Temperature adaptation in isolates of Sclerotinia sclerotiorum affects their ability to infect Brassica carinata

Sclerotinia stem rot (Sclerotinia sclerotiorum) is a serious disease in oilseed Brassica crops worldwide. In this study, temperature adaptation in isolates of S. sclerotiorum collected from differing climatic zones is reported for the first time on any crop. Sclerotinia sclerotiorum isolates from oilseed rape (Brassica napus) crops in warmer northern agricultural regions of Western Australia (WW3, UWA 7S3) differed in their reaction to temperature from those from cooler southern regions (MBRS‐1, UWA 10S2) in virulence on Brassica carinata, growth on agar, and oxalic acid production. Increasing temperature from 22/18°C (day/night) to 28/24°C increased lesion diameter on cotyledons of B. carinataBC054113 more than tenfold for warmer region isolates, but did not affect lesion size for cooler region isolates. Mean lesion length averaged across two B. carinata genotypes (resistant and susceptible) fell from 4·6 to 2·4 mm for MBRS‐1 when temperature increased from 25/21°C to 28/24°C but rose for WW3 (2·35 and 3·21 mm, respectively). WW3, usually designated as low in virulence, caused as much disease on stems at 28/24°C as MBRS‐1, historically designated as highly virulent. Isolates collected from cooler areas grew better at low temperatures on agar. While all grew on potato dextrose agar between 5 and 30°C, with maximum growth at 20–25°C, growth was severely restricted above 32°C, and only UWA 7S3 grew at 35°C. Oxalate production increased as temperature increased from 10 to 25°C for isolates MBRS‐1, WW3 and UWA 7S3, but declined from a maximum level of 101 mg g^?1 mycelium at 20°C to 24 mg g^?1 mycelium at 25°C for UWA 10S2.     

Characterization of Alternaria species associated with potato foliar diseases in China

The population structure of Alternaria species associated with potato foliar diseases in China has not been previously examined thoroughly. Between 2010 and 2013, a total of 511 Alternaria isolates were obtained from diseased potato leaves sampled in 16 provinces, autonomous regions or municipalities of China. Based on morphological traits and molecular characteristics, all the isolates were identified as Alternaria tenuissima, A. alternata or A. solani. Of the three species, A. tenuissima was the most prevalent (75·5%), followed by A. alternata (18·6%) and A. solani (5·9%). Phylogenetic analysis based on sequences of the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) of representative Alternaria isolates showed that A. solani was distinct from the two small‐spored Alternaria species. Phylogenetic analysis of the partial coding sequence of the histone 3 gene divided the same collection of isolates into three main clades representing A. tenuissima, A. alternata and A. solani, respectively. The pathogenicity of the isolates on detached leaves of potato cv. Favorite did not differ significantly between the three species or between isolates from different geographical origins. The results indicate that the population structure of Alternaria species associated with potato foliar diseases differs from that reported previously in China. This is the first report of A. tenuissima causing potato foliar diseases in China.     

Formation of chlamydospores and microsclerotia in Alternaria dauci

Alternaria dauci (Kühn) Groves and Skolko, the cause of leaf blight of carrot, was observed to produce chlamydospores and microsclerotia in vitro. Four different isolates produced chlamydospores on potato dextrose agar, V8 agar and Czapek-Dox agar at 10, 18 and 28°C, from neighbouring cells of hyphae of the submerged mycelium, forming long chains. Formation of chlamydospores was completed in a multiple stage process, including swelling of cells, development of secondary septa, rounding up of cells and cell wall thickening. Mature chlamydospores were released from the mycelium mostly in clusters and in short or long chains, and sometimes in pairs. They were globose to ellipsoid, smooth or verrucose, pale brown in colour, and averaged 13.87?×?11.37 μm, (length × width). Microsclerotia were formed on PDA, by only one isolate, at 28°C first at the edges of the colony, by close contact of hyphae that exhibited extensive branching and intermingling. In their final form microsclerotia were multi-celled, closely attached masses of short and broad dark grey cells, less compact woven at the edges of the microsclerotium. This is the first illustration of chlamydospore and microsclerotia formation in Alternaria dauci.     

