Demethylation Inhibitor Research Articles

Effectiveness of fungicides with different modes of action against net blotch disease of two-rowed spring barley

Net blotch disease caused by Pyrenophora teres is one of the most damaging fungal diseases of barley crop. This study screened comparatively the effectiveness of eleven fungicide products applied to malting barley (Hordeum vulgare L. conv. distichum Alef.) cultivar ‘Daciana’ in conditions from Transylvania. After fungicide treatment, the net blotch disease index decreased an average of 70.37-78.04% relative to untreated control. Fungicide application ensured an average of 28.66% grain yield increase, 7.51% thousand grains weight increase and 4.36% grain starch content increase relative to untreated control. Top performance was obtained by using products that contained demethylation inhibitors targeting sterol biosynthesis in combination with other active substances with a different mode of action particularly targeting mitochondrial respiration. It is a difficult task to achieve top performance on all dimensions: strict disease suppression, high quantity and quality grain yields. Preventing the occurrence of pathogen resistance to fungicide and minimizing negative effect on crop as well as remanence in the plant, are the main challenges for fungicide use and should receive further attention.

Status of Fungicide Resistance and Physiological Characterization of Tebuconazole Resistance in Rhizocotonia solani in Sichuan Province, China.

The resistance prevalence of chemical fungicides has caused increasingly serious agro-ecological environmental problems. However, there are few previous reports about resistance to succinate dehydrogenase (SDHI) or sterol demethylation inhibitor (DMI) in Rhizoctonia solani, one of the main agro-diseases. In this study, the fungicide resistance of 122 R. solani isolates in Sichuan Province was monitored by the mycelial growth rate method. Results showed that all isolates were susceptible to hexaconazole and most isolates were susceptible to thifluzamide, except for the field isolate MSRS-2-7 due to a moderate resistance to thifluzamide (16.43-fold resistance ratio, RR), compared to the sensitivity baseline of thifluzamide (0.042 μg/mL EC50 values). On the contrary, many isolates showed moderate or high resistance to tebuconazole (10.59- to 60.78-fold RR), reaching EC50 values of 0.54~3.10 μg/mL, especially for a highly resistant isolate LZHJ-1-8 displaying moderate resistance to epoxiconazole (35.40-fold RR due to a 3.54 μg/mL EC50 value). The fitness determination found that the tebuconazole-resistant isolates showed higher fitness cost with these characteristics, including a lower growth rate, higher relative electric conductivity, an increased ability to tolerate tebuconazole, and high osmotic pressure. Four new mutations of cytochrome P450 sterol 14α-demethylase (CYP51), namely, S94A, N406S, H793R, and L750P, which is the target for DMI fungicides, was found in the tebuconazole-resistant isolates. Furthermore, the lowest binding energy with tebuconazole was also found in the LZHJ-1-8 isolate possessing all the mutations through analyses with Discovery Studio software. Therefore, these new mutation sites of CYP51 may be linked to the resistance against tebuconazole, and its application for controlling R. solani should be restricted in some areas.

Reduced Sensitivity to Tebuconazole in Botryosphaeria dothidea Isolates Collected from Major Apple Production Areas of China.

