Activity of Ten Fungicides against Phytophthora capsici Isolates Resistant to Metalaxyl

Pepper Phytophthora blight (PPB), caused by Phytophthora capsici, is an important disease of pepper in China. The extensive application of metalaxyl has resulted in widespread resistance to this fungicide in field. This study has evaluated the activities of several fungicides against the mycelial growth and sporangium germination of metalaxyl‐sensitive and metalaxyl‐resistant P. capsici isolates by determination of EC₅₀ values. The results showed that the novel carboxylic acid amide (CAA) fungicide mandipropamid exhibited excellent inhibitory activity against PPB both in vitro and in vivo, with averagely EC₅₀ values of 0.075 and 0.004 μg/ml in mycelial growth and sporangium germination, respectively, and over 88% efficacy in controlling PPB. The other three CAA fungicides also provided over 70% efficacy in controlling PPB. The mycelial growth was less sensitive to quinone outside inhibitor (QoI) fungicides azoxystrobin and trifloxystrobin than that of sporangium germination in P. capsici isolates. However, azoxystrobin and trifloxystrobin provided over 80% efficacy in controlling PPB. It was noted that propamocarb and cymoxanil did not exhibit activity against the mycelial growth or sporangium germination of P. capsici isolates in the in vitro tests, with over 70% efficacy in controlling PPB. The new fungicide mixture 62.5 g/l fluopicolide + 625 g/l propamocarb (trade name infinito, 687.5 g/l suspension concentrate (SC)) produced over 88% efficacy in controlling PPB caused by both metalaxyl‐sensitive and metalaxyl‐resistant isolates. The data of this study also proved that there was obviously no cross‐resistance between metalaxyl and the other tested fungicides. Therefore, these fungicides should be good alternatives to metalaxyl for the control of PPB and management of metalaxyl resistance.     

Sensitivity of Chinese Isolates of Plasmopara viticola to Metalaxyl and Dimethomorph

During 2007 and 2008, 392 isolates of Plasmopara viticola were collected from 11 regions in seven provinces in China, and their sensitivities to metalaxyl and dimethomorph were determined by the floating leaf disk technique. Among all isolates, 13% were classified as sensitive, 26% as low‐level resistant, and 61% as resistant to metalaxyl. Of the 392, 85 were from vineyards never treated with carboxylic acid amide fungicides; these isolates were used to determine the baseline sensitivity to dimethomorph, and their EC₅₀ values ranged from 0.01 to 0.21 (mean ± SD, 0.11 ± 0.04) μg/ml. The other 307 isolates were completely inhibited by a single discriminatory dose of 1.6 μg/ml of dimethomorph.     

Rapid detection of Phytophthora nicotianae by simple DNA extraction and real‐time loop‐mediated isothermal amplification assay

Phytophthora nicotianae is a phytopathogenic oomycete with a wide host range and worldwide distribution. Rapid detection and diagnosis at the early stages of disease development are important for the effective control of P. nicotianae. In this study, we designed a simple and rapid loop‐mediated isothermal amplification (LAMP)‐based detection method for P. nicotianae. We tested three DNA extraction methods and selected the Kaneka Easy DNA Extraction Kit version 2, which is rapid and robust for LAMP‐based detection. The designed primers were tested using mycelial DNA from 35 species (81 isolates) of Phytophthora, 12 species (12 isolates) of Pythium, one isolate of Phytopythium and one isolate each from seven other soil‐borne pathogens. All of the 42 P. nicotianae isolates were detected by these primers, and no other isolates gave positive results. Three isolates were tested for the sensitivity of the reaction, and the lowest amounts of template DNA that could be detected were 10 fg for two isolates and 1 fg for the third. The target was detected within 25 min in all tested samples, including DNA extracted from both inoculated and naturally infected plants. In contrast, PCR assays with P. nicotianae‐specific primers failed or showed weakened detection in several samples. Thus, we found that the rapid DNA extraction and LAMP assay methods developed in this study can be used to detect P. nicotianae with high sensitivity, specificity and stability.     