Species composition,toxigenic potential and pathogenicity of Fusarium graminearum species complex isolates from southern Brazilian rice

This study aimed to assess the extent and distribution of Fusarium graminearum species complex (FGSC) diversity in rice seeds produced in southern Brazil. Four species and two trichothecene genotypes were detected among 89 FGSC isolates, based on a multilocus genotyping assay: F. asiaticum (69·6%) with the nivalenol (NIV) genotype, F. graminearum (14·6%) with the 15‐acetyldeoxynivalenol (ADON) genotype, and F. cortaderiae (14·6%) and F. meridionale (1·1%), both with the NIV genotype. Seven selected F. asiaticum isolates from rice produced NIV in rice‐based substrate in vitro, at levels ranging from 4·7 to 84·1 μg g^?1. Similarly, two F. graminearum isolates from rice produced mainly 15‐ADON (c. 15–41 μg g^?1) and a smaller amount of 3‐ADON (c. 6–12 μg g^?1). One F. meridionale and two F. cortaderiae isolates did not produce detectable levels of trichothecenes. Two F. asiaticum isolates from rice and two from wheat (from a previous study), and one F. graminearum isolate from wheat, were pathogenic to both crops at various levels of aggressiveness based on measures of disease severity in wheat spikes and rice kernel infection in a greenhouse assay. Fusarium asiaticum and the reference F. graminearum isolate from wheat produced NIV, and deoxynivalenol and acetylates, respectively, in the kernels of inoculated wheat heads. No trichothecene was produced in kernels from inoculated rice panicles by any of the isolates. These findings constitute the first report of FGSC composition in rice outside Asia, and confirm the dominance of F. asiaticum in rice agroecosystems.     

Occurrence of the F129L mutation in Alternaria solani populations in Germany in response to QoI application,and its effect on sensitivity

Early blight caused by Alternaria solani is a highly destructive disease of potatoes. Control of early blight mainly relies on the use of preventive fungicide treatments. Because of their high efficacy, azoxystrobin and other quinone outside inhibitors (QoIs) are commonly used to manage early blight. However, loss of sensitivity to QoIs has previously been reported for A. solani in the United States. Two hundred and three A. solani field isolates collected from 81 locations in Germany between 2005 and 2011 were screened for the presence of the F129L mutation in the cytochrome b gene; of these, 74 contained the F129L mutation. Sequence analysis revealed the occurrence of two structurally different cytb genes, which differed in the presence (genotype I) or absence (genotype II) of an intron, with genotype I being the most prevalent (63% of isolates). The F129L mutation was detected only in genotype II isolates, where it occurred in 97%. Sensitivity to azoxystrobin and pyraclostrobin was determined in conidial germination assays. All isolates possessing the F129L mutation had reduced sensitivity to azoxystrobin and, to a lesser extent, to pyraclostrobin. Early blight disease severity on plants treated with azoxystrobin was significantly higher for A. solani isolates with reduced fungicide sensitivity in the conidial germination assay compared with sensitive isolates. Data suggest an accumulation of F129L isolates in the German A. solani population over the years 2009–2011. It is assumed that the application of QoIs has selected for the occurrence of F129L mutations, which may contribute to loss of fungicide efficacy.     

Composition and toxigenic potential of the Fusarium graminearum species complex from maize ears,stalks and stubble in Brazil

A large collection (n = 539) of Fusarium graminearum species complex (FGSC) isolates was obtained from Brazilian maize, and collections formed according to geography and maize part: (i) kernel (n = 110) from south and south‐central Brazil; (ii) stalk (n = 134) from Paraná state (south); and (iii) stubble (n = 295) from Rio Grande do Sul state (south). Species composition, identified using a multilocus genotype approach, was assessed separately in each collection due to differences in geographic sampling. Overall, three species were found: F. meridionale (Fmer; 67% prevalence) with the nivalenol (NIV) genotype, F. graminearum (Fgra; 19%) with the 15‐acetyl (A) deoxynivalenol (DON) genotype, and F. cortaderiae (Fcor; 14%) with the NIV (49/74) or the 3‐ADON (25/74) genotype. In kernels, Fmer was spread across all locations and Fgra and Fcor were found mostly at high elevation (>800 m a.s.l.). The majority (97·8%) of stalk isolates was assigned to Fmer; three were assigned to Fgra. In the stubble, Fmer was less dominant (53%), with a shift towards Fcor as the most frequent species at high elevation sites (>600 m a.s.l.). No differences in the mycelial growth rate were observed among isolates from each species grown at 15°C. Fgra grew faster at 25°C and Fmer showed the widest range of variation across the isolates at both temperatures. The survey data suggest that Fmer may outcompete other species on ears and stalks in comparison to stubble. Additional sampling that controls for other factors, as well as direct testing of aggressiveness on ears and stalk tissue, will be needed to fully evaluate this hypothesis.     

