Phoma Exigua Research Articles

Production of Phytotoxins by Phoma exigua var. exigua, a Potential Mycoherbicide against Perennial Thistles

The potential of the different Phoma exigua var. exigua strains for the biocontrol of the perennial weeds Sonchus arvensis and Cirsium arvense, occurring throughout temperate regions of the world, has been evaluated in previous studies. The majority of the above strains produced ascosonchine, a newly discovered enol tautomer of 4- pyridylpyruvic acid, whereas strains C-177 and S-9, though virulent to weeds, did not produce the above metabolite. In this study, it was demonstrated that the above two strains, grown in liquid and solid cultures, produced p-hydroxybenzaldehyde, cytochalasins B, F, Z2 and Z3, and deoxaphomin. When assayed on the leaves of both C. arvense and S. arvensis, p-hydroxybenzaldehyde was inactive, whereas deoxaphomin demonstrated the highest level of toxicity on leaves of S. arvensis. Cytochalasin Z2 appeared to be the less toxic cytochalasan on both plants according to the lack of the secondary hydroxyl group on C-7. Production of cytochalasins by P. exigua var. exigua strains isolated from C. arvense and S. arvensis is discussed in relation to chemotaxonomy and the biocontrol potential of the fungus.

First Report of Ascochyta Leaf Spot Caused by Phoma exigua var. exigua on Common Bean in Greece.

Common bean (Phaseolus vulgaris L.) is cultivated extensively in Greece for dry and fresh bean production. During 2005 and 2006, a disease with typical blight symptoms was observed occasionally on dark red kidney, brown kidney, and black bean plants in most bean-producing areas of Greece. It rarely was destructive unless the crop had been weakened by some unfavorable environmental conditions. Infected leaves had brown-to-black lesions that developed concentric zones 10 to 30 mm in diameter and also contained small, black pycnidia. Concentric dark gray-to-black lesions also appeared on branches, stems, nodes, and pods. Infected seeds turned brown to black. Plants sometimes showed defoliation and pod drop. The fungus was consistently isolated on potato dextrose agar from diseased leaves and pods and identified as Phoma exigua var. exigua Sutton and Waterstone on the basis of morphological characteristics of conidia and pycnidia (1,2). Spores were massed in pycnidia from which they were forced in long, pink tendrils under moist weather conditions. Conidia were cylindrical to oval, allantoid, hyaline, pale yellow to brown, usually one-celled, and 2 to 3 × 5 to 10 μm. To satisfy Koch's postulates, a conidial suspension (1 × 106 conidia per ml) of the fungus was sprayed onto leaves and stems of bean seedlings (first-leaf stage) (cv. Zargana Hrisoupolis). Both inoculated and control seedlings (inoculated with sterile water) were covered with plastic bags for 72 h in a greenhouse at 23°C. Inoculated plants showed characteristic symptoms of Ascochyta leaf spot 12 to 15 days after inoculation. The fungus was reisolated from lesions that developed on the leaves and stems of all inoculated plants. The pathogen is present worldwide on bean. To our knowledge, this is the first report of P. exigua var. exigua on common bean in Greece.

