Pyraclostrobin reduces germ tube growth of QoI‐resistant Mycosphaerella graminicola pycnidiospores and the severity of septoria tritici blotch on winter wheat

The effect of the quinone outside inhibitors (QoI) azoxystrobin and pyraclostrobin on yields of winter wheat where QoI resistant Mycosphaerella graminicola isolates were dominant was investigated in field trials in 2006 and 2007. Pyraclostrobin significantly increased yields by 1·57 t ha^?1 in 2006 and 0·89 t ha^?1 in 2007 when compared to the untreated controls, while azoxystrobin only provided a significant increase of 1·28 t ha^?1 in 2006. These yield increases were associated with reduction in septoria tritici blotch (STB) development as determined by weekly disease assessments over a 7 week interval. The effect of pyraclostrobin on STB was studied in controlled environment experiments using wheat seedlings inoculated with individual M. graminicola isolates. Pyraclostrobin significantly reduced STB symptoms by up to 62%, whether applied 48 h pre‐ or post‐ inoculation with resistant M. graminicola isolates containing the cytochrome b mutation G143A. Extremely limited disease (<1%) was observed on similarly treated seedlings inoculated with an intermediately resistant isolate containing the cytochrome b mutation F129L, while no disease was observed on seedlings inoculated with a wild‐type isolate. Germination studies of pycnidiospores of M. graminicola on water agar amended with azoxystrobin or pyraclostrobin showed that neither fungicide inhibited germination of spores of resistant isolates containing the mutation G143A. However, pyraclostrobin significantly reduced germ tube length by up to 46% when compared with the untreated controls. Although the QoIs can no longer be relied upon to provide effective M. graminicola control, this study provides an insight into why QoIs still provide limited STB disease control and yield increases even in situations of high QoI resistance.     

Leaching behaviour of azoxystrobin and metabolites in soil columns

BACKGROUND: Azoxystrobin [methyl (E)‐2‐{2‐[6‐(2‐cyanophenoxy)pyrimidin‐4‐yloxy]phenyl}‐3‐methoxyacrylate], a strobilurin fungicide, is a broad‐spectrum, systemic and soil‐applied fungicide. Azoxystrobin has been registered for rice cultivation in India, but no information is available on its leaching behaviour in Indian soils. Therefore, leaching behaviour of azoxystrobin was studied in packed and intact soil columns under different irrigation regimes. RESULTS: Azoxystrobin did not leach out of the 300 mm long columns after 126 and 362 mm rainfall. After percolating water equivalent to 362 mm rainfall, azoxystrobin leached down to 10–15 cm (packed columns) and 15–20 cm (intact columns) depth. Azoxystrobin was not detected in the leachate from the packed column leached with 94.5 mL water every week (140 mm rainfall per month) during the 28 weeks of the study period. However, azoxystrobin acid, formed by azoxystrobin degradation, was detected in the leachate after 18 weeks. At the end of the study, azoxystrobin had leached down to 5–10 cm depth, and only 60% of initially applied azoxystrobin was recovered from the soil. CONCLUSION: The results indicate that azoxystrobin is fairly immobile in sandy loam soil, but azoxystrobin acid, a major metabolite of azoxystrobin, is quite mobile and may pose a threat of soil and groundwater contamination. Copyright © 2009 Society of Chemical Industry     

Effects of thiophanate-methyl and azoxystrobin on the composition of Cercospora kikuchii populations with thiophanate-methyl-resistant strains

