Effects of temperature on germination and hyphal growth from ascospores of A-group and B-group Leptosphaeria maculans (phoma stem canker of oilseed rape)

Ascospores of both A‐group and B‐group Leptosphaeria maculans germinated at temperatures from 5–20°C on distilled water agar or detached oilseed rape leaves. After 2 h of incubation on water agar, some A‐group ascospores had germinated at 10–20°C and some B‐group ascospores had germinated at 5–20°C. The percentages of both A‐group and B‐group ascospores that had germinated after 24 h of incubation increased with increasing temperature from 5–20°C. The observed time (Vo₅₀) which elapsed from inoculation until 50% of the spores had germinated was shorter for B‐group than for A‐group ascospores. Germ tube length increased with increasing temperature from 5–20°C for both ascospore groups. Germ tubes from B‐group ascospores were longer than germ tubes from A‐group ascospores at all temperatures tested, but the mean diameter of germ tubes from A‐group ascospores (1.8 μm) was greater than that of those from B‐group ascospores (1.2μm) at 15°C and 20°C. The average number of germ tubes produced from A‐group ascospores (3.8) was greater than that from B‐group ascospores (3.1) after 24 h of incubation at 20°C, on both water agar and leaf surfaces. Germ tubes originated predominantly from interstitial cells or terminal cells of A‐group or B‐group ascospores, respectively, on both water agar and leaf surfaces. Hyphae from A‐group ascospores grew tortuously with extensive branching, whilst those from B‐group ascospores were predominantly long and straight with little branching, whether the ascospores were produced from oilseed rape debris or from crosses between single ascospore isolates, and whether ascospores were germinating on water agar or leaf surfaces.     

The RustPuccinia abruptavar.partheniicola,a Potential Biocontrol Agent of Parthenium Weed: Environmental Requirements for Disease Progress

The rust fungusPuccinia abruptavar.partheniicola,a potential biological control agent of parthenium weed (Parthenium hysterophorus), was evaluated under controlled environmental conditions. A range of spore germination temperatures as well as dew period durations and temperatures were investigated to determine some of the environmental requirements for disease establishment and disease progress. Plants were inoculated with urediniospores and exposed to dew periods between 3 to 12 h at temperatures of 10, 15, or 20°C. For disease expression, the inoculated plants were then grown in a glasshouse at one of two temperature regimes (30/26°C or 18/13°C; day/night). Urediniospores germinated best at 12 ± 1°C, with lower germination rates at 5°C or above 20°C. No infection occurred when the plants were exposed to dew periods of ≤3 h, regardless of the incubation temperature. The disease progressed most rapidly when plants were inoculated and incubated for a dew period of at least 12 h at a temperature of 15 ± 1°C. The disease progressed most slowly following inoculation at dew periods of 6 h or less. Disease progress was more rapid when the plants were exposed to a cool-temperature regime (18/13°C) than when exposed to a warm-temperature regime (30/26°C). This suggests that good infection of parthenium weed could be obtained when the urediniospores arrive on the plants during the afternoon in the cooler months of the central Queensland autumn when relatively long dew periods are expected.     

Germination and Survival of Fusarium graminearum Macroconidia as Affected by Environmental Factors

The effects of temperature (4–20°C), relative humidity (RH, 0–100%), pH (3–7), availability of nutrients (0–5 g/l sucrose) and artificial light (0–494 μmol/m²/s) on macroconidial germination of Fusarium graminearum were studied. Germ tubes emerged between 2 and 6 h after inoculation at 100% RH and 20°C. Incubation in light (205 ± 14 μmol/m/s) retarded the germination for approximately 0.5 h in comparison with incubation in darkness. The times required for 50% of the macroconidia to germinate were 3.5 h at 20°C, 5.4 h at 14°C and 26.3 h at 4°C. No germination was observed after an incubation period of 18 h at 20°C in darkness at RH less than 80%. At RH greater than 80%, germination increased with humidity. Germination was observed when macroconidia were incubated in glucose (5 g/l) or sucrose (concentration range from 2.5 × 10^?4 to 5 g/l) whereas no germination was observed when macroconidia were incubated in sterile deionized water up to 22 h. Macroconidia germinated quantitatively within 18 h at pH 3–7. Repeated freezing (?15°C) and thawing (20°C) water agar plates with either germinated or non‐germinated macroconidia for up to five times did not prevent fungal growth after thawing. However, the fungal growth rate of mycelium was negatively related to the number of freezing events the non‐germinated macroconidia experienced. The fungal growth rate of mycelium was not significantly affected by the number of freezing events the germinated spores experienced. Incubation of macroconidia at low humidity (0–53% RH) suppressed germination and decreased the viability of the spores.     

