Fungicide Sensitivity Research Articles

Roles of the Skn7 response regulator in stress resistance, cell wall integrity and GA biosynthesis in Ganoderma lucidum

The transcription factor Skn7 is a highly conserved fungal protein that participates in a variety of processes, including oxidative stress adaptation, fungicide sensitivity, cell wall biosynthesis, cell cycle, and sporulation. In this study, a homologous gene of Saccharomyces cerevisiae Skn7 was cloned from Ganoderma lucidum. RNA interference (RNAi) was used to study the functions of Skn7, and the two knockdown strains Skn7i-5 and Skn7i-7 were obtained in G. lucidum. The knockdown of GlSkn7 resulted in hypersensitivity to oxidative and cell wall stresses. The concentrations of chitin and β-1,3-glucan distinctly decreased in the GlSkn7 knockdown strains compared with those of the wild type (WT). In addition, the expression of cell wall biosynthesis related genes was also significantly down-regulated and the thickness of the cell wall also significantly reduced in the GlSkn7 knockdown strains. The intracellular reactive oxygen species (ROS) content and ganoderic acids biosynthesis increased significantly in the GlSkn7 knockdown strains. Interestingly, the level of intracellular ROS and the content of ganoderic acids decreased after N-acetyl-L-cysteine (NAC), an ROS scavenger, was added, indicating that GlSkn7 might regulate ganoderic acids biosynthesis via the intracellular ROS level. The transcript level of GlSkn7 were up-regulated in osmotic stress, heat stress and fungicide condition. At the same time, the content of ganoderic acids in the GlSkn7 knockdown strains also changed distinctly in these conditions. Overall, GlSkn7 is involved in stress resistance, cell wall integrity and ganoderic acid biosynthesis in G. lucidum.

Variation in Fungicide Sensitivity Among Rhizoctonia Isolates Recovered from Potatoes in South Africa

Rhizoctonia is a major pathogen of potato causing substantial yield losses worldwide. Control of Rhizoctonia diseases is based predominantly on the application of fungicides. However, little is known about the fungicide response variability of different Rhizoctonia anastomosis groups associated with potato diseases in South Africa. A total of 131 Rhizoctonia isolates were obtained from potato growing regions of South Africa from 2012 to 2014 and evaluated for sensitivity to fungicides in vitro and in vivo. The fungicides comprised six chemical formulations and one bio-fungicide representing seven Fungicide Resistance Action Committee groups. All Rhizoctonia anastomosis groups were sensitive to tolclofos-methyl (EC50: 0.001 to 0.098 μg a.i. ml-1) and fludioxonil (EC50: 0.06 to 0.09 μg a.i. ml-1) and showed variation in sensitivity to pencycuron, iprodione, benomyl, and Bacillus subtilis QST 713. However, for azoxystrobin, Rhizoctonia isolates exhibited variable sensitivity ranging from sensitivity (EC50: <0.09 μg a.i. ml-1) to insensitivity with EC50 values exceeding 5 μg a.i. ml-1. In greenhouse and field trials, tolclofos-methyl and fludioxonil exhibited significantly greater control of stem and black scurf whereas azoxystrobin was the least effective. This work demonstrated variable sensitivity within and among anastomosis groups of R. solani and binucleate Rhizoctonia to different fungicides. Information on fungicide sensitivity of Rhizoctonia isolates is crucial in the development of effective Rhizoctonia control strategies and facilitates monitoring of fungicide insensitive isolates in the pathogen population.

