Development of Pleospora allii on Garlic Debris Infected by Stemphylium vesicarium

Pseudothecia of Pleospora allii developed best on garlic leaf debris infected by Stemphylium vesicarium incubated at low temperature (5–10°C) and relative humidity (RH) close to saturation. RH of less than 96% prevented the formation of pseudothecia, while an incubation temperature of 15–20°C led to the early degeneration of pseudothecia. Under natural conditions, colonization by pseudothecia of unburied garlic leaf debris varied between seasons from 6.0 to 15.5 pseudothecia/mm2, whereas lower colonization levels were recorded when samples were buried. Pseudothecial maturity was reached 1–4mo after the deposition of garlic debris on the soil surface and 15 days after the burial of residues. In the later case, pseudothecia degenerated with degradation of the plant debris. Ascospore release, which required rainfall or dew periods, occurred between late January and late April depending upon the year. A high correlation was found between pseudothecia maturation and four meteorological variables. Two of which, i.e. the number of hours with RH98% and with a mean temperature of 4.5–10.5°C, and the accumulated rainfall, explained most variability (adjusted R2=0.82–0.98 depending upon the year). A multiple regression equation relating the pseudothecia maturity index with these two variables could be used to forecast the epidemic onset of Stemphylium leaf spots in Southern Spain. Temporal progress of pseudothecia maturation was best fitted by a monomolecular model.     

Germination of <Emphasis Type="Italic">Gibberella zeae</Emphasis> ascospores as affected by age of spores after discharge and environmental factors

The effects of age of ascospores (0–18 days after discharge), photon flux density (0–494 mol m^–2 s^–1 PAR), temperature (4–30 °C), frost (–15 °C for 30 min), relative humidity (RH; 0–100%), pH (2.5–6.5) and dryness (0 and 53% RH for up to 40 min) on the germination of the ascospores of the mycotoxin-producing fungus Gibberella zeae (anamorph Fusarium graminearum) were studied. Freshly discharged ascospores germinated within 4 h at 20 °C and 100% RH. The rate of germination and the percentage of viable ascospores decreased over time after the spores were discharged from perithecia. The time course of ascospore germination was not significantly affected by photon flux density. The period of time required to obtain 50% germinated ascospores at 100% RH was 26.90 h at 4 °C, 10.40 h at 14 °C, 3.44 h at 20 °C and 3.31 h at 30 °C. There was no significant effect of frost on the percentage of viable ascospores. A small percentage (6.6 ± 3.8%) of the ascospores germinated at 53% RH. At RH 84% and 20 °C almost 100% of the freshly discharged ascospores germinated. The time course of ascospore germination was affected by pH. The maximum rate of ascospore germination was estimated to be at pH 3.76. Ascospores lost their ability to germinate following exposure to 0% RH almost instantaneously. No germinating spores were detected after an incubation period of 1 min at 0% RH. Incubating the ascospores at 53% RH decreased the percentage of viable spores from 93 to 6% within 10 min. The data demonstrate that age of spores, relative humidity, temperature and pH, but not photon flux density, are key factors in germination of G. zeae ascospores.     

Barley Powdery Mildew Populations on Volunteers and Changes in Pathotype Frequencies During Summer on Artificially Inoculated Field Plots

Pathotype frequencies in barley powdery mildew populations were assessed in artificially inoculated barley plots. The field experiment was organised in eighteen 3 m × 3 m plots with different inoculum compositions, obtained by sequential inoculation with three isolates, gl-1, gl-2 and gl-3, shortly after seedling emergence. In the conidia populations before summer, pathotypes corresponding to the inoculated isolates were detected at frequencies in the range 11–42% for GL1, 0–14% for GL2 and 2–34% for GL3. On the volunteers appearing after harvest these three pathotypes were observed at lower frequencies: 0–37% for GL1, 0–12% for GL2 and 0–23% for GL3. The overall ranking of GL1, GL2 and GL3 frequencies was thus the same before and after summer. The populations on volunteers were influenced by both sexual and asexual populations present in the same field at the end of the previous growing season. However, at the small-scale level no simple correlation was found between the frequencies in the conidia populations on volunteers and those in the airborne population, or the conidia populations on the crop before summer, or the calculated expected frequencies in populations of ascospores. During the summer survival, chance events may also have a large influence on pathotype frequencies leading to a high variation between repeated events of transition from the crop to the volunteers.     