Azoxystrobin was recently registered in Japan for the control of purple seed stain of soybean caused by Cercospora kikuchii, because the pathogen has developed resistance to thiophanate-methyl. To investigate the effects of these fungicides on the frequency of C. kikuchii strains resistant to thiophanate-methyl and on the genotype structure of the population, we sowed purple-stained seeds, approximately 40% of which were infected with resistant strains, as inocula with asymptomatic seeds and applied thiophanate-methyl and azoxystrobin during the reproductive growth of soybeans. The isolation frequency of resistant strains increased more than 99% by thiophanate-methyl but was not significantly increased by azoxystrobin. In amplified fragment length polymorphism (AFLP) DNA fingerprinting, genotypic diversity was significantly decreased by thiophanate-methyl but was not affected by azoxystrobin. In addition, the similarity of the AFLP genotype structure was increased by thiophanate-methyl but not by azoxystrobin. These results suggest that thiophanate-methyl selectively inhibited the proliferation of sensitive strains, which resulted in a small number of genotypes, most of which were resistant strains. Azoxystrobin was found to nonselectively inhibit proliferation of the pathogen, which retained a large number of genotypes including thiophanate-methyl-sensitive or thiophanate-methyl-resistant strains or both. The nucleotide sequence data for the cytochrome b gene are available in the DDBJ/EMBL/GenBank databases under accession number AB231863.     

Quantitative PCR monitoring of the effect of azoxystrobin treatments on Mycosphaerella graminicola epidemics in the field

Quantitative PCR and visual monitoring of Mycosphaerella graminicola epidemics were performed to investigate the effect of curative and preventative applications of azoxystrobin in wheat field crops. A non-systemic protectant and a systemic curative fungicide, chlorothalonil and epoxiconazole, respectively, were used as references. PCR diagnosis detected leaf infection by M graminicola 3 weeks before symptom appearance, thereby allowing a clear distinction between curative and preventative treatments. When applied 1 week after the beginning of infection, azoxystrobin curative activity was intermediate between chlorothalonil (low effect) and epoxiconazole. When applied preventatively, none of the fungicides completely prevented leaf infection. There was some indication that azoxystrobin preventative treatments may delay fungal DNA increase more than epoxiconazole at the beginning of leaf infection. Both curative and preventative treatments increased the time lapse between the earliest PCR detection and the measurement of a 10% necrotic leaf area. Azoxystrobin only slightly decreased the speed of necrotic area increase compared with epoxiconazole. Hence, azoxystrobin activity toward M graminicola mainly resides in lengthening the time lapse between the earliest PCR detection and the measurement of a 10% necrotic leaf area. Information generated in this way is useful for optimal positioning of azoxystrobin treatments on M graminicola.     

Correlation of in planta endo‐beta‐1,4‐xylanase activity with the necrotrophic phase of the hemibiotrophic fungus Mycosphaerella graminicola

The association of the cell wall degrading enzyme endo‐beta‐1,4‐xylanase (EC 3.2.1.8) with pathogenicity of Mycosphaerella graminicola was examined in planta. The enzyme production of two M. graminicola isolates (T0372 and T0491), as well as their ability to infect seedlings of susceptible wheat cv. Scorpion, was first compared. No significant difference was found between the two isolates regarding spore germination rates, mycelial growth on the leaf surface or direct and stomatal penetrations. However, restricted hyphal growth was observed inside leaves inoculated with T0372, whereas successful mesophyll colonization with a strong intercellular fungal growth was found in leaves infected with T0491. Likewise, T0372 was unable to induce lesions bearing pycnidia and to produce endo‐beta‐1,4‐xylanase activity until 22 days post‐inoculation (d.p.i.). On the other hand, significant high increases of both diseased leaf area bearing pycnidia and endo‐beta‐1,4‐xylanase activity were observed between 16 and 22 d.p.i. for T0491 (r = 0·98). The investigation of 24 additional isolates, including the IPO323 and IPO94269 reference isolates, highlighted a strong correlation between endo‐beta‐1,4‐xylanase activity and disease development levels (r = 0·94). This study demonstrates that differences in pathogenicity in M. graminicola are not linked to events on the leaf surface or to frequency of leaf penetration, but to the ability of the fungus to colonize the mesophyll and to produce the cell wall degrading enzyme endo‐1,4‐beta‐xylanase during the necrotrophic phase.     

