Inoculum potential of Pythium arrhenomanes in soils from three crop rotation experiments

Soil from crop rotation experiments at three different sites was tested by using bait plants and a most probable number (MPN) method to estimate the inoculum potential of Pythium arrhenomanes in terms of the density of infective units. Recorded values varied between 0 and 85 infective units/kg soil. In soil from one of the sites the inoculum densities following barley monoculture were significantly higher than those from crop rotations, whether sampled in the spring or autumn. At the other two sites, which had consistently lower inoculum densities, small effects of rotation treatment were also recorded when they were sampled in the autumn. However, the results did not support the tentative conclusions of earlier studies, that inoculum of P. arrhenomanes could be a major cause of yield reduction in barley monoculture. The MPN method cannot be recommended for predicting expected disease levels of P. arrhenomanes , unless the infective unit can be carefully defined and its relation to yield loss reliably established.     

Temporal and spatial dynamics of watermelon gummy stem blight epidemics

The effect of the distance of initial inoculum on the intensity of watermelon gummy stem blight, caused by Didymella bryoniae, was studied in a naturally-infected rainfed commercial field. The shorter the distance from the focus, the sooner was disease onset and the earlier maximum disease levels were achieved. Maximum disease incidences were reached earlier than maximum severities, but eventually destructive levels were observed for both disease incidence and severity. Disease progressed at similar rates, irrespective of the radial distance from the focus. A detailed study of the disease temporal progress was conducted in inoculated rainfed experimental fields with commercial genotypes Crimson Sweet (susceptible, S) and Riviera (moderately resistant, R). The Gompertz model best described the disease progress curves, and estimated apparent infection rates were 0.049 and 0.020 respectively for S and R genotypes. In addition, spatial pattern studies were conducted during the dry season in overhead irrigated experimental plots, inoculated with point-source foci. Disease intensity gradients were better explained by the Exponential model than by the Power Law model. Gummy stem blight distribution was classified as aggregated by the Ordinary Runs procedure. Two different spatial autocorrelation methods (2DCorr and LCOR) revealed strong short distance spatial dependencies. Long distance positive correlations between quadrats were observed along with periods of higher progress rates. The dynamic patterns of the epidemics of gummy stem blight in watermelon described here are consistent with epidemics of polycyclic diseases with splash-dispersed spores.     

Molecular tools to investigate Rhizoctonia solani distribution in soil

Real-time PCR protocols were developed to detect and discriminate 11 anastomosis groups (AGs) of Rhizoctonia solani using ribosomal internal transcribed spacer (ITS) regions (AG-1-IA, AG-1-IC, AG-2-1, AG-2-2, AG-4HGI+II, AG-4HGIII, AG-8) or β-tubulin (AG-3, AG-4HGII, AG-5 and AG-9) sequences. All real-time assays were target group specific, except AG-2-2, which showed a weak cross-reaction with AG-2^tabac. In addition, methods were developed for the high throughput extraction of DNA from soil and compost samples. The DNA extraction method was used with the AG-2-1 assay and shown to be quantitative with a detection threshold of 10⁻⁷ g of R. solani per g of soil. A similar DNA extraction efficiency was observed for samples from three contrasting soil types. The developed methods were then used to investigate the spatial distribution of R. solani AG-2-1 in field soils. Soil from shallow depths of a field planted with Brassica oleracea tested positive for R. solani AG-2-1 more frequently than soil collected from greater depths. Quantification of R. solani inoculum in field samples proved challenging due to low levels of inoculum in naturally occurring soils. The potential uses of real-time PCR and DNA extraction protocols to investigate the epidemiology of R. solani are discussed.     

