Influence of Air Temperature on the Release of Ascospores of Venturia inaequalis

Abstract The influence of air temperature on the release pattern of Venturia inaequalis ascospores was studied by volumetric spore samplers in two spore sampling periods. In the first period (1991–1996; Passo Segni, Ferrara), 15 ascospore dispersal events were considered occurring in daylight, with high spore counts (168–5892 ascospores per m³ air per event), at an average temperature between 8.4 and 20.3°C. Both the length of the ascospore release period and distribution of airborne spores over time were significantly influenced by temperature. A logistic regression model was used to fit the proportion of ascospores trapped from the orchard air as a function of time after the beginning of the discharge event and air temperature. The accuracy of this equation was tested against data collected in the second spore sampling period (1997–2000; Sala Bolognese, Bologna, and Castelfranco, Modena); 16 dispersal events were considered, triggered by rainfall that occurred both in the dark and in daylight, with low to high spore counts (29–458 ascospores per m³ air per event), at an average temperature between 2.8 and 14.3°C. There was a general agreement between the proportion of ascospores trapped from the orchard air during these events and that estimated by using the logistic equation – in most cases, actual and estimated values showed a high coincidence. Statistical comparison showed a significant correlation (r=0.93, P < 0.01) between observed and estimated data.     

Effects of Temperature and Moisture on Development of Fusarium graminearum Perithecia in Maize Stalk Residues

Fusarium graminearum is the predominant component of the Fusarium head blight complex of wheat. F. graminearum ascospores, which initiate head infection, mature in perithecia on crop residues and become airborne. The effects of temperature (T) and moisture on perithecium production and maturation and on ascospore production on maize stalk residues were determined. In the laboratory, perithecia were produced at temperatures between 5 and 30°C (the optimum was 21.7°C) but matured only at 20 and 25°C. Perithecia were produced when relative humidity (RH) was ≥75% but matured only when RH was ≥85%; perithecium production and maturation increased with RH. Equations describing perithecium production and maturation over time as a function of T and RH (R² > 0.96) were developed. Maize stalks were also placed outdoors on three substrates: a grass lawn exposed to rain; a constantly wet, spongelike foam exposed to rain; and a grass lawn protected from rain. No perithecia were produced on stalks protected from rain. Perithecium production and maturation were significantly higher on the constantly wet foam than on the intermittently wet lawn (both exposed to rain). Ascospore numbers but not their dispersal patterns were also affected by the substrate.     

Studies on the epidemiology of Alternaria brassicicola in Brassica oleracea seed production crops

Alternaria brassicicola lesions present on overwintered leaf litter of Brassica oleracea seed production crops produced high concentrations of spores in the spring, these were able to initiate new infections on foliage and subsequently on inflorescences and pods. A vertical disease gradient developed in maturing crops, the lowest pods becoming infected first and infection spreading slowly upwards. Spores were produced abundantly after 20 h leaf wetness at a mean temperature of 13°C or more. Their release was stimulated by a fall in relative humidity but inhibited at a constant high relative humidity resulting in a daily cycle in air spore concentrations with minimum numbers occurring in the early morning and maximum numbers in the early afternoon. For most of the growing season spore movement was restricted to within the crop, however, massive release of spores and subsequent distribution over a wide area occurred when the crop was cut and later threshed. Using semi-selective agar traps spores released at these times were detected up to 1800 m downwind of the parent crop and were instrumental in infecting nearby young crops destined for seed production in the following season.     

Temporal Patterns of Ascospore Release in Leptosphaeria maculans Vary Depending on Geographic Region and Time of Observation

Diurnal patterns of spore release have been observed in a number of fungal pathogens that undergo wind-assisted dispersal. The mechanisms that drive these patterns, while not well understood, are thought to relate to the ability of dispersing spores to survive their journey and infect new hosts. In this paper, we characterise the diurnal pattern of ascospore release by a Western Australian population of Leptosphaeria maculans. Although L. maculans has been previously shown to exhibit diurnal patterns of ascospore release, these patterns appear to vary from region to region. In order to characterise the pattern of release in the Mediterranean climate of Western Australia, we analysed historical data describing the bi-hourly count of airborne ascospores at Mt Barker, Western Australia. Results of this analysis showed diurnal patterns that differ from those previously observed in other countries, with ascospore release in our study most likely to occur in the afternoon. Furthermore, we found that the time of peak release can shift from month to month within any one season, and from year to year. In explaining the hourly pattern of spore release over an entire season, time since rainfall, time since last release, temperature, hour and month were all shown to be significant variables.     

