Absence Of Fungicide Research Articles

Pasmo disease and lodging in flax as affected by pyraclostrobin fungicide, N fertility and year

Vera, C. L., Irvine, R. B., Duguid, S. D., Rashid, K. Y., Clarke, F. R. and Slaski, J. J. 2014. Pasmo disease and lodging in flax as affected by pyraclostrobin fungicide, N fertility and year. Can. J. Plant Sci. 94: 119–126. Severe infection of the fungal disease known as pasmo, caused by Septoria linicola (Speg.) Garassini, reduces seed yield of flax (Linum usitatissimum L.) in western Canada. Pasmo may also indirectly affect seed yield by predisposing flax plants, under favorable weather and soil fertility conditions, to lodge. With the objective of studying the possible association of this disease with the occurrence of lodging and their effect on seed yield, a study was conducted at Melfort, Saskatchewan, Canada, during 4 consecutive years (2009, 2010, 2011 and 2012), with two fungicide regimes (application and no application) and five rates of nitrogen (N) fertilization (0, 33, 66, 100 and 133% of recommended). The application of pyraclostrobin fungicide reduced disease severity and increased seed yield of flax in the 3 yr (2010, 2011 and 2012) that pasmo infection was detected, and prevented or reduced the occurrence of lodging in the 2 yr (2010 and 2012) with favorable weather conditions for the occurrence of lodging. Increasing rates of N resulted in increased seed yield in 2009 and 2012, regardless of whether fungicide was used or not. Increasing rates of N also increased the severity of pasmo disease in 2010, 2011 and 2012, and of lodging severity in 2010 and 2012. The increase in disease severity due to N occurred in the absence of fungicide in 2011 and when fungicide was applied in 2012, while in 2010 just the effect of N on the two fungicide treatments combined was significant. Only low levels of disease severity (near 20%) were observed when fungicide was applied in 2011, while extreme disease severity (near 100%) occurred in 2012 at all levels of N when fungicide was not applied. The increase in lodging, as N rates increased, was observed only in the absence of fungicide, in both years. The occurrence of lodging took place late in the development of flax.

Characterization of fungicide resistant isolates of Phaeoacremonium aleophilum infecting grapevines in Spain

Fungicide resistance in Phaeoacremonium aleophilum, one of the most frequent fungal pathogens associated with grapevine trunk diseases, was investigated and found to exist in some isolates of the pathogen against a commercial formulation, Escudo®. The effect of this compound and its two active substances, carbendazim and flusilazole, was first evaluated on the mycelial growth of P. aleophilum. Escudo®-resistant isolates were estimated at a frequency of 24% in Spanish vineyards. Then, the two active substances were used individually to test their effect on mycelia growth of twelve single-spore cultures originating from six Escudo®-resistant isolates. Flusilazole (DMI-triazole) did not inhibit mycelia growth of any single-spore cultures of P. aleophilum. Carbendazim (benzimidazole) used alone allowed the growth of the same single-spore cultures that were also resistant to Escudo®. AFLP characterization of sensitive and resistant single-spore cultures showed genetic diversity within P. aleophilum isolates but no AFLP markers were associated with resistance. New primers set (L2/R1) were designed to partially amplify the exon 6 of the beta-tubulin gene of P. aleophilum. Two different point mutations resulted in glycine (GGC) or lysine (AAA) replacing the glutamic acid (GAG) at codon 198 of the beta-tubulin gene in some of the resistant single-spore cultures studied. Resistant single-spore cultures of P. aleophilum were shown to have different aggressiveness levels as sensitive single-spore cultures by inoculation of wood segments of Vitis vinifera in the presence and absence of fungicide.