The significance of temperature during sporulation on the biology of pycnidiospores of Mycosphaerella ligiilicola

The germination, infectivity and survival of pycnidiospores obtained from cultures of Mycosphaerella ligulicola grown at 15 and 26 °C were compared. Spores formed at 26° (‘26° spores’) were less able to germinate at low relative humidities and showed a narrower temperature range for maximum germination after 6 h. At high spore densities 26° spores showed self-inhibition of germination and, over a range of lower densities, growth of their germ tubes was checked, which resulted in lower infection of leaf discs compared with 15° spores in which this phenomenon did not occur. The fungus could be recovered from un-sterile compost over a longer period after inoculation with 15° spores. Only after storage at a temperature well below zero was there a difference in viability between 15° and 26° spores. It is thought that the potential advantage of producing larger numbers of spores at 26° would be realized only under optimum conditions for dispersal and infection. The smaller number of spores produced at 15° are likely to be successful under natural conditions.     

Host and environmental effects on the infection of maize by Puccinia sorghi. I. Prepenetration development and peneration

Germination of urediniospores of Puccinia sorghi on leafves and on ager was sminilar over the range 5–25°C, being greatest at 15°C, At this temperature, maximum germination was attained withing 5 h. Germination on cover slips started at around 99% r. h. and increased with of humidity. Urediniospore germination was not affected by leaf age. In generalk, proportionally more spores germinated on the abaxial than on the adaxial surface. Maximum germination was observed on the abxial surface of the tip portion of the leaf. The optimum temperature for infection structure formation was about 15°C, A munimum period of 3–4 h was required for the initiation of infection. Increase in appressorium and sub-stomatal vesicle formation with increase in dew perio ws observed, with the maxima being attained at about 24 h after inculation.     

Conditions for Infection of Apple by Phytophthora syringae

The processes leading to Phytophthora fruit rot were divided into two main stages for the purposes of investigating the effects of temperature and duration of wet periods on pathogen development: oospore germination and infection of fruit by zoospores. It was found that the first stage was markedly affected by temperature over the range 10–20°C and required a wet period of 4–7 days. At 18 and 20°C, activation was low regardless of the length of the wet period. Once oospore germination (first stage) had occurred, free water was necessary for only a few hours for fruit infection (second stage) to occur, but the incidence of infection rose rapidly over the first 48 h, regardless of temperature over the range 10–20°C. From the data obtained, mathematical models were produced relating the incidence of Phytophthora fruit rot to the two weather variables. These models can be used to develop a weather‐based risk assessment system for the disease.     

Temperature and Leaf Wetness Effects on Infection of Sugarcane by Puccinia melanocephala

Brown rust epidemics in sugarcane, caused by Puccinia melanocephala, vary in severity between seasons. To improve the understanding of disease epidemiology, the effects of leaf wetness, temperature and their interaction on infection of sugarcane by the pathogen were studied under controlled conditions. Disease severity was low at 15 and 31°C regardless of leaf wetness duration. No infection occurred with a 4‐h leaf wetness period. Increasing leaf wetness duration from 7 to 13 h lowered the temperature required for disease onset from 21 to 17°C. More infection occurred with 13 compared to 10 h of leaf wetness at 17°C, and severity decreased for all leaf wetness periods at 29 compared to 27°C. Postinfection suboptimal low and high temperatures increased the time required for lesion development and high temperatures decreased maximum disease severity. The observed effects of leaf wetness and temperature on infection by P. melanocephala could help explain the initiation, rate of increase and decline of brown rust epidemics in the field.     