Differential Control of Head Blight Pathogens of Wheat by Fungicides and Consequences for Mycotoxin Contamination of Grain

Fusarium head blight of wheat is caused by a disease complex comprised of toxigenic pathogens, predominantly Fusarium spp., and a non-toxigenic pathogen Microdochium nivale, which causes symptoms visually indistinguishable from Fusarium and is often included as a causal agent of Fusarium head blight. Four field trials are reported here, including both naturally and artificially inoculated trials in which the effect of fungicide treatments were noted on colonisation by Fusarium and Microdochium, and on the production of deoxynivalenol (DON) mycotoxin. The pathogen populations were analysed with quantitative PCR and samples were tested for the presence of the mycotoxin DON. Application of fungicides to reduce Fusarium head blight gave a differential control of these fungi. Tebuconazole selectively controlled F. culmorum and F. avenaceum and reduced levels of DON, but showed little control of M. nivale. Application of azoxystrobin, however, selectively controlled M. nivale and allowed greater colonisation by toxigenic Fusarium species. This treatment also lead to increased levels of DON detected. nobreak Azoxystrobin application two days post-inoculation increased the production of DON mycotoxin per unit of pathogen in an artificially inoculated field trial. This result indicates the potential risk of increased DON contamination of grain following treatment with azoxystrobin to control head blight in susceptible wheat cultivars. This is the first study to show differential fungicidal control of mixed natural pathogen populations and artificial inoculations in field trials.     

Sensitivity profiles of Mycosphaerella graminicola and Phytophthora infestans populations to different classes of fungicides

Prior to the use of fungicides, the baseline sensitivity of individuals in a pathogen population may already differ by a factor of 10 to 100 between the least and the most sensitive isolates. In Mycosphaerella graminicola populations, this factor, measured in vitro, was 5 to 20 for both the strobilurin analogue azoxystrobin (baseline) and the triazole cyproconazole which has been in use for several years. In Phytophthora infestans populations, this factor, measured in a leaf disc assay, was about 100 for azoxystrobin (baseline), up to 1000 for the cyanoacetamide cymoxanil and >10000 for the phenylamide oxadixyl; both of the latter have been used for many years. In M. graminicola, cross-sensitivity was present between all azole fungicides for the majority of the isolates, whereas no correlation was found between triazoles and azoxystrobin. Despite the existence of cross-sensitivity between azoles, ‘box-and-whiskers’ plots revealed large variations in the sensitivity profiles of some triazoles; isolates resistant to triazoles have not been detected in M. graminicola populations. In P. infestans populations, the proportion of the phenylamide-resistant sub-population increased during the season more rapidly in treated than in untreated fields, but it was low at the beginning of the next season in all fields. During disease epidemics, the fitness of phenylamide-resistant P. infestans isolates, as characterised by lesion size, was higher than that of the sensitive isolates, but after the overwintering period, the recovery of resistant isolates was apparently lower. The presence of both A1 and A2 mating types of P. infestans in European populations, although at different frequencies, allows sexual recombination and increased genetic diversity, affecting sensitivity and fitness. Such mixed populations can still be adequately controlled by using sound anti-resistance strategies. ©1997 SCI     

Early stages of septoria tritici blotch epidemics of winter wheat: build‐up,overseasoning, and release of primary inoculum

