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.     

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.     

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.     

Effect of inoculum type and timing of application of Coniothyrium minitans on Sclerotinia sclerotiorum: influence on apothecial production

The effects of different inocula of the mycoparasite Coniothyrium minitans on carpogenic germination of sclerotia of Sclerotinia sclerotiorum at different times of year were assessed. A series of three glasshouse box bioassays was used to compare the effect of five spore-suspension inocula of C. minitans , including three different isolates (Conio, IVT1 and Contans), with a standard maizemeal–perlite inoculum. Apothecial production, as well as viability and C. minitans infection of S. sclerotiorum sclerotia buried in treated soil, were assessed. Maizemeal–perlite inoculum at 10⁷ CFU per cm³ soil reduced sclerotial germination and apothecial production in all three box bioassays, decreasing sclerotial recovery and viability in the second bioassay and increasing C. minitans infection of sclerotia in the first bioassay. Spore-suspension inocula applied at a lower concentration (10⁴ CFU per cm³ soil) were inconsistent in their effects on sclerotial germination in the three box bioassays. Temperature was an important factor influencing apothecial production. Sclerotial germination was delayed or inhibited when bioassays were made in the summer. High temperatures also inhibited infection of sclerotia by C. minitans . Coniothyrium minitans survived these high temperatures, however, and infected the sclerotia once the temperature decreased to a lower level. Inoculum level of C. minitans was an important factor in reducing apothecial production by sclerotia. The effects of temperature on both carpogenic germination of sclerotia and parasitism of sclerotia by C. minitans are discussed.     

Quantitative Aspects of Infection of Sclerotinia sclerotiorum Sclerotia by Coniothyrium minitans – Timing of Application, Concentration and Quality of Conidial Suspension of the Mycoparasite

White mould disease leads to production of sclerotia, which subsequently survive in soil and may be responsible for future epidemics. The effect of the mycoparasite Coniothyrium minitans in decreasing survival of sclerotia of Sclerotinia sclerotiorum was studied. Infection of sclerotia of S. sclerotiorum by C. minitans can be achieved by a single conidium. Under optimal conditions, 2 conidia per sclerotium produced 63% of the maximum infection (ca. 90%) of sclerotia produced by up to 1000 conidia. Similar results were observed on the infection of stem pieces infected by S. sclerotiorum. In field trials, application of conidial suspensions of C. minitans to a bean crop soon after white mould outbreak led to a higher percentage of sclerotial infection than later applications. Ninety per cent infection of sclerotia was obtained within 3 weeks of application by C. minitans suspensions in the range of 5 × 10⁵ and 5 × 10⁶ conidia ml^–1 at 1000 l ha^–1. The concentration of the conidial suspensions and the isolate used were of less importance. The result was marginally affected by the germinability of the conidia (75% against 61% infected sclerotia at 91% and 16% viability of isolate IVT1, respectively). Less apothecia of S. sclerotiorum developed in soil samples collected after 2 months from plots sprayed immediately after disease outbreak than from those treated 11–18 days later. It is concluded that a suspension of 10⁶ conidia ml^–1 in 1000 l ha^–1 (= 10¹² conidia ha^–1) sprayed immediately after the first symptoms of disease are observed, results in > 90% infection of sclerotia of S. sclerotiorum. The infection of sclerotia, which prevents their carry-over, occurs within a broad range of inoculum quality.     

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.     

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.     

Effect of environmental factors and Sclerotium cepivorum isolate on sclerotial degradation and biological control of white rot by Trichoderma

Laboratory assays demonstrated that two isolates of Trichoderma viride and one isolate of Trichoderma pseudokoningii degraded up to 80% of sclerotia of four isolates of Sclerotium cepivorum in a silty clay soil, and also degraded up to 60% of sclerotia in three other soil types. Relationships were defined between the degree of sclerotial degradation by the two T. viride isolates in the silty clay soil and both temperature and soil water potential. Sclerotia were degraded between 10 and 25°C at −0·00012 MPa, but there was little activity of T. viride at 5°C or at −4 MPa. Degradation of S. cepivorum sclerotia also occurred in the absence of Trichoderma at soil water potentials approaching saturation . Experiments using onion seedling bioassays showed that the efficacy of Trichoderma isolates for the control of white rot using the same selection of soils and S. cepivorum isolates was variable, but that there was significant disease control overall. The importance of environmental factors and pathogen isolate in relation to effective biological control of white rot is discussed.     

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.     