The Epidemiology of Purple Leaf Blotch on Leeks in Victoria, Australia

The incidence of purple leaf blotch disease was investigated on seven successive commercial leek crops grown at Cranbourne, Victoria between 1996 and 1997. First symptoms occurred on older leaves, 54–69 days after transplanting. Lesions with typical symptoms were colonised by either Alternaria porri (6%), Stemphylium vesicarium (42%) or mixtures of both pathogens (52%). Purple leaf blotch was caused by a disease complex and was endemic at nobreak Cranbourne due to the continuous cropping of leeks. Disease incidence in all monitored crops increased as plants matured (123–158 days after transplanting) until harvest but never exceeded 11% due to fortnightly applications of mancozeb. Disease levels showed no significant correlation with weekly temperature, precipitation, relative humidity or leaf wetness duration. Disease levels were significantly (P < 0.05) higher on autumn/winter (May/June) 1997 crops when 38 periods of leaf wetness 8 h because of dew and low temperatures (10–13 °C). The weekly rate of increase of disease incidence was significantly (P < 0.01) correlated with days after transplantation. nobreak Concentrations of airborne A. porri and S. vesicarium conidia within leek crops showed a diurnal periodicity and maximum numbers were trapped between 11:00 and 15:00 h. The concentration of airborne S. vesicarium conidia was three to six times the concentration of airborne A. porri conidia. Conidia were more abundant during spring/summer (September–February). Ascospores of Pleospora allii were found during May–September. The greater concentrations of airborne S. vesicarium conidia suggest that it may be the dominant pathogen in the purple leaf blight complex. Fungicide sprays were unnecessary until 8–10 weeks after transplanting, and regular protectant sprays curtailed but did not eradicate purple leaf blight. The results indicated that predictive models, based on temperature and the frequency of leaf wetness periods 8 h, will assist in reducing fungicide inputs as plants mature and, in southern Victoria, fungicide applications on leeks should be timed for autumn/winter when infection periods occur.     

Comparison of Physical, Chemical and Biological Methods of Controlling Garlic White Rot

Treatment of garlic cloves with tebuconazole (at 1ml of Folicur 25% l^–1) achieved a significant reduction in the rate of disease progress and the final incidence of plant death by Sclerotium cepivorum: garlic yields were improved. Although soil solarization provided the best control of garlic white rot, bringing soil populations of S. cepivorum to negligible levels, similar levels of disease control and garlic yields were achieved when tebuconazole was sprayed to stem bases of plants grown from cloves also treated with tebuconazole. This double treatment almost doubled the yield compared with untreated plants and significantly increased bulb quality under high disease pressure conditions. Soil solarization was also highly effective in a second consecutive crop of garlic, with significant improvements in yield and garlic quality. In contrast, lower levels of disease control were obtained when selected isolates of Trichoderma harzianum and Bacillus subtilis were applied to the soil and cloves respectively.     

Ascospores of the Japanese pear scab fungus (<Emphasis Type="Italic">Venturia nashicola</Emphasis> Tanaka &; Yamamoto) are discharged during the day

We investigated the diurnal pattern of ascospore discharge of the Japanese pear scab fungus (Venturia nashicola Tanaka & Yamamoto) in an orchard. Ascospores of V. nashicola were mainly discharged during the day. Most ascospores were discharged from 7:00 to 19:00: 99.6% in 2001, 99.3% in 2002, and 93.8% in 2005. Because the ascospores were discharged only when the fallen diseased leaves were wet from precipitation, the wetness of these leaves is probably imperative for spore discharge. Ascospore discharge began immediately after precipitation in the daytime. When it rained at night, however, ascospore discharge did not begin until the following morning and never began immediately after precipitation. We also investigated other meteorological factors. When fallen diseased leaves were wet, the percentage of ascospore discharge was positively correlated with the amount of solar radiation and atmospheric temperature and negatively correlated with relative humidity. Ascospore discharge was interrupted by a decrease in solar radiation and atmospheric temperature and by increased relative humidity at night. This report is the first that V. nashicola discharges ascospores primarily during the day.     

