Effect of plant roots on the germination of microsclerotia ofVerticillium dahliae

Induction of germination of microsclerotia by exudates from plant roots may be important for the control ofV. dahliae. Laboratory experiments with root observation boxes were carried out to assess the influence of root tips of seven crop species and cultivars on the germination of microsclerotia ofVerticillium dahliae in soil under controlled conditions. The root density of crops was measured in a field experiment. The results of the laboratory experiments and the field experiment were combined to estimate the total effect of crops on the population of microsclerotia in the field. Germination of microsclerotia was stimulated by all crops compared to a control without a crop. Among crops, roots of potato cvs Element and Astarte had a larger stimulation effect on microsclerotia than that of potato Ostara, pea, flax, sugar beet or onion. The number of hyphae per microsclerotium decreased with distance from the root surface regardless of the crop species or cultivar. Differences in root densities, in the affected root zones and in the stimulation effect on germination of microsclerotia caused large differences among crops in the effect on the population of microsclerotia in the soil. However, growing a rop with the special purpose to reduce the level ofV. dahliae inoculum in the soil is an inefficient control measure, because only a small part of the total soil volume is affected by roots and the number of hyphae per microscleroium affected is too low.Abbreviations MS microsclerotia, microsclerotium     

Plant Host Range of Verticillium longisporum and Microsclerotia Density in Swedish Soils

Verticillium longisporum is a soil-borne fungal pathogen causing vascular wilt of Brassica crops. This study was conducted to enhance our knowledge on the host range of V. longisporum. Seven crop species (barley, oat, oilseed rape, pea, red clover, sugar beet and wheat) and five weed species (barren brome, black-grass, charlock, cleavers and scentless mayweed) all common in southern Sweden were evaluated for infection by response to V. longisporum. Oat, spring wheat, oilseed rape, scentless mayweed and charlock inoculated with V. longisporum in a greenhouse showed stunting to various degrees close to the fully ripe stage. Based on the extent of microsclerotia formation, explants were separated into four groups: for pea and wheat, <5% of the samples had formed microsclerotia; for scentless mayweed, 5–10%; for oat, 10–20%; and for charlock and oilseed rape >80%. The results suggest that plant species outside the Brassicaceae can act as reservoirs of V. longisporum inoculum. Soil inoculum densities in nine fields were monitored over a period of 12 months, which ranged from 1 to 48 cfu g⁻¹ soil. Density of microsclerotia was lowest just after harvest, reaching its maximum six months later. No significant correlation between inoculum density in soil and disease incidence on oilseed rape plants was found. However, the data suggest that a threshold of 1 cfu g⁻¹ soil is needed to cause disease on oilseed rape. Species identification based on microsclerotia morphology and PCR analysis showed that V. longisporum dominated in soil of seven, and V. dahliae in two of the nine fields studied.     