Field Evaluation of Some Models Estimating the Seasonal Pattern of Airborne Ascospores of Venturia inaequalis

A 6-year study was carried out to evaluate the accuracy of some models in estimating airborne ascospores of Venturia inaequalis . The proportion of the season's ascospores trapped on each discharge event was compared with the proportion of mature ascospores, estimated by the New Hampshire model or by some related models. The models differed from each other in the degree-day cumulation, accounting or not for the leaf litter wetness caused by rainfall or by deposition of atmospheric humidity. The New Hampshire model did not fit spore trappings well: 59% of the actual values fell outside the range of the estimates, and 83% of them were overestimates. The wide discrepancy between reality and estimates resulted from the effect of dryness: when many consecutive rainless days occurred, the proportion of ascospores trapped was constantly lower than the model estimates, due to a slowed spore maturation. The effect of dryness was evident during the greater part of the ascospore maturity season, irrespective of the proportion of the season's ascospores that had just matured when the dry period began. Models accounting for leaf litter wetness significantly improved estimates. Therefore, in the Po Valley, the accuracy of the New Hampshire model can be improved by accumulating degree-days only when leaf litter is wet.     

Influence of meteorological parameters on Leptosphaeria maculans and L. biglobosa spore release in central and eastern Poland

Leptosphaeria maculans and L. biglobosa are fungal pathogens able to cause allergic reactions in humans and infect plants of Brassica species. The rate of their development and subsequent spore release depend on weather conditions. The aim of this paper was to pinpoint the exact meteorological conditions triggering the release of L. maculans and L. biglobosa ascospores in central and eastern Poland. Multiple regressions indicated that the frequency and amount of rainfall over short periods were important in mediating spore release. The first ascospore event depended mainly on the number of rainy days during the first 10 days of July and the cumulative precipitation during July and September. The most important variables for maximum spore release were cumulative rainfall in the beginning of July and the end of September, as well as the number of days with precipitation events in the first 10 days of August. The results highlighted for the first time the importance of the days preceding the collection of oilseed rape plants from the field. Higher moisture content of senescing but still living stems play a crucial role in the early start of the ascospore season and the maximum release of ascospores. This was not yet considered to date.     

Ascospore aggregation and oxylipin distribution in the yeast Dipodascopsis tothii

Upon cultivation of the yeast Dipodascopsis tothii in its sexual stage, small ascospores are released individually from the ascus tip, which then assemble in sheathed cluster balls. In contrast to Dipodascopsis uninucleata, this yeast produced smooth bean shaped ascospores with sheath-like appendages that assemble in a disordered sheathed ball of ascospores outside the ascus. Strikingly, upon release, the ascus tip contained 3-hydroxy oxylipins, while the released ascospore clusters contained little or no 3-hydroxy oxylipins as indicated by immunofluorescence microscopy. In D. uninucleata, these oxylipins are concentrated on the spore surface and interspore matrix, but not on the ascus tip.     

The aerobiology of the ascospores

Atmospheric ascospores have been monitored using volumetric spore trap. Spore concentration data were analysed using Spearman's correlation. Our results show that the meteorological factor with the greatest effect on spore concentration was the duration of rain. Temperature increase strongly reduced the ascospore concentration; but the length of windless periods resulted in an increase in spore count. The only measurable effect wind perse actually had on spore count, was registered when a strong wind blew after a long windless period. We observed that the count of ascospores during wet weather could surpass the total concentration of dry conidia measured on a typical, highly polluted summer day. Using selected air samples to study the effect of storms, certain aspects of long-distance spore transport were elucidated. We describe here three main strategies for long-range ascospore transport, "splash-off", "secondary emission" and "sporematrix projectiles".     

Temporal Progress of Yellow Sigatoka and Aerobiology of Mycosphaerella musicola Spores

An understanding of the progression of a disease is important in the adoption of control strategies as well as the evaluation of their efficacies. Temporal analysis is especially useful because it integrates the evolution of the interaction between the components of the pathosystem, as expressed by the accumulated data on the incidence and severity of disease and depicted by the disease progression curve. Within a given patho‐system, the dispersed airborne spores are important components in the progress of plant disease epidemics. Our aims were to evaluate the temporal dynamics of yellow Sigatoka in a banana plantation located in Coronel Pacheco, MG, Brazil, and to assess the aerobiology of Mycosphaerella musicola spores throughout the year. During the rainy season, we observed intense disease progression concomitant with high rates of leaf emission, which caused rapid reversal of the severity peaks after the maximum rates were reached. The yellow Sigatoka progress curve showed two peaks of extreme severity. The first, which occurred during the rainy season, was predominantly caused by a high concentration of conidia. The second, which occurred during the dry season, was predominantly caused by a high concentration of ascospores in the air. The ascospore concentrations were correlated with the severity of the disease 29 days later, indicating the average latency period of the disease in that region. The patterns of the severity curves for both peaks fit the monomolecular model, and the progression rates were higher during the rainy season than the dry season. The spore concentrations were the same at the two evaluated heights. In all evaluations, it was observed a higher concentration of ascospores than of conidia, with the greatest ascospore concentrations occurring during the early hours of the day and the greatest conidia concentrations occurring later, after the dew has dropped from the leaves.     

