Effects of temperature on the latent period of the rose powdery mildew pathogen, Sphaerotheca pannosa

The effects of temperature on the length of the latent period of rose powdery mildew, caused by Sphaerotheca pannosa var . rosae , were studied. At constant temperatures over the range of 10–28°C, the length of the latent period ranged from 4 to 10 days. The relationship between temperature and the rate of fungal development during the latent period (expressed as the reciprocal of the latent period) was described by a nonlinear (thermodynamic) model. The resulting curve was asymmetrically bell-shaped with an optimum temperature of about 22°C. The latent period was further subdivided into two periods: incubation period (from inoculation to visible colonies) and postincubation period (from visible colonies to the first sign of conidiophores). The relationship between temperature and the rate of fungal development during the incubation period (expressed as the reciprocal of the incubation period) was also well described by a thermodynamic model. In contrast, the relationship between temperature and the rate of fungal development during the postincubation period (expressed as the reciprocal of the postincubation period) was approximately linear. The latent period under fluctuating temperatures was predicted using a two-stage integration scheme at a step of 24 min by first integrating the incubation rate and then the postincubation rate. The predicted length of the latent period agreed well with the observed values.     

The effects of constant and fluctuating temperatures on the length of the incubation period of apple powdery mildew (Podosphaera leucotricha)

The effects of constant and fluctuating temperatures on the incubation period of apple powdery mildew, caused by Podosphaera leucotricha , were studied. At constant temperatures, incubation periods ranged from 3 to 12 days over temperatures 8°C–30°C, and no visible lesions developed at 32°C. A nonlinear model was developed to describe the relationship between temperature and the rate of mildew colony development. The resulting curve is bell-shaped with an optimum temperature at about 23°C. When this model was used to predict mildew development under fluctuating temperatures at an integration step of 48 min, however, it consistently overestimated development rate for fluctuating periods with average temperatures higher than 20°C. A nonlinear model was also fitted directly to the fluctuating temperature data, thus taking into account the nonlinear effect. The overestimation of development rate by the constant model for high temperatures was confirmed when the two models were compared. This overestimation probably resulted from differences in the levels of relative humidity between constant and fluctuating temperature regimes. Possible practical use of the model is discussed.     

Effects of Temperature on the Length of the Incubation Period of Rose Powdery Mildew (Sphaerotheca pannosa var. rosae)

The effect of temperature on the length of the incubation period of rose powdery mildew, caused by Sphaerotheca pannosa var. rosae, was studied. At constant temperature over the range 8–28°C, the length of the incubation period ranged from 3 to 10 days; no visible colonies developed at 30°C after 19 days. The relationship between temperature and the rate of development of mildew colonies within the incubation period under constant temperature was described by two alternative non-linear models (exponential and thermodynamic). The resulting curves were asymmetrically bell-shaped with an optimum temperature of c. 23°C. The two constant-temperature models predicted the development of powdery mildew under fluctuating temperatures with similar accuracy, even though the exponential model fitted the constant temperature data less well than the thermodynamic model. The thermodynamic model failed to fit the fluctuating-temperature data directly, whereas the exponential model fitted those data directly and the fit was similar to the corresponding model from the constant-temperature data. Fitting the models to the combined (constant and fluctuating temperature) data gave results that were nearly identical to those based on the constant-temperature data alone.     

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.     

Effects of temperature and moisture on disease and fruit body development of Mycosphaerella pinodes on pea (Pisum sativum)

The effects of temperature (5–30°C) and the duration of moisture on the development of ascochyta blight ( Mycosphaerella pinodes ) on pea seedlings, grown under controlled conditions, were investigated. The optimum temperature for monocyclic processes was 20°C. At this temperature, pycnidiospores germinated after 2 h, appressoria formed after 6 h and the germ-tube penetrated the leaf cuticle after 8 h. Disease symptoms were evident after 1 day of incubation and the first pycnidia formed after 3 days. Longer wetting periods were required for disease development and pycnidial formation at non-optimal temperatures. Disease severity and the number of pycnidia formed on leaves increased with temperature from 5 to 20°C, then decreased between 20 and 30°C. Polynomial equations were fitted to predict the stages of infection, incubation, latency and disease development as functions of temperature and duration of moisture. These equations allow comparisons of pathogen spread with plant development and could be incorporated into disease development models used for crop management programmes.     

