Dutch elm disease is caused by the fungal pathogen Ophiostoma novo-ulmi which is transmitted by the native elm bark beetle, Hylurgopinus rufipes. We have found that four semiochemicals (the monoterpene (−)-β-pinene and the sesquiterpenes (−)-α-cubebene, (+)-spiroaxa-5,7-diene and (+)-δ-cadinene) from diseased American elms, Ulmus americana, synergistically attract H. rufipes, and that sesquiterpene emission is upregulated in elm trees inoculated with O. novo-ulmi. The fungus thus manipulates host trees to enhance their apparency to foraging beetles, a strategy that increases the probability of transportation of the pathogen to new hosts.

Species-specific primers for four species of Neofabraea associated with apple and pear fruit diseases were used in multiplex PCR assays to identify 29 putative Neofabraea isolates, primarily isolated from bull's-eye rot on pears from packing houses in Washington and Oregon. Apples were inoculated with these isolates, and tentative identifications based on morphology of conidia forming on wound-inoculated fruits were compared with identifications made using species-specific primers for different Neofabraea species. PCR-based identifications were successful for 26 of 29 isolates, and in all cases were consistent with tentative identifications based on spore morphologies. The results revealed that in addition to the two species of Neofabraea already known to occur in the Pacific Northwest, N. malicorticis and N. perennans, a third species, N. alba, is also prevalent.

Three fungal species responsible for anthracnose canker, perennial canker, and bull's-eye rot of apple have been considered members of the genus Pezicula for a number of years. Recent studies, however, have provided evidence to (re-)classify these species as Neofabraea. There has been a long historical debate regarding the taxonomy of two of these fungi. In Europe, both Neofabraea malicorticis and N. perennans have generally been considered N. malicorticis, while in North America a species distinction has been maintained. Phylogenetic analyses of Neofabraea isolates were based on DNA sequences of the internal transcribed spacer region of nuclear rDNA (38 isolates), the mitochondrial rDNA small subunit (partial: 48 isolates), the beta -tubulin gene (partial: 25 isolates), and a combined data set (21 isolates). Our work provides evidence for the existence of four distinct Neofabraea apple pathogens including N. malicorticis, N. perennans, N. alba, and a putative new Neofabraea species that was isolated in both Europe and eastern North America. Our results indicate that the primary Neofabraea species causing tree cankers and bull's-eye rot in North America are N. malicorticis and N. perennans in the west and N. alba in eastern Canada. N. perennans. N. alba, and the undescribed Neofabraea species were found in Europe but the presence of N. malicorticis was not confirmed by our limited sampling. inclusion of Rosa spp. in the host range of N. malicorticis is merited.

The effect of bean root residues on populations of known isolates and unidentified Pythium species in soils was assessed, with special regard to herbicide treatment of bean seedlings. The general Pythium population in a muck soil was significantly increased 6 days after foliar treatment of bean seedlings with glyphosate, and by soil amendment with heat-killed bean roots, but not by amendment with roots of healthy bean seedlings. The enhanced populations returned to near initial levels 4 days later. Isolated populations of Pythium ultimum (a glyphosate-synergistic isolate) and Pythium coloratum (a non-synergistic isolate) in a sandy loam soil were enhanced approximately 10-fold over control by roots of healthy bean seedlings and by roots of seedlings killed with either glyphosate or paraquat. Distinct peaks in the population responses of P. ultimum and P. coloratum occurred at 9 and 18 days after treatment, respectively. Strong positive correlations were observed between the population estimates obtained by dilution plating and damping off of sunflower for both P. ultimum and P. coloratum. These results suggests that herbicide treatment of plants can cause temporary increases in both Pythium populations and damping off potential of soils.

Several possible mechanisms for the glyphosate-induced predisposition of bean roots (Phaseolus vulgarisL.) to colonization byPythiumspp. were investigated. Glyphosate at 0.1 and 1.0 μg ml-1from the surfactant-containing formulation Roundup® and the non surfactant-containing formulation Accord® did not affect mycelial growth ofPythium ultimumandPythium sylvaticumon water agar and cornmeal agar. One microgram per millilitre of glyphosate from both formulations significantly stimulated germination of sporangia ofP. ultimum. Germination and growth of germ tubes ofP. ultimumwere significantly greater in root exudates from bean plants whose primary leaves had been treated with glyphosate than in exudates from non-treated plants. The lignin content of roots was increased significantly whenP. ultimumorP. sylvaticumwas added to the hydroponic system in which the roots were growing. When glyphosate was applied 2 days prior toPythium, deposition of lignin in response toPythiumin the bean roots was significantly reduced. These results suggest that predisposition by glyphosate of bean roots to colonization byPythiumspp. may involve changes in root exudates that enhance germination and growth of pathogen propagules, and suppression of a pathogen-induced lignification response by plant roots.