Yield performance and pest resistance among peanut genotypes when grown without fungicides or insecticides

Yield trials without fungicides or insecticides were conducted four consecutive years (2007–10) at the University of Georgia, Coastal Plain Experiment Station, Tifton Campus to evaluate for peanut (Arachis hypogaea L.) pest resistance. The most endemic diseases in the southeast U.S. are tomato spotted wilt (TSW) caused by Tomato spotted wilt virus, white mold (WM) caused by Sclerotium rolfsii Sacc, and both early and late leaf spot caused by Cercospora arachidicola Hori and Cercosporidium personatum (Berk & Curt.) Deighton, respectively. The most endemic insects are tobacco thrips (Frankliniella fusca Hinds) and potato leafhopper (Empoasca fabae Harris). Results from these replicated field tests showed significant differences (P ≤ 0.05) among advanced Georgia breeding lines and cultivars. Three Georgia cultivars ‘Georgia-01R’, ‘Georgia-05E’, and ‘Georgia-10T’ consistently produced among the best yields with high levels of resistance to TSW, white mold, leafhoppers, and leaf spot each year. Georgia-01R is a multiple-pest-resistant, runner-type cultivar with late maturity; whereas, Georgia-05E is a multiple-pest-resistant, virginia-type cultivar with medium-late maturity. Georgia-10T is a new runner-type cultivar with high level of TSWV and white mold resistance and late-maturity, similar to Georgia-01R, one of its parents. However, Georgia-10T does not appear to have the high level of leaf spot and leafhopper resistance as Georgia-01R, but it still had high yields in the absence of fungicides or insecticides.

Sensitivity of Fusarium graminearum causing head blight of wheat in Brazil to tebuconazole and metconazole fungicides

Fusarium head blight (FHB) of wheat, is caused by fungal populations belonging to the Fusarium graminearum species complex. Although triazole fungicides are widely used in FHB management, information about the sensitivity of F. graminearum populations to such fungicides is scarce, particularly in Brazil. This work aimed at determining the sensitivity of pathogenic populations of F. graminearum to metconazole and tebuconazole. Fungal isolates were obtained from diseased wheat heads collected in commercial fields at the northern region of Rio Grande do Sul state, Brazil, along the 2009 and 2010 growing seasons. Fifty isolates obtained from eleven locations were grown in media amended with increasing fungicide dosages and the effective concentration reducing mycelial growth to 50% (EC50) was determined. The median of the EC50 values was 0.001 mg/L (<0.001-0.152 mg/L) and 0.037 mg/L (<0.001-0.324), for tebuconazole and metconazole, respectively. Cross-resistance was detected based on significance of correlation analysis (rs = 0.46; P < 0.0001) between the log-transformed EC50 of the two triazoles. The mean EC50 for a group of ten isolates classified as more differed statistically from the ten isolates representing the less group. Nevertheless, these groups did not differ in terms of mycelial growth on PDA in the absence of fungicides. It was concluded that pathogenic populations of F. graminearum are sensitive to the triazoles at various levels due to the intrinsic characteristics of the isolates or to selection pressure by the increasing fungicide use.

Effect of fungicides on plant growth promoting activities of phosphate solubilizing Pseudomonasputida isolated from mustard (Brassica compestris) rhizosphere

This study was navigated to examine the effects of fungicide-stress on the activities of plant-growth-promoting rhizobacterium Pseudomonasputida with inherent phosphate solubilizing activity. The fungicide-tolerant and phosphate solubilizing P.putida strain PS9 was isolated from the mustard rhizosphere and tentatively identified following standard morphological, physiological and biochemical tests. To further consolidate the identity of the strain PS9, the 16S rDNA sequence analysis was performed. Following the BLAST program, the strain PS9 was identified as P.putida. In the presence of the varying concentrations (0–3200μgmL−1; at a two fold dilution interval) of four fungicides of different chemical families (tebuconazole, hexaconazole, metalaxyl and kitazin) amended in minimal salt agar medium, the P.putida strain PS9 showed a variable tolerance levels (1400–3200μgmL−1) against the tested fungicides. The strain PS9 produced plant-growth-promoting (PGP) substances in significant amount in the absence of fungicides. In general, fungicides applied at the recommended, two and three times of the recommended rates, decreased the PGP attributes of P.putida the strain PS9 and affected the PGP activities in concentration-dependent manner. Fungicides at the recommended dose had minor reducing effect while the doses higher than the recommended dose significantly reduced the PGP activities (phosphate solubilization, salicylic acid, 2,3-dihydroxy benzoic acid, and indole-3-acetic acid production except exo-polysaccharides, hydrogen cyanate and ammonia production). Of the four fungicides, tebuconazole generally, showed maximum toxicity to the PGP activities of the strain PS9. This study inferred that fungicides must be examined in vitro for their possible adverse effects on soil micro flora before their application in agricultural fields. Moreover, the results also suggested the prerequisite of application of fungicide-tolerant PGPR strains as bioinoculants so that their PGP activities may not be suppressed under fungicide stress.

