) sur le niveau d'infection par le Cercospora carotae. Le niveau d'infection a été estimé en comptant le nombre de lésions cm-2 de surface foliaire et en mesurant la période d'incubation. Le niveau d'infection a diminué au fur et à mesure que l'âge des plants augmentait de 39 à 60 j, puis est resté faible chez les plants âgés de 60 à 71 j. La période d'incubation a augmenté de 9 à 16,6 j avec l'âge des plants. Le niveau d'infection a diminué avec l'augmentation de l'âge des feuilles de 1 à 36 j, toutefois la variation entre les feuilles était grande. La période d'incubation a augmenté de 9 à 18,3 j avec l'âge des feuilles, quoique certaines lésions sont apparues tardivement. Le niveau d'infection n'a pas varié en fonction de la position des feuilles sur des plants de 10 et 13 semaines. Toutes les feuilles, à l'exception des deux plus jeunes, furent représentatives de l'infection surtout le plant. L'influence de la position des feuilles sur la période d'incubation fut différente pour les plants de 10 et de 13 semaines et pour les deux répétitions. Des plants de moins de 60 j au stade sept à huit feuilles devraient être utilisés lors d'études sur le développement initial de la brûlure cercosporéenne de la carotte.A greenhouse study was conducted to determine the effects of plant age, leaf age and leaf position on infection of carrot (Daucus carota var. sativa) by Cercospora carotae. Infection was quantified as the number of lesions cm-2 of leaf surface and the length of incubation period. The relative number of lesions decreased linearly with increasing plant age from 39- to 60-d-old plants, and remained low from 60- to 71-d-old plants. The incubation period increased from 9.0 to 16.6 d, with increasing plant age. Relative number of lesions decreased with increasing leaf age from 1 to 36 d, but the variation among leaves was high. The incubation period increased from 9.0 to 18.3 d with increasing leaf age, but lesions on a few young leaves appeared relatively late. Generally, differences in relative number of lesions for leaves on different positions for 10- and 13-wk-old plants were not significant. Infection on all leaves except the two youngest was representative of infection on whole plant. Effect of leaf position on incubation period was different for the 10- and 13-wk-old plants and for the two trials. Plants younger than 60 d old, in the seven-to eight-leaf stages should be used for experiments on the initial development of Cercospora blight of carrots

Fungal communities isolated from dead apple leaves from orchards in Quebec

Le champignon causant la tavelure du pommier, Venturia inaequalis, hiverne dans les feuilles mortes de pommier (Malus pumila) sous forme de pseudothèces. Les objectifs de cette étude étaient de monter une collection de champignons afin de vérifier subséquemment leur résistance au froid et leur potentiel antagoniste contre V. inaequalis et d'acquérir des connaissances sur la microflore des feuilles mortes de pommiers. Des champignons ont été isolés sur des feuilles mortes de pommiers récoltées au printemps et à l'automne de 1993. Au total, 345 isolats fongiques provenant de 49 genres ont été identifiés. Quinze genres sont rapportés pour la première fois comme colonisateurs des feuilles de pommiers en Amérique du Nord.Venturia inaequalis, the causal agent of apple scab, overwinters in apple (Malus pumila) leaves on the orchard floor by producing pseudothecia. The objectives of this survey were to make a collection of fungi to be subsequently tested for their potential as psychrophile biocontrol agents against V. inaequalis and to acquire knowledge on the diversity of the microflora of dead apple leaves. Fungi were recovered from dead apple leaves collected in the spring and fall of 1993. A total of 345 isolates from 49 genera were identified. Fifteen gene were not previously recorded as colonizers of apple leaves in North America

Comparison of decision methods to initiate fungicide applications against cercospora blight of carrot