Five Pythium species, P. ultimum, P. sylvaticum, P. irregulare, P. coloratum, and Pythium 'HS' group, were identified using morphological characteristics out of 65 isolates obtained from roots of glyphosate-treated bean seedlings grown in five different soils. Various genotypes within the Pythium species were determined from RFLP patterns of total DNA. There were six RFLP types represented in P. sylvaticum, three in P. ultimum, and two each for P. irregulare, P. coloratum and P. 'HS' group. The potential of a representative isolate from each RFLP group to enhance the herbicidal action of glyphosate was quantified by estimating glyphosate LD,, values on bean seedlings growing in sterilized soils amended with each isolate separately. The LD,, values were computed by logistic regression using plant mortality data gathered 4 wk after treatment of 2-wk-old seedlings with different doses of glyphosate. Twelve of the 15 isolates of Pythium tested were glyphosate synergists on beans. The efficacy of the different isolates as glyphosate synergists varied both between species and among different RFLP types within the same species. The pathogenicity of the representative isolates to beans without glyphosate treatment was also determined. All Pythium species tested were pathogenic to varying degrees on germinating bean seeds and on 2-wk-old bean seedlings. The results indicate that several Pythium species can function as glyphosate synergists and that five different soils all yielded glyphosate synergistic Pythium isolates.

A total of 39 Pythium isolates representing 14 species of Pythium was assessed for host specificity as glyphosate synergists. This was done using three groups of Pythium isolates from roots of glyphosate-treated bean (PBI), wheat (PWI), and isolates from various glyphosate-untreated hosts. PBI consisted of 15 isolates and included P. ultimum, P. sylvaticum, P. coloratum, P. irregulare and P. group 'HS'. PWI consisted of 14 isolates representing the first four of these species. Pythium from glyphosate-untreated hosts included single isolates representing P. aphanidermatum, P. spinosum, P. paroecandrum, P. hypogynum, P. splendens, P, sulcatum, P. vanterpooli, P. acanthicum, P. arrhenomanes and P. coloratum. The glyphosate synergistic potential of the Pythium isolates was determined by treating a-week-old seedlings growing in soil infested with individual isolates of Pythium with different doses of glyphosate. LD(50) values associated with each isolate were estimated by logistic regression analysis of plant mortalities recorded 4 wk after treatment with glyphosate, and compared with LD(50) values for plants grown in the absence of Pythium. Host specificity was assessed by comparing the glyphosate synergistic potential of PBI on bean and PWI on wheat seedlings, with the potential of these same isolates of PBI on wheat and PWI on bean seedlings. Glyphosate synergistic potential of PBI was also estimated on sunflower and pepper, to test whether PBI were capable of glyphosate synergistic interaction (GSI) on other unrelated dicot yledonous species. Pythium isolates from glyphosate-untreated hosts were tested on bean to determine if Pythium species not represented in the PBI and PWI groups were capable of GSI. The glyphosate synergistic potentials of the PBI and PWI on wheat seedlings were low and inconsistent compared to those observed on dicot plants. AU PWI and 12 of the 15 PBI were glyphosate synergists on beans, and all the PBI were glyphosate synergistic on sunflower and pepper seedlings. All Pythium isolates from glyphosate-untreated sources tested were also glyphosate synergists on bean seedlings. These various tests of glyphosate synergistic potential of Pythium isolates from diverse sources on various plant species revealed no evidence of host specificity among the isolates and species tested.

The invasiveness of Velticillium albo-atrum in roots was compared in clonal populations of phenotypically resistant and susceptible alfalfa plants. Entire root systems from inoculated plants were surface sterilized and plated onto water agar immediately and at weekly intervals for 6 weeks following inoculation. Colonies of Verticillium albo-atrum growing from roots were assessed at 3, 6, and 9 days after plating and were scored as point source or continuous, based on the length of root from which the colony emanated. Point source colonies were defined as colonies emanating from a length of root less than 1 mm long. Point source colonies may reflect the plant's ability to restrict pathogen invasiveness within the root system. Using the ratio of point sourer to total colonies as a criterion, the root assessments over the full 6-week period distinguished the plants as resistant and susceptible and provided a similar differentiation, although not as reliable, to the differentiation provided by conventional foliar symptom assessment. The results suggest that mechanisms for resistance to V. albo-atrum in these two clonal populations operate in both foliar and root tissues. The root assessment technique offers insights into the operation of resistance mechanisms in alfalfa to V. albo-atrum and may offer similar insights in other vascular host-pathogen systems.