Survey on the prevalence of Rhizoctonia spp. in European soils and determination of the baseline sensitivity towards sedaxane

The prevalence of Rhizoctonia spp. in European soils was determined by analysing soil samples from 282 locations. Rhizoctonia spp. were found in 68% of these samples from France, Germany, the UK, Poland, Italy, Spain, Hungary and the Czech Republic. Samples from 136 locations were further analysed by pyrosequencing. Seventy‐six percent of the isolates were Rhizoctonia solani and 24% binucleate Rhizoctonia spp. Rhizoctonia solani anastomosis group (AG) 5 was detected most frequently (25%), followed by AG 9 (16%) and AG 4 (13%). For the binucleate Rhizoctonia spp., AG E was most prevalent (13%). Rhizoctonia cerealis was not detected in soil samples. Soil type or cropping history had no effect on the type of Rhizoctonia observed. Rhizoctonia solani AG 5 was the most frequently detected AG irrespective of the previous crop. The spectrum of AGs detected was similar for France, Germany and Poland but was significantly different for the UK (P = 0·0016). Finally, the baseline sensitivity towards sedaxane, a new active ingredient for seed treatment, was analysed for all isolates. The results indicate a low baseline sensitivity (average EC₅₀ of 0·028 p.p.m.) for all Rhizoctonia AGs. No difference in sensitivity was observed with the isolates obtained from different countries.     

The effect of environmental factors on infection of blueberry fruit by Colletotrichum acutatum

Anthracnose fruit rot of blueberries caused by Colletotrichum acutatum is a serious problem in humid blueberry‐growing regions of North America. In order to develop a disease prediction model, environmental factors that affect mycelial growth, conidial germination, appressorium formation and fruit infection by C. acutatum were investigated. Variables included temperature, wetness duration, wetness interruption and relative humidity. The optimal temperature for mycelial growth was 26°C, and little or no growth was observed at 5 and 35°C. The development of melanized appressoria was studied on Parafilm‐covered glass slides and infection was evaluated in immature and mature blueberry fruits. In all three assays, the optimal temperature for infection was identified as 25°C, and infections increased up to a wetness duration of 48 h. Three‐dimensional Gaussian equations were used to assess the effect of temperature and wetness duration on the development of melanized appressoria (R² = 0·89) on Parafilm‐covered glass slides and on infection incidence in immature (R² = 0·86) and mature (R² = 0·90) blueberry fruits. Interrupted wetness periods of different durations were investigated and models were fitted to the response of melanized appressoria (R² = 0·95) and infection incidence in immature (R² = 0·90) and mature (R² = 0·78) blueberry fruits. Additionally, the development of melanized appressoria and fruit infection incidence were modelled in relation to relative humidity (R² = 0·99 and 0·97, respectively). Three comprehensive equations were then developed that incorporate the aforementioned variables. The results lay the groundwork for a disease prediction model for anthracnose fruit rot in blueberries.     

High genotype diversity and lack of isolation by distance in the Alternaria solani populations from China

A genomic library was used to develop seven SSR markers for studying the population genetics of Alternaria solani, a pathogenic fungus causing early blight disease of potato and tomato worldwide. Population genetic analysis of 268 isolates of A. solani sampled from four locations, each representing one of four potato production systems in China, indicates that these SSR markers are moderately diverse, selectively neutral and possibly unlinked. Population genetic analysis also indicated that genetic variation of A. solani in China is high. About 2/3 of 123 genotypes were detected only once and genotype diversity measured by the standardized Shannon index ranged between 0·82 and 0·92 in the populations. Although clones were detected in multiple populations separated by thousands of kilometres, random association among SSR loci was found in half of the populations assayed. On average, nearly six copies of genetic material were exchanged among these populations each generation and no isolation by distance was detected. It is hypothesized that the joint effects of cryptic sexual reproduction and human‐mediated gene flow may account for the observed population genetic structure of A. solani in China.     