Effect of benomyl on soil fungi associated with rye. 1. Effect on the incidence of sharp eyespot caused by Rhizoctonia cerealis

Effects of benomyl on incidence of pathogens affecting the culm base of rye were studied in field trials and growth chamber experiments. Spraying of the crop with the fungicide at a high dosage (2.4 kg.ha^–1) resulted in a tenfold increase of sharp eyespot caused byRhizoctonia cerealis and reduced foot rot symptoms caused by fusaria by 50%. In a field trial at a low dosage (0.24 kg.ha^–1) a slight increase of sharp eyespot was observed. In one year, probably because of wet conditions during the infection period, sharp eyespot did not occur in either benomyl-treated or untreated plots, but eyespot caused byPseudocercosporella herpotrichoides was abundant. Its occurrence was reduced from 74% affected culm bases in untreated plots to 8% and 1% in plots that received 0.24 and 2.4 kg.ha^–1 of the fungicide, respectively.In growth chambers seedlings were grown in two sandy soils inoculated withR. cerealis. The soil was kept dry at about 35% of the moisture holding capacity. In plots with benomyl (1 mg.kg^–1; moisture content 11% of fresh weight), fewer seedlings emerged than in plots without the fungicide. This result was highly significant (P<0.01) for one soil but not for the other. The number of seedlings that remained free of disease symptoms was higher (P<0.01) in untreated than in fungicide-treated plots of both soils.Isolates of pathogens obtained from diseased culms were tested for their sensitivity to benomyl. Growth of all of them includingR. cerealis was inhibited, although not always completely suppressed, at 10 g.ml^–1 on potato-dextrose agar. ED₅₀ values of most isolates ofR. cerealis were between 2.2 and 3.1 g.ml^–1. The fungus was slightly but consistently less sensitive thanF. culmorum. Mycelial growth ofF. nivale was appreciably more sensitive than that of the otherFusarium spp. from cereals.P. herpotrichoides andF. nivale were the most sensitive pathogens tested with ED₅₀ values of <1 g.ml^–1. Accordingly,F. nivale was absent on culms from treated plots. In a growth chamber experiment, seedlings were protected from infection by supplying the fungicide (1 mg.kg^–1) to previously inoculated soil.In a laboratory assay the effect of benomyl on microbial antagonism toR. cerealis was estimated for rhizosphere soil. Enhanced incidence of sharp eyespot in treated crops was associated with adverese effects of the fungicide on microbial antagonism. There is presumptive evidence thatR. cerealis is suppressed by bacteria after wet periods during the vegetation period of the crop and by fungi after dry periods. Only fungal antagonism, which may be less effective, is affected by benomyl. The response to benomyl of the microflora in different soils varied. Reasons for this inconsistency are suggested.Samenvatting In veldproeven en in een klimaatkamer werd de invloed van benomyl op het optreden van voetziekten in rogge onderzocht. In veldjes die bespoten waren met een hoge dosis van het fungicide (in totaal 2.4 kg.ha^–1) bleken tienmaal zoveel halmen met scherpe oogvlekken, veroorzaakt doorRhizoctonia cerealis, voor te komen dan in onbespoten veldjes. Daarentegen was voetrot veroorzaakt doorFusarium-soorten met 50% verminderd. In een volgende veldproef, waarbij een voor de praktijk geadviseerde dosis (0.24 kg.ha^–1) was toegepast, werd een lichte toename van scherpe oogvlekken waargenomen.In een ander jaar trad scherpe oogvlekkenziekte in het geheel niet op, ook niet in met benomyl behandelde veldjes. De vochtige omstandigheden tijdens de infectieperiode zijn daarvan waarschijnlijk de oorzaak. Daarentegen werd de oogvlekkenziekte, welke doorPseudocercosporella herpotrichoides werd veroorzaakt, veel aangetroffen. In de onbehandelde veldjes waren 74% van de halmen aangetast tegen 8 en 1% in de veldjes die met het fungicide waren behandeld in doseringen van 0.24 en 2.4 kg.ha^–1.De invloed van het fungicide op de aantasting van kiemplanten werd in klimaatkamerproeven onderzocht. Daartoe werden twee zandgronden metR. cerealis geënt. De grond werd droog gehouden (op 35% van het waterhoudend vermogen). In grond met fungicide (1 mg.kg^–1) was de opkomst minder dan in grond zonder fungicide. Dit was zeer significant (P<0.01) voor één van de beide zandgronden, maar niet voor de andere. Het aantal gezonde kiemplanten was in beide gevallen duidelijk hoger (P<0.01) voor de onbehandelde grond.De isolaten van ziekteverwekkers uit aangetaste halmen werden op hun gevoeligheid voor het fungicide getoetst. Op aardappel-glucoseagar werden alle isolaten in hun groei geremd bij een benomyl-concentratie van 10 g.ml^–1.R. cerealis was iets minder gevoelig danF. culmorum. Voor het overgrote deel van de isolaten vanR. cerealis lag de ED₅₀ waarde tussen 2,2 en 3,1 g.ml^–1. De myceliumgroei vanF. nivale werd meer geremd dan die van de andereFusarium-soorten.P. herpotrichoides enF. nivale waren met een ED₅₀ waarde van <1 g.m.^–1 de gevoeligste pathogenen die uit de halmvoeten werden geïsoleerd. Dat de populatie vanF. nivale in benomylhoudende grond wordt onderdrukt, blijkt uit (1) het feit dat de schimmel niet voorkwam op halmen uit behandelde veldjes en (2) de bescherming tegen infectie van kiemplanten als aan de besmette grond fungicide (1 mg.kg^–1) was toegevoegd.In laboratoriumproeven werd de invloed van benomyl op het microbiële antagonisme in rhizosfeergrond tegenR. cerealis bepaald. Een toename in het optreden van scherpe oogvlekkenziekte in behandelde gewassen bleek gepaard te gaan met een remming van het antagonisme tegen de ziekteverwekker. Er zijn sterke aanwijzingen datR. cerealis na vochtige perioden tijdens de vegetatieperiode door bacteriën wordt onderdrukt en na droge perioden door schimmels. Het antagonisme van de laatste groep lijkt minder effectief te zijn en alleen dit antagonisme wordt door benomyl verlaagd. Tenslotte wordt een mogelijke oorzaak aangegeven voor de ongelijke respons op het fungicide van het microbieel antagonisme in verschillende gronden.     

