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.     

Effects of temperature and wetness duration on conidial infection, latent period and asexual sporulation of Pyrenopeziza brassicae on leaves of oilseed rape

Experiments in controlled environments were carried out to determine the effects of temperature and leaf wetness duration on infection of oilseed rape leaves by conidia of the light leaf spot pathogen, Pyrenopeziza brassicae . Visible spore pustules developed on leaves of cv. Bristol inoculated with P. brassicae conidia at temperatures from 4 to 20°C, but not at 24°C; spore pustules developed when the leaf wetness duration after inoculation was longer than or equal to approximately 6 h at 12–20°C, 10 h at 8°C, 16 h at 6°C or 24 h at 4°C. On leaves of cvs. Capricorn or Cobra, light leaf spot symptoms developed at 8 and 16°C when the leaf wetness duration after inoculation was greater than 3 or 24 h, respectively. The latent period (the time period from inoculation to first spore pustules) of P. brassicae on cv. Bristol was, on average, approximately 10 days at 16°C when leaf wetness duration was 24 h, and increased to approximately 12 days as temperature increased to 20°C and to 26 days as temperature decreased to 4°C. At 8°C, an increase in leaf wetness duration from 10 to 72 h decreased the latent period from approximately 25 to 16 days; at 6°C, an increase in leaf wetness duration from 16 to 72 h decreased the latent period from approximately 23 to 17 days. The numbers of conidia produced were greatest at 12–16°C, and decreased as temperature decreased to 8°C or increased to 20°C. At temperatures from 8 to 20°C, an increase in leaf wetness duration from 6 to 24 h increased the production of conidia. There were linear relationships between the number of conidia produced on a leaf and the proportion of the leaf area covered by 'lesions' (both log₁₀-transformed) at different temperatures.     

Germination of Alternaria linicola conidia on linseed: effects of temperature, incubation time, leaf wetness and light regime

Conidia of Alternaria linicola germinated on both water agar and linseed leaves (detached or attached) over a wide range of temperatures (5–25°C) by producing one to several germ tubes. At temperatures between 10°C and 25°C and under continuous wetness in darkness, germination started within 2 h after inoculation and reached a maximum (100%) by 8 to 24 h, depending on temperature. At 5°C, the onset of germination was later and the rate of germ tube elongation was slower than that at 10–25°C. During germination, conidia of A. linicola were sensitive to dry interruptions of wet periods and to light. Short (2 h) or long (12 h) dry interruptions occurring at any time between 2 and 6 h after inoculation stopped conidial germination and germ tube elongation. With continuous wetness, light periods 2 to 12 h long immediately after inoculation inhibited conidial germination, which was resumed only when a dark period followed subsequently. However, germination and germ tube elongation of A. linicola conidia stopped and the viability of the conidia was lost during exposure to dry light periods immediately after inoculation with spore suspensions. Penetration of leaves by A. linicola was evident after 12 h and occurred mainly through epidermal cells (direct) with or without the formation of appressoria.     

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.     

Effects of temperature, relative humidity and duration of wetness period on germination and infection by conidia of the pear scab pathogen (Venturia nashicola)

Experiments were conducted to determine the effects of temperature, relative humidity (RH) and duration of wetness period on in vitro germination of conidia and infection of detached pear leaves by Venturia nashicola , the causal agent of pear scab. Conidia germinated only in near-saturation humidity (RH > 97%). The final percentage germination (24 h after inoculation) at 100% RH without free water was less than half that in free water. Conidia germinated over the range of temperatures tested (5–30°C); the optimum temperature for germination was ≈21°C. Changes in percentage germination of conidia over time were fitted by logistic models at each individual temperature. Polynomial models satisfactorily described the relationships between two (rate and time to 50% of maximum germination) of the three logistic model parameters and temperature. The minimum length of the wetness period for successful infection of detached pear leaves by conidia was observed at several temperatures. The shortest length of wetness period required for infection was 7 h at 22°C. Two polynomial models fitted well the relationship between the minimum wetness duration required for infection, and temperature.     