Effect of inoculum density and soil tillage on the development and severity of rhizoctonia root rot

Rhizoctonia spp. cause substantial yield losses in direct-seeded cereal crops compared with conventional tillage. To investigate the mechanisms behind this increased disease, soils from tilled or direct-seeded fields were inoculated with Rhizoctonia spp. at population densities from 0.8 to 250 propagules per gram and planted with barley (Hordeum vulgare). The incidence and severity of disease did not differ between soils with different tillage histories. Both R. solani AG-8 and R. oryzae stunted plants at high inoculum densities, with the latter causing pre-emergence damping-off. High inoculum densities of both species stimulated early production of crown roots in barley seedlings. Intact soil cores from these same tilled and direct-seeded fields were used to evaluate the growth of Rhizoctonia spp. from colonized oat seeds. Growth of R. oryzae was not affected by previous tillage history. However, R. solani AG-8 grew more rapidly through soil from a long-term direct-seeded field compared to tilled soils. The differential response between these two experiments (mixed, homogenized soil versus intact soil) suggests that soil structure plays a major role in the proliferation of R. solani AG-8 through soils with different tillage histories.     

Effect and underlying mechanisms of pea-cereal intercropping on the epidemic development of ascochyta blight

Field experiments were conducted in western France for two consecutive years to investigate the effect of pea-cereal intercropping on ascochyta blight, a major constraint of field pea production world-wide. Disease pressure was variable in the experiments. Intercropping had almost no effect on disease development on stipules regardless of disease pressure. In contrast, disease severity on pods and stems was substantially reduced in the pea-cereal intercrop compared to the pea monocrop when the epidemic was moderate to severe. Therefore, a pea-cereal intercrop could potentially limit direct yield loss and reduce the quantity of primary inoculum available for subsequent pea crops. Disease reduction was partially explained by a modification of the microclimate within the intercrop canopy, in particular, a reduction in leaf wetness duration during and after flowering. The effect of intercropping on splash dispersal of conidia was investigated under controlled conditions using a rainfall simulator. Total dispersal was reduced by 39 to 78% in pea-wheat canopies compared to pea canopies. These reductions were explained by a reduction in host plant density and a barrier or relay effect of the non-host plants.     

Effects of the mycoparasite Verticillium biguttatum on barley stunt disease caused by Rhizoctonia solani anastomosis group 8 in a model system

The height of barley stunted by Rhizoctonia solani anastomosis group 8 was significantly increased by up to 72·8% after incubation for 8 days at 20°C in seedling tray tests following application of the mycoparasite Verticillium biguttatum. The pathogen and mycoparasite were applied at the rate of 1% Perlite maizemeal inoculum (w/w potting mixture) resulting in propagule densities of approximately 24·0 and 6·6 × 10⁵ colony-forming units (cfu) per g potting mixture, respectively. Sieving (2 mm) R. solani inoculum prior to dilution in potting mixture increased the recovery of propagules from 1·2 × 2·1 × 10³ cfu per g inoculum compared with recovery when inoculum was sieved after dilution. Applications of a V. biguttatum isolate from the UK (vbl) and a Dutch isolate (M73) reduced stunting to a similar extent but did not stimulate the growth of healthy plants. The height of stunted plants was significantly increased after application of V. biguttatum inoculum after 6 days if inoculated trays were preincubated for 1 day prior to planting but a similar increase was only detected after 7 days if seeds were planted immediately. The number of stunted plants which emerged after 4 days was significantly increased by treatment with V. biguttatum but preincubation had no additional effect. These results suggested that control of R. solani was effected both before and after the initiation of disease.     

Spatiotemporal relationships between disease development and airborne inoculum in unmanaged and managed Botrytis leaf blight epidemics