Relation of Japanese <Emphasis Type="Italic">Xanthomonas campestris</Emphasis> pv. <Emphasis Type="Italic">vesicatoria</Emphasis> with worldwide strains revealed with three specific monoclonal antibodies

Strains of Xanthomonas campestris pv. vesicatoria Dye 1978 (Xcv), the causal agent of bacterial spot, have been classified into two groups based on their ability to hydrolyze starch. Three monoclonal antibodies (MAbs), 7AH10, 5HB3, and 4AD2, were produced immunized against the living bacteria and were specific to and could distinguish Xcv strains able or unable to hydrolyze starch (Amy⁺ or Amy^–). The MAb 7AH10, obtained against strain UPB141(Amy^–) reacted in an enzyme-linked immunosorbent assay with all the Amy^– strains (n = 19) and 1 of 11 Amy⁺ strains. Against Xcv 2625, an Amy^– unusual phenotype strain, MAb 5HB3, recognized 97% of our worldwide collection of Xcvs (n = 30). Also against that strain, the MAb 4AD2 reacted with none of the homologous Amy^– phenotypes and with 90% (n = 11) of the heterologous Amy⁺ phenotypes. For all the MAbs, cross reactions with other pathovars or species were less than 4% (n = 67). By assaying a Japanese collection of strains against the three MAbs, the Amy⁺ strains were distinguished from the Amy^– strains, and their relation with other world strains could be demonstrated. All the MAbs reacted with the lipopolysaccharide fraction of the bacterial cell wall during immunoblotting.     