Septoria tritici blotch (STB), caused by Mycosphaerella graminicola, is the most prevalent disease of wheat worldwide. Primary inoculum and the early stages of STB epidemics are still not fully understood and deserve attention for improving management strategies. The inoculum build‐up and overseasoning involves various fungal structures (ascospores, pycnidiospores, mycelium) and plant material (wheat seeds, stubble and debris; wheat volunteers; other grasses). Their respective importance is assessed in this review. Among the mechanisms involved in the early stages of epidemics and in the year‐to‐year disease transmission, infection by ascospores wind‐dispersed from either distant or local infected wheat debris is the most significant. Nevertheless, infection by pycnidiospores splash‐dispersed either from neighbouring wheat debris or from senescent basal leaves has also been inferred from indirect evidence. Mycosphaerella graminicola has rarely been isolated from seeds so that infected seed, although suspected as a source of primary inoculum for a long time, is considered as an epidemiologically anecdotal source. Mycosphaerella graminicola can infect a few grasses other than wheat but the function of these grasses as alternative hosts in natural conditions remains unclear. Additionally, wheat volunteers are suspected to be sources of STB inoculum for new crops. This body of evidence is summarized in a spatio‐temporal representation of a STB epidemic aimed at highlighting the nature, sources and release of inoculum in the early stages of the epidemic.     

Impact of tricyclazole and azoxystrobin on growth,sporulation and secondary infection of the rice blast fungus,Magnaporthe oryzae

BACKGROUND: Rice blast, caused by Magnaporthe oryzae B. Couch sp. nov., is one of the most destructive rice diseases worldwide, causing substantial yield losses every year. In Italy, its management is based mainly on the use of two fungicides, azoxystrobin and tricyclazole, that restrain the disease progress. The aim of this study was to investigate and compare the inhibitory effects of the two fungicides on the growth, sporulation and secondary infection of M. oryzae. RESULTS: Magnaporthe oryzae mycelium growth was inhibited at low concentrations of azoxystrobin and relatively high concentrations of tricyclazole, while sporulation was more sensitive to both fungicides and was affected at similarly low doses. Furthermore, infection efficiency of conidia obtained from mycelia exposed to tricyclazole was affected to a higher extent than for conidia produced on azoxystrobin‐amended media, even though germination of such conidia was reduced after azoxystrobin treatment. CONCLUSIONS: This study presents for the first time detailed azoxystrobin and tricyclazole growth–response curves for M. oryzae mycelium growth and sporulation. Furthermore, high efficacy of tricyclazole towards inhibition of sporulation and secondary infection indicates an additional possible mode of action of this fungicide that is different from inhibition of melanin biosynthesis. Copyright © 2012 Society of Chemical Industry     

Effects of relative humidity on infection,colonization and conidiation of Magnaporthe orzyae on perennial ryegrass

Grey leaf spot, caused by Magnaporthe oryzae, causes severe damage on perennial ryegrass (Lolium perenne) turf. In this study, the effects of relative humidity (RH, 88 to 100% at 28°C) on infection, colonization and conidiation of M. oryzae on perennial ryegrass were investigated in controlled humidity chambers. Results showed that the RH threshold for successful M. oryzae infection was ≥92% at 28°C. The advancement of infection on the leaf tissue was further examined with a green fluorescent protein (GFP)‐tagged M. oryzae strain. No appressorium formation was found when the inoculum was incubated at RH ≤ 88%. Additionally, the GFP‐tagged staining provided a rapid method to quantitatively compare the fungal colonization from leaf tissue at different levels of RH. The fluorescence intensity data indicated that the fungal biomass was highest at 100% RH and there was no fluorescence intensity observed at 88% RH or below. Conidiation was only observed when RH was ≥96%, with the most abundant conidiation occurring 8 days after inoculation. Reduced conidiation was associated with decreasing RH, and no conidiation occurred at RH ≤ 92%. This study indicates that infection and conidiation of M. oryzae on perennial ryegrass required different thresholds: 92% and 96% RH for infection and conidiation, respectively. The quantitative data from this research will assist in prediction of grey leaf spot disease outbreaks and of secondary infection of perennial ryegrass.     