Scanning electron microscopy of differentiating and non-differentiating uredosporelings of wheat stem rust fungus (Puccinia graminis f. sp. tritici) on an artifical substrate

Germination of uredospores of the wheat stem rust fungus (Puccinia graminis f. sp. tritici) on a Millipore membrane and differentiation of sporelings into hyphae and infection structures induced by a heat (30 °C) shock treatment are described. Development of infection structures on an agar medium was generally similar to those formed in vivo although some variations were also observed. Anastomoses among branches of the same hypha and between different hyphae were common. Uredospores germinated on Millipore membranes without the heat shock treatment, produced only undifferentiated long germ tubes; however, differentiation occurred when the spores were germinated on the agar medium by subjecting to the non-differentiating treatment (20 °C/3.5 hr) and incubating at 24 °C.     

Environmental factors influencing the infection of wheat by Puccinia graminis

Germination rate and total germination of Puccinia graminis uredospores were directly related to pustule age and duration between spore collections. Partial drying of the spores enhanced germination rate; keeping them for 18 h at 100% r.h. reduced both rate and total germination. Spores germinated in polystyrene dishes between 4 and 29 °C and optimally between 15 and 23 °C Light (3 cal/cm²/h) had little effect on germination on moist surfaces but inhibited germination on the leaf. In Hybrid 229/8 wheat this effect was more pronounced than in var. Little Club. The number of primary infections increased linearly with duration of surface wetness with a narrow temperature optimum at 23.5 °C. Two phases of infection could be distinguished: germination (requiring darkness and capable of taking place over a wide temperature range) and penetration (requiring light and slightly higher temperature than for germination). Stomatal closure caused by subjecting the plants to water stress led to proporational reductions in infection. The results are discussed in relation to dew formation.     

Host and environmental effects on the penetration of oats by Puccinia graminis avenae and Puccinia coronata avenae

The frequency of penetration from appressoria of Puccinia graminis avenae and P. coronata avenae varied among Avena species and between oat cultivars, although both rusts produced susceptible infection type pustules on the cultivars tested. Penetration on cv. Garry was significantly less than that on the Avena species (A. barbata, A.fatua and A. sterilis) studied and penetration of these Avena species was significantly less than on the cvs Algerian and Fulmark. When the rusts were allowed to develop into pustules on seedlings which had been inoculated with fixed amounts of inoculum, there was a direct relationship between number of pustules produced and penetration frequency. The effects of temperature, light and dew period on penetration from appressoria of ‘single race’ and ‘mixed race’ inocula was also studied on these cultivars and species. Penetration by P. graminis avenae was greatest at 30–35 °C and at light intensities of 5625 lux and above, whereas that by P. coronata avenae was greatest at 20 °C and was unaffected by artificial light intensities up to n 250 lux. Maximal penetration by P. graminis avenae and P. coronata avenae was observed after inoculated plants had been exposed to dew periods of 16 and 12 h respectively. Some penetration was observed after a dew period of 8 h. The time taken for each rust to attain maximum penetration varied from 36 to 52 h after inoculation, depending on the environment, and was usually less for P. coronata avenae than for P. graminis avenae.     