Etiology of asparagus replant-bound early decline

Asparagus replant-bound early decline (ARED) was characterized and its etiology was elucidated in experiments under greenhouse and field conditions. Selective soil treatments were used to differentiate between autotoxic compounds and soil-borne pathogens as causal agents. In greenhouse experiments, there were symptoms of ARED within 12—15 weeks. Asparagus plants grown in soil formerly used for asparagus (asparagus soil) showed brown lesions on primary and secondary roots, and many secondary roots had rotted. Root weights of plants grown in asparagus soil were lower than those of plants grown in fresh soil.Fusarium oxysporum f. sp.asparagi (Foa) was by far the most common species among the fungi isolated from roots with lesions. Under greenhouse and field conditions, there were similar symptoms, which indicates that the results obtained under greenhouse conditions are similar to those in the field. The vertical distribution of the ARED-causing factor(s) was studied in a greenhouse experiment in which plants were grown in soil from three layers: 0–30, 30–60, and 60–90 cm. For all four asparagus soils tested, there were ARED symptoms and similar disease severity in samples from all three depths. The causal factor persisted at least 11 years after soil was no longer used for asparagus. When asparagus soil was diluted with fresh soil to give mixtures with 100%, 80%, 50%, 20% and 0% asparagus soil, disease severity did not decrease with increasing dilution of the asparagus soil from 100% to 20%. Disease severity of all mixtures with asparagus soil was significantly higher than that for fresh soil. The results imply that ARED is caused by a pathogen colonizing the soil rather than inhibition by autotoxins released from residues of the preceding asparagus crop. This conclusion is supported by the results of greenhouse and outdoor experiments with heat and fungicide treatments of soil. ARED was nullified by heat treatments of 30 min at 55 or 60 °C but not 45 and 50 °C, eliminating autotoxins as an important cause of ARED because they are heat-stable. Foa is eliminated by a 30-min soil treatment at 55–60 °C but not 50 °C. Prochloraz, known for its toxicity toF. oxysporum, also nullified ARED. Disease severity level was related to the density of Foa in soil. The results provide conclusive evidence thatF. oxysporum f. sp.asparagi is the main cause of ARED in the Netherlands, which largely removes the need to discriminate between early decline and replant-bound early decline, because Foa is the main cause of both diseases.     

Anastomosis Groups, Pathogenicity and Sensitivity to Fungicides of Rhizoctonia solani Isolates Collected on Potato Crops in France

A collection of 241 isolates of Rhizoctonia solani obtained from potato plants grown in different areas in France was characterized for anastomosis grouping, symptomatology on tubers of different cultivars and sensitivity to three fungicides. Most isolates collected belonged to (anastomosis groups (AGs)) AG 3, but 2% and 4% of the isolates were AG 5 and AG 2-1. AG 3 and AG 2-1 isolates were mostly obtained from sclerotia on tubers, but all AG 5, some AG 3 and some AG 2-1 isolates were recovered from superficial tuber alterations, like deformations, corky or scabby lesions. Sclerotia were formed on tubers produced by healthy stem cuttings grown in soil artificially infested with AG 3, but not on tubers grown in soil infested with either AG 5 or AG 2-1. No variation in susceptibility to sclerotial formation was observed among five potato cultivars. In all cases, a large proportion of tubers showed superficial corky lesions, often associated with deformations. The proportion of tubers with lesions and deformations was highest in soil infested with AG 2-1 and significantly lower on cv. Samba in all treatments. All isolates were highly sensitive to flutolanil, iprodione and pencycuron, except the AG 5 isolates, moderately sensitive to pencycuron. These results show that, although AG 3 is the most common R. solani group on potato in France, AG 5 and AG 2-1 may be present. Isolates differed for pathogenicity. In vitro sensitivity to fungicides varied among AGs.     

The influence of cyst nematodes and drought on potato growth. 1. Effects on plant growth under semi-controlled conditions

Potatoes were grown under a permanent rain shelter in mobile containers in soil with or without potato cyst nematodes (Globodera pallida). The plants were subjected to an early drought stress period starting at planting until 43 days after planting, to a late drought stress period starting at 43 days until senescence at 92 days and to a drought control. Dry matter weight and characteristics of leaves, stems, stolons and roots were determined at periodic harvests. The early drought stress and nematode infection affected many plant organ characteristics in similar ways. Numbers of leaves, specific leaf area, plant height, specific stem weight, leaf area ratio, mean tuber weight and harvest index were reduced by both stress factors at early stages of growth.Later on, interactions between both stress factors which influence the development rate of the plants led to more diverse plant reactions. Plants of all treatments rapidly senesced at about 90 days after planting. Uninfected plants had then depleted the soil nutrient supply whereas the plants grown in the inoculated soil senesced as a result of the potato cyst nematode infection.     