Relationships between temperature and latent periods of rust and leaf-spot diseases of groundnut

The effect of temperature on the latent periods of rust, late leaf spot and early leaf spot diseases of groundnut caused by Puccinia arachidis, Phaeoisariopsis personata and Cercospora arachidicola , respectively, was studied. The latent periods (LP) of rust, late leaf spot and early leaf spot ranged from 12–49 days, 13–38 days and 13–39 days, respectively, between 12°C and 33°C An equation relating the rate of pathogen development (1/LP) to temperature was fitted using daily mean temperatures to provide three cardinal temperatures: the minimum (T_mln), optimum (T_opl), and maximum (T_max), T_mln was about 12°C for rust and about 10°C for the two leaf-spot diseases. T_opt, for all three diseases was close to 25°C. T_max was 31°C for early leaf spot, and extrapolated values for late leaf spot and rust were about 35 and 40°C, respectively.
For P. personata , a temperature response curve was fitted using data only from controlled environment experiments. This curve was used to simulate latent periods from both mean daily and mean hourly temperatures in the field. There was substantially better agreement between observed and simulated latent period with hourly temperatures, provided the developmental rate of the pathogen was determined at a constant temperature.     

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.     

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.     

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.     

Effects of temperature on the development of light leaf spot (Pyrenopeziza brassicae) on oilseed rape (Brassica napus)

The effects of temperature on the development of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape were investigated in controlled-environment experiments. The proportion of conidia which germinated on leaves, the growth rate of germ tubes, the severity of light leaf spot and the production of conidia increased with increasing temperature from 5 to 15 C. The time to 50% germination of conidia and the incubation and latent periods of light leaf spot lesions decreased when temperature increased from 5 to 15°C. At 20°C, however, light leaf spot severity and production of conidia were less and the incubation and latent periods were longer than at 15 C. There were differences between P brassicae isolates and oilseed rape cultivars in the severity of light leaf spot, the production of conidia and the length of the incubation period but not in the length of the latent period. The responses to temperature for lesion severity and incubation and latent periods appeared to be approximately linear over the temperature range 5-15°C and could be quantified using linear regression analysis.     

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.     

Effect of temperature on the transmission of Potato mop-top virus from seed tuber and by its vector, Spongospora subterranea

Potato mop-top virus (PMTV) causes disease in both the growing plant and tubers (spraing) of potato and is transmitted by the plasmodiophorid Spongospora subterranea , the cause of powdery scab. The effect of temperature during plant growth on the transmission of PMTV from infected seed tubers and from infested growing media was investigated in a series of glasshouse experiments. Symptoms developed on foliage of plants derived from infected seed tubers but none developed when PMTV was transmitted by S. subterranea in soil. The incidence of foliar symptoms was greatest on plants grown at 12°C, less at 16°C, few at 20°C and absent at 24°C. The transmission of PMTV from infected seed tubers was not significantly affected by temperatures between 12 and 24°C, but when the virus was transmitted by S. subterranea , minimal tuber infection occurred at 24°C and no differences were recorded at temperatures between 12 and 20°C. The incidence of powdery scab on tubers was greatest at 12 and 16°C and very low at 20 and 24°C. However, the incidence and severity of root galling caused by S. subterranea , was greatest at 20 and very low at 24°C. The incidence of powdery scab was greater on tubers of plants derived from infected seed tubers grown in a fluctuating temperature regime of 12 h at 20°C followed by 24 h at 12°C than on those grown at a constant 20°C, whereas the incidence of tuber infection by PMTV and spraing was similar for both regimes. This demonstrates that infection of roots can occur at a higher temperature than that for powdery scab on tubers and that this root infection can enable the transmission of PMTV into the potato plant.     