Leaf blotch severity on spring barley infected by isolates of Rhynchosporium secalis under different temperature and humidity regimes

The infection efficiency and severity of leaf blotch on spring barley inoculated with three pathotypes of Rhynchosporium secalis from central Norway were studied under different temperature and humidity regimes. Seedlings of the cultivar Arve were subjected to two constant temperatures, 13° or 18°C. Dry periods of 8 h or longer before or after a wet period of 4 h, carried out in the first 48 h postinoculation, reduced disease severity assessed 16 days after inoculation. The effect of dry periods of up to 24 h was nullified when plants were subjected to high humidity for 48 h after the dry treatment. The disease developed most rapidly when the wet period was 48 h and the temperature 18°C. At or near the optimum temperature for R. secalis (18°C), leaf wetness duration as short as 2 h resulted in considerable disease. Isolates reacted differently to temperature. The most aggressive isolate caused severe disease irrespective of temperature (56–70% of the leaf area infected); however, disease severity caused by the least aggressive isolate was significantly higher at the optimum temperature compared with a lower temperature (13°C). This information can facilitate evaluation of weather data in relation to predicting leaf blotch for advisory purposes.     

Infection of onion leaves by Alternaria porri and Stemphylium vesicarium and disease development in controlled environments

Infection of onion by Alternaria porri and Stemphylium vesicarium was investigated under a range of controlled temperatures (4–25°C) and leaf wetness periods (0–24 h). Conidia of A. porri and S. vesicarium germinated within 2 h when incubated at 4°C. Terminal and intercalary appressoria were produced at similar frequencies at or above 10°C. The maximum number of appressoria was produced after 24 h at 25°C. Penetration of leaves by both pathogens was via the epidermis and stomata, but the frequency of stomatal penetration exceeded that of epidermal penetration. There was a strong correlation ( R ² > 90%) between appressorium formation and total penetrations at all temperatures. Infection of onion leaves occurred after 16 h of leaf wetness at 15°C and 8 h of leaf wetness at 10–25°C, and infection increased with increasing leaf wetness duration to 24 h at all temperatures. Interruption of a single or double leaf wetness period by a dry period of 4–24 h had little effect on lesion numbers. Conidia of A. porri and S. vesicarium separately or in mixtures caused similar numbers of lesions. Alternaria porri and S. vesicarium are both potentially important pathogens in winter-grown Allium crops and purple leaf blotch symptoms were considered to be a complex caused by both pathogens.     

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 wetness period and temperature on development of dark pod spot (Alternaria brassicae) on oilseed rape (Brassica napus)

In controlled environment experiments, when oilseed rape pods or leaves were inoculated with spore suspensions of Alternaria brassicae, the maximum disease incidence (proportion of pods or leaves diseased) increased as wetness period after inoculation increased from 4 to 24 h and as temperature increased to 20°C. There was a clear relationship between disease incidence on pods and incidence on leaves with the same wetness/temperature conditions. Logistic equations described the effects of wetness period after inoculation on disease incidence (number of pods or leaves infected) or disease severity (number of lesions on pods or leaves) using temperature-dependent and tissue-dependent parameters. The time from inoculation to the appearance of the first lesions was shorter on pods than on leaves at temperatures ≤15°C and wetness periods ≤12 h. Two-dimensional response surface equations or simple interpolations from one-dimensional equations were used to develop contour maps of expected disease incidence and severity, respectively, on leaves or pods to estimate the effects of different combinations of wetness period during infection and temperature on disease development.     

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.     

Effect of temperature and leaf wetness duration on infection of sweet oranges by Asiatic citrus canker

Asiatic citrus canker, caused by Xanthomonas smithii ssp. citri , formerly X. axonopodis pv. citri , is one of the most serious phytosanitary problems in Brazilian citrus crops. Experiments were conducted under controlled conditions to assess the influence of temperature and leaf wetness duration on infection and subsequent symptom development of citrus canker in sweet orange cvs Hamlin, Natal, Pera and Valencia. The quantified variables were incubation period, disease incidence, disease severity, mean lesion density and mean lesion size at temperatures of 12, 15, 20, 25, 30, 35, 40 and 42°C, and leaf wetness durations of 0, 4, 8, 12, 16, 20 and 24 h. Symptoms did not develop at 42°C. A generalized beta function showed a good fit to the temperature data, severity being highest in the range 30–35°C. The relationship between citrus canker severity and leaf wetness duration was explained by a monomolecular model, with the greatest severity occurring at 24 h of leaf wetness, with 4 h of wetness being the minimum duration sufficient to cause 100% incidence at optimal temperatures of 25–35°C. Mean lesion density behaved similarly to disease severity in relation to temperature variation and leaf wetness duration. A combined monomolecular-beta generalized model fitted disease severity, mean lesion density or lesion size as a function of both temperature and duration of leaf wetness. The estimated minimum and maximum temperatures for the occurrence of disease were 12°C and 40°C, respectively.     