Effect of variable dew temperatures on infection of green foxtail by Pyricularia setariae,Drechslera gigantea,and Exserohilum rostratum

A thermogradient apparatus was used to investigate the effect of variable dew temperatures on infection of green foxtail by the indigenous pathogen Pyricularia setariae (Ps) and the exotic pathogens Drechslera gigantea (Dg), and Exserohilum rostratum (Er) from the southern USA that showed bioherbicide potential against several grassy weeds. This device is capable of creating multiple diurnal temperature cycles, mimicking daily temperature fluctuations that occur under field conditions. Seven temperature regimes, i.e., 15/10 °C, 20/5 °C, 20/15 °C, 25/10 °C, 25/20 °C, 30/15 °C, and 30/25 °C (maximum/minimum), were used with temperature cycling from maximum to minimum and then back up to maximum in a 24 h period. Ps and Dg were much more virulent than Er on green foxtail, resulting in higher levels of disease and weed control. Dg was little affected by the dew temperatures in terms of plant infection and was more efficacious than Ps under cooler dew temperatures (15/10 °C and 20/5 °C), causing twice as much disease. This greater amount of disease coincided with higher conidial germination, appressorial formation and infection-hypha frequency by Dg at the lower temperatures. The efficacy of Ps improved as dew temperature increased, accompanied by a higher percentage of germination and more frequent appressorial production. Dg caused severe disease 2 d after inoculation whereas Ps required 4 d to initiate disease symptoms. These observations suggest that Dg is a superior candidate than Ps for green foxtail control on the Canadian prairies.     

Development of ABA Antagonists to Overcome ABA- and Low Temperature-Induced Inhibition of Seed Germination in Canola,Lentil, and Soybean

ABA antagonists have potential application as growth regulators to improve germination and seedling growth at low temperatures for oilseeds and pulses grown in regions with short seasons such as those in western Canada. Towards development of practical ABA antagonists, a series of 3′-substituted ABA analogs were synthesized and screened in seed germination assays in canola (Brassica napus), lentil (Lens culinaris), and soybean (Glycine max) at low temperature and in overcoming exogenous ABA. The most promising analog, ABA 1009, was selected for further germination testing of dose responses in canola, lentil, and soybean. Analog ABA 1009 at 100 µM was effective in overcoming ABA (10 µM)-induced inhibition for canola, lentil, and soybean germination at ambient temperature, and also promoted germination at low temperature for canola (5 °C).

     

Preservation of Phakopsora pachyrhizi Uredospores

This study compared different temperatures and dormancy‐reversion procedures for preservation of Phakopsora pachyrhizi uredospores. The storage temperatures tested were room temperature, 5°C, ?20°C and ?80°C. Dehydrated and non‐dehydrated uredospores were used, and evaluations for germination (%) and infectivity (no. of lesions/cm²) were made with fresh harvested spores and after 15, 29, 76, 154 and 231 days of storage. The dormancy‐reversion procedures evaluated were thermal shock (40°C/5 min) followed or not by hydration (moist chamber/24 h). Uredospores stored at room temperature were viable only up to a month of storage, regardless of their hydration condition. Survival of uredospores increased with storage at lower temperatures. Dehydration of uredospores prior to storage increased their viability, mainly for uredospores stored at 5°C, ?20°C and ?80°C. At 5°C and ?20°C, dehydrated uredospores showed increases in viability of at least 47 and 127 days, respectively, compared to non‐dehydrated spores. Uredospore germination and infectivity after storage for 231 days (7.7 months), could only be observed at ?80°C, for both hydration conditions. At this storage temperature, dehydrated and non‐dehydrated uredospores exhibited 56 and 28% of germination at the end of the experiment, respectively. Storage at ?80°C also maintained uredospore infectivity, based upon levels of infection frequency, for both hydration conditions. Among the dormancy‐reversion treatments applied to spores stored at ?80°C, those involving hydration allowed recoveries of 85 to 92% of the initial germination.     