First report of Phoma exigua as a pathogen of hop in Slovenia

In August 2005, necrotic lesions on cones and leaves were observed on hop (Humulus lupulus) cvs Bobek, Magnum, and Merkur in the Koroška and Podravje regions of Slovenia. The symptoms on the leaves were initially small circular spots, which later increased in size (1–3 cm in diameter) and developed into oval, greyish white lesions. The affected cones showed reddish-brown necrotic areas on the tips of the bracts and bracteoles, which in some cases progressed throughout the entire cone. Microscopic examination of the affected tissue revealed the presence of irregularly scattered globose pycnidia (80–200 µm wide) with a distinct ostiolum. Conidia were hyaline, ellipsoid, occasionally one-septate, and measured 6·5 (4·5–8) × 2·5 (2–4) µm. Necrotic lesions were surface disinfected (1 min in 2% NaOCl), plated onto oatmeal agar (OA) and incubated at room temperature (~20°C). Isolated colonies produced scarce pycnidia of a Phoma species. On the basis of the characters in vitro on OA, malt agar (MA) and cherry-decoction agar (CA) (Boerema et al., 2004), the pathogen was identified as Phoma exigua. The identification was confirmed by sequencing the ITS region spanning ITS 1, 5·8S rDNA and ITS 2 of two isolates (2PEX and 3PEX) and one Phoma exigua var. exigua isolate (CBS 431·74). All isolates yielded the same 523 bp long sequences, which were identical (E=0·0) to Phoma exigua ITS sequences in GenBank (Altschul et al., 1997). The nucleotide sequences have been assigned GenBank Accession Nos. EF136399 (2PEX) and EF136400 (CBS 431·74). Pathogenicity tests were performed by spraying leaves and mature cones of detached lateral shoots (cvs Bobek and Merkur) with a spore and mycelial suspension (106 CFU per mL). Four bunches, each containing two lateral shoots, were used for each treatment. Bunches were covered with plastic bags and incubated in a growth chamber at 20ºC with relative humidity of 80%. The first lesions developed on leaves and cones of both cultivars 6 days after inoculation. Controls sprayed with distilled water showed no symptoms. The fungus was re-isolated from the lesions on leaves and cones. This is the first report of P. exigua causing severe damage to the leaves and cones of hop. We thank Mr M. Zolnir for assistance and information on the incidence of disease.

Phoma exigua Desm. var. exigua によるゴボウ根黒斑病（新称）

A new disease of edible burdock (Arctium lappa L.) was observed in Okayama Prefecture in 2002. The fungus consistently isolated from symptomatic roots was identified as Phoma exigua Desm. var. exigua on the basis of morphological characteristics of the fungus, color reaction with 1 N-NaOH on malt agar and pathogenicity on roots of edible burdock. Phoma black spot (Ne kokuhan-byo in Japanese) of edible burdock (Arctium lappa L.) is proposed for the new disease name.

Phylogenetic analysis of Phoma species

The cosmopolitan Phoma genus contains mainly phytopathogenic, opportunistic parasites, and saprophyte fungal species. Up to now, the characterization of Phoma species and other taxa of Phoma has been determined on the basis of morphology on standardized media, and gene sequence analysis was only used as a confirmative or distinctive complement.In this study, we tried to find molecular markers which can be used as phylogenetics markers in the molecular based classification in the Phoma genus.We employed a part of the translation elongation factor 1 subunit alpha (EF-1α=tef1) containing both introns and exons and ITS region containing the internal transcribed spacer regions 1 and 2 and the 5.8S rDNA, as potential genetic markers to infer phylogenetic relationships among different Phoma taxa. Twelve different Phoma species sequences were analysed together with the closely related Ascochyta ones. The constructed phylogenetic trees, based on tef1 and ITS sequences, do not support the traditional Phoma sections based on morphological characterization. However, we managed to distinguish between the Phoma strains and Ascochyta species by comparing their tef1 sequences through parsimony analysis. We proved that a tef1 can be a useful phylogenetic marker to resolve phylogenetic relationships at species level in Phoma genus.Both parsimony sequence analyses confirmed that the Phyllosticta sojicola species is identical to the Phoma exigua var. exigua species as Kövics et al. (1999) claimed. However, the evolutionary distance by ITS sequences within Phoma species is too small to get well based consequences for the phylogenetic relationships of Phoma genus.Further investigations would be necessary to clarify whether the tef1 and ITS sequences as phylogenetic molecular markers are well suited for the classification of Phoma species.