China has the largest acreage and is the greatest producer of apples in the world. Apple ring rot (ARR) caused by Botryosphaeria dothidea is one of the major diseases affecting apple fruit production. Tebuconazole, a sterol demethylation-inhibiting (DMI) fungicide, has been increasingly applied to manage ARR in China. Resistance to tebuconazole in field populations of B. dothidea may be selected and accumulate to higher degrees over time. Establishment of tebuconazole sensitivity monitoring programs is critically important for resistance management and effective ARR control. However, the resistance level of B. dothidea to tebuconazole in China remains largely unknown. In this study, invitro mycelial growth assays of B. dothidea in media amended with tebuconazole were conducted, and the sensitivity of B. dothidea to tebuconazole was determined with a set of 390 isolates collected from the major apple production provinces in China between 2006 and 2014. Results showed that the 50% effective concentration (EC50) value ranged from 0.011 to 0.918, 0.040 to 1.621, and 0.052 to 1.925 μg ml-1 with a median value of 0.194, 0.386, and 0.782 μg ml-1 in the isolates collected in 2006, 2010, and 2014, respectively. The frequency distribution of EC50 for tebuconazole was a nonnormal distribution (P < 0.05), suggesting that subpopulations with reduced sensitivity to tebuconazole had emerged in these B. dothidea isolates. The frequency distribution of the B. dothidea isolates collected in 2006 fit a unimodal curve and could be regarded as the baseline sensitivity to tebuconazole. The resistance levels increased over time with the average occurrence frequency of 43.3% and resistance index of 0.38. Positive cross-resistance was observed between tebuconazole and metconazole, which is another DMI fungicide, but multiple resistance was not detected between tebuconazole and non-DMI fungicides. Our results demonstrated that regular long-term resistance monitoring combining with prudent fungicide use should be implemented to prolong the lifespan of tebuconazole in management of ARR in apples.

Baseline Sensitivity and Control Efficacy of Two Quinone Outside Inhibitor Fungicides, Azoxystrobin and Pyraclostrobin, Against Ustilaginoidea virens.

Rice false smut caused by the filamentous fungus Ustilaginoidea virens is a devastating grain disease in rice. Fungicides have been an important measure for the control of this disease. In this study, baseline sensitivities of 179 isolates of U. virens to the quinone outside inhibitor (QoI) fungicides azoxystrobin and pyraclostrobin were established. The distribution of the 50% effective concentration (EC50) values of each fungicide was unimodal. The frequency distribution of logarithmically transformed EC50 values fit or fit closer to a normal distribution. The ranges of EC50 values for azoxystrobin and pyraclostrobin were 0.001 to 0.864 and 0.001 to 0.569 μg/ml, with means and standard errors of the mean values of 0.203 ± 0.012 and 0.079 ± 0.006 μg/ml, respectively. There was a statistically significant and moderately positive correlation (n = 100, r = 0.469, P = 0.001) in sensitivity between these two fungicides. No cross-resistance was found between azoxystrobin, pyraclostrobin, and carbendazim or sterol demethylation inhibitor fungicides. Each fungicide had a significantly higher mean preventive efficacy compared with its curative efficacy. Field assays showed that the control efficacy of pyraclostrobin against rice false smut was greater than that of azoxystrobin. Pyraclostrobin had the best control of rice false smut in three rice varieties, with the control efficacy ranging from 81.5 to 95.5%, whereas azoxystrobin decreased the disease index by 64.1 to 69.2% under the same conditions. These results provide us a reference point in the management of U. virens and future QoI fungicide resistance monitoring programs.

Efficacy evaluation of demethylation inhibitors and mixtures against Colletotrichum spp. causing strawberry anthracnose

Efficacy evaluation of demethylation inhibitors and mixtures against Colletotrichum spp. causing strawberry anthracnose

A new highly effective hybrid fungicide containing difenoconazole and tea tree oil for managing scab of apple, pecan and almond trees and as a tool in resistance management

AbstractManagement of scab diseases of apple, pecan and almond is mostly based on fungicide applications according to host phenological stage and wetness events. Resistance to widely used locally systemic fungicides including the methyl benzimidazoles, demethylation inhibitors (DMIs) and quinone outside inhibitors (QoIs) has been reported. We evaluated the activity of a new premixed hybrid fungicide containing the DMI difenoconazole and essential tea tree oil (TTO) against mycelial growth of Venturia inaequalis in vitro and against scab of apple, pecan and almond in field trials. Mycelial growth of 24 isolates of V. inaequalis collected from commercial apple orchards varied in their sensitivity to difenoconazole (EC50 values of 0.0002–1.29 mg/L). Difenoconazole–TTO was significantly more inhibitory to some of the isolates than difenoconazole alone, primarily to those that were less sensitive to difenoconazole. Protective foliar applications of difenoconazole–TTO in field trials were highly effective in controlling scab of apple and generally provided significantly higher disease control than difenoconazole alone. On apple and almond, difenoconazole–TTO treatments were similarly or more effective than applications with other synthetic fungicides belonging to the DMI, QoI and succinate dehydrogenase inhibitor (SDHI) groups, or their mixtures. Difenoconazole–TTO completely prevented scab on pecan trees. Results suggest that difenoconazole–TTO can be integrated into scab control programmes for these crops and be used as a strategic approach in fungicide resistance management in orchards.