A Model for Sclerotinia sclerotiorum Infection and Disease Development in Lettuce,Based on the Effects of Temperature,Relative Humidity and Ascospore Density

The plant pathogen Sclerotinia sclerotiorum can cause serious losses on lettuce crops worldwide and as for most other susceptible crops, control relies on the application of fungicides, which target airborne ascospores. However, the efficacy of this approach depends on accurate timing of these sprays, which could be improved by an understanding of the environmental conditions that are conducive to infection. A mathematical model for S. sclerotiorum infection and disease development on lettuce is presented here for the first time, based on quantifying the effects of temperature, relative humidity (RH) and ascospore density in multiple controlled environment experiments. It was observed that disease can develop on lettuce plants inoculated with dry ascospores in the absence of apparent leaf wetness (required for spore germination). To explain this, the model conceptualises an infection court area containing microsites (in leaf axils and close to the stem base) where conditions are conducive to infection, the size of which is modified by ambient RH. The model indicated that minimum, maximum and optimum temperatures for ascospore germination were 0.0, 29.9 and 21.7°C respectively and that maximum rates of disease development occurred at spore densities >87 spores cm⁻². Disease development was much more rapid at 80–100% RH at 20°C, compared to 50–70% RH and resulted in a greater proportion of lettuce plants infected. Disease development was also more rapid at 15–27°C compared to 5–10°C (85% RH). The model was validated by a further series of independent controlled environment experiments where both RH and temperature were varied and generally simulated the pattern of disease development well. The implications of the results in terms of Sclerotinia disease forecasting are discussed.     

Airborne inoculum dynamics of Polystigma amygdalinum and progression of almond red leaf blotch disease in Catalonia,NE Spain

The dynamics of airborne ascospores and disease progress of red leaf blotch (RLB) of almond, caused by Polystigma amygdalinum, and their correlations with weather variables were studied from 2019 to 2021 in two almond orchards located in Lleida, NE Spain. Airborne ascospores were detected and quantified by real-time qPCR using species-specific primers for P. amygdalinum. Ascospores were detected mainly from April to June, with a high variability between the yearly cumulative concentrations. Positive significant correlations were found between the weekly proportion of airborne ascospores and the number of wet and mild days—either combined or separated— accumulated rainfall, number of rainy days, accumulated low temperatures on wet days, and mean and maximum relative humidity. In contrast, several thermal variables (maximum temperature, VPD, and number of warm days) were negatively correlated with ascospore catches. Positive significant correlations were found between the cumulative proportion of ascospores and RLB incidence and severity. Weekly variations in RLB incidence and severity showed significant positive correlations with the number of warm days while negative with the number of mild days. Severity was also positively correlated with several thermal variables (mean, maximum, and minimum temperature, and VPD), and negatively correlated with the number of cold days and wet and mild days. Stronger correlations were generally found with ascospore catches or disease progress when using concurrent weekly weather data. Gompertz, monomolecular, and logistic growth models were evaluated to describe RLB disease progress.     

Measuring summer patterns of ascospore release by saltmarsh fungi

One potentially important type of flux from standing-decaying marshgrass is the production and release of ascospores. The most extensive measurements of ascospore release from the principal marshgrass (Spartina alterniflora, smooth cordgrass) of saltmarshes of the eastern coastal United States involved an arbitrary, weeklong period of wet incubation of leaf-blade samples. We examined the possibility that shorter incubations would yield higher estimates of hourly rates of ascospore release, testing wet incubations of 3 to 71 h, using standing-decaying leaf blades of smooth cordgrass from low on living shoots and high on dead shoots. Incubations of 31 h appeared to be optimal. Species compositions of ascospores expelled from the two leaf types were distinctly different: high leaves yielded primarily aMycosphaerella species orPhaeosphaeria halima; low leaves yielded primarilyPhaeosphaeria spartinicola or theMycosphaerella species. All of these species consistently exhibited high coefficients of variation (>100%) for their mean rates of release of ascospores. Only theMycosphaerella species on high leaves gave evidence of a delayed onset of ascospore expulsion during incubation, and this evidence was equivocal. Grand mean rates of ascospore release forP. spartinicola and theMycosphaerella species were, respectively, 106 and 238 spores cm⁻² abaxial leaf area h⁻¹.     