Isolation and characterization of novel soilborne lytic bacteriophages infecting Dickeya spp. biovar 3 (‘D. solani’)

Nine bacteriophages infecting Dickeya spp. biovar 3 (‘Dickeya solani’) were isolated from soil samples collected in different regions in Poland. The phages have a typical morphology of the members of the order Caudovirales, family Myoviridae, with a head diameter of c. 90–100 nm and tail length of c. 120–140 nm. In host range experiments, phage ?D5 expressed the broadest host range, infecting members of all Dickeya spp., and phage ?D7 showed the narrowest host range, infecting isolates of Dickeya dadantii and ‘D. solani’ only. None of the phages was able to infect Pectobacterium spp. isolates. All phages were prone to inactivation by pH 2, temperature of 85°C and by UV illumination for 10 min (50 mJ cm^?2). Additionally, phages ?D1, ?D10 and ?D11 were inactivated by 5 m NaCl and phage ?D2 was inactivated by chloroform. Phages ?D1, ?D5, ?D7 and ?D10 were characterized for optimal multiplicity of infection and the rate of adsorption to the bacterial cells. The latent period was 30 min for ?D1, 40 min for ?D5, 20–30 min for ?D7 and 40 min for ?D10. The estimated burst size was c. 100 plaque‐forming units per infected cell. The bacteriophages were able to completely stop the growth of ‘D. solani’ in vitro and to protect potato tuber tissue from maceration caused by the bacteria. The potential use of bacteriophages for the biocontrol of biovar 3 Dickeya spp. in potato is discussed.     

A new ‘forma specialis’ of Fusarium solani causing leaf yellowing of Phalaenopsis

The pathogenicity of 35 Fusarium solani isolates obtained from diseased leaves of greenhouse‐grown Phalaenopsis plants in Taiwan was tested on different orchids, including Phalaenopsis sp., Cymbidium spp., Oncidium sp., Dendrobium sp. and Cattleya sp., plus pea (Pisum sativum), chrysanthemum (Chrysanthemum indicum) and cucurbit [melon (Cucumis melo) and cucmber (C. sativus)] plants. Isolates of F. solani from Phalaenopsis spp. caused severe leaf yellowing on Phalaenopsis and mild symptoms on Cymbidium spp., but no visual symptoms on Oncidium sp., Dendrobium sp., Cattleya sp., pea, chrysanthemum or melon. Fusarium solani isolates collected from Phalaenopsis, pea and cucurbits were molecularly characterized by internal transcribed spacer (ITS), intergenic spacer (IGS) and β‐tubulin gene analyses. Phylogenetic trees constructed by distance and parsimony methods indicated that isolates from Phalaenopsis were grouped into one type based on ITS, IGS and β‐tubulin sequences with high bootstrap value (> 84%) support, compared to ‘formae speciales’ of F. solani from the other hosts. These analyses show that isolates of F. solani from Phalaenopsis are distinct from F. solani isolates from other hosts in Taiwan. Therefore, it is proposed that F. solani isolates that incite Phalaenopsis leaf yellowing be designated F. solani f. sp. phalaenopsis.     

Characterization and epidemiology of Colletotrichum acutatum sensu lato (C. chrysanthemi) causing Carthamus tinctorius anthracnose