Evaluation of an action threshold‐based IPM wheat model in Rheinland (Germany) in 1999/2001*

Under the specific agricultural and climatic conditions of Rheinland (DE), 48 field trials in three years demonstrated the practicability of the action threshold‐based fungicide strategy of the IPM wheat model. The underlying data for action thresholds and dosage of fungicides applied are outlined. The cereal pathogens concerned, predominantly Mycosphaerella graminicola and Puccinia recondita, were controlled in the early stages of epidemic development using reduced rates of fungicides. The IPM wheat model gave a high efficacy of control, in terms of disease incidence and disease severity, under both low and high disease pressure conditions. The yield levels resulting from this effective reduction in diseases were nearly the same as those of the disease‐free variant. Total yield increase varied between 15% and 30% with an overall average of around 20%, and resulted in monetary benefits of 50–100 EUR ha^?1 in the years considered.     

Comparative efficacy of strobilurin fungicides against downy mildew disease of pearl millet

Three commercial formulations of strobilurins, viz., azoxystrobin, kresoxim-methyl, and trifloxystrobin were evaluated for their efficacy against pearl millet downy mildew disease caused by Sclerospora graminicola. In vitro studies revealed inhibition of S. graminicola sporulation, zoospore release, and zoospore motility at 0.1-2 μg ml⁻¹ of all the three fungicides. The fungicides were evaluated for phytotoxic effects on seed quality parameters and for their effectiveness against downy mildew disease by treating pearl millet by: (1) seed dressing, (2) seed dressing followed by foliar spray, and (3) also by foliar spray alone. The highest non-phytotoxic concentrations of 5, 10, and 10 μg ml⁻¹ for azoxystrobin, trifloxystrobin, and kresoxim-methyl, respectively, were selected for further studies. Under greenhouse conditions, these fungicides showed varying degrees of protection against downy mildew disease. Among the three fungicides, azoxystrobin proved to be the best by offering disease protection of 66%. Further, seed treatment along with foliar application of these fungicides to diseased plants showed enhanced protection against the disease to 93, 82, and 62% in treatments of azoxystrobin, kresoxim-methyl and trifloxystrobin respectively. Foliar spray alone provided significant increase in disease protection levels of 91, 79, and 59% in treatments of azoxystrobin, kresoxim-methyl, and trifloxystrobin, respectively. Disease curative activity of azoxystrobin was higher compared to trifloxystrobin and kresoxim-methyl. Tested fungicides showed weaker translaminar activity, as the disease inhibition was marginal when applied on adaxial leaf surface. Partial systemic activity of azoxystrobin was evident by root uptake, while trifloxystrobin and kresoxim-methyl showed lack of systemic action in pearl millet. A trend in protection against downy mildew disease similar to greenhouse results was evident in the field trials. Grain yield was significantly increased in all strobilurin fungicide treatments over control and maximum increase in yield of 1673 kg ha⁻¹ was observed in combination treatments of seed treatment and foliar spray with azoxystrobin.     

Effects of Azoxystrobin on Mycotoxin Production in a Carbendazim-Resistant Strain ofFusarium sporotrichioides