Patterns of ascospore release in relation to phoma stem canker epidemiology in England (<Emphasis Type="Italic">Leptosphaeria maculans</Emphasis>) and Poland (<Emphasis Type="Italic">Leptosphaeria biglobosa</Emphasis>)

Experiments over five growing seasons at Rothamsted (1998/99–2002/03), four seasons at Boxworth (1998/99, 1999/2000, 2001/02, 2002/03) in England (Leptosphaeria maculans) and three seasons (1998/99–2000/01) at Poznan in Poland (Leptosphaeria biglobosa) suggest that differences in the development of phoma stem canker epidemics between England and Poland relate to differences in weather patterns between the two countries. The duration of ascospore release was longer in England, where winter weather is mild and wet, than in Poland, where winters are cold and often with snow cover, but there was little difference between two sites in England (Rothamsted and Boxworth). Wetness provided by rainfall was essential for release of ascospores of both L. maculans in England and L. biglobosa in Poland. Temperature did not affect release of ascospores over the range 5–20 °C. Diurnal periodicity in release of ascospores of L. maculans in England and L. biglobosa in Poland was similar. The timing (date) of first release of ascospores of L. maculans or L. biglobosa in autumn was related to rainfall in August and September; with increasing rainfall the date was earlier. The incubation periods from first release of ascospores to first appearance of phoma leaf spots for both L. maculans in England and L. biglobosa in Poland, and from first leaf spots to first stem base canker in England, were described using a thermal time (degree-day) approximation.     

Epidemiology of Fusarium Infection and Deoxynivalenol Content in Winter Wheat in the Rhineland, Germany

Details of our long-term research programme concerning the epidemiology of Fusarium spp. and mycotoxin production are summarized. Evaluation of the occurrence of Fusarium spp., mainly on winter wheat (Triticum aestivum), was carried out by investigating Fusarium infection and mycotoxin contamination. Two to 15% of grains were infested during 1995–1998 at three climatologically differing localities of the Rhineland, Germany. Disease progress was accelerated by rainfall during the flowering season. The species most frequently isolated were Fusarium avenaceum, F. poae, F. culmorum and F. graminearum. The mean deoxynivalenol (DON) content varied from 19gkg^–1 (1995) to 310gkg^–1 (1998) and was not always correlated with disease severity. Organic farming systems showed lower rates of infection with ear blight and lower mycotoxin contamination than conventional farming systems.     

Germination and invasion by ascospores and pycnidiospores of <Emphasis Type="Italic">Leptosphaeria maculans</Emphasis> on spring-type <Emphasis Type="Italic">Brassica napus</Emphasis> canola varieties with varying susceptibility to blackleg

The infection processes of ascospores and pycnidiospores of Leptosphaeria maculans were studied on cotyledons of six cultivars of spring-type Brassica napus: one with resistance controlled by a single dominant gene (cv. Surpass 400), three with polygenic resistance (cvs. Dunkeld, Grouse, and Outback), and two susceptible cultivars (Westar and Q2). On all cultivars, ascospore germination, penetration, and development of symptoms on cotyledons were much earlier than that with pycnidiospores. At 2h after inoculation ascospores began to germinate, by 4h about 50% had germinated, and by 6–8h 85%–90% had germinated. In contrast, pycnidiospores began to germinate 1 day after inoculation (dai) and reached only 50% germination by 3 dai. Ascospores began germinating from terminal cells and then later from the interstitial cells. Pycnidiospores germinated predominantly from one end and sometimes from both ends. Germ tubes from ascospores penetrated stomata as early as 4h after inoculation, whereas those from pycnidiospores penetrated at 2 dai. Symptom development with ascospores was 2 days earlier than that with pycnidiospores. Symptoms on Surpass 400 were evident as early as 3–5 dai with ascospores and 5–7 dai with pycnidiospores. However, on other cultivars, symptoms were not evident until 10 dai with ascospores and 12 dai with pycnidiospores. This report is the first on differences in the infection processes by the two spore types. Ascospore and pycnidiospore attachment, germination, and penetration did not differ between resistant and susceptible cultivars, but there were major differences after penetration. Under high humidity, 80%–90% of stomata of susceptible Westar and Q2 had aerial hyphae emerging from stomatal pores. However, fewer stomata (5%–10%) had aerial hyphae on Surpass 400 by 10 dai with ascospores and 12 dai with pycnidiospores, but even these were usually poorly developed. Host differences in spring-type B. napus in relation to production of aerial hyphae have not previously been reported. In Surpass 400, rapid necrosis of guard cells occurred within a few hours of penetration by either type of spore, and subsequently one or a few cells immediately adjacent to the penetration site died. This necrosis then spread to the cells around the penetration site to form a hypersensitive response (in the form of a small, dark lesion) to both ascospores and pycnidiospores. This is the first detailed report on interactions between spring-type B. napus and L. maculans in relation to single dominant gene-based resistance. Neither the cultivars with polygenic resistance nor the susceptible cultivars had such a response.     