Mowing Frequency and Plant Growth Regulator Effects on Dollar Spot Severity and on Duration of Dollar Spot Control by Fungicides.

Management of dollar spot (incited by Sclerotinia homoeocarpa) on golf course fairways is increasingly challenging. The objectives of this study were to determine the influence of mowing frequency and plant growth regulators (PGRs) on dollar spot severity and on the residual efficacy of fungicides for control of dollar spot. Two 4-month-long studies were conducted on 'Putter' creeping bentgrass (Agrostis stolonifera) maintained as a fairway at the University of Connecticut. Treatments were arranged in a three-by-three-by-five factorial that assessed the influence of mowing frequency (2, 4, or 6 days week-1) and PGRs (paclobutrazol, trinexapac-ethyl, or none) on dollar spot control by five fungicide treatments (boscalid, chlorothalonil, iprodione, propiconazole, or none). Turf was mowed in the afternoon hours to minimize the confounding effect of mowing frequency on leaf wetness duration. Treatments were initiated in the late spring of 2007 and 2008, and each fungicide treatment was reapplied only when dollar spot exceeded a threshold of five infection centers plot-1. In the absence of fungicides, dollar spot severity was reduced by 63 to 90% in plots treated with paclobutrazol and by 13 to 55% in plots treated with trinexapac-ethyl. Dollar spot severity was 23 to 50% lower in plots mown 2 days week-1 compared with those mown 6 days week-1. In cases where a significant interaction was observed between mowing frequency and PGRs, dollar spot was reduced on most rating dates in plots treated with trinexapacethyl that were mown 2 days week-1 compared with those mown 6 days week-1. Survival analysis of days until threshold was met revealed that duration of control of fungicides in plots receiving paclobutrazol were 28 to 84% longer compared with plots not receiving PGR. Duration of control by fungicides was generally similar between plots treated with trinexapac-ethyl and no PGR. In general, mowing frequency did not influence duration of control. Results from this study indicate that paclobutrazol could be used to increase the treatment interval of fungicides and that mowing frequency in the absence of dew is likely to have little influence on fungicide residual efficacy. When used without fungicides, PGRs and less frequent mowing may reduce dollar spot in situations where fungicide use is limited.

Response of progeny bred from Bolivian and North American cultivars in integrated management systems for leaf spot of peanut ( Arachis hypogaea)

Early and late leaf spot (caused by Cercospora arachidicola and Cercosporidium personatum, respectively) are major yield-reducing diseases of peanut ( Arachis hypogaea L.) in the southeastern U.S. Effective suppression of both early and late leaf spot, collectively called ‘leaf spot’, can be reached with integrated disease management (IDM) systems utilizing peanut cultivars with moderate levels of resistance to these pathogens. A Bolivian landrace cultivar, Bayo Grande, and a series of progeny breeding lines were evaluated under two IDM systems consisting of various reduced application fungicide regimes and/or tillage treatments in field experiments over two years. In the absence of fungicides or when averaged across fungicide regimes, Bayo Grande and breeding lines had better leaf spot resistance when compared to cultivars Georgia Green, MDR-98 and C-99R in all experiments across years. The addition of three or four fungicide applications to Bayo Grande and several of the breeding lines suppressed leaf spot to levels similar to those genotypes and Georgia Green under the full regime. Conservation tillage reduced the need for fungicides in most genotypes in both years. Yields were higher for Bayo Grande and the breeding lines compared to Georgia Green in three of the four experiments across years. When compared across genotype, yields were greatest for the full season fungicide regime, except in 2002 when yields were similar for all treatments. Leaf spot severity was reduced in conservation tillage compared to conventional tillage in most varieties in the absence of fungicides. However, yields tended to be lower in conservation tillage treatments than in conventional tillage. Under conservation tillage, yields of Bayo Grande and two of the breeding lines were negatively impacted in one year and were overall lower in the other year. Several of these new Bolivian-derived genotypes show promise for use in a reduced fungicide and/or conservation tillage IDM system with the potential to lessen fungicide use compared to standard production practices while maintaining low leaf spot levels and high yields.