Carbendazim-resistant (RS) and control (CS) strains ofFusarium sporotrichioides Sherb., previously developed in our laboratory, were exposed to graded concentrations of azoxystrob in in broth media under shake-culture conditions for 2, 3, 4 and 8 days. Azoxystrobin concentrations were 0, 1, 10 and 100 mg 1^-1 broth and cultures were incubated at a constant 25°C. Mycelial growth was significantly affected by strain (P<0.01), azoxystrobin concentration (P<0.001) and incubation time (P<0.001). Combined results for the four incubation times showed that CS yielded higher mycelial mass than RS (P<0.01) only in the absence of azoxystrobin. At fungicide additions of 1, 10 and 100 mg P^-1 mycelial growth was reduced (P<0.001) with minimal strain differences (P>0.05) at all three doses of azoxystrobin. Significant (P<0.05 or better) strain-fungicide interactions were recorded in trichothecene production following exposure to azoxystrobin. At 4 and 8 days of incubation, the 10 mg 1^-1 addition of azoxystrobin stimulated T-2 toxin synthesis (P<0.05) only in RS cultures. In contrast, T-2 toxin enhancement in CS cultures occurred only on day 8 but at a lower level of azoxystrobin (1 mg1^-1). Thus, the stimulation of T-2 toxin synthesis depended upon strain and azoxystrobin level. Production of diocetoxyscirpenol (DAS) was affected by a more complex set of interactions. Overall means showed that, in comparison with initial values (on day 2 or 3), DAS output maximized significantly(P<0.05) on day 4 in RS cultures and on day 8 in CS. Marked strain effects were observed on exposure to the 10 mg 1^-1 level of azoxystrobin. At this level, DAS production was enhanced in RS only after 4 (P<0.01 ) and 8 (P<0.05) days of incubation, while in contrast, CS reduced DAS production. As with T-2 toxin, DAS production in CS was stimulated (P<0.05 or better) only at low exposure levels of azoxystrobin. In the case of neosolaniol (NEO), however, the main effect of strain was significant (P<0.05), with CS producing consistently more of the mycotoxin than RS on day 4 of the experiment. At this point, the NEO:T-2 toxin ratio was also higher in CS (0.63) than in RS (0.12), a feature reported by us previously. In conclusion, the present investigation has shown for the first time that the development of resistance to one fungicide can affect trichothecene production inF. sporotrichioides on exposure to a second fungicide. These results have been incorporated into a new classification scheme for fungicide efficacy which is also presented in this paper. http://www.phytoparasitica.org posting Oct. 7,2001.     

The use of rainfall and accumulated mean temperature to indicate fungicide activity in the control of leaf diseases of winter wheat in the UK1

During 1988/1990, a series of 21 experiments was established in commercial crops of winter wheat. Chlorothalonil, fenpropimorph and propiconazole were chosen as protectant, eradicant or curative fungicides active against leaf diseases of winter wheat in the UK. To test their properties each one was applied once only to separate plots during a period of 7–8 consecutive weeks in May and June (GS 32–39). Disease progress was assessed weekly on adjacent unsprayed control and sprayed plots up to GS 85. Septoria tritici leaf blotch (Mycosphaerella graminicola) was the disease that occurred most frequently and severely across the 21 sites. Powdery mildew (Erysiphe graminis), brown rust (Puccinia recondita) and yellow rust (P. striiformis) occurred at fewer sites and were sufficiently severe to distinguish differences between the active ingredients at only two or three sites. Analysis of the disease progress curves for 75% control of each disease at one site only indicated that chlorothalonil possessed very good protectant but shorter-term eradicant activity against M. graminicola and P. striiformis. Fenpropimorph exhibited only short-term eradicant control of M. graminicola, but gave excellent protectant and eradicant control of E. graminis and P. striiformis; against P. recondita only eradicant activity was apparent. Propiconazole showed activity similar to that of fenpropimorph against P. recondita and excellent protectant and eradicant activity against M. graminicola and P. striiformis; against E. graminis, it gave good protectant and eradicant control. From disease progress curves, it was possible to calculate the period of protectant and eradicant activity in thermal time (accumulated degree days above zero) for each of the three active ingredients and to identify the most effective timing(s) for fungicide application in relation to rainfall or imputed infection.     

Development of a Fusarium oxysporum f. sp. melonis functional GFP fluorescence tool to assist melon resistance breeding programmes