Comparison of Current Peanut Fungicides Against Athelia rolfsii Through a Laboratory Bioassay of Detached Plant Tissues.

Southern stem rot of peanut, caused by Athelia rolfsii, is an important fungal disease that impacts peanut production worldwide. Foliar-applied fungicides are used to manage the disease, and several fungicides have been recently registered for southern stem rot control in peanuts. This study compared fungicidal, residual, and potential systemic activity of current fungicides against A. rolfsii using a laboratory bioassay. Peanut plants grown in the field were treated with eight fungicides approximately 90 days after planting, and plants were collected for the laboratory bioassay weekly for 5 weeks following application. Peanut plants were separated into the newest fully mature leaf present at sample collection, the second newest fully mature leaf present at the time of fungicide application, the upper stem, and the crown tissues. Each plant tissue was inoculated with A. rolfsii then incubated at 30°C for 2 days. Lesion length was measured, and percent inhibition of fungal growth by each fungicide relative to the control was calculated. All fungicides provided the greatest inhibition of A. rolfsii on leaf tissues that were present at the time of fungicide application, followed by the newly grown leaf and upper stem. Little inhibition occurred on the crown. Fungal inhibition decreased at similar rates over time for all fungicides tested. Succinate dehydrogenase inhibitors provided less basipetal protection of upper stems than quinone outside inhibitor or demethylation inhibitor fungicides. Properties of the fungicides characterized in this study, including several newly registered products, are useful for developing fungicide application recommendations to maximize their efficacy in controlling both foliar and soilborne peanut diseases.

Sensitivity and Resistance Risk Assessment of Fusarium graminearum from Wheat to Prothioconazole.

Fusarium head blight (FHB), caused mainly by Fusarium graminearum, is one of the most devastating diseases of wheat. Prothioconazole is a broad-spectrum demethylation inhibitor fungicide with excellent efficacy against FHB. In this study, 235 strains of F. graminearum collected from different regions of Henan Province of China in 2016, 2017, and 2018 were randomly selected. The sensitivity of F. graminearum to prothioconazole was determined by the mycelial growth inhibition method. The results showed that the half maximal effective concentration (EC50) values of F. graminearum to prothioconazole ranged from 0.4742 to 3.4403 μg/ml, and the average EC50 value was 1.7758 ± 0.6667 μg/ml. The sensitivity frequency distribution presented a consequent unimodal curve, and thus the average EC50 value can be established as the baseline sensitivity of F. graminearum to prothioconazole. Ten strains of prothioconazole-resistant mutants were obtained by fungicide taming, and the resistance factor of the mutants ranged from 5.71 to 12.32. The genetic stability assay showed that resistance can be inherited stably for 10 generations. All mutants displayed different degrees of defects in vegetative growth, conidia formation, and pathogenicity compared with the parental strain. These results indicated that F. graminearum has a low risk of resistance to prothioconazole. Cross-resistance assay showed that no cross-resistance was found between prothioconazole and carbendazim, tebuconazole, phenamacril, and pydiflumetofen. Among all mutants, sequence analysis showed that no mutation site was found in cyp51A and cyp51B. Real-time PCR assays showed that the expression levels of cyp51A and cyp51B of the mutants were significantly increased after prothioconazole treatment for 24 h. In summary, our study provided a theoretical basis for the resistance risk assessment of F. graminearum to prothioconazole and scientific application of prothioconazole in controlling FHB.

