Influence of relative humidity and temperature on development of Didymella rabiei on chickpea debris

Didymella rabiei grew saprophytically on pieces of artificially and naturally infected chickpea stem debris under artificial incubation conditions, and formed pseudothecia and pycnidia. The extent of growth was not significantly affected by temperature of incubation within the range 5–25°C, but was significantly reduced as relative humidity (RH) decreased from 100% to 86%, when no growth occurred. Pseudothecia matured at 10°C and constant 100% RH, or at 5 and 10°C and alternating 100%/34% RH. Under these conditions, pseudothecial maturation, assessed by a pseudothecia maturity index, increased over time according to the logistic model. For temperatures higher than 10°C or RH lower than 100%, pseudothecia either did not form ascospores, or ascopores did not mature and their content degenerated. When pseudothecia that initially developed to a given developmental stage were further incubated at a constant 100% RH, temperature became less limiting for complete pseudothecial development as the developmental stage was more advanced. Pycnidia of the fungus developed and formed viable conidia in all environmental conditions studied, except at 86% RH. However, the density of pycnidia formed and the number of viable conidia per pycnidium were significantly influenced by temperature, RH and the type of debris (artificially or naturally infected) used.     

Effects of temperature on ascospore germination and penetration of oilseed rape (Brassica napus) leaves by A- or B-group Leptosphaeria maculans (phoma stem canker)

Ascospores of both A-group and B-group Leptosphaeria maculans germinated at temperatures from 5 to 20°C on leaves of oilseed rape. Germination of ascospores of both groups started 2 h after inoculation and percentage germination reached its maximum about 14 h after inoculation at all temperatures. Both the percentage of A-/B-group ascospores that had germinated after 24 h incubation and germ tube length increased with increasing temperature from 5 to 20°C. Germ tubes from B-group ascospores were longer than those from A-group ascospores at all temperatures, with the greatest difference at 20°C. Hyphae from ascospores of both groups penetrated the leaves predominantly through stomata, at temperatures from 5 to 20°C. A-group ascospores produced highly branched hyphae that grew tortuously, whereas B-group ascospores produced long, straight hyphae. The percentage of germinated ascospores that penetrated stomata increased with increasing temperature from 5 to 20°C and was greater for A-group than for B-group L. maculans after 40 h incubation.     

Saprophytic Activity and Sporulation of Cryphonectria parasitica on Dead Chestnut Wood in Forests with Naturally Established Hypovirulence

ABSTRACT Sustainable biological control of the chestnut blight fungus Crypho-nectria parasitica with hypovirulence depends on the production and dissemination of hypovirus-infected propagules of the pathogen. We investigated the ability of C. parasitica to sporulate and produce hypo-virus-infected spores on recently dead chestnut wood in coppice stands in southern Switzerland where hypovirulence has been naturally established. The number and type (active, inactive, or none) of cankers was assessed on experimentally cut and stacked stems, firewood stacks, and natural dead wood. Hypovirus-free and hypovirus-infected strains readily survived for more than 1 year in the chestnut blight cankers of the stacked stems. Sporulation of C. parasitica was observed on the surface of preexisting inactive and active cankers, as well as on newly colonized bark areas and was significantly more abundant than on comparable cankers on living stems. On all types of dead wood, we observed more stromata with perithecia than with pycnidia; however, a large proportion of the stromata was not differentiated. All perithecia examined yielded only hypovirus-free ascospores. The incidence of pycnidia that produced hypovirus-infected conidia ranged from 5% on natural dead wood to 41% on the experimental stacks. The mean virus transmission rate into conidia was 69%. Our study demonstrates a considerable saprophytic activity of C. parasitica on recently dead chestnut wood and supports the hypothesis of a role of this saprophytic phase in the epidemiology of hypovirulence.     