Comparatively little quantitative information is available on both the spatial and temporal relationships that develop between airborne inoculum and disease intensity during the course of aerially spread epidemics. Botrytis leaf blight and Botrytis squamosa airborne inoculum were analyzed over space and time during 2 years (2002 and 2004) in a nonprotected experimental field, using a 6 x 8 lattice of quadrats of 10 x 10 m each. A similar experiment was conducted in 2004 and 2006 in a commercial field managed for Botrytis leaf blight using a 5 x 5 lattice of quadrats of 25 x 25 m each. Each quadrat was monitored weekly for lesion density (LD) and aerial conidium concentration (ACC). The adjustment of the Taylor's power law showed that heterogeneity in both LD and ACC generally increased with increasing mean. Unmanaged epidemics were characterized in either year, with aggregation indices derived from SADIE (Spatial Analysis by Distance Indices). For LD, the aggregation indices suggested a random pattern of disease early in the season, followed by an aggregated pattern in the second part of the epidemic. The index of aggregation for ACC in 2002 was significantly greater than 1 at only one date, while it was significantly greater than 1 at most sampling dates in 2004. In both years and for both variables, positive trends in partial autocorrelation were observed mainly for a spatial lag of 1. In 2002, the overall pattern of partial autocorrelations over sampling dates was similar for LD and ACC with no significant partial autocorrelation during the first part of the epidemic, followed by a period with significant positive autocorrelation, and again no autocorrelation on the last three sampling dates. In 2004, there was no significant positive autocorrelation for LD at most sampling dates while for ACC, there was a fluctuation between significant and non-significant positive correlation over sampling dates. There was a significant spatial correlation between ACC at given date (t(i)) and LD 1 week later (t(i + 1)) on most sampling dates in both 2002 and 2004 for the unmanaged and managed sites. It was concluded that LD and ACC were not aggregated in the early stage of epidemics, when both disease intensity and airborne conidia concentration were low. This was supported by the analysis of LD and ACC from a commercial field, where managed levels of disease were low, and where no aggregation of both variables was detected. It was further concluded that a reliable monitoring of airborne inoculum for management of Botrytis leaf blight is achievable in managed fields using few spore samplers per field.     

Effect of soil moisture content on the suppression of Rhizoctonia stem canker on potato by the nematode Aphelenchus avenae and the springtail Folsomia fimetaria

The effect of soil moisture content on the suppression of Rhizoctonia stem canker on potato by mycophagous soil animals was studied in growth chambers. Three soil moisture levels were established in two bioassays, in which potato sprouts grew through a 15-cm soil layer inoculated with sclerotia of Rhizoctonia solani (AG-3). In one experiment two levels of R. solani inoculum were applied. The effect on plant disease of mycophagous soil fauna was assessed by adding the springtail Folsomia fimetaria and/or the nematode Aphelenchus avenae to the soil. In the absence of mycophagous organisms, Rhizoctonia disease severity on potato stems was highest in dry soil. A. avenae and F. fimetaria reduced Rhizoctonia stem canker when applied at populations found in the field. They were effective over a broad range of soil moistures. The stimulatory effect of dry soil conditions on Rhizoctonia stem canker was counteracted by a greater efficacy of the mycophagous soil fauna under these conditions. Mild drought stress did not seem to be a limiting factor in the biological control of stem canker by these two organisms.     

Effects of different 3-year cropping systems on soil microbial communities and rhizoctonia diseases of potato