Seed dormancy dynamics and germination characteristics of Solanum nigrum

Four experiments were conducted to study seed dormancy and germination requirements in Solanum nigrum . In Expt 1, seeds were stratified at different constant and stepwise rising temperatures and their germinability was tested at three germination regimes at weekly intervals. In Expts 2–4, seeds dry stored at 4°C and stratified at 5 and 15°C were tested at constant temperatures, as well as fluctuating temperatures with constant and increasing amplitudes. Results suggest that the rate of dormancy release increased with increasing temperatures ranging from 4.5 to 18.6°C. However, prolonged stratification at higher temperatures caused subsequent induction of dormancy. When tested at constant temperatures, stratified seeds germinated between 18 and 34°C, with the optimum between 26 and 30°C, while dry-stored seeds showed no germination. Fluctuating temperatures, with amplitudes ranging from 5 to 15°C, promoted germination of seeds from all treatments. The dormancy dynamics and germination characteristics of the species will have implications for its survival and establishment. This information can be used to predict time of emergence and, thus, improve control of the species in weed management systems.     

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.     

Factors affecting the incubation period of dark leaf and pod spot (Alternaria brassicae) on oilseed rape (Brassica napus)

Experiments to investigate the factors affecting the incubation period of dark leaf and pod spot (Alternaria brassicae) on leaves and pods of oilseed rape (Brassica napus) were done in controlled environment (constant temperatures) and glasshouse conditions (fluctuating temperatures). The length of the incubation period of dark leaf and pod spot decreased as infection and incubation temperatures increased from 6 to 20 °C. The incubation period decreased as wetness period increased from 2 to 12 h, as inoculum concentration increased from 80 to 2 × 10³ spores ml^–1 and as leaf age increased from 4 to 10 days. Asymptotes of leaf age and inoculum concentration, above which the length of the incubation period did not decrease, were 10 days and 2 × 10³ spores ml^–1, respectively. The shortest and longest incubation periods were 1 and 11 days. The mechanism by which the infection conditions influenced the incubation period of dark leaf and pod spot on oilseed rape seemed to be linked to lesion density. Usually, the length of the incubation period decreased greatly with increasing lesion density.     

Effects of bean line pattern mosaic virus on the monocyclic components of rust and angular leaf spot of Phaseolus bean at different temperatures

Monocyclic components (development rate during the incubation period or latent period, lesion density, lesion size and disease severity) of rust ( Uromyces appendiculatus ) and of angular leaf spot ( Phaeoisariopsis griseola ) in two bean ( Phaseolus vulgaris ) cultivars (Rosinha G-2 and Carioca), pre-infected or not with bean line pattern mosaic virus (BLPMV), were determined. Trials were conducted at temperatures in the range from 9 to 27°C for rust and from 12 to 30°C for angular leaf spot. Regardless of viral pre-infection, the effect of temperature on the four monocyclic components followed an optimum curve and could be described by a generalized beta function. Generally, angular leaf spot was favoured by higher temperatures with an optimum for disease severity between 24.2 and 28.3°C compared with 15.9–18.5°C for rust. Pre-infection with BLPMV did not change the shape of the optimum curves for all components, but significantly reduced lesion density and disease severity on both cultivars. The development rates during incubation and latent periods for both fungal diseases were not affected by BLPMV. Pre-infection with virus did not alter the ranking of cultivars with respect to resistance to both fungal diseases.     