Factors affecting the severity of bacterial canker of pear caused by Pseudomonas syringae pv. syringae

Several factors affecting the severity of bacterial canker of pear were studied. In the orchard, infection of shoots by Pseudomonas syringae pv. syringae occurred only when the inoculum dose exceeded 10⁶ colony-forming units/shoot. However, under favourable conditions in a growth chamber, cankers formed on detached shoots inoculated with 5 cfu/shoot. A second-order polynomial relationship was established between log₁₀ transformed canker length and log₁₀ transformed inoculum dose. In orchard and growth chamber experiments, shoots were susceptible from the time of bud swell until after fruit harvest. The severity of Pseudomonas canker of detached shoots increased if they were frozen at – 10°C for 24 h before inoculation. Shoots were most susceptible when inoculated immediately after wounding, and no cankers developed in the orchard when 3-day-old wounds were inoculated. Additionally, no cankers resulted from inoculation of leaf scars at leaf drop. Actively growing, current-season shoots were more susceptible than shoots that had set a terminal bud. The practical implications of these results are discussed as a basis for control of bacterial canker of pear.     

Comparative epidemiology of Pyrenopeziza brassicae (light leaf spot) ascospores and conidia from Polish and UK populations

Despite differences in climate and in timing of light leaf spot epidemics between Poland and the UK, experiments provided no evidence that there are epidemiological differences between populations of Pyrenopeziza brassicae in the two countries. Ascospores of Polish or UK P. brassicae isolates germinated on water agar at temperatures from 8 to 24°C. After 12 h of incubation, percentages of ascospores that germinated were greatest at 16°C: 85% (Polish isolates) and 86% (UK isolates). The percentage germination reached 100% after 80 h of incubation at all temperatures tested. The rate of increase in germ tube length increased with increasing temperature from 8 to 20°C but decreased from 20 to 24°C, for both Polish and UK isolates. Percentage germination and germ tube lengths of UK P. brassicae ascospores were less affected by temperature than those of conidia. P. brassicae produced conidia on oilseed rape leaves inoculated with ascospores or conidia of Polish or UK isolates at 16°C with leaf wetness durations from 6 to 72 h, with most sporulation after 48 or 72 h wetness. Detection of both mating types of P. brassicae and production of mature apothecia on leaves inoculated with mixed Polish populations suggest that sexual reproduction does occur in Poland, as in the UK.     

Effects of environmental factors on discharge and germination of ascospores of Venturia nashicola

Experiments were conducted under controlled environmental conditions to study the effects of temperature, duration of wetness, relative humidity (RH) and light on the discharge and germination of ascospores of Venturia nashicola , the causal agent of pear scab in China. Discharge of ascospores from pseudothecia required free water or 100% RH. A period of soaking in water as short as 10 s was sufficient to initiate the discharge of ascospores. Temperatures from 10 to 30°C did not significantly affect the temporal trend of ascospore discharge. A greater proportion of ascospores was discharged under light than in the dark. However, a period of light as short as 10 min, either during the initial wetting of pseudothecia or interrupting the darkness, was sufficient to reduce the inhibitory effect of darkness on ascospore discharge. Ascospores were discharged within 10 min after pseudothecia were wetted and most ascospores ( c. 80%) were discharged within the first hour. The temporal pattern of ascospore discharge could be well described by a logistic model, which estimated that 50% of ascospores were discharged within half an hour of wetting. Ascospores germinated over a wide range of temperatures from 5 to 30°C, with an optimum at c . 20°C. Temporal dynamics of ascospore germination at six temperatures (5, 10, 15, 20, 25 and 30°C) were satisfactorily described by logistic models.     

Maturation and Seasonal Discharge Pattern of Ascospores of Anisogramma anomala

ABSTRACT Maturation and release of ascospores of Anisogramma anomala were monitored over a 6-year period (1988 to 1995) in European hazelnut orchards located in western Oregon. Perithecia of A. anomala were dissected from stromata collected monthly from September to May to determine spore maturation. Spore maturation began in late summer; by January, >90% of the spores were morphologically mature. Similarly, both the number of mature ascospores per perithecium and the proportion of ascospores that germinated increased through autumn. After January, the number of spores per perithecium declined until May, when few viable spores remained. Each of the 6 years, rain catch-type spore traps were placed under cankers in diseased trees from 15 September to 30 June. Based on spore collection periods of 1 to 4 weeks, three patterns for the seasonal release of A. anomala ascospores were observed: in the 1988-1989 season, >80% of the seasonal ascospore release occurred between September and January; in the 1989-1990 season, 32 to 42% of the seasonal ascospore release occurred after mid-April; and in the other 4 years, monthly releases of ascospores were relatively uniform over the 9-month seasonal period. Timing and amount of precipitation were the most important variables accounting for the differences among the yearly patterns of ascospore release. Over all years and sites, the cumulative proportion of total ascospores collected in each orchard was highly correlated (R(2) = 0.90) with cumulative precipitation. This relationship was confirmed in mist chamber experiments. A regression model was developed relating cumulative ascospore release to cumulative hours of precipitation. The model provides an estimate of the proportion of ascospores remaining to be released after budbreak, which coincides with the period of highest susceptibility to infection.     