Baseline sensitivity of Botrytis cinerea to fluazinam and cross‐resistance

Grey mould, caused by the fungus Botrytis cinerea Pers ex Fr., is a very destructive and important disease worldwide. Fluazinam is a phenylpyridinamine fungicide with broad‐spectrum activities. The baseline sensitivity of B. cinerea to fluazinam is yet to be established in Henan Province, China. In this study, a total of 117 field isolates of B. cinerea were collected from 49 commercial greenhouses in different locations of Henan Province, in 2016, and the sensitivities of these isolates to fluazinam were determined based on mycelial growth. The effective concentration for 50% (EC₅₀) values ranged from 0.0038 to 0.0441 μg/ml, and the mean EC₅₀ value was 0.0201 ± 0.0081 μg/ml for mycelial growth. The frequency distribution range presented a unimodal curve. To define the cross‐resistance relationships, the linear correlation coefficients of the EC₅₀ values between fluazinam and carbendazim, procymidone, pyrimethanil or boscalid were analysed. The results showed that no correlation was observed between fluazinam and the other tested fungicides. These results provide important information to growers for the prevention and control of grey mould.     

Investigation of the genetic diversity of Phytophthora capsici in China using a universal fluorescent labelling method

Phytophthora capsici is an important oomycete pathogen threatening the vegetable production in China, but very little is known about its population structure. The objective of the present study was to evaluate the genetic diversity of 49 P. capsici isolates obtained from 2007 to 2014 at nine provincial locations in China. Isolates were assessed for mating type, metalaxyl resistance and simple sequence repeat (SSR) genotype. Mating‐type analyses of the isolates showed that both mating types were present in all of the sampled production regions, and the mating‐type frequency in the total Chinese population did not deviate significantly from a 1:1 ratio. Responses of isolates to the fungicide metalaxyl indicated the presence of intermediate resistance to metalaxyl among the field population. A universal fluorescent labelling method was adapted in this study to improve the efficiency of SSR genotyping. Microsatellite genotyping of the isolates using seven SSR markers revealed 44 unique multilocus genotypes. Genetic analyses indicated the existence of two genetic clusters within Chinese P. capsici collection. Clonal reproduction may play a more prominent role in Yunnan Province, but non‐existence of repeated genotypes and existence of both mating types throughout all regions suggest outcrossing and sexual recombination likely play an important role in the overall epidemiology in China. Future studies would include expanded scale sampling at single regions over multiple years to better define the genetic diversity of P. capsici in China.     

Emergence of 13_A2 Blue Lineage of Phytophthora infestans was Responsible for Severe Outbreaks of Late Blight on Tomato in South‐West India

Prior to 2007, late blight was not reported as a serious threat to tomato cultivation in India although the disease has been known on potato since 1953. During the July–December cropping season of 2009 and 2010, severe late blight epidemics were observed in Karnataka state of India, causing crop losses up to 100%. Nineteen Phytophthora isolates, recovered from late blight affected tomato tissues from different localities in Karnataka state between 2009 and 2010, were identified as Phytophthora infestans based on morphology, a similarity search of ITS sequences at GenBank and species‐specific PCR using PINF/ITS5 primer pair. The isolates were further assessed for metalaxyl sensitivity, mating type, mitochondrial DNA (mtDNA) haplotype, DNA fingerprinting patterns based on simple sequence repeats (SSR) and RFLPs using the RG57 probe and aggressiveness on tomato. All isolates were metalaxyl resistant, A2 mating type, mtDNA haplotype Ia and had identical SSR and RG57 fingerprints and highly aggressive on tomato. The phenotypic and genotypic characters of isolates examined in this study were found to be similar to that of 13_A2 genotype of P. infestans population reported in Europe. Thus, appearance of new population similar to 13_A2 genotype was responsible for severe late blight epidemics on tomato in South‐West India.     