Plasma-activated water inhibits in vitro conidial germination of Colletotrichum alienum,a postharvest pathogen of avocado

Cold plasma, an ionized gas produced by applying an electrical current to air, can be used to produce plasma-activated water (PAW), which has excellent antimicrobial properties. In this study PAW was applied to conidia of Colletotrichum alienum to investigate its impact on conidial germination in vitro. PAW was produced by treating tap, deionized, or distilled water with cold plasma for 30 or 60 min to produce PAW30 or PAW60, each of which was then incubated for up to 24 hr with a conidial suspension of C. alienum in a ratio of 1:1, 1:2, or 1:3 (conidial suspension:PAW), and the percentage germination measured. The greatest reduction in germination occurred when conidia were incubated with PAW60 produced from deionized water or distilled water, for all ratios. For PAW30, deionized water was the most effective for all three ratios, and on this basis, deionized water was selected for all further experiments. PAW produced from smaller volumes of water and at shorter distances from the cold plasma source was more effective at reducing germination. Treatment of conidia with acidified water was not as effective as PAW at inhibiting germination. Nitrates and nitrites were present in the PAW in varying concentrations and may have contributed to the inhibition of germination. PAW retained activity and reduced germination even after storage for 15 days. These findings demonstrate the potential of PAW as a novel treatment for postharvest fungal pathogens.     

Optimizing conditions for growth and sporulation of Pyrenophora semeniperda

The effects of photoperiod, light quality, wounding and sealing of culture plates on in vitro growth and sporulation of Pyrenophora semeniperda were examined to define conditions conducive to the abundant and rapid production of conidia. For maximum growth and sporulation, the leaf-spotting and seedborne plant pathogen required an alternating light/dark sequence. Fewer conidia were produced under conditions of constant illumination or constant darkness. Growth was enhanced by light of wavelengths longer than 500 nm, while sporulation was enhanced by light of wavelengths shorter than 500 nm. Sporulation was enhanced under conditions of alternating temperature with a diurnal photoperiod and by mycelial wounding. Sealing of Petri plates had an inhibitory effect on sporulation. Significantly more conidia of higher quality formed when unsealed cultures, wounded after 7 days, were exposed to 23°C during the light phase and 19°C during darkness. Under these optimal conditions, conidial numbers were increased by 800% or more compared with unwounded cultures grown at constant 25°C in 12 h alternating cool-white light.     

Effects of humidity, leaf wetness, temperature and light on conidial production by Phaeoisariopsis personata on groundnut

Controlled-environment studies of conidial production by Phaeoisariopsis personata on groundnut are described. With constant relative humidity (RH), conidia were only produced above a threshold (94·5% RH) and there was a linear increase between 94·5% RH and 100% RH. Conidial production was less with continuous leaf wetness (resembling heavy dew) than with continuous 98–99% RH, but it was similar with intermittent leaf wetness and intermittent 98–99% RH (8 h at 70% RH each day). With alternate high (≥97% RH) and low humidity, daily conidial production depended both on the duration of high RH and on the low RH value. With 99% RH at night (12 h), night-time conidial production decreased with the previous daytime RH. After conidial production had started, small numbers of conidia were produced even when the RH was well below the threshold (94·5%). Conidia were produced in continuous light when the photon flux density was 2 μmol/m²/s, but production was completely inhibited with 60 μmol/m²/s. With constant RH, more conidia were produced with a 12 h photoperiod than in continuous darkness. However, more than 75% of the conidia were produced in the dark. With continuous darkness, more conidia were produced during the night (18.00–06.00 h) than during the day, but this biological rhythm was overcome with a (light-night)/(dark-day) regime. With constant 98–99% RH there was a linear increase in conidial production with temperature between 10 and 28°C, and virtually no conidia were produced at 33°C. The daily production of conidia increased with time for 2 to 6 days, depending on the treatment.     

Survival of Oidium anacardii on cashew (Anacardium occidentale) in southern Tanzania

During March and April of 1993 and 1994, surveys on the incidence and severity of cashew powdery mildew ( Oidium anacardii ) were conducted in the Newala, Mtwara, Nachingwea and Tunduru areas of southern Tanzania to determine the variation in perennation between localities. Only immature cashew shoots, panicles and fruit can be infected by O. anacardii conidia. Cashew trees at sites in each district were assessed for shoot and panicle production and cashew powdery mildew. Survival of O. anacardii between seasons, in any area, was determined by the degree of production of shoots that were within the canopy and by the incidence of infection. Immature shoots produced from the main branches within the tree canopy were the main source of active powdery mildew in all districts; trees in the Newala district had the highest numbers of infected immature shoots in comparison with survey sites in the other areas. During the 1994 cashew-growing season (June–August), powdery mildew developed more rapidly and affected more shoots on the inside of the tree canopy than on the outside. Germination of conidia was reduced after aqueous suspension for 3 h. Germination on cashew leaves submerged under 2 mm of water was not affected. Appressorial and hyphal formation by germinating conidia on leaves decreased with increasing duration under water. Germination of conidia on glass slides at 100% r.h. was higher at 25 and 30°C than at 15°C and there was no germination at 35°C.     

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.     