Effect of crop rotation on the incidence of soil-borne fungal diseases of potato

Effects of crop rotation on the incidence of soil-borne pathogens and on the performance of potato were investigated in five field experiments. Rotations differed in cropping frequency of potato and in crop sequence.Incidence of stem canker caused byRhizoctonia solani was strongly influenced by the cropping frequency of potato and not by crops with which the potato was alternated in the rotation. Cropping frequency of potato also affected the occurrence of black scurf, but less pronounced than for stem canker. The antagonistVerticillium bigutatum slightly reducedR. solani (black scurf) in plots on sandy soil continuously cropped with potato. Incidence of stem canker was also strongly affected by granular nematicides applied to the soil, nitrogen level and the cultivar grown.     

Selection and Screening of Endorhizosphere Bacteria from Solarized Soils as Biocontrol Agents Against Verticillium dahliae of Solanaceous Hosts

Verticillium dahliae antagonistic endorhizosphere bacteria were selected from root tips of tomato plants grown in solarized soils. Fifty-three out of the 435 selected bacterial isolates were found to be antagonistic against V. dahliae and several other soilborne pathogens in dual cultures. Significant biocontrol activity against V. dahliae in glasshouse trials was demonstrated in three of 18 evaluated antagonistic isolates, provisionally identified as Bacillus sp. Although fluorescent pseudomonads were also isolated from root tips of tomato plants, none of the tested isolates exercised any significant antagonistic activity against V. dahliae in dual cultures. So these isolates were not tested in glasshouse trials in this study. Finally, two of the most effective bacterial isolates, designated as K-165 and 5-127, were shown to be rhizosphere colonizers, very efficient in inhibiting mycelial growth of V. dahliae in dual cultures and successfully controlling Verticillium wilt of solanaceous hosts. In glasshouse experiments, root dipping or soil drenching of eggplants with bacterial suspension of 10⁷cfu ml^–1 resulted in reduced disease severity expressed as percentage of diseased leaves (40–70%) compared to the untreated controls under high V. dahliae inoculum level (40 microsclerotia g^–1 soil). In heavily Verticillium infested potato fields, experiments with potato seeds dusted with a bacterial talc formulation (10⁸cfu g^–1 formulation), showed a significant reduction in symptom development expressed as percentage of diseased potato plants and a 25% increase in yield over the untreated controls. As for their effectiveness in increasing plant height, both bacterial isolates K-165 and 5-127 produced indolebutyric, indolepyruvic and indole propionic acids. Both antagonists are considered as plant growth promoting rhizobacteria bacteria since significantly increased the height of treated plants compared with the untreated controls. Chitinolytic activity test showed that both isolates were able to produce chitinase. Testing rhizospheric and endophytic activity of the antagonists it was shown that although the bacteria are rhizosphere inhabitants they also preferentially colonize the endorhizosphere of tomatoes and eggplants. Fatty acid analysis showed that isolate K-165 could belong to Paenibacillus alvei while 5-127 to Bacillus amiloliquefaciens.     

Incidence of Verticillium wilt of artichoke in eastern Spain and role of inoculum sources on crop infection

Surveys of 94 artichoke fields throughout the artichoke production areas of Comunidad Valenciana (eastern Spain) were conducted from 1999 to 2002 to determine the incidence and distribution of Verticillium wilt.Verticillium dahliae was isolated from 80.9% of the sampled fields, and detected in all artichoke-growing areas, with a mean disease incidence of 53.8% infected plants. The disease was found to cause severe damage to cv. ‘Blanca de Tudela’, which is the most important artichoke cultivar grown in Spain, and was also observed on the seed-propagated cv. ‘Imperial Star’. In field trials to study the role of infected planting material and soil inoculum on infection of artichoke plants during the cropping season,V. dahliae was transmitted from infected stumps to the plants, confirming that the use of infected stumps could have greatly contributed to the dissemination of the pathogen. Inoculum density ofV. dahliae in soil had an effect on crop infection, in that a higher number of microsclerotia per gram of soil resulted in a higher percentage of infected plants. In addition, yield of cv. Blanca de Tudela was significantly affected byV. dahliae infection, showing that a higher percentage of infection corresponded with lower yield. http://www.phytoparasitica.org posting July 21, 2005.     