Development and Validation of Conventional and Quantitative Polymerase Chain Reaction Assays for the Detection of Storage Rot Potato Pathogens, Phytophthora erythroseptica, Pythium ultimum and Phoma foveata

AbstractThe diseases pink rot, watery wound rot and gangrene are important storage rot diseases of potato associated predominantly with Phytophthora erythroseptica (P.), Pythium ultimum (Py.) and Phoma exigua (Phoma) var. foveata respectively. Reliable molecular‐based diagnostic tests are required that will not only allow unequivocal identification of symptoms but will further advance epidemiological studies of these potato diseases to increase our understanding and contribute to more effective management and control strategies to the potato industry. Primers and probes were designed in specific regions of the internal transcribed spacer (ITS) regions to develop conventional and real‐time quantitative polymerase chain reaction (PCR) assays able to detect all possible fungal and oomycete pathogens causing pink rot, watery wound rot and gangrene. The specificity of each diagnostic assay was rigorously tested with over 500 fungal/oomycete plant pathogen isolates from potato and reference culture collections, and both conventional and real‐time PCR methods produced similar results. In terms of sensitivity, the detection limits for real‐time PCR went below ag DNA levels compared with pg DNA levels with conventional PCR. The real‐time PCR assays developed to detect Phoma foveata and Py. ultimum on tubers were suitable for the comparative Ct method (ΔΔCt) of quantification using the cytochrome oxidase gene of potato as a normalizer assay; an advantage as the need for a standard curve is eliminated. Each assay detected Phoma species (var. foveata or exigua) from naturally infected tubers showing symptoms of gangrene, and P. erythroseptica or Py. ultimum were also detected following inoculation of Russet Burbank tubers. Each diagnostic assay developed could reliably detect and distinguish between the pink rot, watery wound rot and gangrene‐causing potato pathogens.

Interceptação de Phoma exigua var. foveata, praga exótica e quarentenária para o Brasil, em germoplasma de batata procedente da França

No Laboratório de Quarentena Vegetal da Embrapa Recursos Genéticos e Biotecnologia, foram realizadas análises fitossanitárias em tubérculos de batata, introduzidos no país para pesquisa, procedentes da França. Alguns desses tubérculos, em exame direto sob microscópio estereoscópio, apresentaram lesões deprimidas de aspecto encharcado, coloração marrom-arroxeada, casca amolecida e picnídios negros sobre as lesões. Em lâminas, sob microscópio de luz, foram observadas características morfológicas do fungo Phoma exigua var. foveata, praga exótica e de quarentena A1 para o Brasil. Para o teste de patogenicidade foram utilizados tubérculos sadios de batata inoculados com cultura pura do fungo isolado dos tubérculos sintomáticos. Sintomas de apodrecimento foram observados após 3 dias da inoculação, evoluindo até o 8º dia, quando o fungo foi re-isolado dessas lesões, em BDA, e confirmado o agente etiológico da doença. Os tubérculos apresentando sintomas foram incinerados e aqueles aparentemente sadios foram tratados com fungicida sistêmico, plantados em quarentenário por duas gerações e re-analisados. Após confirmação da sanidade dos tubérculos, os mesmos foram enviados ao requisitante.

Effects of culture media, temperature, pH, and light on growth, sporulation, germination, and bioherbicidal efficacy of Phoma exigua, a potential biological control agent for salal (Gaultheria shallon)

In order to evaluate the potential use of Phoma exigua isolate PFC 2705 (PFC2705) as a biological control agent for salal (Gaultheria shallon), effect of cultural and environmental parameters on growth, conidia production, and pathogenicity of P. exigua were characterized in studies conducted under laboratory and greenhouse conditions. Within a range of 5–30°C, the optimum growth and germination temperature range was 20–25°C. The effect of pH on mycelial growth and conidial germination was not significant from pH 5 to 10. Fluorescent light significantly enhanced sporulation of the fungus on most agar media tested, yet was not necessary for growth. The type of culture media significantly affected mycelium growth, sporulation, and conidia germination. Age of mycelia used as inoculum affected the disease severity on salal. PFC2705 suppressed the growth of mature salal plant by inciting lesions on leaves, branch tips, and axillary buds and caused 56% death of the total biomass above ground. Characteristics such as easy inoculum production, wide range of growth environments, and high infectivity on salal increased the potential of P. exigua as a biocontrol agent for management of salal.

Phoma Basal Rot of Romaine Lettuce in California Caused by Phoma exigua: Occurrence, Characterization, and Control.