Un modèle prévisionnel a été comparé, en 1991 et 1992, à d'autres méthodes pour son efficacité à déterminer le début des applications de fongicides utilisés pour lutter contre la brûlure cercosporéenne de la carotte, causée par le Cercospora carotae. Le premier traitement fongicide était appliqué lorsque : 1) les plants avaient atteint 15 cm de hauteur (méthode conventionnelle); 2) les feuilles intermédiaires de 50 % des plants présentaient des symptômes (méthode du 50 % d'incidence); 3) lorsque la valeur cumulative d'équivalence d'infection (CE) avait atteint 14 (méthode de prévision avec la valeur CIE = 14); 4) lorsque la valeur CIE avait atteint 18 (méthode de prévision avec la valeur CIE = 18). Pour toutes les méthodes, les applications subséquentes de fongicides ont été faites aux 10 jours lorsqu'il n'y avait pas de pluie, et aux 7 jours en présence de pluie. Les valeurs CIE ont été calculées en fonction de la durée d'humectation du feuillage et de la température durant cette période, tout en tenant compte des périodes d'humidité élevée et d'interruptions des périodes d'humectation du feuillage, et étaient cumulatives depuis l'émergence des plants. Pour les méthodes avec prévision, que ce soit pour une valeur CIE de 14 ou de 18, aucune perte de rendement n'a été observée et moins de traitements ont été nécessaires comparativement à la méthode conventionnelle et à celle du 50 % d'incidence. Dans une autre étude, les valeurs CIE ont été comparées aux pourcentages de champs qui avaient atteint des niveaux d'incidence de 50, 80 et 100% afin d'établir les seuils de risque léger (11 < CIE < 15) et de risque élevé (16 < CIE < 20). Seulement 3% des champs avaient déjà atteint l'incidence de 50 % lorsqu'une valeur CIE a atteint le seuil préconisé pour un risque faible et, 19 % des champs, une incidence de 80% pour un risque élevé.In 1991 and 1992, two thresholds of a forecasting model were compared with two other decision methods for effectiveness in timing the first fungicide application against Cercospora blight of carrot induced by Cercospora carotae. The first fungicide application was made when : 1) the plants reached 15 cm in height (conventional method); 2) the intermediate (middle) leaves of 50% of the plants were diseased (50% disease incidence threshold method); 3) the cumulative infection equivalence (CE) was 14 (forecasting model CE 14); and 4) the CE was 18 (forecasting model CE 18). In all four treatments, subsequent applications of fungicide were made at 10-d intervals when there was no rain, and at 7-d intervals when there was rain. The CE was calculated based on duration of leaf wetness and temperature during the wet period, corrected for high humidity and interrupted wet periods, and was cumulative starting at crop emergence. For thresholds of CE 14 and CE 18, no yield losses were observed and the total number of fungicide applications needed was lower compared to conventional and 50% disease incidence threshold methods. In a separate study, the CE thresholds were related to the percentage of commercial fields that reached disease incidence thresholds of 50, 80 and 100% to establish low risk (CE 11-15) and high risk (CE 16-20) thresholds. The forecasting of low and high risk CE thresholds were too late for 3 and 19% of the commercial fields because those fields had more than 50 and 80% of the plants diseased, respectively

Cross-resistance between myclobutanil and tebuconazole and the genetic basis of tebuconazole resistance in Venturia inaequalis

BACKGROUND: Myclobutanil is one of the most widely used demethylation inhibitor (DMI) fungicides for the management of apple scab, caused by Venturia inaequalis. Strains of V. inaequalis resistant to myclobutanil have been reported across the world. Tebuconazole, another DMI fungicide, has been proposed as an alternative to myclobutanil, and the extent of cross-resistance with myclobutanil therefore needs to be evaluated. The sensitivity to tebuconazole and myclobutanil of a total of 40 isolates was determined. Half the isolates came from an isolated orchard which had never been sprayed with fungicides and half from orchards sprayed regularly with myclobutanil, but still with disease control problems. The progeny of a tebuconazole resistant (R) × sensitive (S) V. inaequalis cross were analysed in order to improve understanding of the genetic control of tebuconazole sensitivity. RESULTS: There is cross-resistance between myclobutanil and tebuconazole (r=0.91; P < 0.001). Sensitivity to tebuconazole of the progeny of a R×S cross varied quantitatively in a pattern which implied at least two gene loci differing between the parental strains. In addition, the asymmetric distribution of the sensitivity in the progeny implied possible epistatic effects. CONCLUSION: Resistance to myclobutanil and tebuconazole is strongly correlated. At least two genes are involved in the control of tebuconazole resistance in V. inaequalis

Infection efficiency of Phaeoisariopsis personata and the influence of different wetness patterns on germ-tube growth of the pathogen