Suppressive effect of potassium silicate on powdery mildew of strawberry in hydroponics

From 1996 to 1997, potassium silicate (SiO₂) was tested at 0, 25, 50, and 100mgl^–1 in hydroponics to control powdery mildew. Other elements were added in the usual amounts, and the strawberries were cultivated hydroponically in a greenhouse for 4 months (from October to January). The powdery mildew spread in the control plot, but little mildew developed in the plot with 25mgl^–1 silicate, and none in plots with more than 50mgl^–1 silicate. The suppressive effect lasted for about 4 months on fruits and even longer on leaves. On analysis of mineral content in the leaves, only the silicate content differed markedly between the control and treated plants. Nitrogen, phosphate, potassium, and calcium contents did not differ greatly. The maximum silicate content was about 24 times that of the control, and disease severity decreased significantly when the content was more than 1.5% in the leaves. The hardness of the strawberry leaves, measured by a creep meter, was increased by the silicate treatment.     

Growth and sporulation of <Emphasis Type="Italic">Stemphylium vesicarium</Emphasis>, the causal agent of brown spot of pear,on herb plants of orchard lawns

The inoculum sources of ascospores of Pleospora allii and of conidia of its anamorph Stemphylium vesicarium were investigated in relation to the brown spot disease epidemiology on pear. Dead and living leaves of three pear varieties (Abate Fétel, Conference and William), seven grasses (Poa pratensis, Festuca rubra, Festuca ovina, Lolium perenne, Digitaria sanguinalis and Setaria glauca) and Trifolium repens, which are used in pear orchard lawns, were inoculated with conidia of Stemphylium vesicarium virulent on pear and incubated under controlled-environment. Stemphylium vesicarium was always re-isolated from dead leaves of the considered plants, but not from symptomless green or yellowish living leaves. The fungus was occasionally re-isolated from leaf segments showing unspecific necrosis. Inoculation of pear leaves with isolates from grasses demonstrated that the fungus did not lose pathogenicity. Pseudothecia, ascospores and conidia were produced on all the dead inoculated leaves; differences between specimens were found for phenology of pseudothecia, their density and size, and for the number of conidia produced. Pseudothecia were produced faster in the lawn species than in pear leaves, and their density was higher, especially for S. glauca, L. perenne and P. pratensis. Ascospore maturation and ejection was more concentrated for the pseudothecia developed on pear leaves than for those on F. ovina and S. glauca. All the lawn species produced more conidia than pear leaves.     