Beginning in 2000 and continuing into 2004, a previously undescribed disease caused significant damage to romaine lettuce (Lactuca sativa) in the coastal Salinas Valley of California. Symptoms were brownish black, sunken cavities on the crown and upper taproot. Cavities were firm, lacked signs of fungal growth, and resulted in cracking and weakening of the crown. Affected plants were stunted, uneven in growth, and unmarketable. Crisphead lettuce also developed these symptoms, although disease incidence was always lower than that for romaine. A fungus was consistently isolated from cavities. Using morphological features and molecular methods, the fungus was identified as Phoma exigua. Pathogenicity of isolates from romaine and crisphead lettuce was demonstrated on both of these lettuce types. Replicated field studies showed that azoxystrobin, boscalid, and cyprodinil + fludioxonil fungicides effectively controlled the disease. This is the first report of a crown disease caused by P. exigua in the United States. A similar disease, Phoma basal rot on greenhouse grown lettuce in the United Kingdom, had been previously observed, but no report has been published.

First Report of Leaf Spot Caused by Phoma exigua on Hydrangea macrophylla in Italy.

Hydrangea macrophylla is grown in Italy as a potted plant and also for landscaping. During the fall of 2005 in a nursery located in Lazio (central Italy), a severe foliar disease was observed on 12-month-old potted plants of cv. Hanabi. Small necrotic spots surrounded by chlorotic haloes were observed on the upper side of infected leaves. At temperatures near 20°C and relative humidity ranging between 80 to 90%, spots enlarged to form round areas 2 to 7 cm in diameter that were well defined by a brown margin. Severely infected leaves became chlorotic and abscised. Heavily affected plants were defoliated. Infected plants rarely died, but the presence of lesions on mature plants decreased aesthetic quality and subsequent market value. The disease occurred on 30% of the plants in one nursery. Stems and flowers were not affected by the disease. From infected leaves, a fungus was consistently isolated on potato dextrose agar with 25 mg/liter of streptomycin added. The fungus was grown on leaf extract agar, 30 g of leaves per liter, and maintained at 22°C (12 h of light and 12 h of dark). After 30 days, black pycnidia 275 to 255 μm in diameter developed, releasing conidia that were hyaline, elliptical, nonseptate, and measuring 4.6 to 7.6 (average 6.0) × 1.4 to 4.2 (average 2.6) μm. On oatmeal agar, the addition of a drop of concentrated NaOH caused a positive reaction, turning the medium red (2). On the basis of its morphological characteristics, the fungus was identified as a Phoma sp. The ITS (internal transcribed spacer) region of rDNA was amplified using the primers ITS4/ITS6 (3) and sequenced. BLASTn analysis (1) of the 560 bp obtained showing an E-value of 0.0 with Phoma exigua. The nucleotide sequence has been assigned GenBank Accession No. DQ384612. Pathogenicity tests were performed by spraying leaves of healthy 6-month-old potted H. macrophylla (cvs. Hanabi and Zaffiro) plants with a spore and mycelial suspension (105 CFU/ml). Plants without inoculation served as controls. Five plants were used for each treatment. Plants were covered with plastic bags for 5 days after inoculation and kept in a growth chamber at 20°C with relative humidity at 80 to 90%. The first lesions developed on leaves of cv. Hanabi 12 days after inoculation, while control plants remained healthy. Lesions did not develop on inoculated cv. Zaffiro plants. The fungus was consistently reisolated from the lesions of cv. Hanabi. The pathogenicity test was carried out twice. The presence of P. exigua on H. macrophylla has been reported in the United States (4). In Italy, the disease can be found in a limited area.

Clavibacter michiganensis subsp. sepedonicus

Clavibacter michiganensis<i> subsp. </i>sepedonicus

Lung mass caused byPhoma exigua

We describe a well documented case of a fatal Phoma exigua lung infection in a patient with acute myeloid leukaemia and diabetes. The infection was histology and culture proven and a PCR assay was developed for detecting P. exigua DNA in deparaffinized lung-biopsy material. Confirmation of rare fungal pathogen infections requires a multidisciplinary approach involving clinical observations, mycology, histopathology and radiology.