ABSTRACT Eight different 3-year cropping systems, consisting of soybean-canola, soybean-barley, sweet corn-canola, sweet corn-soybean, green bean-sweet corn, canola-sweet corn, barley-clover, and continuous potato (non-rotation control) followed by potato as the third crop in all systems, were established in replicated field plots with two rotation entry points in Presque Isle, ME, in 1998. Cropping system effects on soil microbial community characteristics based on culturable soil microbial populations, single carbon source substrate utilization (SU) profiles, and whole-soil fatty acid methyl ester (FAME) profiles were evaluated in association with the development of soilborne diseases of potato in the 2000 and 2001 field seasons. Soil populations of culturable bacteria and overall microbial activity were highest following barley, canola, and sweet corn crops, and lowest following continuous potato. The SU profiles derived from BIOLOG ECO plates indicated higher substrate richness and diversity and greater utilization of certain carbohydrates, carboxylic acids, and amino acids associated with barley, canola, and some sweet corn rotations, indicating distinct differences in functional attributes of microbial communities among cropping systems. Soil FAME profiles also demonstrated distinct differences among cropping systems in their relative composition of fatty acid types and classes, representing structural attributes of microbial communities. Fatty acids most responsible for differentiation among cropping systems included 12:0, 16:1 omega5c, 16:1 omega7c, 18:1 omega9c, and 18:2omega6c. Based on FAME biomarkers, barley rotations resulted in higher fungi-to-bacteria ratios, sweet corn resulted in greater mycorrhizae populations, and continuous potato produced the lowest amounts of these and other biomarker traits. Incidence and severity of stem and stolon canker and black scurf of potato, caused by Rhizoctonia solani, were reduced for most rotations relative to the continuous potato control. Potato crops following canola, barley, or sweet corn provided the lowest levels of Rhizoctonia disease and best tuber quality, whereas potato crops following clover or soybean resulted in disease problems in some years. Both rotation crop and cropping sequence were important in shaping the microbial characteristics, soilborne disease, and tuber qualities. Several microbial parameters, including microbial populations and SU and FAME profile characteristics, were correlated with potato disease or yield measurements in one or both harvest years. In this study, we have demonstrated distinctive effects of specific rotation crops and cropping sequences on microbial communities and have begun to relate the implications of these changes to crop health and productivity.     

Automated image processing framework for analysis of the density of fruiting bodies of Leptosphaeria maculans on oilseed rape stems

Understanding the transmission of plant pathogen inoculum during the periods when the host plants are not present is crucial for predicting the initiation of epidemics and optimizing mitigation strategies. However, inoculum production at the end of the cropping season, survival during the intercrop period, and the emergence or release of inoculum can be highly variable, difficult to assess, and generally inferred indirectly from symptom data. As a result, a lack of large datasets hampers the study of these epidemiological processes. Here, inoculum production was studied in Leptosphaeria maculans, the cause of phoma stem canker of oilseed rape. The fungus survives on stubble left in the field, from which ascospores are released at the beginning of the next cropping season. An image processing framework was developed to estimate the density of fruiting bodies produced on stem pieces following incubation in field conditions, and a quality assessment of the processing chain was performed. A total of 2540 standardized RGB digital images of stems were then analysed, collected from 27 oilseed rape fields in Brittany over four cropping seasons. Manual post-processing removed 16% of the pictures, e.g. when moisture-induced darkening of the oilseed rape stems caused overestimation of the area covered with fruiting bodies. The potential level of inoculum increased with increasing phoma stem canker severity at harvest, and depended on the source field and the cropping season. This work shows how image-based phenotyping generates high-throughput disease data, opening up the prospect of substantially increased precision in epidemiological studies.     

Mapping rhizoctonia patch in consecutive cereal crops in Western Australia

Rhizoctonia patches in a field were mapped during consecutive seasons of cropping with wheat, oats or barley sown directly without tillage into the previous season's stubble. Distribution, size, shape and number of patches varied considerably between seasons. Less than a quarter of patches were circular and the others tended to be elongated parallel to seed-rows. Barley was most severely affected, followed by wheat and then oats.
It is proposed that changes in patch distribution, configuration and number between seasons is due to shifts in a balance of suppressive or conducive soil factors interacting with Rhizoctonia solani inoculum.     

Interactions Between the Soybean Cyst Nematode and Fusarium solani f. sp. glycines Based on Greenhouse Factorial Experiments

ABSTRACT The soybean cyst nematode, Heterodera glycines, and the fungus that causes sudden death syndrome (SDS) of soybean, Fusarium solani f. sp. glycines, frequently co-infest soybean (Glycine max) fields. The interactions between H. glycines and F. solani f. sp. glycines were investigated in factorial greenhouse experiments with different inoculum levels of both organisms on a soybean cultivar susceptible to both pathogens. Measured responses included root and shoot dry weights, H. glycines reproduction, area under the SDS disease progress curve, and fungal colonization of roots. Both H. glycines and F. solani f. sp. glycines reduced the growth of soybeans. Reproduction of H. glycines was suppressed by high inoculum levels but not by low levels of F. solani f. sp. glycines. The infection of soybean roots by H. glycines did not affect root colonization by the fungus, as determined by real-time polymerase chain reaction. Although both pathogens reduced the growth of soybeans, H. glycines did not increase SDS foliar symptoms, and statistical interactions between the two pathogens were seldom significant.     