Nucleic adaptability of heterokaryons to fungicides in a multinucleate fungus, Sclerotinia homoeocarpa

Sclerotinia homoeocarpa is the causal organism of dollar spot in turfgrasses and is a multinucleate fungus with a history of resistance to multiple fungicide classes. Heterokaryosis gives rise to the coexistence of genetically distinct nuclei within a cell, which contributes to genotypic and phenotypic plasticity in multinucleate fungi. We demonstrate that field isolates, resistant to either a demethylation inhibitor or methyl benzimidazole carbamate fungicide, can form heterokaryons with resistance to each fungicide and adaptability to serial combinations of different fungicide concentrations. Field isolates and putative heterokaryons were assayed on fungicide-amended media for in vitro sensitivity. Shifts in fungicide sensitivity and microsatellite genotypes indicated that heterokaryons could adapt to changes in fungicide pressure. Presence of both nuclei in heterokaryons was confirmed by detection of a single nucleotide polymorphism in the β-tubulin gene, the presence of microsatellite alleles of both field isolates, and the live-cell imaging of two different fluorescently tagged nuclei using laser scanning confocal microscopy. Nucleic adaptability of heterokaryons to fungicides was strongly supported by the visualization of changes in fluorescently labeled nuclei to fungicide pressure. Results from this study suggest that heterokaryosis is a mechanism by which the pathogen adapts to multiple fungicide pressures in the field.

Fungicide Dip Treatments for Management of Botrytis cinerea Infection on Strawberry Transplants

Botrytis cinerea, the causal agent of Botrytis fruit rot, is annually introduced into Florida strawberry fields by infected transplants. The disease can be managed by fungicide sprays throughout the season; however, previous studies have demonstrated that several chemical classes are no longer effective owing to B. cinerea resistance. The objectives of this study were to evaluate the effectiveness of preplant fungicide-dip treatments of strawberry transplants on reducing B. cinerea colonization and to evaluate fungicide sensitivity of surviving B. cinerea isolates. Transplants of two strawberry cultivars, Winterstar ‘FL 05-107’ and ‘Florida Radiance’, were dipped in 11 different fungicides. After the plant establishment period, eight leaves per plot were collected before and 14 days after treatment to evaluate B. cinerea incidence. Isolates (n = 139) obtained from the transplants were tested for fungicide sensitivity. Plant diameter was measured 47 days after planting. All treatments, including the controls, reduced B. cinerea incidence by at least 77% compared with predip incidence. However, fungicide-resistant isolates were recovered from all treatments tested.

Pathological characterization of Corynespora cassiicola isolates from traditional and non-traditional rubber growing areas

Rubber ( Heveabrasiliensis ) is one of the major economically important estate crops and generates the third largest export income of Sri Lanka. Rubber plantations established mostly in Wet Zone and certain regions in Intermediate Zone and the cultivated areas are known as traditional areas. However, presently rubber cultivation has been expanded to the dry zone of country and the cultivated areas are known as non-traditional areas. Corynesporacassiicola is the most destructive foliar pathogen of the rubber plant causing Corynespora leaf fall disease (CLFD) and the disease has caused a major devastation in rubber industry resulting in a remarkable economic loss. This study aimed to determine the variability of C. cassiicola isolates from traditional and non-traditional rubber growing areas using pathological factors. Ten isolates of C. cassiicola which had been isolated from diseased leaves of different clones grown in traditional (five isolates) and non-traditional (five isolates) areas were used for characterization. Variability in pathogenicity, temperature sensitivity, growth rate, conidia production, fungicide sensitivity to two fungicides; mancozeb and carbendazim and toxin production were examined. Data were statistically analyzed and the final analytical output revealed a statistically significant difference (p <0.05) between the isolates, but not between two geographical regions for all parameterstested except for toxin production and sensitivity to carbendazim. Though isolates of C.cassiicola show significant difference in pathological factors among isolates irrespective of geographical location, they do not behave differently in different climatic regions. Keywords: Corynesporacassiicola , CLFD, toxins, traditional and non-traditional areas