South American leaf blight of Hevea brasiliensis: spore dispersal of Microcyclus ulei

Trapping of ascospores and conidia of Microcyclus ulei among young trees of Hevea brasiliensis in Trinidad from May 1973 to May 1975 snowed that ascospores occurred throughout the year whilst conidia were present only during the wet season. Peak ascospore concentrations occurred in August and November during the wet season, the latter peak being more marked and the former coinciding with the period of maximum conidium liberation. In dry weather the number of ascospores increased during the night to a maximum at 06.00 h, and decreased to a low level during the day. On rainy days heavy ascospore discharge also occurred during the day. Ascospore concentration decreased significantly after dawn on sunny days whilst on overcast days the concentration remained high most of the day. Conidium production was highest around 10.00 h and decreased towards the evening to a low level during the night, reaching a minimum at 07.00 h.     

Effects of humidity and temperature on ascospore discharge of <Emphasis Type="Italic">Graphostroma platystoma</Emphasis> in the laboratory and in the field

The effects of air humidity and temperature on the ascospore discharge of Graphostroma platystoma were experimentally investigated. The ascospores were not discharged from the stromata in air at 100% relative humidity (RH). However, they were discharged from the wetted stromata at 3°, 10°, and 24°C under 100% RH or nearly so. The amount of the discharged ascospore was large at 24°C, medium at 10°C, and small at 3°C. The ascospores in the rainwater that washed down the stromata were counted after rainfall in the field. The discharge was observed from September to the following May.     

Ultrastructure of asci,ascospores, and spore release inLophodermella sulcigena (Rostr.) v. Hohn.

Summary The croziers were formed from large multinucleate cells at the base of the hysterothecium. The diploid ascus had basal and apical vacuoles and there was prominant endoplasmic reticulum near the extending tip of the ascus. The spore delimiting membranes were continuous with the plasmalemma and possibly arose from it. The spore walls were formed between the two membranes. The ascus had a simple apical ring around a thinner region of the wall which became the pore through which the spores were released. Just before spore release the outer layer of the ascospore wall became vesiculated and eventually mucilagenous. The long clavate ascospores were released one at a time, stretching the neck of the ascus as they emerged.     

Environmental Factors Influencing the Dispersal of Venturia inaequalis Ascospores in the Orchard Air

A 6-year study was carried out in an apple-growing region of North Italy by trapping airborne ascospores of Venturia inaequalis with a volumetric spore trap operated continuously during the ascospore season, with the aim of better defining the weather conditions that allow ascospores both to discharge and to disperse into the orchard air. A total of more than 60 ascospore trapping events occurred. Rain events were the only occurrences allowing ascospores to become airborne (a rain event is a period with measurable rainfall ≥0.2 mm/h – lasting one to several hours, uninterrupted or interrupted by a maximum of two dry hours); on the contrary, dew was always insufficient to allow ascospores to disperse into the air at a measurable rate, in the absence of rain. In some cases, rain events did not cause ascospore dispersal; this occurred when: (i) rain fell within 4–5 h after the beginning of a previous ascospore trapping; (ii) rain fell at night but the leaf litter dried rapidly; (iii) nightly rainfalls were followed by heavy dew deposition that persisted some hours after sunrise. Daytime rain events caused the instantaneous discharge and dispersal of mature ascospores so that they became airborne immediately; for night-time rainfall there was a delay, so that ascospores became airborne during the first 2 h after sunrise. This delay did not always occur, and consequently the ascospore trapping began in the dark, when: (i) the cumulative proportion of ascospores already trapped was greater than 80% of the total season's ascospores; (ii) more than one-third of the total season's ascospores was mature inside pseudothecia and ready to be discharged.     