Baseline sensitivity and bioactivity of tetramycin against Sclerotium rolfsii isolates in Huanghuai peanut-growing region of China

Peanut stem rot caused by Sclerotium rolfsii is a serious soil-borne disease and poses a threat to the peanut production. The antibiotic fungicide tetramycin has a broad antifungal spectrum against multiple pathogens and possess low environmental risks. In current study, a total of 250 isolates collected from Huanghuai peanut-growing region of China (Henan, Shandong and Hebei Province) were used to establish the baseline sensitivity of S. rolfsii to tetramycin. The baseline sensitivity curve was unimodal and distributed from 0.01 to 0.36 mg/L, with a mean EC50 (50% effective concentration) value of 0.11 ± 0.06 mg/L. Tetramycin also had strong inhibitory activity on the formation and germination of sclerotia. There was no significant correlation of S. rolfsii sensitivity to tetramycin and other commonly used SDHI (succinate dehydrogenase inhibitor), QoI (quinone outside respiration inhibitor) and DMI (demethylation inhibitor) fungicides. Moreover, tetramycin significantly increased the cell membrane permeability and reduced the oxalate acid content. Greenhouse experiments showed that tetramycin has both protective and curative efficacy against S. rolfsii, while protective efficacy was higher than curative efficacy. Anyhow, the bioactivity of tetramycin is similar (curative efficacy) or higher (protective efficacy) than the control fungicide validamycin. In terms of application method, root drench may be more suitable for tetramycin than spraying, because root drench of tetramycin obtained a higher efficacy. These results indicated that tetramycin may be a potential alternative fungicide for the efficient control of peanut stem rot.

SeedborneCercospora beticolaCan Initiate Cercospora Leaf Spot from Sugar Beet (Beta vulgaris) Fruit Tissue

Cercospora leaf spot (CLS) is a globally important disease of sugar beet (Beta vulgaris) caused by the fungus Cercospora beticola. Long-distance movement of C. beticola has been indirectly evidenced in recent population genetic studies, suggesting potential dispersal via seed. Commercial sugar beet "seed" consists of the reproductive fruit (true seed surrounded by maternal pericarp tissue) coated in artificial pellet material. In this study, we confirmed the presence of viable C. beticola in sugar beet fruit for 10 of 37 tested seed lots. All isolates harbored the G143A mutation associated with quinone outside inhibitor resistance, and 32 of 38 isolates had reduced demethylation inhibitor sensitivity (EC50 > 1 µg/ml). Planting of commercial sugar beet seed demonstrated the ability of seedborne inoculum to initiate CLS in sugar beet. C. beticola DNA was detected in DNA isolated from xylem sap, suggesting the vascular system is used to systemically colonize the host. We established nuclear ribosomal internal transcribed spacer region amplicon sequencing using the MinION platform to detect fungi in sugar beet fruit. Fungal sequences from 19 different genera were identified from 11 different sugar beet seed lots, but Fusarium, Alternaria, and Cercospora were consistently the three most dominant taxa, comprising an average of 93% relative read abundance over 11 seed lots. We also present evidence that C. beticola resides in the pericarp of sugar beet fruit rather than the true seed. The presence of seedborne inoculum should be considered when implementing integrated disease management strategies for CLS of sugar beet in the future.

Optimizing Use of DMI Fungicides for Management of Apple Powdery Mildew Caused by Podosphaera leucotricha in New York State.