Virus transmission by aphids in potato crops

This paper reviews the contribution of vector activity and plant age to virus spread in potato crops. Determining which aphid species are vectors is particularly important for timing haulm destruction to minimize tuber infection by potato virus Y (PVY). Alate aphids of more than 30 species transmit PVY, and aphids such asRhopalosiphum padi, that migrate in large numbers before flights of the more efficient vector,Myzus persicae, appear to be important vectors. Differences in methodology, aphid biotypes and virus strains prevent direct comparisons between estimates of vector efficiencies obtained for aphids in different countries in north western Europe. M. persicae is also the most efficient vector of potato leafroll virus (PLRV), but some clones ofMacrosiphum euphorbiae transmit PLRV efficiently toNicotiana clevelandii and potato test plants. The removal of infected plants early in the season prevents the spread of PLRV in cool regions with limited vector activity. The proportion of aphids acquiring PLRV from infected potato plants decreases with plant age, and healthy potato plants are more resistant to infection later in the season. Severe symptoms of secondary leafroll developed on progeny plants of cv. Maris Piper derived from mother plants inoculated with PLRV in June or July of the previous year. Progeny plants derived from mother plants inoculated in August showed only mild symptoms, but the concentration of PLRV in these plants was as high as that in the plants with severe symptoms.     

Assessment of recent outbreaks of <Emphasis Type="Italic">Dickeya</Emphasis> sp. (syn. <Emphasis Type="Italic">Erwinia chrysanthemi</Emphasis>) slow wilt in potato crops in Israel

Suspected Dickeya sp. strains were obtained from potato plants and tubers collected from commercial plots. The disease was observed on crops of various cultivars grown from seed tubers imported from the Netherlands during the spring seasons of 2004–2006, with disease incidence of 2–30% (10% in average). In addition to typical wilting symptoms on the foliage, in cases of severe infection, progeny tubers were rotten in the soil. Six strains were characterised by biochemical, serological and PCR-amplification. All tests verified the strains as Dickeya sp. The rep-PCR and the biochemical assays showed that the strains isolated from blackleg diseased plants in Israel were very similar, if not identical to strains isolated from Dutch seed potatoes, suggesting that the infection in Israel originated from the Dutch seed. The strains were distantly related to D. dianthicola strains, typically found in potatoes in Western Europe, and were similar to biovar 3 D. dadanti or D. zeae. This is the first time that the presence of biovar 3 strains in potato in the Netherlands is described. One of the strains was used for pathogenicity assays on potato cvs Nicola and Mondial. Symptoms appeared 2 to 3 days after stem inoculation, and 7 to 10 days after soil inoculation. The control plants treated with water, or plants inoculated with Pectobacterium carotovorum, did not develop any symptoms with either method of inoculation. The identity of Dickeya sp. and P. carotovorum re-isolated from inoculated plants was confirmed by PCR and ELISA.     

Identification of rhizomania-infected soil in Europe able to overcome <Emphasis Type="Italic">Rz1</Emphasis> resistance in sugar beet and comparison with other resistance-breaking soils from different geographic origins

Rhizomania, caused by Beet necrotic yellow vein virus (BNYVV), is vectored by Polymyxa betae. The disease can only be controlled by growing partially resistant sugar beets, which quantitatively reduce virus replication and spread. None of the known major resistance genes (Rz1, Rz2, Rz3), alone or in combination, are able to prevent BNYVV infection entirely. Here we report for the first time the identification of a Spanish soil, containing an A-type BNYVV with RNA 1-4, displaying Rz1 resistance-breaking abilities comparable to soils from the USA and to those from France containing the French (Pithiviers) P-type BNYVV with RNA 5. A resistance test with several soil samples vs. different sugar beet cultivars was conducted under standardised conditions. Sugar beets were analysed after 12 weeks of greenhouse cultivation for taproot weight, BNYVV and relative P. betae content. The soil samples from Spain, France and the USA produced high virus contents and strong rhizomania symptoms in Rz1 plants, indicative of resistance-breaking abilities. In addition, all resistance-breaking soil samples produced detectable virus concentrations in plant lateral roots of the Rz1 + Rz2 cultivar, and plants grown in the Spanish soil sample also had reduced taproot weight and displayed severe rhizomania disease symptoms. Additionally, the main pathogenicity factor P25, responsible for the formation of BNYVV symptoms, showed high sequence variability in the amino acid tetrad at position 67–70. The results suggest the geographically independent selection of BNYVV resistance-breaking isolates following the uniform cultivation of Rz1-containing sugar beet cultivars.     