Effect of temperature of production of Botrytis allii conidia on their pathogenicity to harvested white onion bulbs

Botrytis allii colonies incubated at low temperatures have been reported to produce larger conidia that germinate faster and give rise to longer germ-tubes than those grown at room temperature. The present study compared the effect of conidia produced at 20°C and at 0 and –2°C on their pathogenicity to artificially inoculated white onion bulbs, and the effect of conidial concentration (5×10³ and 5×10⁴ conidia/mL) on disease incidence, lesion area, incubation and latent period during storage at 20, 5 and 0°C. At all storage temperatures and periods tested conidia produced at −2°C caused a higher disease incidence and larger areas of rot than those produced at higher temperatures. When the conidial production temperature was raised to 20°C, the duration of incubation on the bulbs inoculated with 5×10⁴ conidia/mL was more than doubled during storage at 0°C, tripled at 5°C, and took 50% longer at 20°C. The incubation period was not significantly affected by conidial concentration at 20°C, and only slightly at 5 and 0°C, but at low temperatures the latent period was longer because of the delay induced in sporulation. These data are consistent with the packers' opinion that cross-infection of spring onions by long-term refrigerated onions in grading lines caused earlier and heavier rotting.     

Sporulation of Pyrenopeziza brassicae (light leaf spot) on oilseed rape (Brassica napus) leaves inoculated with ascospores or conidia at different temperatures and wetness durations

In controlled environment experiments, sporulation of Pyrenopeziza brassicae was observed on leaves of oilseed rape inoculated with ascospores or conidia at temperatures from 8 to 20°C at all leaf wetness durations from 6 to 72 h, except after 6 h leaf wetness duration at 8°C. The shortest times from inoculation to first observed sporulation ( l ₀), for both ascospore and conidial inoculum, were 11–12 days at 16°C after 48 h wetness duration. For both ascospore and conidial inoculum (48 h wetness duration), the number of conidia produced per cm² leaf area with sporulation was seven to eight times less at 20°C than at 8, 12 or 16°C. Values of Gompertz parameters c (maximum percentage leaf area with sporulation), r (maximum rate of increase in percentage leaf area with sporulation) and l ₃₇ (days from inoculation to 37% of maximum sporulation), estimated by fitting the equation to the observed data, were linearly related to values predicted by inserting temperature and wetness duration treatment values into existing equations. The observed data were fitted better by logistic equations than by Gompertz equations (which overestimated at low temperatures). For both ascospore and conidial inoculum, the latent period derived from the logistic equation (days from inoculation to 50% of maximum sporulation, l ₅₀) of P. brassicae was generally shortest at 16°C, and increased as temperature increased to 20°C or decreased to 8°C. Minimum numbers of spores needed to produce sporulation on leaves were ≈25 ascospores per leaf and ≈700 conidia per leaf, at 16°C after 48 h leaf wetness duration.     

The effect of temperature before or after inoculation with Phoma exigua var. foveata on the development of gangrene in potato tubers

Potato tubers of five cultivais were incubated at five temperatures between 2 and 20°C for 1 month and then inoculated with Phoma exigua var. foveata and stored at 5°C. Larger rots usually developed on tubers previously incubated at the higher temperatures but cultivar differences were greatest where tubers had been incubated at 2°C. In another experiment, rot size was related to the duration of incubation at 20°C before inoculation.
In a series of experiments, tubers were stored at different temperatures after inoculation; other factors (e.g. wound type or r.h.) were also investigated in some of these. Combining the results of all experiments a linear decrease of % wounds infected (logit transformation) with storage temperature was demonstrated. The size of rots was not simply related to temperature, but tended to increase over the range 2–10°C and then stabilized or declined at higher temperatures. Relative humidity treatments of 75% v. 95% did not consistently affect either the incidence of rotting or rot size. The other factors investigated did not usually interact significantly with temperature.
The relevance of the results for resistance testing is discussed and a simple equation relating disease incidence to inoculum levels, type of damage and temperature is proposed.     