Sensitivity to Fludioxonil of Botrytis cinerea Isolates from Tomato in Henan Province of China and Characterizations of Fludioxonil‐resistant Mutants

Grey mould, caused by Botrytis cinerea Pers ex Fr., is one of the most common diseases of tomato worldwide. Fludioxonil belongs to the phenylpyrrole fungicides, which have high activity against B. cinerea. The sensitivity of fludioxonil was evaluated on the basis of the level of inhibition of mycelium growth in 274 B. cinerea isolates collected from different locations (untreated with this fungicide) in Henan Province, China. The EC₅₀ values for fludioxonil ranged from 0.0033 to 0.0415 mg/l, and the average EC₅₀ values were 0.0156 ± 0.0078 mg/l. Three fludioxonil‐resistant mutants were obtained by subculturing fludioxonil‐sensitive wild‐type isolates on continuously increasing fludioxonil concentrations. For the cross‐resistance assay, fludioxonil revealed positive cross‐resistance with procymidone but did not reveal cross‐resistance with pyrimethanil, boscalid and trifloxystrobin. Mycelial growth, conidial production, hyphal dry weight and pathogenicity were diminished significantly between the fludioxonil‐resistant mutants and their sensitive wild‐type parental isolates. This study shows for the first time that fludioxonil‐resistant isolates of B. cinerea are still not present in Henan Province because this fungicide is an attractive and effective fungicide for chemical control. Recommendations can be made to growers to use fludioxonil to control grey mould and to consider the potential moderate resistance risk of using this fungicide.     

Baseline sensitivity and control efficacy of fluazinam against Sclerotinia sclerotiorum in Henan Province,China

Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, is a devastating disease in Henan Province, of the main rapeseed production areas in China. Fluazinam belongs to the broad‐spectrum phenylpyridinamine fungicides, which have high activity in inhibiting the mycelial growth of S. sclerotiorum. In this study, 191 field isolates were obtained from different oilseed rape fields in Henan Province, before being exposed to fluazinam in 2015. The baseline sensitivity of S. sclerotiorum to fluazinam was established. The effective concentration for 50% inhibition of mycelial growth (EC₅₀) ranged from 0.0019 to 0.0337 μg/ml, and the mean EC₅₀ value was 0.0084 ± 0.0055 μg/ml. The range of the frequency distribution was narrow. The results of a cross‐resistance assay revealed no cross‐resistance between fluazinam and carbendazim, dimethachlone, boscalid or fludioxonil. Field efficacy tests showed that the control efficacies of fluazinam (50% WG) applied at 150, 225 and 300 g ai ha^?1 were 67%, 73% and 88%, respectively. In contrast, the control efficacies of boscalid (50% WG) and carbendazim (50% WP) applied at 225 and 1,500 g ai ha^?1 were 71% and 52%, respectively.     

Baseline sensitivities to azoxystrobin and tebuconazole in Sclerotinia sclerotiorum isolates from sunflower in Iran related to sensitivities to carbendazim and iprodione

In this study, sensitivities of 156 Sclerotinia sclerotiorum isolates collected from sunflower fields of West Azarbaijan province, Iran, were assessed to carbendazim and iprodione, and the baseline sensitivities were established for azoxystrobin and tebuconazole. Resistance to carbendazim and iprodione was observed in 53.85% and 4.49% of the isolates, respectively. The 50% effective concentration (EC₅₀) values of azoxystrobin for the isolates ranged from 0.017 to 3.515 μg/ml with a mean of 0.330 μg/ml, and 8.97% of the strains showed low levels of resistance to the fungicide. However, in the presence of salicylhydroxamic acid, all isolates were sensitive to azoxystrobin and EC₅₀ values ranged from 0.015 to 0.263 μg/ml with a mean of 0.086 μg/ml. All isolates were found to be sensitive to tebuconazole, and EC₅₀ values ranged from 0.003 to 0.177 μg/ml with a mean of 0.036 μg/ml. Among the multiple-resistant isolates, the strains exhibiting resistance to both carbendazim and iprodione were detected in the highest frequency (4.49%). No correlation was observed between mycelial growth and aggressiveness with fungicide sensitivity of the isolates suggesting the absence of fitness cost associated with resistance to the studied fungicides. The results indicated that iprodione, azoxystrobin and tebuconazole could be effectively used in rotation or mixture in spray programmes to manage S. sclerotiorum in the region. The baselines established for azoxystrobin and tebuconazole would be useful in monitoring the fungal populations in the province to assess possible shifts in fungicide sensitivity of S. sclerotiorum isolates in the future.     