Characterization of Sclerotinia minor populations in Texas peanut fields

In the summer of 2004 an epidemic of sclerotinia blight of peanut, a disease caused by Sclerotinia minor, occurred in Texas in fields where the disease was never previously detected. The disease was observed on many plants within one of the fields (&gt;3000 disease foci), although most foci were &lt;1 m. It is hypothesized that these observations were inconsistent with the recent introduction of a monocyclic pathogen, even if disease developed under conducive environmental conditions. The pattern of disease is most suggestive of the presence of foliar (ascospore) infections, although air temperature was above the known limits for apothecia development if the pathogen had arrived in the field in 2004 peanut seed. To further examine this epidemic, 232 isolates were collected, across a variety of spatial scales spanning this field and other Texas peanut fields, and evaluated for aggressiveness, fungicide sensitivity and genotypic diversity. There was wide variation among isolates for the phenotypic characteristics measured, but there was no evidence that a genotypically unique, highly aggressive, and fungicide resistant isolate had been introduced or evolved. The predominant genotype, TX1, which contained 154 isolates, was found in every county and field population.

Triazole Fungicides Sensitivity of Sclerotinia homoeocarpa in Korean Golf Courses.

Chemical management of dollar spot in turf may lead to the development of Sclerotinia homoeocarpa populations with reduced fungicide sensitivity. The objective of this study was to investigate resistance of S. homoeocarpa isolates to triazole fungicides and to test cross-resistance among three triazole fungicides. A total of 66 isolates of S. homoeocarpa were collected from 15 golf courses across Korea, and tested via in vitro sensitivity assay against hexaconazole, propiconazole and tebuconazole. EC50 values of the isolates to these fungicides were distributed in the range of 0.001–1.1 a. i. μg ml−1. Based on the EC50 values, twelve representative strains were selected as sensitive isolates including control and insensitive isolates with respect to each fungicide. At a concentration of 0.1 a. i. μg ml−1 for all fungicides, the selected strains were distinguished as sensitive or resistant isolates with the mycelial growth inhibition rate of 50% as the criterion. The EC50 values of resistant strains exposed to hexaconazole, propiconazole and tebuconazole were 20–50 times, 50–70 times, and 77 times greater, respectively, than that of the control strains. Two isolates of S. homoeocarpa S0–41 and Sh14-2-1 showed sensitivity toward all the fungicides used, while two other isolates Sh7-5-1 and Sh2-1-1 showed resistance to all fungicides. Each isolate showed similar resistance to the three types of triazole fungicides, whereby cross-resistance of isolates was confirmed in the present study; all three triazole fungicide combinations displayed significant correlation coefficients equivalent to or greater than 0.8.

Phenotypical and Molecular Characterisation of Fusarium circinatum: Correlation with Virulence and Fungicide Sensitivity

Fusarium circinatum, causing pine pitch canker, is one of the most damaging pathogens of Pinus species. This study investigated the use of phenotypical and molecular characteristics to delineate groups in a worldwide collection of isolates. The groups correlated with virulence and fungicide sensitivity, which were tested in a subset of isolates. Virulence tests of twenty isolates on P. radiata, P. sylvestris and P. pinaster demonstrated differences in host susceptibility, with P. radiata most susceptible and P. sylvestris least susceptible. Sensitivity to the fungicides fludioxonil and pyraclostrobin varied considerably between isolates from highly effective (half-maximal effective concentration (EC50) &lt; 0.1 ppm) to ineffective (EC50 &gt; 100 ppm). This study demonstrates the potential use of simply acquired phenotypical (cultural, morphological) and molecular metrics to gain a preliminary estimate of virulence and sensitivity to certain fungicides. It also highlights the necessity of including a range of isolates in fungicide tests and host susceptibility assays, particularly of relevance to tree breeding programmes.

Azole sensitivity in Leptosphaeria pathogens of oilseed rape: the role of lanosterol 14\u03b1-demethylase