Factors affecting mummification and sporulation of pome fruit infected by Monilinia fructigena in Dutch orchards

A 2-year field experiment (1997–98, 1998–99) was conducted to study mummification and subsequent sporulation in spring of apple (cvs James Grieve, Golden Delicious) and pear (cv . Conference) fruits infected by Monilinia fructigena . Most mummified fruits were found in James Grieve and Conference, whereas in late-infected Golden Delicious, fruits were still soft when examined in April. In the first year, these late-infected fruits had a significantly higher sporulation intensity per sporulating fruit ( P  = 0·05) compared with Golden Delicious fruits infected 9 and 5 weeks before harvest maturity, which were partly mummified. It was concluded that early- and late-infected fruits contributed to primary inoculum in the next season. In a postinfection regime of 25°C and 65–75% relative humidity under controlled conditions, the number of Conference fruits sporulating decreased rapidly, and after 12 weeks' incubation sporulation had completely ceased. After 8 weeks' incubation, sporulation intensity in the postinfection regime at 10°C was significantly higher than that at 20 and 25°C in a first experiment with inoculated unripe fruit ( P  = 0·05). Results of a second experiment with ripe fruit were less clear. These results are discussed in relation to orchard disease management.     

Production and immunodetection of ascospores of Mycosphaerella brassicicola: ringspot of vegetable crucifers

Pseudothecia containing abundant ascospores of Mycosphaerella brassicicola were produced in vitro on Brussels sprout decoction agar at 15°C under a 16-hour photoperiod of different light regimes. Spermogonia containing spermatia were also produced on the decoction agar. Ascospores were released when cultures were misted with SDW and placed under continuous light. Germination of ascospores was highest between 20°C and 25°C and spores remained viable at relative humidities above 93.5%. Exposure of ascospores to 55% relative humidity for 24 h reduced their germination to 75%. A polyclonal antiserum raised against whole ascospores was used to detect, by immunofluorescence, the ascospore and mycelial wall of M . brassicicola , following reaction with anti-rabbit IgG FITC conjugate. Autofluorescence of spore and mycelial components of other fungal species could be eliminated using the counterstains Evan's blue and eriochrome black at 0.2% and 0.5%, respectively, in phosphate buffered saline (pH 7.2). A procedure was developed to detect, by immunofluorescence, ascospores of M . brassicicola on artificially inoculated Melinex spore tape. Coating of the spore tape with bovine serum albumin provided a suitable support medium and blocking agent for detection of ascospores in the field. The potential use of the system for selective detection of ascospores of M . brassicicola in infected crops of vegetable brassicas in the presence of other ascosporic fungi is discussed. Keywords : ascospores, immunofluorescence, Mycosphaerella brassicicola , spore production, spore trapping .     

Latent infection of winter oilseed rape by Leptosphaeria maculans

Plants of oilseed rape, cultivars Primer and Jet Neuf, were grown in a glasshouse and inoculated at G.S. 2.4–2.7 with pycnidiospores or ascospores of Leptosphaeria maculans. The plants were kept for a further 2–4 weeks at 14°C and then transferred, together with uninoculated plants, to a polythene tunnel in winter. The majority of stems of inoculated plants did not have macroscopic symptoms of L. maculans infection 6 weeks after inoculation. Examination of whole mounts of peripheral tissue and transverse sections of fixed and embedded portions of these stems revealed intercellular septate fungal hyphae, often deep in non-necrotic cortical tissue, in symptomless inoculated plants but not in uninoculated plants. L. maculans was recovered following surface sterilization of adjacent portions of the same stems. When symptomless inoculated plants were transferred to a glasshouse at 18–20°C, cankers soon developed. The significance of these latent mycelial infections to canker development in the field is discussed.     