Association of Binucleate Rhizoctonia with Soybean and Mechanism of Biocontrol of Rhizoctonia solani

ABSTRACT The association of binucleate Rhizoctonia (BNR) AG-K with soybean and the interaction of BNR, R. solani AG-4, and soybean seedlings were investigated to elucidate the mechanism of biocontrol of R. solani by BNR. Sixty-hour-old seedlings were inoculated and incubated in a growth chamber at 24 degrees C; plants were examined with light microscopy and with scanning and transmission electron microscopy at various times following inoculation. BNR grew over hypocotyls, roots, and root hairs, but only colonized epidermal cells. Hyphae of BNR appeared to attach to the epidermis and, 5.5 h following inoculation, began penetrating cells by means of penetration pegs without forming distinct appressoria or infection cushions. There was evidence of cuticle degradation at the point of penetration. Infection hyphae moved to adjacent epidermal cells by direct penetration of epidermal radial walls. There were epidermal and cortical cell necrosis, beginning with the fragmentation of the tonoplast and followed by the disintegration of cytoplasm, organelles, and plasma membranes. Cell necrosis was also observed in adjacent cells where there was no evidence of BNR hyphae. Cell walls were not destroyed. After 144 h, there was noevidence of BNR hyphae in cortical cells. Attempted penetrations were observed, but papillae formed on the inside of cortical cell walls. Pre-inoculation of soybean seedlings with BNR 24 or 48 h before inoculation with R. solani (1 cm between inocula) affected the growth of R. solani on soybean tissue. There were fewer hyphae of R. solani, the hyphae branched sparingly, and infection cushions were rare when compared with hyphal growth on soybean inoculated only with R. solani. These effects were observed before the BNR hyphae began to intermingle with the hyphae of R. solani on the surface of the inoculated host. Preinoculation of soybean seedlings 24 h before inoculation with R. solani significantly (P = 0.05) reduced disease incidence and severity caused by R. solani AG-4. The lesions caused by R. solani always appeared distally, not proximally, to the BNR inoculum. The interactions of intermingling hyphae of BNR and R. solani were examined in vitro and on the surface of the host. There was no evidence of lysis, mycoparasitism, inhibition of growth, or any other form of antagonism between hyphae. The results of these studies strongly suggest that induced resistance is the mechanism of biocontrol of R. solani on soybean by BNR. The inhibition of hyphal growth of R. solani on the surface of soybean tissue preinoculated with BNR appears to be a novel characteristic of induced resistance.     

A comparison of controlled environment and field trials for detection of resistance in cereal cultivars to root rot caused by Rhizoctonia solani

A screening test was developed to study the effect of different quantities of inoculum of Rhizoctonia solani on root rot and growth of cultivars of wheat, barley, oats, rye and triticale in a controlled environment and the field in two seasons. The ranking of relative susceptibility to disease of cultivars within a cereal differed between the controlled environment and field experiments, and between seasons in the field. The ranking of relative susceptibility between cereals also differed between experiments. In the controlled-environment experiment, wheat had the highest disease rating, followed by oats, barley, triticale and rye, but in the field barley had the highest rating followed by wheat and then oats. The overall differences between the cereals were small.     