Lanosterol 14-α demethylase is a key enzyme intermediating the biosynthesis of ergosterol in fungi, and the target of azole fungicides. Studies have suggested that Leptosphaeria maculans and L. biglobosa, the causal agents of phoma stem canker on oilseed rape, differ in their sensitivity to some azoles, which could be driving pathogen frequency change in crops. Here we used CYP51 protein modelling and heterologous expression to determine whether there are interspecific differences at the target-site level. Moreover, we provide an example of intrinsic sensitivity differences exhibited by both Leptosphaeria spp. in vitro and in planta. Comparison of homologous protein models identified highly conserved residues, particularly at the azole binding site, and heterologous expression of LmCYP51B and LbCYP51B, with fungicide sensitivity testing of the transformants, suggests that both proteins are similarly sensitive to azole fungicides flusilazole, prothioconazole-desthio and tebuconazole. Fungicide sensitivity testing on isolates shows that they sometimes have a minor difference in sensitivity in vitro and in planta. These results suggest that azole fungicides remain a useful component of integrated phoma stem canker control in the UK due to their effectiveness on both Leptosphaeria spp. Other factors, such as varietal resistance or climate, may be driving observed frequency changes between species.

Fungicide sensitivity, growth rate, aggressiveness and frost hardiness of Monilinia fructicola and Monilinia laxa isolates

Monilinia fructicola, the most destructive pathogen of the genus Monilinia, has recently been introduced into Serbia and many other European countries. Since then, many studies have been conducted to evaluate the characteristics of Monilinia species that have a role in the establishment and survival of the pathogen in new areas. The present study assessed the capacity of M. fructicola to repress and replace Monilinia laxa in Serbia based on: fungicide sensitivity, growth rate and aggressiveness at different temperatures, as well as frost hardiness of the isolates of both species. The results showed that the isolates of M. fructicola, compared to M. laxa, were significantly less sensitive to the following fungicides: iprodione, tebucanozole, chlorothalonil, azoxystrobin, fluopyram, and boscalid. In addition, M. laxa isolates exhibited little variation in sensitivity to all of the tested fungicides, whereas M. fructicola isolates displayed a wide range of sensitivity. The temperature of 5°C favored M. laxa growth and aggressiveness, while at 30°C M. fructicola grew faster and had higher lesion expansion rate. These results support an assumption that M. fructicola will continue to spread in Serbian orchards in coming years, particularly on stone fruits harvested during hot summer weather.

Sensitivity variation and cross-resistance of Zymoseptoria tritici to azole fungicides in North America

Though delayed relative to Europe, fungicide resistance in Zymoseptoria tritici is now present in North America. We assessed azole fungicide sensitivity and cross-resistance in Zymoseptoria tritici isolates from the Willamette Valley of western Oregon, USA. Isolates were grown in the presence of 12 serial dilutions of four different azole fungicides (cyproconazole, propiconazole, tebuconazole, and prothioconazole) and their EC50 values calculated. Mean sensitivity of Z. tritici to the four fungicides differed by three orders of magnitude. Significant correlation coefficients ranging from to 0.42 to 0.75 were found among the three triazole fungicides (cyproconazole, propiconazole, and tebuconazole). Prothioconazole, a triazolinthione and more recently introduced fungicide in Oregon, showed evidence of weaker cross-resistance with the three triazoles (r ranging from 0.26 to 0.42). Our results suggest that, in the early stages of fungicide evolution, growers may be able to conserve azole fungicides through conscientious and carefully selected fungicide applications and management practices.

Regulatory roles of introns in fungicide sensitivity of Fusarium graminearum.

Although the roles of introns have been much debated in eukaryotic organisms, none of them have been functionally characterized in Fusarium graminearum. In this study, we characterized the roles of introns in regulation of fungicide-sensitivity of F. graminearum. β2 tub, cyp51A and myosin-5 are important target genes of benzimidazoles, triazoles and cyanoacrylates respectively. To explore the sensitivity regulation functions of introns in target genes, several detailed deletion studies were completed on the intronic regions of β2 tub, cyp51A and myosin-5. Phenotypic analyses showed that deletion of the fourth intron from β2 tub gene (designated β2 Δi4), the sole intron from cyp51A gene (cyp51A-Δi) and the second intron from myosin-5 gene (myo5-Δi2) exhibited an increased sensitivity to corresponding fungicides. In contrast, deletion of the first or second intron from β2 tub gene exhibited a decreased sensitivity to carbendazim. qRT-PCR showed that the mRNA transcript levels of target genes were significantly downregulated in β2 Δi4, cyp51A-Δi and myo5-Δi2 respectively. Meanwhile, Western blot assays revealed that the protein expression levels of β2 tub was also dramatically reduced in β2 Δi4, but accumulated in β2 Δi1 and β2 Δi2. Overall, our results indicate that introns in target genes significantly regulate the fungicide-sensitivity by influencing expression of the corresponding resident genes in F. graminearum.