Environmental Influences on the Release of Ophiosphaerella agrostis Ascospores Under Controlled and Field Conditions

ABSTRACT Ophiosphaerella agrostis, the causal agent of dead spot of creeping bentgrass (Agrostis stolonifera), can produce prodigious numbers of pseudothecia and ascospores throughout the summer. The environmental conditions and seasonal timings associated with O. agrostis ascospore release are unknown. The objectives of this research were to (i) determine the influence of light and relative humidity on ascospore release in a controlled environment, (ii) document the seasonal and daily discharge patterns of ascospores in the field, and (iii) elucidate environmental conditions that promote ascospore release under field conditions. In a growth chamber, a sharp decrease (100 to approximately 50%; 25 degrees C) in relative humidity resulted in a rapid (1- to 3-h) discharge of ascospores, regardless of whether pseudothecia were incubated in constant light or dark. In the field, daily ascospore release increased between 1900 and 2300 h and again between 0700 and 1000 h local time. The release of ascospores occurred primarily during the early morning hours when relative humidity was decreasing and the canopy began to dry, or during evening hours when relative humidity was low and dew began to form. Few ascospores were released between 1100 and 1800 h when the bentgrass canopy was dry. The release of ascospores also was triggered by precipitation. Of the ascospores collected during precipitation events, 87% occurred within 10 h of the beginning of each event.     

Studies on the epidemiology of the tar spot disease complex of maize in Mexico

During the period 1986–1988 field studies were conducted on the epidemiology of the tar spot disease complex (TDC) of maize ( Zea mays ) caused by Phyllachora maydis, Monographella maydis and Coniothyrium phyllachorae. Under field conditions we found that P. maydis symptoms always appeared first, followed by symptoms of either M. maydis or C. phyllachorae. M. maydis causes leaf necrosis and has the most devastating effect. The primary symptoms covered about 12% of the leaf area below the ear leaf, whereas the total necrotic leaf area amounted to 30–60%, here considered as a secondary effect. Maximum TDC severity occurred during the winter season of 1988, which was characterized by a temperature range of 17–22°C, a mean RH >75%, and > 7h of leaf wetness per night. The highest numbers of windborne ascospores of P. maydis were trapped at an RH > 85% and at temperatures of 17 to 23°C in the winter of 1987 and 1988, although large numbers were also caught at temperatures of >23°C and RH <70%. Spore release was strongly influenced by light conditions and followed a similar diurnal curve throughout three seasons, reaching a maximum at 17.00–21.00 hours. The spread off. maydis within the field was very homogeneous. The incubation period of P. maydis was 12 to 15 days, and most of the ascospores were released within 3 weeks after formation of the ascostromata. M. maydis inoculum in plant debris was reduced by 90% within 3 to 4 months.     

Prestorage heat treatment reduces pathogenicity of Penicillium expansum in apple fruit

Penicillium expansum is one of the main postharvest pathogens of apples in Israel. Heating apple fruit inoculated with P. expansum for 96 h at 38°C completely inhibited decay development. Fruit held for 24 h at 42°C or 12 h at 46°C had significantly reduced decay after an additional 14 days incubation at 20°C, compared with unheated inoculated control fruit. Mycelial growth and percentage spore germination in vitro were inversely proportional to length of time of exposure to various temperatures. The ET₅₀ for spore germination was 42, 34 and 20 h at 38, 42 and 46°C, respectively, while the ET₅₀ for mycelial growth was 48, 44 and 36 h at those temperatures. When Penicillium spores were incubated on crude extract prepared from the peel of apple fruits held 4 days at 38°C, germ tube elongation was significantly reduced, while the walls of the tubes were thicker, compared with germ tubes from spores incubated on crude extract prepared from peel of non-heated fruit. The evidence presented here supports the hypothesis that the effect of heating on the decay of apples caused by P. expansum is not only the result of direct inhibition of fungal germination and growth by high temperature, but is also partly due to the formation of an inhibitory substance in the heated peel.     