Fusarium wilt, caused by Fusarium oxysporum f. sp. melonis (Fom), is one of the most widespread and devastating melon diseases. This vascular disease is caused by the colonization of melon xylem vessels by any of the four Fom races reported (r0, r1, r2 and r1,2, subdivided into r1,2w and r1,2y). The macroscopic evaluation of disease symptoms (disease rating, DR) at several days post‐inoculation (dpi) with Fom spores has been the traditional method to determine the resistance of melon accessions to this fungal pathogen. In this study, one isolate from each Fom race was transformed by Agrobacterium tumefaciens to constitutively express the green fluorescent protein (GFP). Fom‐GFP transformants, as virulent as the corresponding wildtype races, were selected to develop an inoculation assay based on the non‐invasive evaluation of the fluorescence emitted by Fom‐GFP. It was determined that melon root neck was the appropriate area to follow Fom‐GFP and a fluorescence signal rating (FSR) was established in parallel to DR determination. This method allowed the evaluation of GFP signal in the root neck of inoculated melon seedlings at 11–15 dpi. The GFP signal was scored in 62 melon accessions/breeding lines inoculated with different Fom‐GFP, followed by evaluation of the macroscopic DR in the aerial part of melon seedlings at 20–28 dpi. Correlation analysis demonstrated a direct and significant relationship between FSR and DR. This method has shown to be an effective and reliable tool that can assist Fom resistance breeding programmes in melon.     

Effects of initial nematode population density and water regime on resistance and tolerance to the rice root-knot nematode Meloidogyne graminicola in African and Asian rice genotypes

The rice root-knot nematode (RKN), Meloidogyne graminicola, is an important pathogen affecting rice production in South and Southeast Asia. Efficacy of resistance and tolerance in selected M. graminicola-resistant African rice genotypes TOG5674, TOG5675 and CG14 and -susceptible Asian rice genotypes IR64 and UPLRi-5 were examined under a range of initial population densities (Pi) and water regimes. Resistance to M. graminicola in resistant rice genotypes was not broken with increasing pathogen pressure (Pi = 15,000 to 60,000 J2/plant). Resistant rice genotypes were even tolerant to the damage and yield loss caused by high pathogen pressure. On the other hand, increasing Pi levels caused more damage on susceptible rice genotypes. Final nematode population densities in the root systems of resistant and susceptible rice genotypes were significantly lower under flooded conditions than under upland and drought conditions. TOG5674, TOG5675 and CG14 were more tolerant to M. graminicola infection even when grown under upland and drought conditions while IR64 and UPLRi-5 were highly sensitive.     

利用绿色荧光蛋白GFP研究稻瘟病菌与水稻的互作

稻瘟病菌Magnaporthe oryzae严重威胁水稻的产量与质量,明确稻瘟病菌与水稻互作过程及机理,对防治稻瘟病具有重要意义。本研究利用稻瘟病菌常用致病菌株GUY11和ZB25,构建了绿色荧光蛋白GFP的过量表达菌株,并通过荧光显微观察菌株侵染寄主水稻过程中侵染结构的形成与发育,包括孢子萌发、附着胞形成、侵染钉形成、侵染菌丝增殖、坏死斑形成及产孢。另外,通过比较过量表达菌株对稻瘟病高抗水稻和易感水稻的侵染过程,发现侵染过程的差异主要集中于侵染钉的穿透和侵染菌丝的定殖。本研究为分析稻瘟病菌对寄主水稻的定殖规律提供了一种有效工具。     

Tan spot: a new disease of pyrethrum caused by Microsphaeropsis tanaceti sp. nov.

The isolation frequency of Microsphaeropsis sp. in spring in association with necrotic lesions on leaves in Tasmanian pyrethrum (Tanacetum cinerariifolium) fields has increased substantially since first identification in 2001. Examination of morphological features and sequencing of the internal transcribed spacer region (ITS) resulted in the identification of a new species, herein described as Microsphaeropsis tanaceti sp. nov. The pathogenicity of three M. tanaceti isolates to two pyrethrum cultivars was confirmed by inoculating glasshouse‐grown plants in three experiments. No significant differences in the susceptibility of the two cultivars to infection by M. tanaceti were found. Symptoms were tan‐coloured spots which coalesced around the margins of the leaves. Therefore, the name ‘tan spot’ is proposed for this new disease of pyrethrum. The sensitivity of seven M. tanaceti isolates to difenoconazole and azoxystrobin, commonly used fungicides for the management of foliar diseases in spring, was assessed under in vitro conditions. Sensitivity testing for difenoconazole was conducted using a mycelial growth assay on potato dextrose agar, whilst testing for sensitivity to azoxystrobin used a conidial germination assay on water agar. Microsphaeropsis tanaceti was found to be more sensitive to azoxystrobin than difenoconazole, with complete inhibition of conidial germination at concentrations above 0·625 µg a.i. mL^?1. By comparison, concentrations of 50 µg a.i. difenoconazole mL^?1 or greater were required for significant inhibition of mycelial growth. It therefore appears likely that there is currently some control of tan spot as a result of the use of azoxystrobin and to a lesser extent, difenoconazole, for the control of other diseases.     