A Comparison of Different Estimation Methods for Fungicide EC50 and EC95 Values

EC₅₀ and EC₉₅ (the effective concentrations to cause inhibitions by 50 and 95%, respectively) are commonly used to express fungicide potency. Different methods are currently employed to calculate EC₅₀ and EC₉₅ values. In this study, EC₅₀ and EC₉₅ values for fungicide epoxiconazole against 34 isolates of Sclerotinia sclerotiorum were calculated with seven different methods. Results showed that for both EC₅₀ and EC₉₅ calculations, there was no significant difference among three statistical programs IBM spss ^®, GraphPad Prism^® and dps ^® (P ≥ 0.066). Methods linear log (linear regression of mycelial growth inhibition vs. logarithmic concentration) and interpolation log (linear interpolation from inhibition and logarithmic concentration data) were not significantly different (P ≥ 0.058) from IBM spss in EC₅₀ calculations. These results indicate that among the seven methods, the three statistical programs IBM spss , GraphPad Prism, dps and linear log method are appropriate for EC₅₀ calculations. But for EC₉₅ calculations, only the three statistical programs are recommended, and GraphPad Prism is likely to give a little higher values than spss and dps .     

In vitro growth response of <Emphasis Type="Italic">Phytophthora cactorum</Emphasis>, <Emphasis Type="Italic">P. nicotianae</Emphasis> and <Emphasis Type="Italic">P.</Emphasis> × <Emphasis Type="Italic">pelgrandis</Emphasis> to antibiotics and fungicides

The reactions of isolates of Phytophthora cactorum, P. nicotianae and P. × pelgrandis to metalaxyl, mancozeb, dimethomorph, streptomycin and chloramphenicol were tested to obtain information about the variability of resistance in these pathogens. Distinct genetic groups showed significant differences in resistance to all tested substances except streptomycin. In response to streptomycin, the growth inhibition rates of distinct groups did not differ significantly. The most remarkable differences were detected in the reactions to chloramphenicol and metalaxyl. Discriminant analysis evaluating the effect of all substances confirmed the differences among the groups, which are in agreement with the differences revealed by earlier DNA analyses.     

The effect of chemical and biological treatment on root and crown rot disease caused by Phytophthora cryptogea Pethybr. & Lafferty in Gerbera

Phytophthora root and crown rot (Phytophthora cryptogea) on gerbera is difficult to manage because most gerbera cultivars are susceptible to P. cryptogea. This study was conducted in order to determine the in vivo (pot experiment) efficacy of some fungicides and biofungicides. In pot experiments, fungicides were applied 7 days after inoculation with P. cryptogea, while biofungicide was applied 7 days before inoculation. In this study, soil drenches of five fungicides were tested. “Ametoctradin+dimethomorph (100 ml/day),” “mandipropamid+difenoconazole (60 ml/day),” “propamocarb+fosetyl‐Al (200 ml/day),” “mancozeb+metalaxyl‐M (250 g/day)” and “azoxystrobin+difenoconazole (100 ml/day)” active substances were used. Similarly, one biofungicide Bacillus amyloliquefaciens syn. MBI 600 (50 g/100 L) was applied by soil drenching. Efficacy of treatments was assessed according to the percentage of the root system which was visibly rotten at the end of the experiment. Root and crown rot severity was rated on a scale of 0 = 0% root system necrotic, 1 = 1%‐25% necrotic, 2 = 26%‐50% necrotic, 3 = 51%‐75% necrotic and 4 = 76%‐100% necrotic from 12 to 21 days. In this experiment, “azoxystrobin 200 g/L + difenoconazole 125 g/L” exhibited the highest efficacy against P. cryptogea with a ratio of 43.75%. The other fungicides and biofungicides ametoctradin 300 g/L + dimethomorph 225 g/L, mandipropamid 250 g/L + difenoconazole 250 g/L, propamocarb 530 g/L + fosety‐Al 310 g/L, mancozeb 64%+metalaxyl‐M 4% and Bacillus amyloliquefaciens syn. MBI 600 11% were ineffective. Importance should be given to management strategies of P. cryptogea of and more experiments should be carried out for a better understanding of the use of registered fungicides and biofungicides.     