Purple nutsedge tuber sprouting

Purple nutsedge ( Cyperus rotundus L.) tubers remain viable for several years and serve as its principal means of survival. The maintenance of high moisture content is essential to tuber survival. Apical dominance influences bud dormancy within a tuber and in a chain of tubers, and dormancy increases with tuber age. Several growth inhibitors were identified in tubers, but their role in tuber dormancy has not been established. Moisture levels in soil must increase to a critical level before sprouting occurs, but excess soil moisture deters sprouting. Oxygen may be a limiting factor for tuber sprouting in waterlogged soils. Although light is not a requirement for sprouting, it has promoted sprouting. Temperature regulates sprouting; no sprouting occured below 10°C and above 45°C. Optimum sprouting occurred between 25 and 35°C when provided with constant temperatures. However, daily alternating temperatures greatly stimulated sprouting. A daily short duration (0.5 h) of high temperature increased sprouting to nearly 100%, whereas less than 50% sprouting occurred without the daily high temperature pulse. Bud break occurred readily for most tubers at 20°C and in nearly 100% of the tubers with a single 0.5 h exposure to a high temperature (35°C) pulse. However, most buds did not elongate if the tuber remained at 20°C. Bud elongation occurred at higher temperatures, and daily alternating temperatures stimulated shoot elongation up to eightfold greater than at the respective mean constant temperatures. Daily soil temperature fluctuation may be a major signal for purple nutsedge emergence, such as when the plant canopy is removed, or when soils are solarized. Future research is needed to determine tuber sprouting for different ecotypes, and on the role of the rhizome chain. Systems to manipulate sprouting may provide new strategies for purple nutsedge management.     

Effects of temperature on maturation of pseudothecia of Leptosphaeria maculans and L. biglobosa on oilseed rape stem debris

Effects of temperature on maturation of pseudothecia of Leptosphaeria maculans and L. biglobosa , closely related species which coexist on UK oilseed rape, were investigated. Stages in pseudothecial maturation on naturally infected oilseed rape debris were examined, both in controlled environments (5, 10, 15 or 20°C) under continuous wetness and in natural conditions (debris exposed in September and December 2000, and July, September and November 2002). Pseudothecia sampled weekly were assigned to maturation classes A (asci undifferentiated), B (asci differentiated), C (ascospores differentiated) or D (ascospores mature). Progress in pseudothecial maturation (assessed by time until 50% of pseudothecia reached each class) was similar for L. maculans and L. biglobosa at 15–20°C, but L. biglobosa matured more slowly at < 10°C. Maturation time decreased almost linearly with temperature from 5 to 20°C under continuous wetness but was longer in natural conditions, especially when periods of dry weather occurred. Differences in pseudothecial maturation are likely to contribute to epidemiological differences between L. maculans and L. biglobosa , which may explain their coexistence. It is appropriate to use the degree-day approximation to assess pseudothecial maturation at temperatures between 5 and 20°C, providing debris is wet.     

The effect of heat on the growth and recovery of Aspergillus spp. from the mycoflora of onion seeds

Temperature optima for growth of Aspergillus niger and A. flavus on agar lay between 30°C and 35°C; the optimum for A. fumigatus was 40°C. A. flavus grew less rapidly in culture than the other two species. These fungi were recovered when a single sample of onion seeds, produced in Sudan, was plated out onto agar and incubated over a range of temperatures from 15°C to 45°C. In line with the growth optima of the fungus, the recovery of A. niger was greatest between 25°C and 35°C; recovery of A. flavus was greatest between 30°C and 35°C and recovery of A. fumigatus greatest between 40°C and 45°C. Hot-water treatment for durations of up to 60 min at 50°C failed to reduce the incidence of recovery of seedborne A. niger and A. flavus from seeds incubated at 30°C on agar; A. fumigatus was not recovered from seeds treated in this way. However, when seeds were hot-water treated at 60°C and incubated on agar at 30°C, A. niger was virtually eliminated by a treatment duration of 15 min or more; the incidence of recovery of A. fumigatus was significantly increased compared with the 50°C treatment and there was no change in the incidence of A. flavus. Hot-water treatment at 60°C for more than 30 min significantly reduced seed germination.     

The effect of temperature on hatching and penetration of chickpea roots by Pratylenchus thornei

Egg hatch of Pratylenchus thornei was influenced by temperature. It took place at all temperatures within the range 10–25°C and was optimal at 20°C. Root penetration increased steadily with increasing time of incubation up to the end of the experiment 11 days after inoculation. Temperature affected penetration rate in chickpea ( Cicer arietinum ) cultivars UC 27 and JG 62 but not in line P 2245, being significantly higher at 20–25°C than that at 15°C. At the end of the experiment, roots of line P 2245 held at 15°C contained more P. thornei than cultivars UC 27 and JG 62. No difference in percentage penetration among host genotypes was observed at 20 or 25°C. All migratory stages of P. thornei penetrated roots of chickpea from the first to 11th days after inoculation.