Assessing the Risk for Resistance and Elucidating the Genetics of Colletotrichum truncatum That Is Only Sensitive to Some DMI Fungicides.

The genus Colletotrichum contains a wide variety of important plant pathogens, and Colletotrichum truncatum is one of the most prevalent species of Colletotrichum on chili in China. Demethylation-inhibitor fungicides (DMIs) are currently registered chemical agents for the management of the anthracnose disease caused by Colletotrichum spp. To assess the risk for DMI resistance development, 112 C. truncatum isolates were collected from infected pepper in 13 regions of China. The sensitivity of C. truncatum isolates to five DMI fungicides was determined based on mycelial growth inhibition assay. C. truncatum was sensitive to prochloraz, epoxiconazole, and difenoconazole, but not to tebuconazole or myclobutanil. Baseline sensitivity using the 112 C. truncatum isolates was established for the first three effective DMIs. Prochloraz, epoxiconazole, and difenoconazole EC50 values were 0.053 ± 0.023, 1.956 ± 0.815, and 1.027 ± 0.644 μg/ml, respectively. Eleven stable DMI-resistant mutants all exhibited lower fitness levels than their wild-type parents, suggesting a low risk of DMI resistance in C. truncatum. By inducing gene expression, CtCYP51 expression increased slightly in the resistant mutants as compared to wild-types when exposed to DMI fungicides and thus contributed at least partially to resistance. Molecular docking with CYP51 structure models was used to explain differential sensitivity of the DMI fungicides in C. truncatum. Our results suggest that the M376L/H373N mutations in CYP51 changed the conformation of DMIs in the binding pocket. These changes prevented the formation of the Fe – N coordinate bond between the heme iron active site and tebuconazole or myclobutanil, and apparently contributed to tebuconazole and myclobutanil insensitivity of C. truncatum.

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

AbstractThe 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.

Phylogenetic relationship and fungicide sensitivity of members of the Colletotrichum gloeosporioides species complex from apple

The members of the Colletotrichum gloeosporioides species complex (CGSC), the dominant pathogens of apple bitter rot in Nagano prefecture, Japan, were reidentified and the relationship between the species and fungicide sensitivity was revealed. Based on phylogenetic analysis of the ApMat locus with the neighbor-joining (NJ) method, isolates from apple contained three species of the CGSC; C. fructicola, C. aenigma, C. siamense, and three clades of the CGSC: Clade V, S and K. Colletotrichum fructicola and Clade S dominated in Nagano Prefecture. Isolates of C. siamense, C. aenigma and Clade V, S and K remained sensitive to benomyl and quinone outside inhibitor (QoI) fungicides, while C. fructicola often developed resistance to benomyl and QoI fungicides. These results suggest that the development of fungicide resistance differs among members of the CGSC.

Distribution and Fungicide Sensitivity of Colletotrichum Species Complexes from Rubber Tree in Hainan, China.