White rust of chrysanthemums

Teleutospores of Puccinia horiana Henn. germinate and discharge sporidia between 4 and 23 °C. At the optimum temperature of 17 °C sporidia discharge starts within 3 h. Maximum germination of the sporidia takes place within 2·5 h between o and 30 °C, there being no clear optimum. High humidity and a film of moisture appear to be necessary for germination of both teleutospores and sporidia. Sporidia can penetrate either leaf surface of chrysanthemum to cause infection between 4 and 24 1°C and within the optimum temperature range, 17–24 °C, effectively penetrate within 2 h. The sporidia are very sensitive to desiccation at below 90 % relative humidity. Methods are described, using leaf discs and whole plants, for screening chrysanthemum cultivars for susceptibility to white rust. Cultivars were placed in five classes ranging from susceptible to immune. Leaf discs of immune cultivars can be distinguished within 30 h by a brown discolouration at the point of inoculation. The early stages of development of the fungus in susceptible, resistant and immune hosts are described. The incubation period in susceptible plants is normally 7–10 days, teleutospores being formed a few days later. Leaves become less susceptible with age but the oldest leaves on 5-month-old plants could still be infected. The maximum survival time of teleutospores in the sori on detached leaves was 8 weeks but was considerably less under moist conditions or buried in soil. Low doses of a mancozeb with zineb fungicide controlled infection by preventing penetration rather than by inhibiting sporidial germination.     

Modelling the effects of temperature and wetness on the polycyclic phase of Stemphylium vesicarium,the pathogen causing purple spot on asparagus (Asparagus officinalis L.)

The polycyclic phase of Stemphylium vesicarium is the key factor for the forecast and integrated control of purple spot on asparagus. The annual dynamics of airborne conidia were determined under field conditions by conidia traps. From 2013 to 2015, conidia became airborne at the earliest at mid‐July, but the number trapped was considerably enhanced only after mid‐August, early September. The cumulative percentage of trapped conidia was best described using a logistic function depending on the daily temperature sum (base 0°C) accumulated only on days with >0.2 mm of rainfall (R² = .81). The germination of conidia was modelled by a generalized beta‐modified Chapman Richards function, and the germ tube length was modelled by a generalized beta‐power function. Conidia germinated in a wide temperature range, with an optimum at 23.3°C, whereas germ tube length had a narrow nearly optimum temperature range around 28.7°C, which indicates that infection by conidia is more restricted by germ tube growth than by germination. The effect of temperature on the number of lesions produced by two strains on green asparagus spears had the narrowest optimum range (optimum at 21.9°C) of all parts of the polycyclic phase. In plant tissue, the spread of the fungus depends on the mycelium growth. The mycelium growth of the four strains, which was modelled with data from a petri dish experiment, had an optimum temperature at 24.7°C.     

Effect of short‐duration high temperatures on weed seed germination

Thermal soil disinfestation techniques are effective reducers of weed seedbank and weed emergence. Two experiments (Expt 1 and Expt 2) were conducted to test the effect of brief exposure to varying temperatures on the seed germination of Amaranthus retroflexus, Echinochloa crus‐galli, Galinsoga quadriradiata, Portulaca oleracea, Setaria viridis and Solanum nigrum. To this end, species seeds were moistened with loamy‐sand soil and placed into test tubes. The tubes were heated rapidly and then cooled by dipping them into a hot water bath until target temperatures were achieved. Expt 1 temperatures ranged between 55°C and 85°C at 5°C intervals and Expt 2 ranged between 48°C and 86°C at 2°C intervals. Thereafter, the tubes were dipped into a cooling (1°C) water bath. Exposure to target temperatures ranged between 2 s and 5 s. Soil temperatures were monitored using embedded thermocouples. A log‐logistic dose–response model described the effect of heating on seed germinability; temperatures required for 99% reductions were calculated. On the basis of the predictive model equation used, weed species' germination sensitivity to high temperature exposure can be ranked as follows: E. crus‐galli (79.6°C), S. viridis (75.8°C), S. nigrum (74.6°C), P. oleracea (72.2°C), A. retroflexus (70.9°C) and G. quadriradiata (68.1°C). The interval between no effects to complete seed devitalisation occurred at temperatures varying from 6.5°C to 15.7°C. Seed size and weight varied directly with heat tolerance. Study results not only inform the timing and optimal adjustment for effective thermal soil treatment, but also demonstrate a relatively simple and generalizable methodology for use in other studies.     