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.     

Effects of leaf wetness duration, relative humidity, light and dark on infection and sporulation by Didymella rabiei on chickpea

No infection occurred at less than 95% relative humidity (r.h.) when chickpea plants were dried after inoculation with conidia of Didymella rabiei. Infection was significant when the dry leaves were exposed to 98% r.h. for 48 h. When inoculated plants were subjected to different leaf wetness periods, some infection occurred with 4 h wetness, and disease severity increased with wetness duration according to an exponential asymptote, with a maximum value after about 18 h. Germination of conidia and germ tube penetration increased linearly with increasing wetness periods when recorded 42 h after inoculation. With a 24-h wetness period, germination of conidia was first observed 12 h after inoculation and increased linearly with time up to 52 h (end of the experiment). Dry periods immediately after inoculation, followed by 24-h leaf wetness, reduced disease severity; as the dry period increased the severity decreased. Disease severity increased with increasing periods of darkness after inoculation. The number of pycnidia and the production of conidia on infected leaves increased only slightly with high r.h. (either in the light or in the dark), but large increases occurred over an 8-day period when the leaves were kept wet.     

Differences Between Ascospores and Conidia of Didymella rabiei in Spore Germination and Infection of Chickpea

ABSTRACT Studies were performed to compare the germination and infection of ascospores and conidia of Didymella rabiei under different temperature and moisture conditions. Germination of ascospores and conidia on cover glasses coated with water agar began after 2 h, with maximum germination (>95%) occurring in 6 h at 20 degrees C. No germination occurred at 0 and 35 degrees C. Ascospores germinated more rapidly than conidia at all temperatures. Germination declined rapidly as the water potential varied from 0 to -4 MPa, although some germination occurred at -6 MPa at 20 and 25 degrees C. Ascospores germinated over a wider range of water potentials than conidia and their germ tubes were longer than those of conidia at most water potentials and temperatures. The optimum temperature for infection and disease development by both ascospores and conidia was around 20 degrees C. Disease severity was higher when ascospores were discharged directly onto plant surfaces from naturally infested chickpea debris compared with aqueous suspensions of ascospores and conidia sprayed onto plants Disease severity increased as the length of the wetness period increased. When dry periods of 6 to 48 h occurred immediately after inoculation, disease severity decreased, except for the shorter periods which had the opposite effect. Disease severity was higher with ascospore inoculum when no dry periods occurred after inoculation.     

Early development of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) in relation to temperature and leaf wetness.

In controlled environment experiments to study early development of light leaf spot, lesions developed with leaf wetness durations of 16 to 48 h after inoculation of oilseed rape with conidial suspensions of Pyrenopeziza brassicae at 12 or 18°C, but not with leaf wetness durations of 0 to 13h. The incubation period was 21 to 22 days at 12°C and 14 to 18 days at 18°C for leaf wetness durations of 16 to 48 h. The latent period was 21 to 23 days at 12°C and 18 to 19 days at 18°C, and the total number of lesions increased with increasing leaf wetness duration at both temperatures. In field experiments, light leaf spot always developed on oilseed rape with a leaf wetness duration of 48 h after inoculation in both 1990/1991 and 1991/1992, but the percentage leaf area affected was less on plants placed in an oilseed rape crop than on those placed in a glasshouse. Plants moved to an oilseed rape crop immediately after inoculation nearly always developed light leaf spot symptoms when they were inoculated between 19 October 1990 and 1 March 1991 or between 27 September 1991 and 14 February 1992, but plants inoculated between 31 August and 16 October 1990 or on 20 September 1991, when estimated leaf wetness duration was less than 16 h for several days after they were placed in crops, did not develop symptoms. The latent period of light leaf spot on plants transferred to the oilseed rape crop was 15 to 40 days, and there was an approximately linear relationship between 1 (latent period) and mean temperature during this period. The accumulated temperature during the latent period ranged from c. 150 to 250 day-degrees. The severity of lesions on these plants increased with increasing temperature from 5 to 15°C.     