In 2012, Colletotrichum isolates were collected from field‐grown safflower (Carthamus tinctorius) crops in central Italy from plants exhibiting typical anthracnose symptoms. Colletotrichum isolates were also collected from seed surfaces and from within seeds. The genetic variability of these isolates was assessed by a multilocus sequencing approach and compared with those from Colletotrichum chrysanthemi and Colletotrichum carthami isolates from different geographic areas and other Colletotrichum acutatum sensu lato‐related isolates. Phylogenetic analysis revealed that all of the strains isolated from C. tinctorius belonged to the species described as C. chrysanthemi, whereas all of the strains belonging to C. carthami had been isolated from Calendula officinalis. Phenotypic characterization of isolates was performed by assessing growth rates at different temperatures, morphology of colonies on potato dextrose agar (PDA) and the size of conidia. All C. chrysanthemi isolates from safflower had similar growth rates at different temperatures, comparable colony morphologies when grown on PDA and conidial sizes consistent with previously described C. chrysanthemi isolates. Pathogenicity tests were performed by artificially inoculating both seeds and plants and confirmed the seedborne nature of this pathogen. When inoculated on plants, C. chrysanthemi caused the typical symptoms of anthracnose on leaves. This is the first record of this pathogen on C. tinctorius in Italy, and it presents an updated characterization of Colletotrichum isolates pathogenic to safflowers in Europe.     

Stem rot of jewel orchids caused by a new forma specialis,Fusarium oxysporum f. sp. anoectochili in Taiwan

Stem rot of Anoectochilus formosanus (Af) caused by Fusarium oxysporum (Fo) is a major limiting factor to jewel orchid production in Taiwan. Fo causes discoloration in vascular tissues. However, some newly collected Fo isolates from Af stem rot do not cause vascular discoloration, suggesting changes may have occurred in the pathogen. Among recent Fo isolates from Af there are two colony types, the cottony alba (CA) and the sporodochial (S). In order to confirm that both colony types cause Af stem rot, 200 isolates were obtained from diseased stems in Nantou County and characterized by colony type and whether or not the infected plants had vascular discoloration. Isolates of both the CA and S types caused stem rot of Af; some isolates in each colony type caused vascular discoloration whilst others did not. Pathogenicity tests with 22 isolates resulted in stem rot disease severity ratings on Af of 3·1–4·0 and 2·1–4·0 with CA and S type colonies, respectively. The same isolates failed to cause disease on Cattleya, Dendrobium or Phalaenopsis plants. Phylogenetic analysis of partial intergenic spacer sequences showed that these isolates were distinguishable from other formae speciales of Fo and could be separated into two groups correlated with the CA or S type colonies with high bootstrap. Based on pathogenic, morphological and molecular characterizations, the Fo that causes stem rot of Af is proposed to be a new forma specialis, F. oxysporum f. sp. anoectochili, with different pathotypes.     

Comparison of the development <Emphasis Type="Italic">in planta</Emphasis> of a pyrrolnitrin-resistant mutant of <Emphasis Type="Italic">Botrytis cinerea</Emphasis> and its sensitive wild-type parent isolate

Botrytis cinerea is able to build-up resistance to pyrrolnitrin, an antibiotic produced by diverse biocontrol agents, possibly compromising the durability of this method of disease control. The development of two near-isogenic lines of B. cinerea differing in their level of resistance to pyrrolnitrin was compared in tomato plants and on PDA medium. In tomato plants, significant differences in the percentage of infected petioles 1 day after inoculation and in symptom progression on petioles and stems were observed between the resistant mutant and the sensitive wild-type parent, suggesting a difference in their level of aggressiveness. Cytohistological investigations revealed that conidia of both near-isogenic lines germinated 6 h after inoculation and mycelium developed within petiole tissues 12 h after inoculation. However, while the wild-type parent isolate spread throughout the petiole and rapidly invaded the stem tissues via the leaf-abscission zone 72 h after inoculation, the pyrrolnitrin-resistant mutant failed to extend beyond petiole tissues to invade the stem. Moreover, 72 h after inoculation, the mycelial development of the pyrrolnitrin-resistant mutant was accompanied by abnormal glycogen accumulation and chlamydospore-like cell formation. In contrast, wild-type parent mycelium was normally structured with intensive colonization of stem tissues. Additionally, on PDA medium the mycelium of the pyrrolnitrin-resistant mutant was less vigorous than the wild-type isolate. These results suggest that the acquisition of pyrrolnitrin-resistance in B. cinerea is accompanied by changes in mycelial structure and reduction in mycelial growth, leading to a noticeable loss of aggressiveness on tomato plants.     