Differential accumulation of monolignol-derived compounds in elicited flax (Linum usitatissimum) cell suspension cultures

Lignin and lignans share monolignols as common precursors and are both potentially involved in plant defence against pathogens. In this study, we investigated the effects of fungal elicitors on lignin and lignan metabolism in flax (Linum usitatissimum) cell suspensions. Cell suspension cultures of flax were treated with elicitor preparations made from mycelium extracts of Botrytis cinerea, Phoma exigua and Fusarium oxysporum F ssp lini. Elicitors induced a rapid stimulation of the monolignol pathway, as confirmed by the increase in PAL (phenylalanine ammonia-lyase, EC 4.1.3.5), CCR (cinnamoyl-CoA reductase EC 1.2.1.44) and CAD (cinnamyl alcohol dehydrogenase EC 1.1.1.195) gene expression and PAL activity. At the same time, CCR activity only increased significantly in F. oxysporum-treated cells 24 h post elicitation. On the other hand, CAD activity measured for coniferyl alcohol formation was transiently decreased but a substrate-specific activation of CAD activity was observed in F. oxysporum-treated cells when using sinapyl alcohol as substrate. The accumulation of monolignol-derived products varied according to the elicitor used. B. cinerea or P. exigua-elicited cell cultures were characterised by a reinforcement of the cell wall by a deposit of 8-O-4'-linked non-condensed lignin structures and phenolic monomers, while at the same time no stimulation of 8-8'-linked lignan or 8-5'-linked phenylcoumaran lignan accumulation was observed. Additionally, elicitation of cell cultures with F. oxysporum extracts even triggered a strong incorporation of monolignols in the non condensed labile ether-linked lignin fraction concomitantly with a decrease in lignan and phenylcoumaran lignan accumulation. Several hypotheses are proposed to explain the putative role of these compounds in the defence response of flax cells against pathogens.

First Report of Phoma exigua var. heteromorpha Causing Oleander Dieback in Spain.

During the autumn of 2003, a new disease was detected in oleander (Nerium oleander L.) nurseries in Valencia and Murcia in eastern Spain. Affected leaves showed ovoid or ellipsoid necrotic spots. Necrotic lesions were also observed on stems and lateral shoots that resulted in severe blight and defoliation. In some cases, severe infections caused the death of plants. Isolations from symptomatic leaves and stems onto potato dextrose agar (PDA) supplemented with 0.5 g liter-1 of streptomycin sulfate (PDAS) consistently yielded dark olivaceous fungal colonies. For sporulation, these isolates were transferred to potato carrot agar (PCA) and incubated at 25°C for 15 days with a 12-h photoperiod. Abundant pycnidia (200 μm in diameter) developed superficially or immersed in the culture medium. Conidia were hyaline, ellipsoid or cylindrical, guttulate, and occasionally, one septate. Conidial dimensions were 6.1 to 9.6 × 2.2 to 3.2 μm (average 6.2 × 2.8 μm). The addition of a drop of concentrated NaOH to the cultures gave a blue-green pigmentation to the agar changing to brown-red. On the basis of cultural characteristics and fungal morphology, the isolates were identified as Phoma exigua Desmaz. This identification was confirmed by sequencing the complete internal transcribed spacer regions 1 and 2, including the 5.8S ribosomal DNA of isolate Pho 6 (GenBank Accession No. AY899262). This sequence was identical to sequences in GenBank from well-characterized strains of P. exigua (1). Pathogenicity tests were conducted on 9-month-old oleander plants (cv. Splendens Gigantium) using three isolates of P. exigua from different locations. Two methods of inoculation were used. Leaves were spray inoculated with an aqueous suspension (1.5 × 105 conidia per ml) of conidia harvested from 15-day-old cultures grown on PCA, or a 5-mm-diameter agar disc, cut from the margin of an 8-day-old culture growing on PCA, was inserted mycelium side down in a stem wound made with a sterile scalpel and sealed with Parafilm. Controls were inoculated with sterile distilled water or sterile PCA discs. There were five replicates for each isolate and inoculation method with an equal number of uninoculated plants. After inoculation, all plants were covered separately with plastic bags for 48 h to maintain high humidity. Plants were maintained in a greenhouse at 20 to 30°C. Within 5 to 15 days after inoculation, symptoms developed that were similar to those observed in the nurseries. The fungus was reisolated from the stems and leaves of all inoculated plants, completing Koch's postulates. On the basis of ITS sequence data and the host from which they were isolated, our isolates were identified as P. exigua var. heteromorpha (Sch. et Sacc.) Noordeloos et Boerema (2,3). This disease has been previously reported to cause severe damage to oleander in France, California, Italy, and the Netherlands. To our knowledge, this is the first report of P. exigua var. heteromorpha on oleander in Spain.