A review of white rust (Puccinia horiana Henn.) disease on chrysanthemum and the potential for its biological control with Verticillium lecanii (Zimm.) Viégas

The biology, aetiology and epidemiology of Puccinia horiana, the cause of white rust disease of chrysanthemum (Chrysanthemum spp.) is reviewed in relation to current environmental, cultural and chemical methods for its control. Importantly, basidiospore release, germination and infection can take as little as 5 h at optimum r.h. (96%) and temperature (between 17–24°C). Recent developments using the fungus Verticillium lecanii for the control of insects on glasshouse-grown all-year-round chrysanthemums rely upon the maintenance of r.h. during night periods in excess of 95%, thus predisposing plants to white rust attack. However, V. lecanii is unusual in that it can also parasitise spores and fruiting structures of a range of rust fungi including P. horiana. This mycoparasitic ability is also reviewed, and against this background, the potential for an integrated insect and white rust control programme on all-year-round chrysanthemums is assessed.     

Effect of temperature and dark pretreatment on the germination of three species of Jamesonia (Pteridaceae,Polypodiopsida)

To study the influence of temperature on the germination ability of three species of Jamesonia (Jamesonia imbricata, Jamesonia scammaniae and Jamesonia rotundifolia), spores were cultured at 10°C, 15°C and 20°C. A temperature of 15°C was selected as representative of the natural annual average temperature of the paramo environment that Jamesonia species inhabit. In addition, a dark pretreatment of 2 days was tested to verify if germination was enhanced. The results indicated that germination of Jamesonia, considering the three species as a whole, is affected by temperature, but is independent of the dark treatment. All species showed higher and faster germination at 20°C, and exhibited a threshold minimum temperature around 10°C, below which germination is avoided or extremely low and delayed. This could suggest that spore germination in Jamesonia is adapted to establish gametophyte populations during frost‐free periods.     

The biology of Peronospora viciae on pea: laboratory experiments on the effects of temperature, relative humidity and light on the production, germination and infectivity of sporangia

Germination of Peronospora viciae sporangia washed off infected leaves varied from 20% to 60%. Sporangia shaken off in the dry state gave 11–19% germination. Most sporangia lost viability within 3 days after being shed, though a few survived at least 5 days. Infected leaves could produce sporangia up to 6 weeks after infection, and sporulating lesions carried viable sporangia for 3 weeks. Sporangia germinated over the range 1–24 °C, with an optimum between 4 and 8 °C. Light and no effct. The temperature limits for infection were the same as for germination, but with an optimum between 12 and 20 °C. A minimum leaf-wetness period of 4h was required, and was independent of temperature over the range 4–24 °C. Maximum infectivity occurred after 6h leaf wetness at temperatures between 8 and 20 °C. Infection occurred equally in continuous light or in darkness. After an incubation period of 6–10 days sporangia were produced on infected leaves at temperatures between 4 and 24 °C, with an optimum of 12–20 °C. Exposure to temperatures of 20–24 °C for 10 days reduced subsequent sporulation. Sporangia produced at suboptimal temperatures were larger, and at 20 °C. smaller, than those produce at 12–16 °C. Viability was also reduced. No sporangia were produced in continuous light, or at relative humidities below 91%. For maximum sporulaiton an r.h. of 100% was required, following a lower r.h. during incubation. Oospores wre commonly formed in sporulating lesions, and also where conditons limited or prevented sporulation. The results are discussed briefly in relaiton to disease development under field conditions.     

Rhythmic Behavior in Germination of Cocklebur Seeds

Non-dormant, lower seeds of cocklebur (Xanthium pensylvanicum Wallr.) germinated with unimodal flush after 20 and 36 h from the start of water imbibition at 33 and 23°C, respectively. At 28°C, however, germination occurred bimodally, the time of each peak coinciding with that at 23 and 33°C. This type of germination behavior was induced even at 33°C, when the seeds were contacted with some osmotica. Further, the application of different osmotica at 28°C caused a rhythmic multimodal germination with a period of about 16 h. It was suggested that an endogenous rhythmicity may be involved in the control of cocklebur seed germination.