In vitro toxicity of alternative oxidase inhibitors salicylhydroxamic acid and propyl gallate on Fusicladium effusum

Pecan scab, caused by the fungus Fusicladium effusum (syn. Cladosporium caryigenum and Fusicladosporium effusum), is the most devastating disease of the commercial pecan (Carya illinoinensis) production in Southeastern United States. Disease control depends primarily on multiple applications of fungicides per season. Fungicides available for scab control include those in the quinone outside inhibitor (QoI) group. QoI fungicides have a high level of resistance risk and in vitro sensitivity assays are essential to monitor development of resistance in populations of F. effusum. In vitro assays were conducted to determine sensitivity of F. effusum to the QoI fungicide azoxystrobin. In vitro assays were conducted with and without the addition of salicylhydroxamic acid (SHAM) to inhibit alternative oxidase (AOX) and eliminate the rescue impact of AOX on fungal respiration in the presence of QoI fungicides. When 0.653 mM SHAM was added to the medium, growth of a majority of the isolates was significantly reduced, even in the absence of fungicide. Of the 89 isolates tested, only 21 showed a dose response to azoxystrobin. Growth of the other 68 isolates was insufficient to measure a dose response. Only 18 isolates showed a clear dose response to azoxystrobin both with and without SHAM, but the calculated EC50 values were significantly different (P < 0.0001). Further experiments were performed to investigate the effects of different concentrations of SHAM and another AOX inhibitor, propyl gallate (PG) on hyphal growth derived from mycelial fragments of F. effusum in liquid medium in microtiter plates and hyphal growth derived from germinated conidia on solid medium in the absence of fungicide. Even at the lowest concentrations tested, both SHAM and PG (0.01 and 0.25 mM, respectively) inhibited hyphal growth of F. effusum in liquid medium and on solid medium. These results suggest the role of AOX and possible auxiliary toxicity of these inhibitors in F. effusum physiology. Due to the toxic effects of these AOX inhibitors on F. effusum in vitro, future sensitivity profiling for QoIs will be performed without the addition of these inhibitors.

Plant growth promoting activities of phosphate-solubilizing Enterobacter asburiae as influenced by fungicides

Four fungicides, tebuconazole, hexaconazole, metalaxyl and kitazin were applied at recommended and higher rates to examine their effects on the plant growth promoting activities of the fungicide tolerant Enterobacter asburiae strain PS2 under in vitro conditions. Enterobacter asburiae strain PS2 was isolated from the mustard rhizosphere and was assessed for the fungicide-tolerance and production of plant growth promoting traits (phosphate solubilization, siderophores, indole acetic acid, exo-polysaccharides, hydrogen cyanide and ammonia production) both in the presence and absence of fungicides. Enterobacter asburiae strain PS2 showed plant growth promoting activities even in the presence of fungicides which however, decreased progressively with the increase in fungicide concentration. Fungicides at recommended dose had little effect while the dose higher than the recommended one adversely affected the physiological traits, like, phosphate solubilization, siderophore, and indole acetic acid synthesis. Among all fungicides, tebuconazole in general, showed maximum toxicity to plant growth promoting activities. Fungicide-tolerant E. asburiae strain PS2 with the inherent ability to produce growth regulators even in the presence of fungicides can be exploited as a bio-inoculant to increase the productivity of crops grown in fungicide stressed soils.