In vitro effects of flutriafol and azoxystrobin on Beauvaria bassiana and its efficacy against Tetranychus urticae

BACKGROUND: Testing the compatibility of chemical pesticides and fungal biocontrol agents is necessary if these two agents are to be applied together in the integrated management of plant pests and diseases. In this study, the fungicides azoxystrobin (a strobilurin) and flutriafol (a triazole) were tested in vitro for their effects on germination of conidia and mycelial growth of Beauveria bassiana (Bals.) Vuill. and in bioassay for their effect on fungal activity against Tetranychus urticae Koch. The fungicides were tested at three different concentrations [recommended rate for field use (1 × X) and the dilutions 10^?1× X and 10^?2× X]. RESULTS: Flutriafol inhibited growth of mycelia and germination of the fungal conidia at all concentrations tested in vitro, and also reduced the efficacy of B. bassiana in bioassays against mites. The inhibitive effect of azoxystrobin in vitro varied with the concentration applied. A significant effect was observed at 1 × X and 10^?1× X concentrations on both the germination of conidia and mycelia growth. At 10^?2× X concentration, azoxystrobin showed little effect on B. bassiana. However, when this fungicide was tested in bioassays, none of the concentrations reduced B. bassiana activity against mites. CONCLUSION: Azoxystrobin was most compatible with B. bassiana, while flutriafol was the most harmful. Further studies are required to confirm the negative effect of flutriafol on B. bassiana activity. Copyright © 2010 Society of Chemical Industry     

Overexpression of the sterol 14α-demethylase gene (MgCYP51) in Mycosphaerella graminicola isolates confers a novel azole fungicide sensitivity phenotype

BACKGROUND: The recent evolution towards resistance to azole fungicides in European populations of the wheat pathogen Mycosphaerella graminicola has been caused by the progressive accumulation of mutations in MgCYP51 gene, encoding the azole target sterol 14α‐demethylase. Particular combinations of mutations have been shown specifically to affect the interaction of the MgCYP51 protein with different members of the azole class. Although additional mechanisms, including increased MgCYP51 expression and enhanced active efflux, have been proposed, the genetic changes underlying these mechanisms are unknown. RESULTS: Analysis of the azole sensitivities of recent M. graminicola isolates identified a novel phenotype, seemingly independent of changes in MgCYP51 coding sequence. Characterised by a 7‐16‐fold reduction in in vitro sensitivity to all azoles tested and by growth on seedlings at higher doses of azoles in glasshouse tests compared with isolates carrying the same MgCYP51 variant (L50S, S188N, I381V, ΔY459/G460, N513K), isolates with this phenotype constitutively overexpress MgCYP51 by between 10‐ and 40‐fold compared with the wild type. Analysis of sequences upstream of the predicted MgCYP51 translation start codon identified a novel 120 bp indel, considered to be an insertion, in isolates overexpressing MgCYP51. CONCLUSIONS: The identification of an insertion in the predicted MgCYP51 promoter in azole‐resistant isolates overexpressing MgCYP51 is the first report of a genetic mechanism, other than changes in target‐site coding sequence, affecting sensitivity to multiple azoles in field isolates of M. graminicola. The identification of recent isolates overexpressing MgCYP51 confirms the ongoing evolution and diversification of resistance mechanisms in European populations of M. graminicola. Copyright © 2012 Society of Chemical Industry