Kievitone, phaseollinisoflavan and phaseollin were detected in roots of bean seedlings (Phaseolus vulgaris L.) grown in natural soil. Comparison of phytoalexin production by roots grown in different media indicated that these phytoalexins were probably induced by microorganisms in soil. The influence of common root rot pathogens of bean, Pythium spp., on phytoalexin production was determined. Pythium ultimum elicited kievitone, phaseollinisoflavan and phaseollin in roots grown in sterilized silica sand. P. sylvaticum induced only kievitone and phaseollin in the same growth medium. Glyphosate did not significantly affect the accumulation of phytoalexins within 3 days. However, by day 5, significantly more phaseollin was detected in the roots of Pythium inoculated plants treated with glyphosate than in Pythium inoculated plants not treated with glyphosate. In a hydroponic system, both Pythium spp. elicited accumulation of kievitone and phaseollin in root tissue, and both phytoalexins were exuded into the bathing solution. Glyphosate application did not significantly affect accumulation or exudation of phytoalexins by bean roots in the hydroponic system. The results from this study illustrate the nature and extent of phytoalexin production by bean roots in the absence and presence of microbes.

Sclerotial ontogeny, maturation, and aging in Sclerotium cepivorum are described using light and scanning electron microscopy. On potato dextrose agar, the mycelium spread, branching irregularly. Six days after inoculation sclerotial initials appeared, formed by hyphae branching and looping. From 6 to 8 days, the number and size of initials increased, and mucilagenous material appeared. By day 9, hyphal bundles formed in the mycelium. Between 9 and 11 days, spherical forms developed and the sclerotia grew. By day 12, an acellular matrix appeared, and to day 18 this matrix progressively obscured the surface hyphae and became black. A layer of ovoid rind cells developed at the surface. To examine the reduced survival of laboratory-produced compared with field-collected sclerotia, sclerotia from a variety of sources and conditions were compared. In general, the rind of sclerotia aged in dry conditions had a broken, irregular appearance versus fresh sclerotia or sclerotia aged under moist, axenic conditions. Sclerotia aged dry developed 1 to 4 layers of rind cells, while sclerotia kept moist developed only 1 or 2 layers. The structural and survival differences between laboratory-produced and natural sclerotia are attributable to differences in the moisture conditions under which they matured and aged. Key words: Sclerotium cepivorum, white rot, morphogenesis, sclerotia.

Diallyldisulfide (DADS) and onion seedlings grown from bacterized seeds significantly enhanced the germination of Sclerotium cepivorum sclerotia in muck soil compared with germination in the absence of seedlings or in the presence of seedlings grown from surface-sterilized nonbacterized seeds. Germination was greater for sclerotia on the soil surface than for buried sclerotia. When used for seed bacterization, Bacillus subtilis strain B-2 and four other bacterial strains obtained from rhizospheres of field-grown onions differed in their abilities to enhance germination of sclerotia. Strains B-2, UI-2, and B caused significantly greater germination of sclerotia than did strains UI-1 and W. Sclerotia in soil containing onion seedlings bacterized with B-2, UI-2, and B significantly supported reduced general indigenous bacterial and fungal populations. Treatment of soil with DADS reduced general indigenous bacterial but not fungal populations associated with sclerotia. There were significant inverse correlations between the proportions of sclerotia germinating and populations of bacteria and fungi associated with sclerotia of S. cepivorum. Key words: onion white rot, sclerotia, seed bacterization, mycosphere microflora, germination.