Wheat Genotype-Specific Induction of Soil Microbial Communities Suppressive to Disease Incited by Rhizoctonia solani Anastomosis Group (AG)-5 and AG-8

ABSTRACT The induction of disease-suppressive soils in response to specific cropping sequences has been demonstrated for numerous plant-pathogen systems. The role of host genotype in elicitation of the essential transformations in soil microbial community structure that lead to disease suppression has not been fully recognized. Apple orchard soils were planted with three successive 28-day cycles of specific wheat cultivars in the greenhouse prior to infestation with Rhizoctonia solani anastomosis group (AG)-5 or AG-8. Suppressiveness to Rhizoctonia root rot of apple caused by the introduced isolate of R. solani AG-5 was induced in a wheat cultivar-specific manner. Pasteurization of soils after wheat cultivation and prior to pathogen introduction eliminated the disease suppressive potential of the soil. Wheat cultivars that induced disease suppression enhanced populations of specific fluorescent pseudomonad genotypes with antagonistic activity toward R. solani AG-5 and AG-8, but cultivars that did not elicit a disease suppressive soil did not modify the antagonistic capacity of this bacterial community. When soils were infested prior to the initial wheat planting, all cultivars were uniformly susceptible to R. solani AG-8. However, when pathogen inoculum was added after three growth-cycles, wheat root infection during the fourth growth-cycle varied in a cultivar specific manner. The same wheat cultivar-specific response in terms of transformation of the fluorescent pseudomonad community and subsequent suppression of Rhizoctonia root rot of apple was observed in three different orchard soils. These results demonstrate the importance of host genotype in modification of indigenous saprophytic microbial communities and suggest an important role for host genotype in the success of biological control.     

Biotic changes in relation to local decrease in soil conduciveness to disease caused by <Emphasis Type="Italic">Rhizoctonia solani</Emphasis>

The relationships between biotic changes and local decrease in soil conduciveness in disease patches towards the disease incited by Rhizoctonia solani AG 2-2 in a sugar beet field in France were investigated. Soil samples from healthy and diseased areas were analysed for bacterial and fungal densities, molecular and physiological microbial community structures, and antagonistic abilities of Trichoderma isolates collected from diseased and healthy areas. Although the inoculum density was higher inside the disease patches, the respective soil was less conducive towards disease incited by R. solani AG 2-2. It was concluded that the pathogen was present in healthy areas but did not incite disease in field conditions. Conversely, the response of the microflora to previous development of R. solani in diseased areas prevented further pathogenic activity. Indeed, genetic and physiological structures of the fungal communities and physiological structures of the bacterial communities were modified in disease patches compared to healthy areas. The terminal restriction fragment length polymorphism (T-RFLP) analysis revealed that the peaks corresponding to R. solani and Trichoderma spp. were higher inside the patches than in the healthy areas. Trichoderma isolates from the disease patches were more antagonistic than those from the healthy areas. These results suggest that disease caused by R. solani AG 2-2 induced changes in genetic and physiological structure of microbial populations and development of antagonists. The decreased conduciveness inside the patches may help explain patch mobility in the following season.     

Susceptibility of intercrops to infection with Rhizoctonia solani AG 2‐2 IIIB and influence on subsequently cultivated sugar beet

The susceptibility of intercrop species (Raphanus sativus, Brassica juncea, B. rapa, Sinapis alba and Phacelia tanacetifolia) to the sugar beet pathogen Rhizoctonia solani was investigated in vitro, in the greenhouse and in the field with artificial inoculation. Disease severity in subsequently cultivated sugar beet was monitored in the field. Differences in susceptibility between species were found to be consistent in all experimental systems. All intercrop species were susceptible to R. solani. Brassica rapa and R. sativus were less susceptible than P. tanacetifolia. Compared to fallow, the cultivation of B. rapa and R. sativus reduced disease severity in subsequently grown sugar beet (median ratings of up to 3·0 and 3·5, respectively, depending on environmental conditions). This resulted in higher white sugar yield compared to fallow (up to 210% and 157% for B. rapa and R. sativus, respectively). This study demonstrates that in vitro and greenhouse resistance tests are suitable systems to predict the effects of intercrop species susceptibility in the field on disease severity and white sugar yield in subsequently grown sugar beet. Intercrop breeding programmes might profit from fast and efficient screening tests to provide Rhizoctonia‐resistant intercrops as an additional control measure against R. solani in sugar beet.     