First Report of Phoma exigua as a Pathogen of Salal (Gaultheria shallon) in British Columbia, Canada.

Salal (Gaultheria shallon Pursh.) is an ericaceous, evergreen, and rhizomatous shrub that competes for nutrients and moisture with young conifers in low elevation, coastal British Columbia (BC). A survey was conducted on southern Vancouver Island, BC during the summer of 1999 to find fungal pathogens of salal that might serve as biocontrol organisms (3). Phoma exigua Desmaz. (isolate PFC2705) near Parksville, BC proved to be pathogenic on salal. Identification of PFC2705 at the Centraalbureau voor Schimmelcultures was based on morphology and ITS sequences (GenBank Accession No. AY927784). Pathogenicity was determined with 24 salal seedlings (3-month-old) by inoculating with mycelial suspensions (20% v/v) or conidial suspensions (1 × 106 conidia per ml in 0.5% potato dextrose broth). Inoculated seedlings were placed in plastic bags and incubated in a greenhouse (16 to 23°C with natural light). Plastic bags were removed after 2 days. Initial disease symptoms were observed 2 days after inoculation. Brown, sunken lesions appeared on the surface of young leaves and stems and extended quickly. All seedlings were killed within 14 days. Twelve control plants showed no disease symptoms. With diseased salal leaves incubated at 23°C with 12-h fluorescent light/dark and 100% relative humidity, pycnidia appeared on leaf surfaces within 5 days. Conidia were hyaline, ellipsoid, one-celled, sometimes two- to three-celled, 2.5 to 3.8 × 5 to 12.5 μm, with a rounded base; the colony was gray or dark gray on potato dextrose agar after 5 to 7 days. Reisolation from the inoculated diseased leaves produced a mycelial colony that shared the same growth and morphological characteristics as the initial isolate. Phyllosticta gaultheriae Ellis & Everh., a widely reported foliar pathogen of salal, is distinct morphologically from P. exigua (1). To our knowledge, this is the first report of P. exigua as a pathogen of salal in Canada (2). A voucher specimen has been deposited at the Pacific Forestry Center Herbarium (DAVFP No. 28735).

Effects of potato dextrose broth and gelatin on germination and efficacy of Phoma exigua, a potential biocontrol agent for salal (Gaultheria shallon)

The role of a combination of four gelatin concentration levels (0.0%, 0.2%, 0.5%, and 0.8%) and eight potato dextrose broth (PDB) concentration levels (0.0%, 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%) on water evaporation, water absorption, conidial germination, and biocontrol efficacy of conidia of Phoma exigua isolate PFC2705 for salal was evaluated. PDB and gelatin reduced water evaporation. After 3 h of evaporation at 23 °C and 22% RH, the water content of gelatin–PDB combinations increased compared with PDB alone. The relationships between the amounts of absorbed water and PDB concentration at all gelatin concentration levels were linear (p < 0.001) after 60 min in a 100% RH environment. After 240 min, the mass of absorbed water was 11.8 times the dry mass of the 0.5% gelatin and 1.5% PDB combination, and this relationship did not change significantly over the following 16 h. PDB and gelatin increased conidial germination and might have provided nutrition for conidia of P. exigua PFC2705 at different RH environments. Conidia of PFC2705 did not germinate in sterilized ion-free water, germinated somewhat (4% to 10%) in gelatin suspensions, and germinated well (average 89%) in suspensions of different concentrations of PDB or gelatin–PDB combinations after 24 h at 23 °C. The average conidial germination at 24 h was 76% in 100% RH, 12% in 97.5% RH, and 0.0% at 92.5% RH; at 48 h it was 44.4% in 97.5% RH and 20% in 92.5% RH. No conidia germinated when RH was lower than 92.5% over 48 h. Conidia in the gelatin–PDB combinations smeared on slides stored for 4 weeks at 23 °C with 22% RH had an average conidial germination rate of 20% after 16 h incubation at 100% RH. Gelatin–PDB combinations with 0.5% gelatin and 1.5% to 2.5% PDB caused significantly larger lesions on detached young salal leaves (20 to 25 days old) than other gelatin–PDB combinations.