Occurrence of fenhexamid resistance in Botrytis cinerea from greenhouse strawberries in China

This study assessed the fenhexamid sensitivity of 143 Botrytis cinerea isolates collected from greenhouse strawberries in five regions of China between 2012 and 2013, and identified four isolates with moderate levels of resistance: two from the Xinjiang Uygur Autonomous Region and two from Hebei Province. The baseline fenhexamid sensitivity of B. cinerea exhibited a unimodal distribution with a mean EC₅₀ value of 0.20 ± 0.10 μg/ml (SD). The EC₅₀ values of the fenhexamid‐resistant isolates ranged from 0.05 to 0.40 μg/ml. Molecular analysis of the fenhexamid target gene erg27 revealed that the resistant isolates collected from Xinjiang (163‐6 and 163‐22) contained three mutations that led to amino acid changes (V365A, E368D and A378T) known to be associated with fenhexamid resistance, but that the isolates from Hebei lacked any mutations, indicating that an alternative mechanism could be responsible for their resistance. Most of the biological characteristics of the fenhexamid‐resistant isolates, such as mycelial growth, sclerotia production and pathogenicity, did not significantly differ from those of the sensitive ones (p ≤ .05), but it was noted that some of the resistant isolates exhibited reduced rates of sporulation and spore germination. In addition, the resistant isolates exhibited lower osmotic sensitivity than the sensitive ones. The study found no evidence of cross‐resistance with other fungicides, but that there was negative cross‐resistance with procymidone, iprodione, carbendazim and pyraclostrobin, which indicates that the inclusion of these fungicides within an integrated pest management (IPM) programme could help to minimize the risk of fenhexamid resistance developing in B. cinerea.     

Uneven Distribution of Mating‐Type Alleles Among Togninia minima Isolates,One of the Causal Agents of Leaf Stripe Disease on Grapevines in Northwest Iran

Togninia minima is the main fungal species associated with grapevine leaf stripe disease worldwide. This species is mainly known from its asexual state in nature; nevertheless, a biallelic heterothallic mating strategy has been confirmed for this species based on in vitro crossing studies. There are no data available on the incidence of an active sexual cycle within the populations of this species in many grapevine‐producing countries as well as Iran. The possibility of a clandestine sexual cycle within the Iranian isolates of T. minima was evaluated by analysing the distribution and frequency of the mating‐type alleles on a microspatial and a macrogeographical scales. Towards this aim, a total of 90 T. minima isolates were recovered from grapevines with esca disease from the vineyards in north and north‐western Iran. A multiplex PCR method previously designed by authors was applied for simultaneous identification and determination of the mating‐type alleles in T. minima populations. The results on the screening of mating‐type alleles using multiplex PCR method revealed the mating‐type identity of 77 isolates as Mat1‐2 and 23 isolates as Mat1‐1. Our results showed that both Mat1‐1 and Mat1‐2 isolates are present in a single vineyard and even on single vines. The distribution of mating‐type alleles in the sampled area skewed from the 1 : 1 ratio (77 : 23); however, co‐occurrence of both mating types in a single vineyard and even on single vines is suggestive for the presence of an active sexual cycle for T. minima in north‐western Iran.     