Controlled environment studies with P. personata [Mycosphaerella berkeleyi], the causal agent of late leaf spot disease of groundnut, showed that infection is enhanced if leaves are exposed to alternate wet and dry periods (intermittent wetness) compared with continuous wetness. Detailed investigations to elucidate this phenomenon revealed more germ tubes per conidium and more branching of germ tubes with intermittent wetness than with continuous wetness. With intermittent wetness there was clear evidence of tropic growth of germ tubes and branches towards stomata and subsequent penetration. With continuous wetness, germ tube growth did not appear to be directional and germ tubes commonly passed over the stomatal guard cells, therefore leading to relatively few stomatal penetrations. For both wetness regimes, stomatal penetrations continued to increase with increased leaf wetness for at least 6 d after inoculation and there was a linear relationship between the number of stomatal penetrations and the number of resultant lesions. Infection efficiency was markedly increased when the spore load was reduced to 0.1 conidia/cm² (c. 1 spore/leaflet)

Effects of humidity, leaf wetness, temperature and light on conidial production by Phaeoisariopsis personata on groundnut

Conidial production by P. personata [Mycosphaerella berkeleyi] on groundnut was studied under controlled environment conditions. With constant RH, conidia were only produced above a threshold (94.5% RH) and there was a linear increase between 94.5 and 100% RH. Conidial production was less with continuous leaf wetness (resembling heavy dew) than with continuous (98-99%) RH, but it was similar with intermittent leaf wetness and intermittent (98-99%) RH (8 h at 70% RH each day). With alternate high (≥97% RH) and low humidity, daily conidial production depended both on the duration of high RH and on the low RH value. With 99% RH at night (12 h), night-time conidial production decreased with the previous daytime RH. After conidial production had started, small numbers of conidia were produced even when the RH was well below the threshold level (94.5%). Conidia were produced in continuous light when the photon flux density was 2 µmol/m²/s, but production was completely inhibited with 60 µmol/m²/s. With constant RH, more conidia were produced with a 12-h photoperiod than in continuous darkness. However, >75% of the conidia were produced in the dark. With continuous darkness, more conidia were produced during the night (18.00-06.00 h) than during the day, but this biological rhythm was overcome with a light-night/dark-day regime. With constant (98-99%) RH, there was a linear increase in conidial production with temp. between 10 and 28°C, and virtually no conidia were produced at 33°. The daily production of conidia increased with time for 2-6 d, depending on the treatment

Sporulation rate in culture and mycoparasitic activity, but not mycohost specificity, are the key factors for selecting Ampelomyces strains for biocontrol of grapevine powdery mildew (Erysiphe necator)

To develop a new biofungicide product against grapevine powdery mildew, caused by Erysiphe necator, cultural characteristics and mycoparasitic activities of pre-selected strains of Ampelomyces spp. were compared in laboratory tests to the commercial strain AQ10. Then, a 2-year experiment was performed in five vineyards with a selected strain, RS1-a, and the AQ10 strain. This consisted of autumn sprays in vineyards as the goal was to reduce the number of chasmothecia of E. necator, and, thus, the amount of overwintering inocula, instead of targeting the conidial stage of the pathogen during spring and summer. This is a yet little explored strategy to manage E. necator in vineyards. Laboratory tests compared the growth and sporulation of colonies of a total of 33 strains in culture; among these, eight strains with superior characteristics were compared to the commercial product AQ10 Biofungicide® in terms of their intrahyphal spread, pycnidial production, and reduction of both asexual and sexual reproduction in E. necator colonies. Mycoparasitic activities of the eight strains isolated from six different powdery mildew species, including E. necator, did not depend on their mycohost species of origin. Strain RS1-a, isolated from rose powdery mildew, showed, togetherwith three strains from E. necator, the highest rate of parasitism of E. necator chasmothecia. In field experiments, each strain, AQ10 and RS1-a, applied twice in autumn, significantly delayed and reduced early-season development of grapevine powdery mildew in the next year. Therefore, instead of mycohost specificity of Ampelomyces presumed in some works, but not confirmed by this study, the high sporulation rate in culture and the mycoparasitic patterns became the key factors for proposing strain RS1-a for further development as a biocontrol agent of E. necator