Powdery mildew, caused by the ascomycete Podosphaera leucotricha, is an endemic disease found wherever apples are grown that reduces both tree vigor and fresh market yield. In the absence of durable host resistance, chemical management is the primary means of disease control. Demethylation inhibitor (DMI) fungicides are widely used to manage apple powdery mildew, but members within this fungicide class have been observed to differ in efficacy with respect to disease control. Moreover, debate exists as to the optimal timing of DMI fungicide applications for management of apple powdery mildew. In this regard, the goal of this study was to determine the best-use practices for DMI fungicides to manage apple powdery mildew in New York State. Multiyear trials were conducted to evaluate the potential differential efficacy performance of four common DMI fungicides, and additional trials were conducted to assess optimal application timing. In all years, we observed that treatments of flutriafol and myclobutanil consistently had the lowest incidences of powdery mildew compared with difenoconazole and fenbuconazole. In the 2018 and 2021 trials, the newly registered mefentrifluconazole was more comparable to the difenoconazole program with respect to powdery mildew disease incidence. We hypothesize that differences in DMI efficacy may result from each fungicide's water solubility and lipophilicity characteristics and thus their ability to move systemically in the host or more easily penetrate the surface of germinating conidia. Applications timed between petal fall and first cover resulted in the lowest incidence of powdery mildew on terminal leaves of apple shoots compared with applications timed before petal fall. These observations are contrary to previous studies conducted in regions with differing climates. We also found that the incidence of secondary powdery mildew observed 2 weeks after petal fall was influenced by applications of DMI fungicides during the previous season. For example, management programs consisting of applications of flutriafol or myclobutanil in the previous season tended to have lower incidence of apple powdery in the next spring, presumably because of reductions in overwintering inoculum. Despite reports of DMI resistance in other apple pathosystems, the DMI fungicide class is still relevant for the successful management of apple powdery mildew in New York State.

Emergence of Fungicide Sensitivity in Leptosphaeria maculans Isolates Collected from the Czech Republic to DMI Fungicides

In the Czech Republic, demethylation inhibitors (DMIs) are used both as fungicides in controlling phoma stem canker and as growth regulators. This heavy use can result in the development of resistant isolates. A total of 45 and 286 Leptosphaeria maculans isolates were tested in vitro, using the mycelial growth and microtiter plate assays, respectively. The objective was to determine the sensitivity of L. maculans isolates collected in the Czech Republic to the fungicides tetraconazole, metconazole, and prochloraz. The mean EC50 values with the mycelial growth plate method were 1.33, 0.78, and 0.40 µg mL−1 for tetraconazole, metconazole, and prochloraz, respectively. The mean EC50 values for the microtiter plate assay were 3.01, 0.44, and 0.19 µg mL−1 for tetraconazole, metconazole, and prochloraz, respectively. All three fungicides also had high variation factors that may be due to inserts in the ERG11 promoter region. In addition, cross sensitivity among the three fungicides was observed. Overall, the high variation factors and the PCR (polymerase chain reaction) results observed in this study could signify the presence of resistant isolates in L. maculans Czech populations, especially in isolates tested for sensitivity to tetraconazole.

A strategy for controlling Cercospora leaf spot, caused by Cercospora beticola, by combining induced host resistance and chemical pathogen control and its implications for sugar beet yield

Cercospora beticola populations resistant to methyl benzimidazole carbamate (MBC), quinone outside inhibitor (QoI), and demethylation inhibitor (DMI) fungicides have been reported in Egypt. This pathogen causes a difficult to control variant of Cercospora leaf spot (CLS) on sugar beet. We conducted field experiments over two years to investigate treatment efficacies on MBC- and QoI-resistant, but partially DMI-sensitive, C. beticola from a research farm in Sakha, Egypt. DMI fungicides showed moderate efficacy with half the maximal effective concentration (EC50). Efficacy was 67.2–69.1% and 63.4–63.6% for epoxiconazole (EPO) and propiconazole (PRO), respectively. When each fungicide was mixed with salicylic acid (SA) at EC50 (EPO + SA) and EC50 (PRO +SA), efficacy was 77.5–79.1% and 77.0–78.2%, respectively. The QoI fungicide, azoxystrobin, showed lower efficacy (46.4–49.5%) when applied alone, but enhanced efficacy when combined with SA at EC50/EC50 (77.0–78.2%). Carbendazim alone was only 47.5–45.1% effective, but when mixed with SA, had a 67.1% co-toxicity factor in vitro, indicating antagonism between components. An Artemisia cina aqueous extract (WEA) combined with SA (WEA/SA) showed 76.0–78.2% efficacy, compared with 69.0–71.0% and 49.0–52.7% for WEA and SA, respectively. Single treatments were classified into five sensitivity classes from sensitive (<1) to resistant (>100) based on their EC50 values. Significant increases in insensitivity were observed from 2015/16 to 2017/18. All treatments increased total yield, sucrose percentage, and recoverable white sugar compared with the untreated control. A combination of control strategies could allow for effective disease and fungicide resistance management.