Specific and Sensitive Detection of Phytophthora nicotianae by Nested PCR and Loop‐mediated Isothermal Amplification Assays

Phytophthora nicotianae is an important soilborne plant pathogen. It causes black shank in tobacco and other commercially important crop diseases. Early and accurate detection of P. nicotianae is essential for controlling these diseases. In this study, primers based on the Ras‐related protein gene (Ypt1) of P. nicotianae were tested for their specific detection of the pathogen using nested PCR and LAMP assays. For specificity testing, DNA extracts from 47 P. nicotianae isolates, 45 isolates of 16 different oomycetes and 25 isolates of other fungal species were used; no cross‐reaction with other pathogens was observed. The sensitivity assay showed that the nested PCR and LAMP assays had detection limits of 100 fg and 10 fg genomic DNA per 25‐μl reaction, respectively. Furthermore, the nested PCR and LAMP assays were used for the detection of DNA from naturally P. nicotianae‐infected tobacco tissues and soil. Our results suggest that the LAMP assay has the greatest potential for the specific detection of P. nicotianae in regions that are at risk of contracting tobacco black shank disease and that the Ypt1 gene is a novel and effective target of P. nicotianae LAMP visual detection.     

Baseline sensitivity of Fusarium graminearum from wheat fields in Henan,China, to metconazole and analysis of cross resistance with carbendazim and phenamacril

The filamentous plant pathogenic fungus Fusarium graminearum is one of the most important pathogens causing Fusarium head blight (FHB) in wheat in the Henan Province of China. Metconazole is among the demethylation inhibitor (DMI) fungicides with a higher inhibitory activity on the mycelial growth of F. graminearum. In 2016 and 2017, 119 single spore isolates of F. graminearum, prior to being exposed to metconazole, were recovered from 52 wheat fields near 11 cities in Henan Province. The inhibitory activity of metconazole on the mycelia of the Henan F. graminearum population was determined, and EC₅₀ values were calculated. The range of EC₅₀ values of the Henan F. graminearum population to metconazole was 0.0103 to 0.0775 μg/ml with an average EC₅₀ value of 0.0293 ± 0.0114 μg/ml. The sensitivity frequency distribution curve presented a single peak in a narrow range. No cross-resistance was found between the DMI fungicide metconazole and the benzimidazole fungicide carbendazim or the cyanoacrylate fungicide phenamacril. Therefore, these sensitivity data could be used as the baseline of F. graminearum susceptibility to metconazole in the Henan Province and provide the basis for monitoring metconazole resistance in this area.     

Characterization of Pythium spp. associated with root rot of tobacco seedlings produced using the float tray system in Zimbabwe

The study was undertaken to identify and characterize Pythium isolates associated with root rot disease of tobacco seedlings as a first step towards developing management strategies for the pathogen. A total of 85 Pythium isolates were collected from diseased tobacco seedlings during 2015–2016 tobacco growing season. The isolates were identified to species level using sequencing of the internal transcribed spacer region. Thereafter, a subset of the isolates was tested for sensitivity to the commonly used fungicides, metalaxyl, azoxystrobin and a combination of fenamidone/propamocarbby growing isolates on Potato Dextrose Agar plates amended with the fungicides. The sequence analysis of the ITS‐rDNA identified Pythium myriotylum as the dominant Pythium species associated with the root rot of tobacco seedlings in Zimbabwe. Pythium aphanidermatum and P. insidiosum were also identified albeit at lower frequencies. Phylogenetic analyses of the ITS region of the P. myriotylum isolates showed little sequence diversity giving rise to one distinct clade. The fungicide sensitivity tests showed that metalaxyl provided the best control of P. myriotylum in vitro, as compared to other fungicides. To the best of our knowledge, this is the first comprehensive study to determine and characterize Pythium species associated with root rot of tobacco in the float seedling production system in Zimbabwe.     