Impact and Interaction of Nitrogen and Phytophthora infestans as Yield-limiting and Yield-reducing Factors in Organic Potato (Solanum tuberosum L.) Crops

For organic potato producers, the two main challenges are disease and nutrient management. Both are limited by regulations that on the one hand prohibit the use of chemical fertilizers, especially nitrogen, and on the other hand prohibit most synthetic pesticides. Late blight caused by Phytophthora infestans is commonly thought to be the most yield-reducing factor. However, because there is no really effective fungicide available to control late blight, there are virtually no yield loss data available for organic farming conditions. In this paper, the state of the art of organic potato tuber growth under on-farm conditions with respect to disease and nutrient management is summarized by field trials and on-farm surveys on commercial organic crops carried out in the years 1995–1998. Soil nitrogen (N) levels, plant N uptake, disease development of P. infestans and potato yield were measured. Results indicated that N availability was most important in limiting yields in organic potato crops. From on-farm data, a model including disease development, growth duration of the crops until foliage decay and different parameters related to N status of the crop could explain 73% of the observed variation in yield. Only 25% of this variation in yield could be attributed to the influence of late blight. Differences in N availability explained 48%. In conclusion, several points emerged from the results. In organic farming, yields are mainly limited by nutrient availability in spring and early summer. The effects of late blight on yields may often be overestimated and cannot be deduced from results in conventional farming because of the strong interaction with nutrient status. Depending on N availability, tubers stop growing between mid-July (70–90 kg N ha−1 uptake), the end of July (110–140 kg N ha−1 uptake) and mid-August (140–180 kg N ha−1 uptake) due to N limitations. The higher the N status of a potato crop, the longer the growing period needed to achieve the attainable yield and the higher the probability that late blight stops further tuber growth and becomes the key tuber-yield-limiting factor. In the second part of this paper, the interactive effects of soil N availability and the impact of P. infestans on yield in the presence and absence of fungicides from 1996 to 1998 for mid-early main crops are reported. An empirical schematic model of disease impact depending on N availability was developed.

Comparing artificial and natural selection in rate of adaptation to genetic stress in Aspergillus nidulans

In an experimental study of adaptation to negative pleiotropic effects of a major fungicide resistance mutation in the filamentous fungus Aspergillus nidulans we have investigated the relative effectiveness of artificial selection vs. natural selection on the rate of compensatory evolution. Using mycelial growth rate as a fitness measure, artificial selection involved the weekly transfer of the fastest growing sector onto a fresh plate. Natural selection was approximated by transferring random samples of all the spores produced by the mycelium. Fungicide resistant and fungicide sensitive haploid and diploid strains were used in an evolution experiment over 10 weekly transfers, which is equivalent to 1200 cell cycles. Two different environmental conditions were applied: a constant fungicide-free environment and a weekly alternation between presence and absence of fungicide. Results show that for all strains and conditions used the transfer of a random sample of all spores leads to more rapid adaptation than the transfer of the visually 'fittest' sector. The rates of compensatory evolution in the constant and the alternating environment did not differ. Moreover, haploid strains tend to have a higher rate of adaptation than isogenic diploid strains.

Repeatability estimates of genetic, phenotypic and environmental correlations in oats with and without fungicide application

1 ABSTRACT - Estimates of associations among traits are not very precise when based on annual data since the year factor influence the phenotype strongly. Therefore, objective of this work was to estimate the repeatability coefficients and number of years needed for an accurate prediction of genetic, phenotypic and environmental correlations among six important traits in oat with and without fungicide application (treatments). Nine oat cultivars were evaluated for four years (2000, 2001, 2002, and 2003) with and without fungicide, in a random block design with three replications. In the absence of fungicide, two years of trial evaluation were enough to provide reliable phenotypic and genetic correlation estimates; the same was not true for the experiments with fungicide application where at least seven (phenotypic) and 21 (genetic) years were required. In both treatments the environmental correlation estimates were little predictable.