Single-site root inoculations on eggplant with microsclerotia ofVerticillium dahliae

For many soilborne plant pathogens, disease results from multiple root infections. Studying the infection dynamics of single or multiple propagules of these pathogens applied at one site of the root system may be the basis for understanding the development of disease caused by multiple root infections. The effect of single-site inoculations of roots of eggplant seedlings with microsclerotia of the wilt-causing fungusVerticillium dahliae, was studied. Experiments were conducted using specially designed pots which enabled the incorporation and removal of inoculum in the soil. Inoculations were carried out by placing microsclerotia, firmly embedded in small sections of polypropylene screen filter, directly below the growing tip of the main root of young eggplant seedlings. Three to 4 days after inoculation, the root had grown over the screen filter, and the filter was removed. Root platings showed high infection levels at the inoculation site, but also several (discrete) root infections were noted some distance above and below the site of inoculation. Exposure of the root to the lowest number of microsclerotia (26/inoculation site) was sufficient to lead to up to 65% root infections. Number of plants with root infections declined over time, ranging from a maximum of 65–100% 2–4 wk after inoculation, to 10–29% at 6–7 wk after inoculation. Apparently,V. dahliae died in nonsystemic infections after some time.     

A Greenhouse Test for Screening Sugar Beet (Beta Vulgaris) for Resistance to Rhizoctonia Solani

Rhizoctonia solani Kühn is a serious plant pathogenic fungus, causing various types of damage to sugar beet (Beta vulgaris L.). In Europe, the disease is spreading and becoming a threat for the growing of this crop. Plant resistance seems to be the most practical and economical way to control the disease. Experiments were carried out to optimise a greenhouse procedure to screen plants of sugar beet for resistance to R. solani. In the first experiment, two susceptible accessions were evaluated for root and leaf symptoms, after being grown in seven different soil mixtures and inoculated with R. solani. The fungus infected all plants. It was concluded that leaf symptoms were not reliable for the rating of disease severity. Statistically significant differences between the soil mixtures were observed, and there were no significant differences between the two accessions. The two soil mixtures, showing the most severe disease symptoms, were selected for a second experiment, including both resistant and susceptible accessions. As in the first experiment, root symptoms were recorded using a 1–7 scale, and a significant expression of resistance was observed. The average severity of the disease in the greenhouse experiment generally was comparable with the infection in field experiments, and the ranking of the accessions was the same in the two types of experiments. It was concluded that evaluation procedures in the greenhouse could be used as a rapid assay to screen sugar beet plants for resistance to R. solani.     

Relation between cropping frequency of peas and other legumes and foot and root rot in peas

The relation between the frequency of legume crops in a rotation and the root rot severity in pea was examined in a field survey. Additionally, greenhouse experiments were performed with soil samples from legume rotation trials or from farmers' fields. The frequency of pea crops in current rotations proved to be much less than the recommended value of one in six years. The correlation between pea root rot and the number of years that pea or other legumes were not grown on the field under consideration (called crop interval) was weak. Root rot severity correlated better with the frequency of peas or legumes in general over a period of 18 years, but the frequency still explained only a minor fraction of the variation in disease index. Some experimental data pointed to the occurrence of a highly specific pathogen microflora with continuous cropping of only one legume species, but this phenomenon probably does not occur in farmers' fields. In field samples, root disease index for pea correlated well with that for field bean. The survival of resting structures of pathogens such asAphanomyces euteiches probably explains why the frequency of legume cropping has a higher impact than crop interval on root disease incidence. Pea-free periods and legume frequencies have a poor predictive value for crop management purposes.     