Summary: Résumé: ZusammenfassungSeedlings of wheat (Triticum aestivum L.) and beans (Phaseolus vulgaris L.) were less sensitive to glyphosate when grown in heat‐treated soil than in raw soil. Pythium spp. and Fusarium spp. were not detected in heat‐treated loam or muck soils at the time of glyphosate treatment, although fungi of several other genera were present. The efficacy of glyphosate on wheat or beans grown in heat‐treated loam soil was restored when untreated aqueous soil extracts were added to the heat‐treated soil. Bean seedlings grown in five different soil types varied in their sensitivity to glyphosate. The variation in LD50 among autoclaved soils was lower than that among raw soils. Between 13‐ to 47‐fold more glyphosate was required to kill the bean seedlings in any of the autoclaved soils compared with their corresponding raw soils. This differential effect was not observed on bean seedlings sprayed with either 2, 4‐D or paraquat. LD50 values for glyphosate on apple (Malus domestica Borkh.) seedlings growing in previously sterilized loam soil were reduced by inoculation of the soil with a representative Fusarium sp. or Pythium sp. obtained earlier from apple seedlings treated with glyphosate, but not by a Cylindrocarpon sp. from apple or Pythium ultimum Trow from glyphosate‐treated bean. The efficacy of glyphosate on bean, wheat or apple seedlings can be affected by changes in certain microbial components of the soil. Effet d'un traitement du sol a la chaleur et de la microflore sur l'efficacité du glyphosate sur plantules Des plantules de blé (Triticum aestivum L.) et de haricots (Phaseolus vulgaris L.) ont été moins sensibles au glyphosate quand elles sont cultivées dans des sols traités par la chaleur que dans des sols bruts. Pythium spp et Fusarium spp. n'ont pas été détectés dans la terre traitée à la chaleur ou dans des sols fumés au moment de l'application du glyphosate, bien que des champignons de quelques autres genres aient été présents. L'efficacité du glyphosate sur blé et haricots cultivés en sol traité par la chaleur a été récupérée quand des extraits aqueux de sols non traités ont été additionnés au sol traité. Les plantules de haricots cultivés dans 5 types de sols ont varié dans leur sensibilité au glyphosate. La variation des DL50 pour les sols autoclavés était plus faible en comparaison de celle des sols bruts. Il a fallu de 13 à 47 fois plus de glyphosate pour détruire les plantules de haricot dans tous les sols autoclaves que dans les sols bruts correspondants. Cet effet différentiel n'a pas été observé chez les plantules de haricots traités avec du 2,4 D ou du paraquat. Les valeurs de DL50 pour le glyphosate sur des plantules de pommiers (Malus domestica Borkh.) cultivées sur des sols avec de la terre précédemment stérilisée, ont été réduites par l'innoculation du sol avec un Fusarium sp. ou Pythium sp. obtenu plutôt à partir de plantules de pommier traitées au glyphosate, mais pas avec du Cylindrocarpon du pommier, ou un Pythium ultimum venant d'un haricot traité au glyphosate. L'efficacité du glyphosate sur plantules de haricot, blé ou pommier peut être affecté par des changements de la composition microbiologique du sol. Die Wirkung von Boden‐Hitzesterilisation und Mikroflora auf die Wirksamkeit von Glyphosat auf Keimpflanzen Keimpflanzen von Weizen (Triticum aestivum L.) und Buschbohne (Phaseolus vulgaris L.) waren, in hitzebehandeltem Boden wachsend, gegenüber Glyphosat weniger empfindlich als in unbehandeltem Boden. Pythium spp. und Fusarium spp. wurden in hitzebehandeltem Lehm‐ oder stark organischem Boden zur Glyphosatbehandlung nicht gefunden, obwohl andere Pilzgattungen vorhanden waren. Die Wirksamkeit des Glyphosats auf in hitzebehandeltem Boden gezogenen Weizen‐ oder Bohnenpflanzen stellte sich wieder ein, wenn unbehandelte wäßrige Bodenextrakte dem hitzebehandeltem Boden zugefügt wurden. Bohnenkeimpflanzen zeigten in 5 verschiedenen Bodenarten eine unterschiedliche Empfindlichkeit gegenüber Glyphosat. Die LD50 variierte in autoklavierten Böden weniger als in unbehandelten. Zur Abtötung der Bohnen‐keimpflanzen waren in den autoklavierten Böden 13‐ bis 47mal größere Wirkstoffmengen erforderlich als in den entsprechenden unbehandelten Böden. Diese Wirkungsunterschiede wurden bei mit 2,4‐D oder Paraquat behandelten Bohnenkeimpflanzen nicht beobachtet. Wurden Apfelsämlinge (Malus domestica Borkh.) in sterilisiertem Lehmboden angezogen, so sanken die LD50‐Werte für Glyphosat bei Inokulation des Bodens mit repräsentativen Fusarium‐ oder Pythium‐Arten, die zuvor von Glyphosat‐behandelten Apfelsämlingen gewonnen worden waren, jedoch nicht mit einer Cylindrosporium‐Art von Äpfeln oder Pythium ultimum Trow von Glyphosat‐gehandelteten Bohnen. Die Wirksamkeit von Glyphosat auf Keimpflanzen von Bohnen, Weizen oder Äpfeln kann durch Veränderungen bestimmter mikrobiologischer Bodenkomponenten beeinflußt werden.