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.     

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.     

Effect of temperature on latent period of septoria leaf blotch on winter wheat under outdoor conditions

Batches of two winter wheat cultivars (Riband and Apollo) were inoculated with conidia of Mycosphaerella graminicola at weekly intervals over a 2 year period. Following 72 h incubation, plants were placed in ambient temperatures ranging between −7 and 32°C with mean batch temperatures of 2·9–20·2°C. Latent period until the first visible symptoms ranged between 11 and 42 days. The relationship between development of lesions and accumulated thermal time was described using a shifted cumulative gamma distribution model. The model provided good estimates of lesion development with r ² > 0·92 for both cultivars. Base temperatures, below which the pathogen did not develop, were estimated from the model as approximately −2·4°C for the two cultivars. Latent period was estimated as being 250 and 301 degree-days above the estimated base temperature, when defined as time from inoculation to first lesion and time to 50% of maximal lesions, respectively, for cv. Riband. The values for cv. Apollo were similar, but with estimates of thermal time periods c . 5% higher. The relationship between mean temperature and inverse latent period, expressed as days either to first lesion or to 50% of maximal lesions, was best described by a linear regression with r ² > 0·96 for both cultivars. The opportunity for plants to outgrow disease was reduced when prolonged periods of cold temperature occurred, because the base temperature for growth of the pathogen was less than that for the crop.     

Temperature-mediated responses of flumetsulam and metosulam on Raphanus raphanistrum

The effect of temperature and light after spraying on the activity of flumetsulam and metosulam when applied to seedlings of Raphanus raphanistrum was evaluated under controlled environments. Flumetsulam and metosulam were applied at 0.01–3 times the recommended doses and the plants were subjected to a range of temperatures after spraying. Herbicide activity was estimated from dose–response curves of fresh weight. Varying the temperature after spraying from 1 to 20 °C increased the activity of flumetsulam and metosulam, as determined by comparison of ED₅₀, by a factor of 97 and 7 respectively. Large increases in herbicide activity occurred in the 1–5 °C range and smaller but significant increases at temperatures greater than 5 °C. No significant differences in the activity of the herbicides were found when the plants were subjected to light or dark conditions at 5 °C after spraying. The influence of temperature on activity may lead to opportunities for rate adjustment of flumetsulam and metosulam based on the temperature prevailing around the time of spraying.     

A Glasshouse Cropping Method for Screening Lettuce Lines for Resistance to Sclerotinia Sclerotiorum

A soil-based glasshouse crop procedure was developed to screen lettuce lines for resistance to Sclerotinia sclerotiorum. Six sequential crops of 19 different lettuce lines with a range of cultural morphologies, reported previously to exhibit some form of resistance to S. sclerotiorum, were planted in a glasshouse infested with S. sclerotiorum and natural disease development compared with a standard susceptible commercial butterhead cultivar, Rachel. Concomitantly, the same lettuce lines were planted in pots in a nearby glasshouse, were artificially inoculated with ascospores of S. sclerotiorum, assessed for infection and scored for disease severity. Most of lines exhibited resistance in at least one of the crop or direct inoculation assessments with wild form, PI 251246, and stem lettuce, Taiwan, exhibiting resistance in three of the assessments and wild form, PI 271938 (Lactuca serriola), and Iceberg (crisp) line, 74-1076, exhibiting resistance in all four assessments. Cos line, PI 250427, was less resistant than the standard control in all assessments. The crop based screen with predictable, natural disease development was the most discriminating overall assessment and enabled growth habit to be taken into account during the screening process which was not possible through the direct inoculation procedures. Nevertheless, the novel ascospore inoculation screening process provided information on the type of resistance expressed that could not be identified from the cropping procedure.