Colletotrichum gloeosporioides and C. acutatum species complexes are causal agents of Colletotrichum leaf disease (CLD) of rubber trees worldwide. To determine the geographic distribution of Colletotrichum species complexes associated with CLD of rubber trees in Hainan, China, and their sensitivity to fungicides used in the region, a total of 275 Colletotrichum isolates were collected from 52 rubber tree plantations in 11 counties. These isolates were identified based jointly on morphological characteristics and PCR-based methodology. Of these isolates, 78 and 22% belonged to the C. gloeosporioides species complex (CGSC) and the C. acutatum complex (CASC), respectively. The incidence of CGSC isolates was greater than the CASC in all counties sampled. The incidence of CASC isolates appeared to be lower in the western and central south of Hainan than in other regions. There was no association in their presence at a given plantation between the two species complexes. The in vitro sensitivity of these two species complexes to carbendazim, chlorothalonil, and four demethylation inhibitor (DMI) fungicides (difenoconazole, propiconazole, myclobutanil, and prochloraz) was determined. Carbendazim was effective against CGSC but not against CASC with mean ED50 values of 0.176 and 2.182 µg/ml, respectively. CASC isolates were more sensitive to difenoconazole, propiconazole, and myclobutanil (mean ED50 values of 0.177, 0.129, and 1.424 µg/ml, respectively) than CGSC isolates (mean ED50 values of 0.710, 0.348, and 3.496 µg/ml, respectively). Mean ED50 values of CGSC against chlorothalonil and prochloraz were 173.341 and 0.035 µg/ml, respectively; corresponding values for CASC were 151.441 and 0.040 µg/ml. These results suggest that prochloraz, propiconazole, and difenoconazole are effective against both species complexes.

Involvement of Penicillium digitatum PdSUT1 in fungicide sensitivity and virulence during citrus fruit infection

A putative sucrose transporter PdSUT1 included in the same clade that Sut1p from Schizosaccharomyces pombe was identified in Penicillium digitatum, the major citrus postharvest pathogen. PdSUT1 gene was characterized using target gene disruption and gene overexpression. The ΔPdSUT1 mutants generated by gene elimination showed reduction in fungal virulence during citrus fruit infection assayed in mature fruit at 20°C. However, the overexpression mutants did not increased disease severity neither in the mutants coming from a high virulent nor from a low virulent P. digitatum progenitor strains.Moreover, fungicide sensitivity was affected in the deletant mutants but not in the overexpression transformants. The expression analysis of several genes involved in fungicide resistance showed an intensification of MFS transporters and a decrease of sterol demethylases transcriptional abundance in the ΔPdSUT1 mutants compare to the parental wild type strain.PdSUT1 appear not to be directly involved in fungicide resistance although can affect the gene expression of fungicide related genes. These results indicate that PdSUT1 contribute to P. digitatum fungal virulence and influence fungicide sensitivity through carbohydrate uptake and MFS transporters gene activation.

Seasonal dynamics and fungicide sensitivity of organisms causing brown patch of tall fescue in North Carolina

ABSTRACTBrown patch, caused by multiple species of Rhizoctonia and Rhizoctonia-like fungi, is the most severe summer disease of tall fescue in home lawns across the southeastern United States. Home lawns were surveyed in central North Carolina from 2013 to 2015 to determine the organisms present during typical epidemics of brown patch in tall fescue. Isolates of Rhizoctonia and Rhizoctonia-like fungi were obtained by sampling 147 locations in July 2013 and May and July 2014. In addition, 11 sites were sampled once a week for 12 consecutive weeks from late May to the end of July 2015. All isolates were identified to species and anastomosis group with nuc rDNA internal transcribed spacer (ITS) sequence analysis. Isolations from brown patch lesions in May 2014 predominately yielded Ceratobasidium cereale (77% of the organisms recovered), whereas the organisms recovered in July 2013 and 2014 were R. solani AG 2-2-IIIB (44%), R. solani AG 1-IB (37%), and R. zeae (14%). In 2015, Ceratobasidium cereale was isolated from all 11 locations in May but was replaced by Rhizoctonia species in June and July. Sensitivity of the May 2014 isolates to multiple concentrations of the fungicides azoxystrobin, flutolanil, fluxapyroxad, and propiconazole was compared with sensitivity of isolates collected in 2003, to determine whether multiple years of exposure to fungicides applied for brown patch control had altered fungicide sensitivity. Historical isolates of R. solani, which had never been exposed to fungicide applications for brown patch control, were also included for comparison. Mean EC50 values (concentration of fungicide needed to inhibit mycelial growth by 50%) varied across fungicides and species, but no resistance was observed, and there was no apparent shift in sensitivity over the years. An additional 94 isolates from 2015 were screened against azoxystrobin, flutolanil, fluxapyroxad, and propiconazole, and fungicide insensitivity was not observed.