A New Approach: Determining cyt b G143A Allele Frequency in Zymoseptoria tritici by Digital Droplet PCR.

Simple SummaryDroplet digital polymerase chain reaction (ddPCR) is an innovative technique for quantifying a target DNA in a diluted target sample, based on the partition of PCR reaction in a large number of sub-reactions (droplets). In recent years there has been an increase in digital PCR (dPCR) utilization in several different fields, and the application of ddPCR in fungicide resistance studies is very recent. Zymoseptoria tritici is the causal agent of Septoria Tritici Blotch (STB), one of the most devasting foliar diseases of wheat grown in temperate climates. STB control relies mostly on fungicide applications and mutations conferring fungicides resistances are concerning phenomena. G143A substitution in fungal cytochrome bc1 confers resistance toward Quinone outside Inhibitor (QoIs) fungicides. In Italy, QoIs are currently sprayed in STB control programs. To the best of our knowledge, we have developed the first ddPCR assay for G143 and A143 alleles detection in samples of gDNA from Z. tritici monocondial cultures. We have also investigated G143 and A143 alleles frequency in Italian Z. tritici populations representative of different fungicide management strategies. The detection of very low G143A substitution percentages in Z. tritici populations is essential for monitoring the emergence of QoIs resistance in field and effectively control STB.Z. tritici first appeared in Italy later than in northern-central European countries. QoIs fungicides currently play a role in STB control, used in combination with Demethylation Inhibitors (DMIs) or Succinate dehydrogenase Inhibitors (SDHIs). In this study, we set up a fast, sensitive, and accurate ddPCR protocol in order to investigate the presence and frequency of G143A substitution, causing a reduction in strobilurins’ efficacy in Z. tritici. The best PCR conditions for the clear separation of positive and negative droplets were identified. The lowest wild-type and resistant alleles frequencies were accurately determined on samples consisting of mixed DNAs from monoconidial cultures of Z. tritici and were expressed as fractional abundance. The protocol was tested by determining the copy number and frequency of alleles on gDNA purified in three Italian Z. tritici field populations representative of different fungicide management strategies. For the first time, the determination of allele concentration and the frequency of a mutation involved in Z. tritici fungicide resistance was carried out by employing digital PCR. This new approach provides a diagnostic tool that is rapid and able to detect very low G143A substitution percentages, which is very useful for fungicide resistance detection at early stages, thus, informing field management strategies for contrasting STB disease.

Challenges in Ramularia collo-cygni Control

Ramularia leaf spot (RLS), caused by the fungus Ramularia collo-cygni, has recently become widespread in Europe. Succinate dehydrogenase inhibitor (SDHI) and demethylation inhibitor (DMI) fungicides are mainly applied for disease control on barley fields, but pathogen isolates with a reduced sensitivity can cause difficulties. There is an urgent need for new spring barley cultivars that are more resistant to RLS development and can inhibit R. collo-cygni epidemics.