Sensitivity of Zymoseptoria tritici Isolates from Tunisia to Pyraclostrobin,Fluxapyroxad, Epoxiconazole,Metconazole, Prochloraz and Tebuconazole

Sensitivity of 159 isolates of Zymoseptoria tritici collected from durum wheat fields in Tunisia in 2012 was analysed towards pyraclostrobin, fluxapyroxad, epoxiconazole, metconazole, prochloraz and tebuconazole using microtiter tests. All isolates were found to be highly sensitive to pyraclostrobin with EC₅₀ <0.01 mg/l with the exception of three isolates from the same field with higher EC₅₀ values (>0.5 mg/l). These three isolates carried a mutation in the cytochrome b gene encoding the G143A substitution. This is the first report of quinone outside inhibitors (QoI) resistance in Z. tritici in Tunisia. Sensitivity towards r fluxapyroxad was in a narrow range with EC₅₀ values ranging between 0.013 and 0.125 mg/l, which can serve as baseline sensitivity data for the future. Demethylation inhibitors sensitivity varied across a broad range with the data indicating a slight shift in sensitivity when compared to a previous study on the 2010 population. No highly sensitive strains were isolated from samples from fields, which had received three or four DMI applications.     

Detection and molecular characterization of carbendazim-resistant Colletotrichum truncatum Isolates causing anthracnose of soybean in Thailand

A loss of fungicide efficacy, particularly for carbendazim, was noted in soybean fields in Thailand and was considered to be due to the development of Colletotrichum truncatum resistance. The carbendazim sensitivity of C. truncatum populations isolated from various soybean fields in Thailand was thus evaluated with in vitro sensitivity assays and molecular characterization of mutations in the sequences of the ß₂-tubulin (TUB2) gene that confer carbendazim resistance in the pathogen. Among 52 isolates, 46 isolates were classified as highly resistant (HR) to carbendazim (EC₅₀ > 1,000 µg/ml). All HR isolates grew on PDA amended with carbendazim at 1,000 µg/ml. Six isolates were classified as carbendazim sensitive (S) (EC₅₀ < 1 µg/ml). Mycelial growth on PDA amended with 1 µg/ml carbendazim was inhibited by over 50% compared with growth on PDA alone. When a partial TUB2 gene from the isolates was amplified and analysed using predicted amino acid sequences, an alteration from glutamic acid to alanine at codon 198 (E198A) was found in 45 HR isolates for which the EC₅₀ was higher than 2000 µg/ml. This mutation resulted from a nucleotide substitution from adenine to cytosine (GA G → GC G). The other HR isolate, CtPhS_1, with EC₅₀ of 1,127 µg/ml, had an alteration at codon 200 (F200Y) (TT C → TA C).     

Sensitivity to boscalid in field isolates of Sclerotinia sclerotiorum from rapeseed in Henan Province,China

Sclerotinia stem rot caused by Sclerotinia sclerotiorum is an important disease of oilseed rape in Henan province of China. Boscalid belongs to succinate dehydrogenase inhibitor (SDHI) fungicides, many of which have strong antifungal activity against S. sclerotiorum. In 2015, a total of 175 isolates of S. sclerotiorum were collected from diseased oilseed rape plants in seven different regions of Henan Province. The EC₅₀ values of 175 isolates of S. sclerotiorum to boscalid ranged from 0.0073 to 0.3880 μg ml^?1, and the mean EC₅₀ value was 0.15 ± 0.09 μg ml^?1. The frequency distribution was unimodal. There was no cross‐resistance between boscalid and carbendazim, procymidone, iprodione, dimethachlone, fludioxonil or fluazinam. Field experiments showed that control efficacies of treatments with boscalid (50% WG) at 225, 300 and 375 g ai ha^?1 were 71%, 81% and 90%, respectively. In contrast, the control efficacy of carbendazim (50% WP) at 1,500 g ai ha^?1 was only 52%.