Components of quantitative resistance to powdery mildew (Erysiphe pisi) in pea (Pisum sativum)

Components of quantitative resistance in pea ( Pisum sativum ) to Erysiphe pisi , the pathogen causing powdery mildew, were investigated. Conidium germination, infection efficiency, latent period and conidium production dynamics on cv. Quantum (quantitatively resistant) were compared with those on Pania and Bolero (susceptible). There was an additional comparison in conidium germination experiments with the resistant cv. Resal. Quantitative resistance in Quantum did not affect conidium germination, but infection efficiency of conidia on this cultivar was 34% less than on the susceptible Pania. More conidia germinated on 5-day-old leaflets than on 15-day-old leaflets but the age of the plant did not affect percentage germination or infection efficiency. The length of the latent period did not differ between cultivars. Total conidium production (AUC) per unit leaflet area on Quantum was 25% less than on Pania. The maximum conidium production per day (CMAX) per unit leaflet area on Quantum was 33% less than on Pania. The time to maximum conidium production per day (TMAX) was 10% longer on Quantum than on Pania. The cv. Bolero, reported to be susceptible, also showed some degree of quantitative resistance, but this differed from that of Quantum. Total conidium production was less on Bolero than on Quantum, but the conidia on Bolero were produced sooner, and for a shorter period, than on Quantum. The stability of these responses was tested by analysing components in three different temperature regimes and testing for interactions with temperature, and with leaflet age. Temperature affected all conidium production variables. AUC per leaflet area was nearly seven times as great and CMAX nearly 15 times greater at 23°C than at 13°C. TMAX increased by 1.5 times when temperature increased from 13°C to 18°C or 23°C. Several interactions occurred and these are described.     

Use of sclerotia of<Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> for efficient production of conidia of<Emphasis Type="Italic">Coniothyrium minitans</Emphasis> in liquid culture

A study was conducted to determine the feasibility of using sclerotia ofSclerotinia sclerotiorum for producing conidia ofConiothyrium minitans in liquid culture. The medium (SST) was made of water containing 2.0, 1.5, 1.0 or 0.5% (w/v) ground sclerotia ofS. sclerotiorum and 100 μgl ⁻¹ thiamine hydrochloride (HCl). One ml of conidial suspension (2 × 10⁷ conidia ml⁻¹) ofC. minitans LRC 2534 was inoculated into 100 ml of SST medium or control (thiamine HCl in water) and incubated at 20 ± 2°C on a shaker at 200 rpm. Subsamples were removed periodically and examined under a compound microscope. Conidia in the SST media germinated within 24 h, developed into branched hyphae within 48 h, produced pycnidia after 3–4 days, and the pycnidia released mature conidia after 7 days. Production of conidia varied with the concentration of sclerotia in the SST medium. It was lower (3.6 × 10⁶ conidia ml⁻¹) at 0.5% but higher (1.2 × 10⁸ conidia ml⁻¹) at 2%. The new conidia were viable and the colonies developing from them showed the original morphological characteristics. It was concluded that using SST liquid medium as a substrate for mass production of conidia ofC. minitans has potential for use in commercial development of this mycoparasite as a biocontrol product. http:www.phytoparasitica.org posting Jan. 23, 2007.     

Laboratory studies on the growth and reproduction of Cladosporium allii-cepae, the cause of leaf blotch of onion

Cladosporium allii-cepae is a slow-growing pathogen of onion which, on malt extract agar, had a mean colony diameter of 2.7 cm after 28 days at 16°C. Growth and reproduction were greatest on malt onion leaf agar and were poor on synthetic media. The optimum and maximum temperatures for growth were 20 and 28°C, respectively. Sporulation was most abundant at 10–15°C. The fungus grew poorly in buffered Czapek Dox medium at pH 2.2–7.8 and most growth was recorded at pH 6.5. Sporulation was enhanced by exposure of colonies to near ultraviolet light.
A large proportion of spores germinated in distilled water and at 100% r.h. In distilled water germination was greatest at 15–20° and in air at 100% r.h. at 20°C. Of 10 fungicides tested, fentin hydroxide, fentin acetate/maneb and iprodione were the most effective in inhibiting spore germination, growth and reproduction of the fungus.     

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.     

Effects of temperature on germination and hyphal growth from conidia of Ramularia rhei and Ascochyta rhei, causing spot diseases of rhubarb (Rheum rhaponticum)

Rhubarb leaf and petiole spot disease, caused by Ramularia rhei and Ascochyta rhei , has gradually become more noticeable in the UK field crop. Conidial germination and subsequent colony growth of R. rhei and A. rhei were investigated under in vitro conditions on potato dextrose agar and in vivo on leaf discs. Results indicated that the two fungi responded differently to temperature. Ramularia rhei was better adapted to temperatures ≤ 25°C, with an optimum around 20°C, whereas A. rhei was more adapted to temperatures ≥ 15°C, with an optimum > 25°C. Overall, conidia of R. rhei germinated and subsequent colonies grew at greater rates than those of A. rhei on leaf discs at temperatures ≤ 25°C. These results indicated that it is important to identify the causal agent of leaf and petiole spot diseases in rhubarb field crops in order to estimate disease risks accurately.