Gnomonia fragariae, a Cause of Strawberry Root Rot and Petiole Blight

Gnomonia fragariae has been occasionally listed among the fungi associated with diseased strawberry plants. However its pathogenicity has not been established. During the investigation on strawberry decline in Latvia and Sweden, a fungus was repeatedly recovered from discoloured root and crown tissues of severely stunted plants. Attempts to induce sporulation of the isolates grown on several agar media resulted in the formation of mature ascomata only on potato carrot agar and oatmeal agar. On morphological grounds and comparisons with reference herbarium specimens these isolates were identified as Gnomonia fragariae. The pathogenicity of the fungus was evaluated initially in the detached leaf assay and subsequently in three bioassays on strawberry plants. All the bioassays showed that G. fragariae was pathogenic on strawberry and capable of causing severe root rot and petiole blight. The symptoms that developed in the greenhouse experiments closely resembled those observed in the fields. The fungus did not cause rapid plant death but growth and development of inoculated strawberry plants was severely affected. To our knowledge this is the first time when pathogenicity of G. fragariae as a root rot pathogen has been clearly established. Our study shows that G. fragariae is one of the serious pathogens involved in the root rot complex of strawberry in Latvia and Sweden.     

Evaluation of antagonistic bacteria for suppression of bacterial ring rot of potato

Bacterial strains with potential for biological control of bacterial ring rot of potato caused byClavibacter michiganensis subsp.sepedonicus were isolated from the surface of potato tubers. Eighty-eight potential biocontrol candidates, selected on the basis ofin vitro antibiosis toC. m. sepedonicus, produced inhibition zones with radii ranging from 0.5 to 16 mm on test plates. All antagonistic isolates were screened in the greenhouse for biocontrol activity on micropropagated potato plantlets root-inoculated withC. m. sepedonicus. Eight strains consistently prevented infection of plantlets but there was no significant correlation between the width of the inhibition zone in thein vitro assay and ring rot suppression in the plant bioassay. Three strains that showed a high level of biological control potential were identified as a saprophytic enteric bacterium (strain 7G), anArthrobacter sp. (strain 16C), and a soil coryneform bacterium (strain 18A). These were tested in a field plot by co-inoculating cut seed potato tubers withC. m. sepedonicus and antagonists. Strains 7G and 18A significantly increased plant stand whereas 16C decreased disease incidence. The relative number of ostensibly ring rot-free progeny tubers was generally greater when antagonists were present.     

Interactions Between the Potato Cyst Nematode Globodera rostochiensis and Diseases Caused by Rhizoctonia solani AG3 in Potatoes Under Field Conditions

Findings from 2 years of field experiments investigating the relationship between Globodera rostochiensis and Rhizoctonia solani on unique field sites are reported. In 2000, a field experiment was positioned on land that had previously been used for experimental work investigating integrated potato cyst nematode (PCN) management methods. This study had produced an ‘untypical’ mosaic of PCN population densities ranging from 5 to 221 eggs g⁻¹ soil. In 2001, the field experiment was conducted on a different field site and overlaid on a focus of G. rostochiensis population densities ranging from 11 to 108 eggs g⁻¹ soil. In each experiment, potatoes (cv. Désirée) were grown in plots with similar population densities of G. rostochiensis that were either uninoculated or inoculated with R. solani. A series of potato plant harvests were undertaken to investigate the effects of nematode infestation on the incidence and severity of R. solani diseases and the associated development of plants. In both experiments, a clear relationship was found between the density of G. rostochiensis juveniles present in potato roots and the incidence of stolons infected by R. solani, 6 weeks after planting. For the first time this interaction has been determined under field conditions. The results of the study suggest that the interaction between nematode and fungus is indirect and possible mechanisms are discussed.     