Multiple Mutations and Overexpression in the CYP51A and B Genes Lead to Decreased Sensitivity of Venturia effusa to Tebuconazole

Multiple demethylation-inhibiting (DMI) fungicides are used to control pecan scab, caused by Venturia effusa. To compare the efficacy of various DMI fungicides on V. effusa, field trials were conducted at multiple locations applying fungicides to individual pecan terminals. In vitro assays were conducted to test the sensitivity of V. effusa isolates from multiple locations to various concentrations of tebuconazole. Both studies confirmed high levels of resistance to tebuconazole. To investigate the mechanism of resistance, two copies of the CYP51 gene, CYP51A and CYP51B, of resistant and sensitive isolates were sequenced and scanned for mutations. In the CYP51A gene, mutation at codon 444 (G444D), and in the CYP51B gene, mutations at codon 357 (G357H) and 177 (I77T/I77L) were found in resistant isolates. Expression analysis of CYP51A and CYP51B revealed enhanced expression in the resistant isolates compared to the sensitive isolates. There were 3.0- and 1.9-fold increases in gene expression in the resistant isolates compared to the sensitive isolates for the CYP51A and CYP51B genes, respectively. Therefore, two potential mechanisms—multiple point mutations and gene over expression in the CYP51 gene of V. effusa isolates—were revealed as likely reasons for the observed resistance in isolates of V. effusa to tebuconazole.

Fungicide Efficacy on Tar Spot and Yield of Corn in the Midwestern United States

Tar spot of corn caused by Phyllachora maydis has recently led to significant yield losses in the eastern corn belt of the Midwestern United States. Foliar fungicides containing quinone outside inhibitors (QoI), demethylation inhibitors (DMI), and succinate dehydrogenase inhibitors (SDHI) are commonly used to manage foliar diseases in corn. To mitigate the losses from tar spot, 13 foliar fungicides containing single or multiple modes of action (MOA/Fungicide Resistance Action Committee groups) were applied at their recommended rates in a single application at the standard VT/R1 (tassel/silk) growth stage timing to evaluate their efficacy against tar spot in a total of eight field trials in Illinois, Indiana, Michigan, and Wisconsin during 2019 and 2020. The single MOA fungicides included either a QoI or DMI. The dual MOA fungicides included a DMI with either a QoI or SDHI, and fungicides containing three MOAs included a QoI, DMI, and SDHI. Tar spot severity estimated as the percentage of leaf area covered by P. maydis stroma on the ear leaf of the nontreated control at dent growth stage ranged from 1.6 to 23.3% in the trials. Averaged across eight field trials all foliar fungicide treatments significantly reduced tar spot severity, but only Delaro 325SE (prothioconazole + trifloxystrobin), Revytek 3.33LC (mefentrifluconazole + pyraclostrobin + fluxapyroxad), and Veltyma 3.34S (mefentrifluconazole + pyraclostrobin) significantly increased yield over the nontreated control. When comparing groups of fungicide treatments by the number of MOAs, foliar fungicide products that had two or three MOAs decreased tar spot severity over not treating and products with one MOA. In addition, the fungicide group that contained all three MOAs significantly increased yield over not treating with a fungicide or using a single MOA group.

Fungicide Sensitivity of Gaeumannomyces Species Associated with Take-All Root Rot of St. Augustinegrass in Texas

Gaeumannomyces species are soilborne fungi associated with take-all root rot (TARR) of St. Augustinegrass ( Stenotaphrum secundatum) and other warm-season turfgrasses. Penetrant fungicides, such as demethylation inhibitors (DMIs) and quinone outside inhibitors (QoIs), have been labeled and used for TARR management. Reports of turfgrass pathogens resistant to these fungicide groups are increasing. However, there is no fungicide sensitivity information for Gaeumannomyces species in turfgrass. In this study, sensitivities to QoI fungicide azoxystrobin and DMI fungicide tebuconazole were determined for Gaeumannomyces species ( G. graminicola, G. arxii, and G. floridanus) isolated from St. Augustinegrass in Texas. A discriminatory concentration of 0.1 µg a.i. ml−1 was deemed suitable for rapid screening and accurate prediction of the effective concentration of a fungicide that inhibits fungal growth by 50% (EC50) for tebuconazole and azoxystrobin. The EC50 values of Gaeumannomyces species ranged from 0.021 to 0.1 µg ml−1 for tebuconazole (with the exception of one isolate having an EC50 value = 0.296 µg ml−1) and from 0.014 to 0.399 µg ml−1 for azoxystrobin. To our knowledge, this is the first study to investigate fungicide sensitivities of Gaeumannomyces species associated with TARR in St. Augustinegrass to fungicides in the DMI and QoI chemical groups. Most isolates tested in this study can be considered sensitive to tebuconazole and azoxystrobin. This indicates that the fungicide resistance of Gaeumannomyces species from St. Augustinegrass in Texas lawns might not be prevalent at the moment. Future field validation needs to confirm in vitro sensitivities and field efficacy of tebuconazole and azoxystrobin.