British Columbia Regional Meeting, 2004

British Columbia Regional Meeting, 2004

Selected cultural and environmental parameters influence disease severity of dandelion caused by the potential bioherbicidal fungi, Phoma herbarum and Phoma exigua

Selected cultural and environmental variables were investigated for their influence on the efficacy of Phoma herbarum and Phoma exigua to cause disease on dandelion (Taraxacum officinale) under growth room conditions. In both species, mycelial fragments caused significantly greater disease severity on dandelion than spore suspensions. Mycelial age was not an important factor in disease severity caused by P. herbarum, with all cultures causing high disease ratings. However, younger cultures of P. exigua caused the greatest disease severity on dandelion, but significantly less than that caused by P. herbarum. The initial pH of the growth medium (potato dextrose broth) did not affect disease severity caused by either Phoma species. Increasing concentrations of mycelia of P. herbarum were applied to dandelions that were then exposed to various leaf wetness durations. Disease severity increased with increasing leaf wetness duration. For dandelions exposed to no leaf wetness duration, the greater the mycelial concentration, the greater the disease rating. However, for dandelions exposed to all leaf wetness durations, all concentrations of mycelia caused similar disease ratings. As P. herbarum caused high disease ratings on dandelion, it therefore warrants further investigation as a potential bioherbicide for the control of this weed.

Pathogenesis of Potato Gangrene Caused by Phoma exigua var. foveata: II. Activities of some Hydrolases and Dehydrogenases

AbstractThe location of enzyme activity in gangrene‐diseased tubers was determined using the nitrocellulose blotting method. The activity of aminopeptidase and esterase was located in tissues adjacent to dry rot caused by Phoma exigua var. foveata and in other apparently healthy tissues. The activity of glucuronidase, succinic and glucose‐6‐phosphate dehydrogenases (G‐6‐PDH), however, was confined to tissues adjacent to the rotted tissue. The pathogen produces very active α‐ and β‐glycosidases, so their highest activity occurred in rotten tissue that was filled with fungal mycelium. Results suggest that all these enzymes are involved in alteration of cell metabolism and the destruction of diseased tuber tissue.

In vitro culturing of yellow starthistle (Centaurea solstitialis) for screening biological control agents

Abstract In an effort to design a bioassay to screen for biocontrol pathogens and their toxins, procedures were developed to produce calli of yellow starthistle (YST), Centaurea solstitialis (Asteraceae), on solid and liquid media. Three Murashige and Skoog (MS) media with different hormone additives were compared for their effects on the growth of the YST calli. The most effective medium was made of MS salts supplemented with two cytokinins and a low rate of auxin. The growth of the calli slowed when cytokinins were omitted. Similar results were observed in a bioassay setup to compare the reactions of the YST calli with those of detached leaves exposed to potential toxins in the culture filtrates of test fungi. After exposure for 96 h to different concentrations of culture filtrates from three Alternaria alternata isolates, the calli exposed to 100 and 50% concentrations of the filtrates were significantly more damaged than other concentrations tested, based on a visual rating scale. No differences were observed in the damage ratings among the A. alternata isolates tested. An additional bioassay was setup to determine if calli could be used to screen pathogenic fungi. After 48 h, a distinction could be made in a visual rating between calli treated with spores of a pathogen, Phoma exigua , and a nonpathogen, Penicillium sp., confirming that YST calli can be used to screen potential pathogenic fungi against YST.