Weeds are the most economically important of all pests with respect to sales of pesticides and limitations to crop yields (62). Herbicide sales represent more than two thirds of the 436 x 106 kg of pesticides used annually in United States (113), and almost one half of the $21 billion worldwide pesticide market (8). Numerous cases where an herbicide treatment influences disease outcome have been studied by plant pathologists as well as weed scientists. The concepts and the literature on interactions between herbicides, microorganisms, and plant diseases have been extensively reviewed (2-4, 40, 51, 59, 78, 129, 150). Most of the literature on herbicides deals with their direct effects on weeds and the effects of weed control on crops (Figure 1A). The effects of herbicides coming in direct contact with crops are considered secondarily in the weed-management literature (Figure 1B). The protection or predisposition of crops to disease that is sometimes observed following the use of herbicides is of particular interest to plant pathologists. As we document in this review, herbicides can alter soil ecosystems by having a direct effect on various components of the soil microflora, such as

Sclerotium cepivorum, the causal agent of white rot of Allium species, survives in soil by means of small, black, spherical sclerotia formed on the roots and bulbs of infected host plants. Factors responsible for the initiation of sclerotium formation in nature are unknown. When grown in culture on potato dextrose agar at 17 +/- 1-degrees-C, S. cepivorum forms sclerotia about 4 days after full colonization of the surface of the medium, regardless of the elapsed time from inoculation. Restriction of mycelial growth in S. cepivorum after a period of rapid growth and expansion initiates sclerotium formation. Growth may be restricted by physical, nutritional, competitive, or antagonistic factors. Physical injury to the mycelium and staling products do not induce sclerotium formation in S. cepivorum.

(1990). Comparative efficacy of induced resistance for selected diseases of greenhouse cucumber. Canadian Journal of Plant Pathology: Vol. 12, No. 1, pp. 16-24.

Marked strains of Bacillus subtilis B-2 and four selected rhizobacteria were introduced into onion rhizospheres by seed bacterization. Their effects on plant growth and their populations in root surface and root zone soil environments of field-grown onions were estimated 30 days following sowing. Populations of indigenous rhizosphere bacteria and fungi were also assessed by dilution plating. Seed bacterization with B-2 UI-1 and B caused significant increases in shoot height and shoot dry weights of onion seedlings over controls. Only UI-2 caused a significant increase in root dry weight. All of the strains survived in seedling rhizopheres in substantial numbers, but there were marked differences among the strains. Overall, UI-2 and B were comparatively good colonizers, UI-1 and W were intermediate and B-2 was poor. Seed bacterization treatments significantly reduced indigenous bacterial and fungal populations in seedling rhizospheres. Promotion of onion seedling growth by seed bacterization was not correlated with the relative persistance of the introduced bacterium, but may be related to the ability of the introduced bacterium to reduce components of the indigenous rhizosphere microflora.

Populations of a marked strain of the B-2 isolate of Bacillus subtilis (B-2) in the rhizospheres of onion seedlings grown from bacterized seeds in muck soil at various pH, moisture and temperature regimes were monitored for 14 weeks. Irrespective of regime, populations of B-2 in onion rhizospheres declined rapidly within the first 14 days after seeding, from 4.8 × 10 6 cfu per seed on day zero to a mean of 1.1 × 10 3 cfu per plant at day 14, and less rapidly during the next 12 weeks following seeding. An average population of 9.5 × 10 1 cfu per plant was recovered at 14 weeks following seeding. Within the pattern of general decline, survival of B-2 in the rhizosphere was favored by high temperature, high moisture and high pH regimes; temperature appeared to be the most important variable. Seed bacterization significantly increased shoot dry weight (12–94%), root dry weight (13–100%) and shoot height (12–40%) of onion seedlings over controls. Increases in shoot height and shoot weight were greatest at low temperature and high moisture, under all pH regimes. Root weight was similarly affected by temperature and moisture, but was significantly increased at pH 6.5 compared to 5.5 and 4.5. Though B. subtilis B-2 failed to maintain high populations in the onion rhizosphere, it nevertheless caused significant growth effects on bacterized onion seedlings. The observed growth effects were not proportional to rhizosphere populations of B-2.