Implicações da aplicação de fungicida na adaptabilidade e estabilidade de rendimento de grãos em aveia branca

Vinte genótipos de aveia (Avena sativa L.), testados em 13 ambientes, foram submetidos à análise de adaptabilidade e estabilidade com o objetivo de avaliar o efeito da utilização de fungicida nestes parâmetros, através do modelo de regressão segmentado de CRUZ et al. (1989). A aplicação de fungicida afetou os parâmetros de adaptabilidade (b1), responsividade (b1+b2) e estabilidade (&lt;img border=0 id="_x0000_i1026" src="../../../../img/revistas/cr/v34n3/a07img01.jpg"&gt;), indicando que as estimativas devam ser realizadas em separado nos ambientes com e sem fungicida. Além disso, o uso de fungicida proporcionou efeito favorável sobre a estabilidade de rendimento de grãos, sendo que quatro genótipos testados mostraram ser estáveis (UPF 19, UPF 20, OR-3 e OR-4), enquanto que na ausência de fungicida todos os genótipos revelaram instabilidade de rendimento de grãos. Entretanto, nenhuma cultivar testada evidenciou características de genótipo ideal, conforme preconizado pelo modelo adotado.

Phenotype Instability in Botrytis cinerea in the Absence of Benzimidazole and Dicarboximide Fungicides

ABSTRACT Stability of phenotypes of isolates of Botrytis cinerea that were sensitive or resistant to benzimidazole and dicarboximide fungicides was examined in the absence of fungicides in laboratory and growth room experiments. Twelve greenhouse isolates of B. cinerea were subcultured on potato dextrose agar (PDA) for 20 generations and on geranium seedlings for 15 generations. Three isolates of each of the following four phenotypes were used: sensitive to the fungicides thiophanate-methy1 (a benzimidazole) and vinclozolin (a dicarboximide) (S(T)S(V)), resistant to both fungicides (R(T)R(V)), resistant to thiophanate-methy1 and sensitive to vinclozolin (R(T)S(V)), and sensitive to thiophanate-methy1 and resistant to vinclozolin (S(T)R(V)). In three trials on PDA, 36 populations were subcultured; 8 populations changed phenotypes by the end of 20 generations, as determined by conidium germination on fungicide-amended medium. Five of the eight initially were S(T)R(V); the resulting phenotypes were S(T)S(V), R(T)S(V), and R(T)R(V). Populations from eight other isolates exhibited temporary changes in phenotype during intermediate generations on PDA but reverted to initial phenotypes by the twentieth generation; five of these populations changed to phenotype R(T)R(V). In two geranium seedling trials, each of the 12 greenhouse isolates was inoculated onto a set of three seedlings for each generation, and diseased tissue that developed was used to initiate the next generation. Therefore, a total of 72 populations of B. cinerea were subcultured in the two trials; 5 of these populations changed phenotype at the end of 15 generations. Three of the five initially were S(T)R(V); these changed to phenotypes S(T)S(V) or R(T)R(V). In each of the two trials on geranium seedlings, a population subcultured from one S(T)S(V) isolate changed phenotype one to phenotype R(T)R(V) and one to phenotype R(T)S(V). In all trials, no population resistant to thiophanate-methy1 changed to a thiophanate-methy1-sensitive phenotype, and no population changed to phenotype S(T)R(V). Random amplified polymorphic DNA (RAPD) fingerprints were generated with the 12 initial isolates and 49 isolates subcultured on PDA or geranium seedlings. Cluster analyses of RAPD markers showed that subcultured isolates exhibiting the same phenotype clustered together and that subcultured isolates derived from a common greenhouse isolate but with different phenotypes were in different clusters. Some populations that did not change phenotype exhibited considerable differences in RAPD marker patterns. The results of this study indicate that, in the absence of fungicides, sensitive populations of B. cinerea can develop resistance to thiophanate-methy1 and vinclozolin, and this resistance can be maintained in populations through multiple generations. Populations resistant only to vinclozolin (S(T)R(V)) exhibited a high frequency of phenotype change, and populations resistant to both fungicides (R(T)R(V)) were stable.