Pathogenic variation of Mycosphaerella species infecting banana and plantain in Nigeria

Mycosphaerella species that cause the ‘Sigatoka disease complex’ account for significant yield losses in banana and plantain worldwide. Disease surveys were conducted in the humid forest (HF) and derived savanna (DS) agroecological zones from 2004 to 2006 to determine the distribution of the disease and variation among Mycosphaerella species in Nigeria. Disease prevalence and severity were higher in the HF than in the DS zone, but significant (P < 0·001) differences between agroecological zones were only observed for disease severity. A total of 85 isolates of M. fijiensis and 11 isolates of M. eumusae were collected during the survey and used to characterize the pathogenic structure of Mycosphaerella spp. using a putative host differential cultivar set consisting of Calcutta‐4 (resistant), Valery (intermediate) and Agbagba (highly susceptible). Area under disease progress curve (AUDPC) was higher on all cultivars when inoculated with M. eumusae than with M. fijiensis, but significant (P < 0·05) differences between the two species were only observed on Valery. Based on the rank‐sum method, 8·3% of the isolates were classified as highly aggressive and 46·9% were classified as aggressive. About 11·5% of all the isolates were classified as least aggressive, and all of these were M. fijiensis. The majority of M. eumusae isolates (seven out of 11; 64%) were classified as aggressive. A total of nine pathotype clusters were identified using cluster analysis of AUDPC. At least one M. fijiensis isolate was present in all the nine pathotype clusters, while isolates of M. eumusae were present in six of the nine clusters. Isolates in pathotype clusters III and V were the most aggressive, while those in cluster VIII were the least aggressive. Shannon’s index (H) revealed a more diverse Mycosphaerella collection in the DS zone (H = 1·81) than in the HF (H = 1·50) zone, with M. fijiensis being more diverse than M. eumusae. These results describe the current pathotype structure of Mycosphaerella in Nigeria and provide a useful resource that will facilitate screening of newly developed Musa genotypes for resistance against two important leaf spot diseases of banana and plantain.     

Phytophthora infestans populations in central,eastern and southern African countries consist of two major clonal lineages

Limited knowledge is available on Phytophthora infestans populations in Sub‐Saharan Africa (SSA). Therefore, and in response to recent severe late blight epidemics, P. infestans isolates from potato, tomato and Petunia × hybrida from eight SSA countries were characterized. Isolates were characterized with ‘old’ markers, including mating type (176 isolates), mitochondrial DNA haplotype (mtDNA) (281 isolates), glucose‐6‐phosphate isomerase (Gpi) (70 isolates), restriction fragment length polymorphism analysis with probe RG‐57 (49 isolates), and by metalaxyl sensitivity (64 isolates). Most isolates belonged to the US‐1 genotype or its variants (US‐1.10 and US‐1.11). The exceptions were genotype KE‐1 isolates (A1 mating type, mtDNA haplotype Ia, Gpi 90/100 and unique RG‐57 genotype), identified in two fields in Kenya, which are related to genotypes previously identified in Rwanda (RW‐1 and RW‐2), Ecuador and Europe. Metalaxyl‐resistant P. infestans isolates from potato were present in all the countries except Malawi, whereas all the isolates from tomato were sensitive. Genotyping of 176 isolates with seven simple sequence repeat (SSR) markers, including locus D13 that was difficult to score, revealed 79 multilocus genotypes (MLGs) in SSA. When this locus was excluded, 35 MLGs were identified. Genetic differentiation estimates between regional populations from SAA were significant when locus D13 was either excluded (P = 0·05) or included (P = 0·007), but population differentiation was only low to moderate (F_ST = 0·044 and 0·053, respectively).     