Variation in the development of secondary dormancy in oilseed rape genotypes under conditions of stress

Summary A substantial amount of seed is left in the fields before and during harvest of oilseed rape. Although this crop exhibits little or no primary dormancy, the absence of certain environmental cues that promote germination of imbibed seeds induces secondary dormancy. The work reported investigated the extent to which environmental stress conditions, including osmotic stress, low oxygen stress and anaerobiosis, induce secondary dormancy in oilseed rape, and examined the variation in development of secondary dormancy between and within genotypes. Osmotic stress was most effective in inducing dormancy. Anaerobic treatment produced very few dormant seeds, as did an atmosphere low in oxygen and high in nitrogen. The development of secondary dormancy under osmotic stress varied considerably between and within genotypes. Dormancy ranged from almost zero to about 60% for winter genotypes and about 85% for spring types. Within genotypes, variations occurred between seed lots and years of harvest. Temperature variations affected the percentage of dormant seeds. More dormant seeds were likely to be produced with incubation under water stress at 20 °C than at 12 °C. In winter genotypes, fewer dormant seeds were produced when incubation temperature and germination test temperatures differed. Thus, incubating at 20 °C and 12 °C, followed by germination tests at 20 °C and 12 °C, respectively, produced most dormant seeds. Also, in the winter genotypes, the potential development of secondary dormancy was positively correlated with the pattern and speed of germination of untreated seeds.     

Effects of the sowing depth and temperature on the seedling emergence and early growth of wild barley (Hordeum spontaneum) and wheat

The experiment was conducted under a controlled environment to study the effects of different temperature regimes (15/10°C, 20/15°C, and 25/20°C day/night) and sowing depths (0, 2, 4, and 6 cm) on the seedling emergence and early growth (height gain) of wheat (cv. Marvdasht) and wild barley ( Hordeum spontaneum ). The cumulative emergence and plant height gain over time were modeled with the use of a logistic function. For a particular temperature regime, the maximum percentage emergence (E _max ) of wheat was higher than that of wild barley across all sowing depths. The maximum and minimum E _max values for both species occurred at 20/15°C and 25/20°C, respectively. The time taken to reach 50% of the E _max (i.e. E₅₀) increased with the sowing depth in both species under all temperature regimes. The E₅₀ of wild barley was greater than that of wheat for all temperature regimes, with maximum differences observed at 20/15°C. The greatest maximum plant height (H _max ) was observed at the surface planting for both plants. The H _max was reduced at temperatures either lower or higher than 20/15°C, with a more notable reduction in wild barley. At all temperature regimes, the time taken to reach 50% of the H _max (i.e. H₅₀) increased linearly with the sowing depth but, at higher temperatures, the accelerated growth rate reduced the H₅₀. The wild barley seedling emergence and height gain rate, as expressed relative to those of wheat, revealed the highest superiority of wheat over wild barley at 25/20°C and the sowing depth of 4 cm.     

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.     

Growth in vitro and infectivity of Colletotrichum coccodes on potato tubers at different temperatures

To investigate the ability of black dot symptoms to develop on infected potato tubers during storage, the growth of Colletotrichum coccodes was followed in vitro on malt agar at temperatures ranging from 5–27°C, and in vivo on artificially infected potato tubers kept at 5, 10 and 15°C. In vitro , 13 isolates from different geographical origins grew at all temperatures tested; growth started with a delay of 10 days at 5°C and of 4 days at 10°C, and was fastest at 27°C. All isolates had similar growth patterns and produced conidia and sclerotia at all temperatures. Minitubers were successfully infected at 5, 10 and 15°C by depositing either a mycelial plug or a drop of conidial suspension on the tuber surface. Sclerotia were observed after 7 days at the point of inoculation. Symptoms extended in all cases, although more slowly at 5 and 10 than at 15°C. Latent infections were detected in up to 21% of tubers without black dot symptoms at harvest. These results show that latent infections by C. coccodes are probably quite frequent, and that the pathogen is able to develop at low temperatures in controlled conditions. This suggests that black dot symptoms can increase during storage if stores are not adequately managed.