Effect of Microsphaeropsis sp. Strain P130A on Germination and Production of Sclerotia of Rhizoctonia solani and Interaction Between the Antagonist and the Pathogen

Microsphaeropsis sp. strain P130A was evaluated for the control of tuber-borne inoculum of Rhizoctonia solani based on the viability of sclerotia produced in vitro and on both the viability and production of tuber-borne sclerotia. The interactions between the antagonist and the pathogen, as well as the effect of the toxins produced by the antagonist on mycelial growth of R. solani were studied using transmission electron microscopy. On sclerotia produced in vitro, for all incubation periods (1 to 42 days), Microsphaeropsis sp. significantly reduced germination. Percent germination of sclerotia treated with Microsphaeropsis sp. decreased with increasing incubation period from an average of 82.0% after 1 day to stabilize at an average of 5.8% after 35 days. Similarly, percent germination of tuber-borne sclerotia was significantly lower when tubers were treated with Microsphaeropsis sp. Both 2% formaldehyde and Microsphaeropsis sp. treatments significantly reduced sclerotia germination to approximately 10% after 42 days of incubation at 4 degrees C. Furthermore, on tubers treated with the antagonist, the number of sclerotia per square centimeter decreased from 1.6 to 0.5 during the 8 months of storage at 4 degrees C, whereas an increase from 1.2 to 7.8 sclerotia per square centimeter was observed on untreated tubers. Microsphaeropsis sp. (strain P130A) colonized hyphae of R. solani within 4 days after contact on culture media. Transmission electron microscopic observations showed that the antagonist induced a rupture of the pathogen plasma membrane and that a chitin-enriched matrix was deposited at sites of potential antagonist penetration. Host penetration was not associated with pathogen cell wall alterations, which occurred at the time of progress of the antagonist in the pathogen cytoplasm. In the presence of a crude extract of Microsphaeropsis sp., cells of R. solani showed cytoplasm disorganization and breakdown of plasma membranes. Antibiosis and mycoparasitism were involved in the antagonism of R. solani by Microsphaeropsis sp., but the sequence by which these events occur, as well as the significance of wall appositions produced by R. solani, is yet to be established.     

Incidence of soil-borne plant pathogens isolated from barley and winter wheat, and other crops in the rotation, on Prince Edward Island

Barley and winter wheat were sampled over 3 years to characterize soil-borne organisms involved with cereal crown and root disease complexes. Winter wheat crowns were infected more often by Fusarium avenaceum and F. sambucinum than barley crowns. Bipolaris sorokiniana and F. graminearum were more common in crown tissue of barley underseeded with clover than in winter wheat crowns. In roots, the incidence of F. avenaceum was highest in winter wheat. In soil, populations of Rhizoctonia solani and F. sambucinum were higher in winter wheat than barley underseeded with ryegrass. Stunt nematodes (Tylenchorhynchus spp.) were greatest in winter wheat and barley underseeded with ryegrass. The incidence of F. avenaceum in roots of winter wheat correlated positively with the severity of crown and root rot symptoms. The severity of barley crown and root rot correlated positively with the incidence of R. solani in crowns, F. avenaceum in crowns and roots, B. sorokiniana in roots, and numbers of stunt nematodes in the soil. The incidence of soil-borne organisms was also recorded in annual ryegrass. soybean, potato, pea, and clover which were grown in rotation with barley and winter wheat. The incidence of R. solani and F. avenaceum was highest in clover crown tissue, and F. sambucinum was prevalent in soybean hypocotyls. Soil population levels of R. solani, F. avenaceum , and stunt nematodes were greatest in ryegrass and pea soil just prior to ploughing down or harvesting these crops, respectively.