Biocontrol and population dynamics of Fusarium spp. on wheat stubble in Argentina

The biocontrol effect of Clonostachys rosea (strains 016 and 1457) on Fusarium graminearum, F. avenaceum, F. verticillioides, F. langsethiae, F. poae, F. sporotrichioides, F. culmorum and Microdochium nivale was evaluated on naturally infected wheat stalks exposed to field conditions for 180 days. Experiments were conducted at two locations in Argentina, Marcos Juarez and Río Cuarto. Antagonists were applied as conidial suspensions at two inoculum levels. Pathogens were quantified by TaqMan real‐time qPCR. During the first year at Marcos Juarez, biocontrol was observed in one antagonist treatment for F. graminearum after 90 days (73% reduction) but after 180 days, the pathogen decreased to undetectable levels. During the second year, biocontrol was observed in three antagonist treatments for F. graminearum and F. avenaceum (68·3% and 98·9% DNA reduction, respectively, after 90 days). Fusarium verticillioides was not controlled at Marcos Juarez. At Río Cuarto, biocontrol effects were observed in several treatments at different intervals, with a mean DNA reduction of 88·7% for F. graminearum and F. avenaceum, and 100% reduction for F. verticillioides in two treatments after 180 days. Populations of F. avenaceum and F. verticillioides were stable; meanwhile, F. graminearum population levels varied during the first 90 days, and low levels were observed after 180 days. The other pathogens were not detected. The study showed that wheat stalks were important reservoirs for F. avenaceum and F. verticillioides populations but less favourable for F. graminearum survival. Clonostachys rosea (strain 1457) showed potential to reduce the Fusarium spp. on wheat stalks.     

Effects of granular nematicides on growth and microbial antagonism to Rhizoctonia solani

Effects of nematicides on growth and microbial antagonism toRhizoctonia solani were investigated as part of a study on the mechanisms involved in the increased incidence of this pathogen in nematicide-treated potato crops.Ethoprophos inhibited mycelial growth ofR. solani on potato dextrose agar (PDA), Czapek Dox agar (CDA) and on water agar (WA). Aldicarb stimulated its growth on PDA up to 14% but not on CDA and WA. Oxamyl inhibited mycelial growth on CDA and WA, but not on PDA.Ethoprophos and aldicarb stimulated development of the mycoparasiteVerticillium biguttatum on cultures ofR. solani. The effect was dependent on the medium on which the host fungus was grown. ForRhizoctonia cultures on PDA, growth of the mycoparasite was highly promoted by aldicarb and to a lesser extent by ethoprophos. WhenR. solani was grown on CDA, the development of the mycoparasite was not affected by aldicarb, slightly stimulated by ethoprophos and slightly inhibited by oxamyl. On water agar, its development on the host mycelium was not affected.In field trials on sandy soil, nematicides encouragedV. biguttatum probably by increased availability of substrate (i.e.Rhizoctonia mycelium) perhaps through reduced activity of the mycophagous fauna.Soil fungistasis was increased by ethoprophos and to a lesser extent by aldicarb at very high doses. At normal field rates, no effects can be expected on fungistasis. So the increased stem and stolon infection of potatoes in nematicide-treated fields was not caused by a direct effect of the nematicides on growth ofR. solani or by suppressing the microbial antagonism.Samenvatting De invloed van granulaire nematiciden op de groei vanRhizoctonia solani en op het microbiële antagonisme tegen deze schimmel werd onderzocht in het kader van een studie over de mechanismen die een rol spelen bij de toename van de aantasting in een met nematiciden behandeld aardappelgewas.Ethoprofos remde de myceliumgroei vanR. solani op aardappeldextrose agar (PDA), Czapek Dox agar (CDA) en op wateragar (WA). Aldicarb stimuleerde op PDA de groei met maximaal 14%. Op CDA en WA werd geen effect van aldicarb waargenomen. Oxamyl veroorzaakte groeiremming op CDA en WA, maar niet op PDA.Ethoprofos en aldicarb stimuleerden de ontwikkeling van de mycoparasietVerticillium biguttatum op cultures vanR. solani. De mate van groeistimulering was afhankelijk van de voedingsbodem waarop de waard,R. solani, werd gekweekt. De groei vanV. biguttatum werd sterk gestimuleerd door aldicarb en in geringere mate door ethoprofos, wanneer de waard gekweekt werd op PDA. Aldicarb had geen effect op de mycoparasiet wanneerR. solani op CDA gekweekt werd, terwijl ethoprofos de groei wel iets stimuleerde en oxamyl een gering remmend effect had. Op WA werd geen effect van de nematiciden op het mycoparasitisme vastgesteld.In veldproeven op zandgrond stimuleerden de nematiciden het voorkomen vanV. biguttatum op de stolonen. Het effect werd waarschijnlijk veroorzaakt door een verhoogde substraat beschikbaarheid (d.w.z. mycelium vanR. solani). De verhoogde beschikbaarheid van dit mycelium kan samenhangen met een door nematiciden gereduceerde activiteit van de fungivore bodemfauna.De bodemfungistase werd verhoogd door ethoprofos en, in geringere mate, door aldicarb bij hogere doseringen. Bij de in de praktijk aanbevolen doseringen kan echter geen effect op de fungistase verwacht worden. De toename in stengel- en stolonaantasting van aardappelen, geteeld in met granulaire nematiciden behandelde percelen, kon niet worden toegeschreven aan een direct effect van de nematiciden op de groei vanR. solani of aan een vermindering van het microbiële antagonisme.     