Plant waste-based composts suppressive to diseases caused by pathogenic Fusarium oxysporum

The suppressive ability of three plant residue-based composts that could serve as components of soilless media for several vegetable crops was tested on four different formae speciales of Fusarium oxysporum: melonis, basilici, radicis-lycopersici and radicis-cucumerinum. The composts were prepared under controlled conditions from a mixture of separated cow manure (SCM) with orange peels (OP), wheat straw (WS), or dried tomato plants that had been removed from the greenhouse after the end of the season (TP). Disease development in melon, tomato and cucumber seedlings growing in the three composts was significantly less than that observed in peat. Plant inoculation was achieved by conidia produced in culture, conidia naturally produced on infected stems and soil inoculum produced by enriching the soil with infected tissues. Pathogen colonization of the roots and stems of infected melon plants grown in TP–SCM and OP–SCM composts was significantly lower than that of peat-grown plants. Sterilization by gamma irradiation reduced the suppressive capability of TP–SCM and OP–SCM composts, whereas it did not affect the disease development and final disease incidence in peat. Tested formae speciales exhibited differing decline rates of the conidia incorporated in the composts, compared with the rate in the peat control, which suggests that different mechanisms may be involved in the suppression of the different pathogens. The present study shows that composts based on plant-waste residues suppress diseases caused by different formae speciales of Fusarium oxysporum.     

Characterization ofRhizoctonia solani isolates from potatoes in turkey and screening potato cultivars for resistance to AG-3 isolates

A total of 304Rhizoctonia solani isolates and 60 binucleateRhizoctonia-like fungi were recovered from stems and tubers of infected potato plants over a 2-yr period in northeast Turkey.R. solani isolates were identified to 11 anastomosis groups (AGs): AG-1 (0.66%), AG-2-1 (5.6%), AG-2-2 (0.99%), AG-3 (83.9%), AG-5 (4.6%), AG-6 (0.66%), AG-8 (1.32%), AG-9 (0.33%), AG-10 (1.32%), AG-12 (0.33%), and AG-13 (0.33%). In the greenhouse tests, most of the AG-3 isolates were significantly more virulent than isolates belonging to other AGs on potato cv. Batum. Isolates of other anastomosis groups differed in their virulence. Results indicated that AG-3 is an important pathogen on potatoes grown in the study area. Five of 22 commercial and local potato cultivars evaluated for their reaction toR. solani AG-3 isolates (TP-2) under greenhouse conditions were highly resistant; the remaining cultivars exhibited different levels of susceptibility to the pathogen isolate. http://www.phytoparasitica.org posting July 14, 2005.     

Deleterious effects of fungi isolated from paddy soils on seminal root of rice

Soil samples were collected from rice paddies at 22 locations in northeastern Honshu, Japan. In 20 of the samples, seedling growth of rice was improved by soil pasteurization (aerated steaming at 60°C for 30 min), although no typical disease symptoms were observed in the seedlings grown in the untreated soil samples. In most locations, rice seedlings grew better in a potting medium containing root material from plants grown in the pasteurized portion of a soil sample than in a medium with root material from plants grown in the unpasteurized portion of the same sample. The results suggest that microorganisms that restrained rice seedling growth may be common in the soils of rice paddies. Approximately 800 isolates were obtained from seedling roots grown in unpasteurized soils and grouped by cultural and microscopic morphologies. The deleterious effects of 79 isolates from 21 major groups, most of which were soil-dwelling taxa, were examined after direct inoculation of the seminal roots. Isolates of Curvularia sp., Cirrenalia sp., Eppicoccum nigrum, Fusarium graminearum, F. oxysporum, Gliocladium virens, Humicola sp., Penicillium sp., Rhizoctonia oryzae-sativae, Sclerotium hydrophilum, Trichoderma aureoviride, and T. harzianum inhibited root growth, suggesting that deleterious root-infecting fungi were more common in paddy soil than previously thought. These fungi may be involved in the restraint of rice seedling growth.