Epidemiology of Grey Mould in Annual Waiting-bed Production of Strawberry

The epidemiology of Botrytis cinerea was studied in five annual strawberry crops using waiting-bed transplants, a system widely adopted in the Netherlands. On dead leaves of transplants the incidence of B. cinerea varied from 26.7% to 52.6%, but the leaf area with potential sporulation was low (3.5–15.6%). During each crop cycle, the availability of necrotic leaf substrate for spore production of B. cinerea was generally low and varied between seasons and with the quality of transplants. B. cinerea sporulated on a maximum of 15.5 cm² of leaf area per plant, measured as potential sporulation. The aerial concentration of B. cinerea conidia in untreated plots did not differ from the concentration in plots where all dead leaves had been removed, nor from the concentration at 25–50 m distance from the strawberry plots. B. cinerea incidence on flowers ranged from 5% to 96%, but no correlation was found with the potential spore production on necrotic leaves. Grey mould at harvest varied from 1.4% to 11.3% and was correlated with the average precipitation during the harvesting period but not with B. cinerea incidence on flowers. Post-harvest grey mould ranged from 2.1% to 32.6% and was correlated with petal colonisation by B. cinerea. The results suggest that in the annual cropping system with waiting-bed transplants, necrotic leaves are not a significant source of B. cinerea inoculum, unlike in other strawberry production systems. Therefore, control measures of grey mould in this annual system should focus on protection of flowers and young developing fruits, and not on the reduction of inoculum production on leaf debris.     

Epidemiological aspects of Didymella bryoniae,the cause of stem and fruit rot of cucumber

The survival ofDidymella bryoniae and the incidence of ascospores in glasshouses, outdoors and under controlled conditions were studied. The fungus was able to overwinter in the open as dormant mycelium. Dry and undecomposed crop residues remained a source of infection for more than one year. Moisture and a minimum temperature between 5 and 10°C were needed for fructification. For ascospore release a high relative humidity was not sufficient, the substrate had to be moist during a short period. Ascospores could be trapped throughout day and night both outdoors and in glasshouses, but there was a marked peak during a period of 3 h in the evening. Both on days with and without rain about the same numbers of ascospores were trapped from crop residues in the open. Ascospore release was favoured by watering the plants in the glasshouse. Under controlled conditions the release of ascospores was determined by humidity and not by light or darkness. In a cucumber crop in the glasshouse the first ascospores were trapped at about the same time the first symptoms on the plants appeared. In the glasshouse with introduced diseased plant debris, particularly when the debris became wet when the plants were watered, the disease was more severe and yield was less than in a glasshouse without introduced plant debris. Airborne ascospores may cause the primary infection of a cucumber crop. Therefore, hygienic measures must be taken to eliminate plant debris as source of infection, both in glasshouses and outdoors.     

Studies on the selective fungitoxicity of aliphatic amines

A comparison of the uptake of simple (C₁–C₄) aliphatic amines byPenicillium digitatum revealed that the fungitoxicity of (–)sec-butylamine [(–)SBA] was not due to its accumulation by hyphae since (+)SBA was accumulated to the same extent and methylamine, which showed negligible antifungal activity, accumulated to twice the level of SBA. Amines with a secondary alkyl structure were resistant to fungal metabolism, whereas primary amines were degraded to a significant extent byP. digitatum during a 4 h incubation period.(–)SBA accumulated in the fresh hyphae ofP. digitatum to a level 24 times higher than that in the culture medium containing 1 mol ml^–1; most of the SBA effluxed from the hyphae when transferred to fresh culture medium minus SBA. SBA did not accumulate when hyphae were incubated in N₂ or in the presence of respiratory inhibitors. The absorption and accumulation of SBA is characteristic of active transport.     

Morphological studies on attachment of spores of <Emphasis Type="Italic">Magnaporthe grisea</Emphasis> to the leaf surface of rice

Rice leaves were inoculated with spores of Magnaporthe grisea, and the number of fluorescence-labeled spores that attached to the leaf surface were counted before and after leaves were dipped and then stirred in water. Just 5% of the spores were retained on the leaf surface 1h after inoculation; the percentage retained then increased rapidly between 1.25 and 1.50h, and most had attached by 2h. Scanning electron microscopy revealed that most conidia were lying on a few wart-like protuberances 2–4µm high. Spores became attached when the germ tubes were long enough to reach the leaf surface, at least 3µm, by mucilaginous substances at the tip. Retained spores swayed when water was added under the cover glass from one side, indicating that the attachment was confined to the tips of germ tubes. Spores are attached to the rough leaf surface by mucilaginous substances – not at the tip of spore as reported on smooth artificial substrates but at the tip of the germ tubes.     