Virulence of ‘Dickeya solani’ and Dickeya dianthicola biovar‐1 and ‐7 strains on potato (Solanum tuberosum)

Studies were conducted to explain the relative success of ‘Dickeya solani’, a genetic clade of Dickeya biovar 3 and a blackleg‐causing organism that, after recent introduction, has spread rapidly in seed potato production in Europe to the extent that it is now more frequently detected than D. dianthicola. In vitro experiments showed that both species were motile, had comparable siderophore production and pectinolytic activity, and that there was no antagonism between them when growing. Both ‘D. solani’ and biovar 1 and biovar 7 of D. dianthicola rotted tuber tissue when inoculated at a low density of 10³ CFU mL^?1. In an agar overlay assay, D. dianthicola was susceptible to 80% of saprophytic bacteria isolated from tuber extracts, whereas ‘D. solani’ was susceptible to only 31%, suggesting that ‘D. solani’ could be a stronger competitor in the potato ecosystem. In greenhouse experiments at high temperatures (28°C), roots were more rapidly colonized by ‘D. solani’ than by biovar 1 or 7 of D. dianthicola and at 30 days after inoculation higher densities of ‘D. solani’ were found in stolons and progeny tubers. In co‐inoculated plants, fluorescent protein (GFP or DsRed)‐tagged ‘D. solani’ outcompeted D. dianthicola in plants grown from vacuum‐infiltrated tubers. In 3 years of field studies in the Netherlands with D. dianthicola and ‘D. solani’, disease incidence varied greatly annually and with strain. In summary, ‘D. solani’ possesses features which allow more efficient plant colonization than D. dianthicola at high temperatures. In temperate climates, however, tuber infections with ‘D. solani’ will not necessarily result in a higher disease incidence than infections with D. dianthicola, but latent seed infection could be more prevalent.     

Pathogenicity of fungi associated with ash dieback towards Fraxinus excelsior

A large part of the area in Europe in which Fraxinus excelsior is native is currently affected by ash dieback, a threatening disease caused by the ascomycetous fungus Hymenoscyphus fraxineus. Fungi other than H. fraxineus also occur in large numbers on stems of the dying ash trees. To clarify their possible role in the dieback process, six fungal species common on dying stems and twigs of ash in Poland, i.e. Cytospora pruinosa, Diaporthe eres, Diplodia mutila, Fusarium avenaceum, F. lateritium and F. solani, were tested for pathogenicity using a test based on artificial wound inoculations of 6‐year‐old F. excelsior plants under field conditions, with H. fraxineus included for comparison. There were significant differences in index of pathogenicity among the fungi tested. Hymenoscyphus fraxineus (mean index 5.78) was the most pathogenic. Diplodia mutila (4.23) and C. pruinosa (4.02) were significantly less pathogenic than H. fraxineus, but significantly more than the other fungi. Diaporthe eres (2.43), F. avenaceum (1.92), F. solani (1.86) and F. lateritium (1.08) were the least pathogenic (P < 0.0001). The extent of disease symptoms caused by F. solani and F. lateritium was statistically similar to the control (P = 0.05). All tested fungi were successfully reisolated from inoculated stems. The contribution of the results to understanding the possible role of these fungi in the ash dieback process in F. excelsior, particularly in trees weakened after primary infection by H. fraxineus, is discussed.