Association of hydrogen peroxide with commercial fungicide formulations in the control of Asian soybean rust

ABSTRACT: In recent years, there have been reductions in the efficacy of the fungicidal control of Phakopsora pachyrhizi, thereby hindering the management of soybean rust and compromising crop yield. This study evaluated the effects of incorporating hydrogen peroxide (H2O2) in commercial fungicide formulations on the control of soybean rust. We conducted two experiments, one of which was performed in a greenhouse environment and the other under field conditions. In both environments, we examined the following four control programs using commercial fungicide formulations: (I) azoxystrobin + cyproconazole (quinone outside inhibitor [QoI] + demethylation inhibitor [DMI]); (II) picoxystrobin + cyproconazole (QoI + DMI); (III) pyraclostrobin + epoxiconazole + fluxapyroxad (QoI + DMI + succinate dehydrogenase inhibitor); and (IV) water (H2O) (program without fungicide application), combined with the incorporation of (i) H2O2; (ii) mancozeb (positive control I); (iii) chlorothalonil (positive control II); or (iv) water (H2O) alone. Analyses of infected leaf area and grain yield revealed that the addition of H2O2 to the formulations of DMI and QoI fungicides led to a reduction in disease severity of between 33% and 44% relative to the effects of these products used alone, as well as an increase in yield and SPAD values. The use of H2O2 and multi-site fungicides alone failed to provide effective control of soybean rust. In addition to enhancing the efficacy of disease control, the use of H2O2 associated with commercial fungicide mixtures was shown to be a potential tool for the management of fungicide resistance and reduction in losses from Asian soybean rust.

Sensitivity of Colletotrichum nymphaeae to Six Fungicides and Characterization of Fludioxonil-Resistant Isolates in China.

Colletotrichum nymphaeae is the dominant species causing anthracnose disease of peach in China. In this study, 140 isolates of C. nymphaeae were assessed for their sensitivity to six fungicides. It was found that C. nymphaeae was highly resistant to carbendazim, procymidone, and boscalid but sensitive to pyraclostrobin and prochloraz. For fludioxonil, the fungus exhibited differential sensitivities (i.e., approximately 14% of isolates were resistant to fludioxonil and the resistance was stable). Fludioxonil-resistant isolates had a mean EC50 value of 2.2380 µg/ml, whereas the mean EC50 value was 0.0194 µg/ml in fludioxonil-sensitive isolates. The mean EC50 values of C. nymphaeae for pyraclostrobin and prochloraz were 0.0083 µg/ml and 0.002 µg/ml, respectively. No cross-resistance was observed between fungicides from different groups. Mycelial growth rate, control efficacy, and osmotic stress responses were significantly different (P < 0.05) between fludioxonil-sensitive (FluS) and -resistant (FluR) isolates, but no significant difference was observed (P > 0.05) in virulence and sporulation between FluS and FluR isolates. No mutation was detected in coding regions of the CnOs-1, Cal, Hk1, Hog1, TPI, and Mrr1 genes. Interestingly, with fludioxonil treatment, the expression of ABC transporter gene atrB was significantly overexpressed in some resistant isolates. However, overexpression of the atrB gene was not detected in one moderately and one highly resistant isolate, indicating that other unknown mechanisms may be involved. Current findings uncovered several effective chemicals and provided the foundation for designing management strategies to practically control peach anthracnose with the most effective demethylation inhibitor fungicides and quinone outside inhibitor fungicides.