Monitoring Fungicide Sensitivity Levels and Mycelial Compatibility Groupings of Sclerotium rolfsii Isolates from Florida Peanut Fields

ABSTRACT Sclerotium rolfsii, the causal agent of peanut stem rot, is a diverse pathogen that has exhibited decreases in sensitivity to fungicides in areas where they are frequently applied. To better understand this pathogen's diversity and its response to various fungicides in Florida, a monitoring survey was done to examine isolates from several peanut producing areas using morphological characteristics, mycelial compatibility groupings and fungicide sensitivity profiles. A high level of morphological diversity was observed among a small number (N = 15) of isolates which was affirmed by both Shannon-Weiner (E = 0.812) and Simpson's (D = 0.280) indices. However, despite this high level of diversity, fungicide sensitivity of these isolates to flutolanil (EC50 = 0.031 ppm) and tebuconazole (EC50 = 0.008 ppm) appears to remain relatively unchanged when compared to a previous baseline study. Utilizing a small number of isolates, this monitoring survey indicated the EC50 values for the products azoxystrobin (EC50 = 0.050 ppm), prothioconazole (EC50 = 0.213 ppm), penthiopyrad (EC50 = 0.016 ppm) and solatenol (EC50 = 0.005 ppm). A trend for hormesis was also noted in this survey (e.g. flutolanil), but further research is necessary to better understand sub-lethal fungicide dose effects on increasing mycelial growth. It is apparent from these results that despite the high levels of phenotypic diversity in S. rolfsii populations, current fungicide management practices should remain effective for disease control.

Sensitivity of Exserohilum turcicum to demethylation inhibitor fungicides

Exserohilum turcicum, the causal agent of northern leaf blight (NLB) of corn (Zea mays), can cause significant yield reductions in Illinois. Metconazole, propiconazole, and prothioconazole are demethylation inhibitor (DMI) fungicides often applied as solo active ingredients or mixed with other fungicide groups to control NLB. Monitoring for shifts in DMI fungicide sensitivity in E. turcicum populations is important for making management decisions and maintaining fungicide efficacy. Sensitivity to metconazole, propiconazole, and prothioconazole were determined for E. turcicum isolates collected prior to DMI use on corn (baseline group collected prior to 1992) and E. turcicum isolates collected between 2009 and 2014. An in vitro mycelial growth assay was used to determine the effective fungicide concentration at which 50% of the fungal growth was inhibited (EC50) for each isolate-fungicide combination. Baseline EC50 value least-square means (lsmeans) for metconazole, propiconazole, and prothioconazole were 0.032, 0.060, and 0.254 μg/mL, respectively. When lsmeans of EC50 values for 2009, 2010, 2011, 2012, 2013, and 2014 E. turcicum isolates were compared to the lsmean of the baseline E. turcicum EC50 values, no significant (P ≤ 0.05) shift towards reduced sensitivity was observed for metconazole, propiconazole, or prothioconazole. Three E. turcicum isolates had EC50 values significantly higher (P ≤ 0.05) than the least sensitive baseline isolate for metconazole, and one isolate had an EC50 value significantly higher (P ≤ 0.05) than the least sensitive baseline isolate for propiconazole. These isolates will require further evaluation to determine if they demonstrate reduced field sensitivity. Isolate prothioconazole EC50 values were positively correlated (P ≤ 0.05) with those of metconazole and propiconazole (r = 0.1690 and r = 0.3269, respectively); however, no correlation was found between metconazole and propiconazole EC50 values. Positive correlations between EC50 values of prothioconazole and the other fungicides suggest the potential for cross-resistance between these DMI fungicides. To date, no loss of NLB control has been observed with the use of metconazole, propiconazole, and prothioconazole in the field.