Sensitivity of fungi from cereal roots to fluquinconazole and their suppressiveness towards take‐all on plants with or without fluquinconazole seed treatment in a controlled environment

The take‐all fungus, Gaeumannomyces graminis var. tritici, was highly sensitive to fluquinconazole (in‐vitro EC50 0·016–0·018 mg L−1), a fungicide developed for use as a seed treatment to control take‐all, and to prochloraz (EC50 0·006 mg L−1). Fungi of other genera that were commonly isolated from cereal roots were sensitive in varying degrees to prochloraz but were relatively insensitive (e.g. Fusarium culmorum, EC50 &gt; 20 mg L−1) or slightly sensitive (e.g. Epicoccum purpurascens, EC50 0·514 mg L−1) to fluquinconazole. Gaeumannomyces graminis var. graminis and G. cylindrosporus, weak parasites that can protect roots against take‐all, and an unnamed Phialophora sp., all closely related to the take‐all fungus, were highly or moderately sensitive to fluquinconazole. Alternaria infectoria and E. purpurascens were most consistently effective in suppressing development of take‐all on pot‐grown wheat plants dual‐inoculated with G. graminis var. tritici and the nonpathogen. Take‐all was decreased more on dual‐inoculated wheat plants grown from seed treated with fluquinconazole or fluquinconazole plus prochloraz than when only an antagonistic fungus (A. infectoria, E. purpurascens, Fusarium culmorum or Idriella bolleyi) or a seed treatment was applied. These fungi were less effective in combination with seed treatments on barley. Gaeumannomyces graminis var. graminis and G. cylindrosporus, tested on wheat, suppressed take‐all only in the absence of fungicides. It is suggested that the performance of seed treatment containing fluquinconazole against take‐all may in some circumstances be enhanced by its partial specificity for the take‐all fungus.

Green leaf area decline of wheat flag leaves: the influence of fungicides and relationships with mean grain weight and grain yield

SummaryA modified Gompertz model was derived to describe the fractional decline in green area of wheat flag leaves in field experiments where green leaf area at time t=100exp[‐exp(‐k(t‐m))]. Curves fitted over time to visual assessments of green leaf area (% of total leaf area) throughout flag leaf life accounted for more than 98% of variation in 45 of 48 wheat cultivar × fungicide treatment (+/−) comparisons. This data set spanned 17 yr and therefore included cultivars of contrasting parentage and age. In the absence of fungicide, green leaf area decline was associated with drought or infection with a number of foliar pathogens including Septoria tritici (sexual stage Mycospherella graminicola), Erysiphe graminis and Puccinia striiformis. Fungicides applied to the flag leaf included propiconazole, propiconazole plus tridemorph, flusilazole or azoxystrobin. Fungicide effects on m (i.e. time to 37% green area) were closely related to fungicide effects (% of untreated) on mean grain weight (variation accounted for (VAF) = 80%) and grain yield (VAF = 85%).

Mechanisms of Resistance to Fenpropimorph and Terbinafine, Two Sterol Biosynthesis Inhibitors, in Nectria haematococca, a Phytopathogenic Fungus

The mechanisms of resistance to terbinafine, a squalene epoxidase inhibitor, and to fenpropimorph, a sterol Δ14-reductase and/or Δ8 → Δ7-isomerase inhibitor, were investigated in laboratory mutants of the phytopathogenic fungus Nectria haematococca. Neither modified fungicide uptake nor fungicide metabolism could explain resistance in the mutants studied. The terbinafine-resistant mutants contained at least 10 times more squalene than the wild-type strain, and when cultivated in presence of terbinafine, they required a 30-times higher fungicide concentration to obtain a very high level of accumulated squalene similar to that of the wild-type strain. Thus, a reduced affinity of the squalene epoxidase toward both the substrate and the fungicide could explain terbinafine resistance in these mutants. In some fenpropimorph-resistant mutants, the effects of fenpropimorph on sterol biosynthesis were similar to those in the wild-type strain, suggesting that resistance could be due to tolerance to sterol Δ8,14-sterol accumulation and to Δ5,7-sterol decrease. In one of them, the major sterol in absence of fungicide was not ergosterol, but ergosta-5,7,22,24(241)-tetraenol, indicating reduced Δ24(241)-reductase activity. The potential role of a modified Δ24(241) reductase in fenpropimorph resistance has yet to be elucidated. In the remaining fenpropimorph-resistant mutants, much higher fenpropimorph concentrations were required to cause ergosterol decreases similar to that in the wild-type strain, suggesting a Δ14-reductase modification. Among them, some mutants accumulated Δ8-sterols as well as Δ8,14-sterols, indicating that the mechanism of resistance may be due to reduced affinity of the Δ14-reductase toward fenpropimorph, whereas the other mutants accumulated only Δ8,14-sterols. Those last mutants contained much larger amounts of 4,4-dimethyl and 4α-methyl sterols than the wild-type strain, in the absence of fungicide. The most abundant methylated sterol was 4,4-dimethylfecosterol, the presumed product of the Δ14-reductase, suggesting that resistance may be due to overproduction of the Δ14-reductase in these mutants.