Induced Resistance to Cyst and Root-knot Nematodes in Cereals by DL-β-amino-n-butyric Acid

Foliar sprays and soil drenches with DL--amino-n-butyric acid (BABA) reduced the number of Heterodera avenae and H. latipons cysts on wheat and barley. Foliar sprays of wheat with 8000mgl^–1 BABA reduced the number of H. avenae cysts by 90%, whereas 2000mgl^–1 BABA was enough to reduce the number of H. latipons cysts by 79%. Multiple spray treatments with 2000mgl^–1 BABA at 10-day intervals reduced the number of H. avenae cysts on wheat and barley. A soil drench of wheat with 125mgl^–1 BABA reduced the number of H. latipons cysts by 93% and H. avenae cysts by 43%. Second-stage juveniles of these nematodes penetrated and formed syncytia in wheat roots soil-drenched with BABA. More adult males of H. avenae were produced in BABA (<250mg1^–1)-treated wheat roots (~76%) than in untreated roots (27%). Soil drenches with higher concentrations of BABA inhibited development of adult males and females. Several chemical elicitors of induced resistance were tested for their ability to reduce the number of H. avenae cysts on wheat. Only BABA was found to be an effective resistance inducer. The number of egg masses of an unidentified Meloidogyne sp. root-knot nematode, which infects only monocots, was also reduced by 95% by a soil drench of wheat with 500mgl^–1 BABA. Development of this nematode inside the BABA-treated roots was also inhibited.     

Sources and seasonal dynamics of inoculum for brown spot disease of pear

The dynamics of the production of Stemphylium vesicarium conidia and Pleospora allii ascospores from different inoculum sources on the ground were compared in a model system of a wildflower meadow mainly composed of yellow foxtail, creeping cinquefoil and white clover. The meadow was either inoculated (each October) or not inoculated with a virulent strain of S. vesicarium, and either covered or not covered with a litter of inoculated pear leaves. Spore traps positioned a few centimetres above the ground were exposed for 170 7-day periods between October 2003 and December 2006. Ascospores and conidia were trapped in 46 and 25% of samples, respectively. Ascospore numbers trapped from the pear leaf litter were about five times higher than those from the meadow, while conidial numbers were similar from the different inoculum sources. The ascosporic season was very long, with two main trapping periods: December–April, and August–October; the former was most important for the leaf litter, the latter for the meadow. The conidial season lasted from April to November, with 92% of conidia caught between July and September. The fungus persistently colonized the meadow: the meadow inoculated in early October 2003 produced spores until autumn 2006. The present work demonstrates that orchard ground is an important source of inoculum for brown spot of pear. Thus, it is important to reduce inoculum by managing the orchard ground all year long.     

Effect of Fungicides on Fusarium Head Blight and Deoxynivalenol Content in Durum Wheat Grain

In 1998–99 and 1999–2000 six trials were conducted to evaluate the effect of fungicides on Fusarium head blight in the field, on infected kernels and deoxynivalenol (DON) concentration in grain. A single application of prochloraz, tebuconazole, epoxiconazole or bromuconazole, applied to durum wheat varieties at the manufacturer's recommended dose at the beginning of anthesis stage, provided good control of the disease when infective pressure in the field was low to medium, and when the main pathogens were F. graminearum and F. culmorum. Kresoxim-methyl showed a low efficacy at controlling the disease. Tebuconazole, prochloraz and bromuconazole were effective at controlling F. graminearum and F. culmorum, while kresoxim-methyl was not effective in reducing Fusarium infected kernels. DON concentration in grain of cultivars inoculated with F. graminearum and F. culmorum was high, averaging 4.2mgkg^–1 (untreated control). Tebuconazole, prochloraz and bromuconazole reduced DON concentration by 43%, while epoxiconazole was ineffective. DON concentration in kernels of naturally infected cultivars was 1.95mgkg^–1, a concentration which exceeds the 1mgkg^–1 maximum level of contamination allowed in the United States. Furthermore prochloraz, bromuconazole and tebuconazole applications, in the naturally inoculated trials, reduced DON concentration from 73% to 96%, while epoxiconazole showed the lowest effectiveness. Moreover, a positive linear correlation between Fusarium infected grains and the DON concentration was observed.