Cucullosporella mangrovei, ultrastructure of ascospores and their appendages

The ultrastructure ofCucullosporella mangrovei ascospores is described. Mature ascospores possess two wall layers, an outer electron-dense episporium and an innermost tripartite mesosporium. Episporial elaborations form electrondense spore wall ornamentations from which extend fibrils that may constitute a highly hydrated exosporium which was not visualised at either the scanning electron microscope or light microscope level. Ascospores possess a hamate appendage at each pole which unfolds in seawater to form a long thread. Ultrastructurally the polar appendage comprises folded fibro-granular electron-dense material and fine fibrils. The fibrils form a matrix around and within the fibro-granular appendage and around the entire unreleased ascospore. These fibrils have not been observed associated with the ascospore appendages in other species of the Halosphaeriales and are a discrete and new appendage component. The fibro-granular appendage and fibrils are bounded by the outer delimiting membrane which is absent around released ascospores. The nature of the spore appendage is compared with that of other marine and freshwater ascomycetes and the taxonomic assignment of the species is discussed.     

The release of elongated, sheathed ascospores from bottle-shaped asci in Dipodascus geniculatus

Yeasts use different mechanisms to release ascospores of different lengths from bottle-shaped asci. Round to oval-shaped ascospores are enveloped in oxylipin-coated compressible sheaths, enabling ascospores to slide past each other when they reach the narrowing ascus neck. However, more elongated ascospores do not contain sheaths, but are linked by means of oxylipin-coated interlocked hooked ridges on the surfaces of neighboring ascospores, thereby keeping them aligned while they are pushed towards the ascus tip by turgor pressure. In this study, we found elongated, oxylipin-coated sheathed ascospores in Dipodascus geniculatus that are released effectively from bottle-shaped asci without alignment. This is possible because the ascus neck and opening have a diameter that is the same as the length of the ascospore, thus allowing the ascospores to turn sideways without blocking the ascus when they are released. We found that increased concentrations of acetylsalicylic acid inhibit both ascospore release and 3-hydroxy oxylipin production in this yeast, thereby implicating this oxylipin in sexual reproduction.     

The meiosis‐specific APC activator FgAMA1 is dispensable for meiosis but important for ascosporogenesis in Fusarium graminearum

Ascospores are the primary inoculum in Fusarium graminearum. Interestingly, 70 of its genes have premature stop codons (PSC) and require A‐to‐I editing during sexual reproduction to encode full‐length proteins, including the ortholog of yeast Ama1, a meiosis‐specific activator of APC/C. In this study, we characterized the function of FgAMA1 and its PSC editing. FgAMA1 was specifically expressed during sexual reproduction. The Fgama1 mutant was normal in growth and perithecium formation but defective in ascospogenesis. Instead of forming four‐celled, uninucleate ascospores, Fgama1 mutant produced oval, single‐celled, binucleated ascospores by selfing. Some mutant ascospores began to bud and underwent additional mitosis inside asci. Expression of the wild‐type or edited FgAMA1 but not the uneditable allele complemented Fgama1. In the Fgama1 x mat‐1‐1 outcross, over 60% of the asci had eight Fgama1 or intermediate (elongated but single‐celled) ascospores, suggesting efficient meiotic silencing of unpaired FgAMA1. Deletion of FgPAL1, one of the genes upregulated in Fgama1 also resulted in defects in ascospore morphology and budding. Overall, our results showed that FgAMA1 is dispensable for meiosis but important for ascospore formation and discharge. In F. graminearum, whereas some of its targets are functional during meiosis, FgAma1 may target other proteins that function after spore delimitation.     

OBSERVATIONS ON THE BIOLOGY OF CLOVER ROT

The field behaviour of clover rot in eastern England indicates that infection takes place by ascospores, and not by soil-borne mycelium developing directly from the sclerotia. To support this view, experimental evidence is presented to show that cutting of red clover at the time when the ascospores are being shed, reduces the percentage infection in the following spring.
Ascospore infection of clover causes a 'non-aggressive' spotting which may develop into an 'aggressive' infection. Sclerotinia trifoliorum has been shown to remain viable in non-aggressive spots for at least 16 weeks. Similar symptoms occur when beans are infected with S. trifoliorum var. fabae. Non-aggressive and aggressive infections are two phases of a sequence in time, and not two alternative types of infection dependent on the initial spore load. The occurrence of non-aggressive and aggressive phases in nature explains (1) the observed delay between ascospore discharge and the appearance of obvious disease symptoms, and (2) the seasonal nature of the disease.
Aggressive infection develops only in a saturated atmosphere and within a temperature range of 5–20° C., and its establishment is favoured by a heavy dosage of inoculum. In most seasons humidity is the main 'limiting' factor; over the 18-year period, 1930–47, there is a significant regression between the severity of clover rot and the previous December plus January rainfall.
Crop rotation and autumn grazing check the disease, but the latter must be practised with discretion because, under certain conditions, its effects are more drastic than those of the disease. The only hopeful method of effective control is the breeding of resistant strains of legumes.