A MATHEMATICAL MODEL FOR FUNGAL POPULATION GROWTH AND THE FUNGICIDE RESISTANCE PROBLEM

A deterministic mathematical model which includes impulses occurring at fixed time periods is proposed in the study of the effects of different dosages on the population dynamics of resistant fungi. The model includes the following parameters which characterize the phenomenon: rS and rR (the apparent infection rates in the sense of Vanderplank for susceptible and resistant fungi), α (change rate from susceptible to resistant populations) and F (the rate by which the population is reduced by fungicide application). As a preliminary step, the model is solved in the absence of fungicide, in order to establish the resistance frequency in the initial population. Numerical simulations have been carried out considering low dosages and high dosages and varying the parameters α, rS, rR. According to the simulations, smaller dosages result in a lower reduced efficacy and delay the instant for total population resistance.

Sterol composition and resistance to DMI fungicides in Erysiphe graminis

AbstractThe effects of the fungicides, flutriafol, triadimenol and an experimental compound (2,2‐dimethyl‐3‐(2‐chlorobenzyl)‐4‐(1,2,4‐triazol‐1‐yl)‐butan‐3‐ol), on conidial viability and lipid composition, have been examined in strains of barley and wheat powdery mildew (Erysiphe graminis DC.) sensitive or resistant to DMI fungicides. In the absence of fungicides, these strains had the same sterol composition, regardless of their sensitivity to DMIs. Following treatment with the compounds, conidia contained eburicol and obtusifoliol, indicating inhibition of the 14α‐demethylase. There was also an accumulation of episterol, suggesting that introduction of the 5,6‐double bond was also inhibited. The changes in sterol composition induced by the fungicides were small. The fatty acid composition of sensitive and resistant strains was also determined. Triadimenol treatment caused an increase in the saturation of C22 and C24 acids in all strains examined, but changes were not correlated with the biological effects of the fungicide. No single mechanism of resistance emerged from the study.

Inhibition of phosphatidylcholine and chitin biosynthesis in Pyricularia oryzae, Botrytis fabae and Fusarium graminearum by edifenphos

Colony growth of the fungi Pyricularia oryzae, Botrytis fabae and Fusarium graminearum was reduced by 50% (ED50) by edifenphos concentrations of 7, 25 and 190 microM respectively; the phosphatidylcholine (PC) content of biomass of P. oryzae, B. fabae, and F. graminearum harvested from fungicide-containing-cultures was reduced by 50% by 6, 95 and 350 microM-edifenphos respectively. By contrast, the activities of membrane-bound chitin synthase preparations isolated from the three fungi were approximately equally sensitive to edifenphos. A direct relationship was observed between PC contents of biomass grown in the presence of ediphenphos and in vivo rates of chitin synthesis (biomass incubated with [3H]GlcNAc in the absence of fungicide). Membrane-bound chitin synthase preparations from P. oryzae grown in medium containing 3 or 6 microM-edifenphos had, at the same fungicide concentration, a lower rate of in vivo chitin synthesis than preparations isolated from biomass grown in the absence of edifenphos. Membrane-bound chitin synthase preparations from P. oryzae grown in the presence and absence of 6 microM-edifenphos had the same Km values for the substrate (UDP-[14C]GlcNAc) but different Vmax values. The results suggest that chitin synthesis is inhibited directly by non-competitive inhibition of chitin synthase activity, and indirectly following inhibition of PC biosynthesis. P. oryzae is very sensitive to edifenphos because inhibition of PC biosynthesis occurs at very low fungicide concentrations, and therefore in this fungus inhibition of PC biosynthesis probably represents the primary mode of action of the fungicide.