Home About us Contact
|
Home About us Contact | ||
|
|
||
Wetness Duration (wetness + duration)
Selected AbstractsMicrobiotic crusts as biomarkers for surface stability and wetness duration in the Negev DesertEARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2009Giora J. Kidron Abstract Microbiotic crusts play an important role in arid and semi-arid regions. Yet, very little information exists regarding the factors that impact their development. In an attempt to assess the main factors that may determine their growth, measurements of the amount of fines (silt and clay), rain, moisture content, wetness duration and wind erosion and deposition were carried out along a 12 station transect within a partially crusted dune field in the western Negev Desert and compared to the crust cover and chlorophyll content. Surface stability was the only variable that exhibited significant relationship with crust cover while daylight wetness duration exhibited strong positive relationship (r2 = 0·92,0·99) with the crust's chlorophyll content. The data point out that microbiotic crusts may serve as a useful biomarker for surface stability. While wetness duration and wind will control crust cover and the crust chlorophyll content in semi-stable habitats (with absolute annual change in sand level of 2,3 mm), stable habitats (absolute change <1 mm) will be controlled primarily by moisture, while habitats with low surface stability (absolute change of tens and hundreds of millimeters) will be primarily controlled by wind. Furthermore, owing to the strong positive relationship between daylight wetness duration and the crust's chlorophyll content, the crust may serve as a useful biomarker for the quantification of surface wetness duration. Copyright © 2009 John Wiley & Sons, Ltd. [source] Measurements of transpiration in four tropical rainforest types of north Queensland, AustraliaHYDROLOGICAL PROCESSES, Issue 26 2007David McJannet Abstract Transpiration of four different rainforest types in north Queensland, Australia, was determined using the heat pulse technique for periods ranging between 391 and 657 days. Despite the complexity of the natural rainforest systems being studied, the relationship between sample tree size and daily water use was found to be strong, thus providing a robust means by which to scale transpiration from individual trees to the entire forest stand. Transpiration was shown to be dependent on solar radiation and atmospheric demand for moisture with little evidence of limitation by soil moisture supply. Total stand transpiration was controlled by forest characteristics such as stem density, size distribution and sapwood area. Annual transpiration for each of the four sites ranged between 353 mm for cloud forest and 591 mm for montane rainforest. In comparison with the international literature, transpiration from Australian rainforests is low; the reasons for this could be related to a combination of differences in forest structure, climatic conditions, canopy wetness duration and tree physiology. Copyright © 2007 John Wiley & Sons, Ltd. [source] Infection of Blackcurrant Leaves by Drepanopeziza ribis in Relation to Weather Conditions and Leaf PositionJOURNAL OF PHYTOPATHOLOGY, Issue 5 2009Xiang Ming Xu Abstract Drepanopeziza ribis causes the leaf spot disease of blackcurrant (Ribes nigrum) and may lead to severe premature leaf-fall. Artificial inoculation studies were carried out to investigate infection of leaves by D. ribis conidia in relation to environmental conditions and leaf position (age) on cvs. Baldwin and Ben Hope in April and July 2007. All leaves on a number of selected extension shoots on potted three-year old plants were inoculated with conidia and then incubated under different conditions: 10, 17.5 and 25°C each with five wet periods (4, 8, 12, 24 and 30 h). Number of infected leaves was determined. The two cultivars differed significantly in their susceptibility to conidial infection: cv. Baldwin was much more susceptible than cv. Ben Hope. Older leaves on extension shoots were more susceptible to conidial infection than younger leaves. Increasing length of wetness duration led to increasing incidence of leaves infected, particularly when inoculated in July. However, the effects of temperature were inconclusive and generally very small in comparison with other factors. Field epidemics were monitored over three years (2005,07). Field data confirmed the main findings from controlled inoculation studies: severe disease was associated with very wet conditions and older leaves. Furthermore, they also suggested that significant disease increase only occurred from late July onwards. [source] Effects of temperature and continuous and interrupted wetness on the infection of pear leaves by conidia of Venturia nashicolaPLANT PATHOLOGY, Issue 3 2005B.-H. Li Experiments were conducted to determine: (i) the effects of temperature and duration of continuous wet periods on the infection of pear seedlings by conidia of Venturia nashicola, the causal agent of pear scab; and (ii) the effects of the length and temperature of dry interrupting periods on the mortality of infecting conidia. Average number of scab lesions per leaf increased with increasing duration of wetness. Logistic models adequately described the change in the average number of scab lesions per leaf at 5, 10, 15, 20 and 25°C over the wetness duration. At 30°C, only a few lesions developed. Simple polynomial models satisfactorily described the relationship of the three logistic model parameters (maximum number of lesions, rate of appearance and the time to 50% of the maximum number of lesions) with temperature. The optimum temperature for infection was found to be approximately 20°C. The relationship between mortality and the length of a dry period interrupting an infection process can be satisfactorily described by an exponential model. The rate of mortality at 10, 16 and 22°C did not differ significantly, but was significantly less than that at 28°C. [source] Environmental conditions influencing Sclerotinia sclerotiorum infection and disease development in lettucePLANT PATHOLOGY, Issue 4 2004C. S. Young The environmental factors that influence infection of lettuce by ascospores of Sclerotinia sclerotiorum, and subsequent disease development, were investigated in controlled environment and field conditions. When lettuce plants were inoculated with a suspension of ascospores in water or with dry ascospores and exposed to a range of wetness durations or relative humidities at different temperatures, all plants developed disease but there was no relationship between leaf wetness duration or humidity and percentage of diseased plants. Ascospores started to germinate on lettuce leaves after 2,4 h of continuous leaf wetness at optimum temperatures of 15,25°C. The rate of development of sclerotinia disease and the final percentage of plants affected after 50 days were greatest at 16,27°C, with disease symptoms first observed 7,9 days after inoculation, and maximum final disease levels of 96%. At lower temperatures, 8,11°C, disease was first observed 20,26 days after inoculation, with maximum final disease levels of 10%. Disease symptoms were always observed first at the stem base. In field-grown lettuce in Norfolk, 2000 and 2001, inoculated with ascospore suspensions, disease occurred only in lettuce planted in May and June, with a range of 20,49% of plants with disease by 8 weeks after inoculation. In naturally infected field-grown lettuce in Cheshire, 2000, disease occurred mainly in lettuce planted throughout May, with a maximum of 31% lettuce diseased within one planting, but subsequent plantings had little (, 4%) or no disease. Lack of disease in the later plantings in both Norfolk and Cheshire could not be attributed to differences in weather factors. [source] Modelling the progress of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) in relation to leaf wetness and temperaturePLANT PATHOLOGY, Issue 2 2001K. Papastamati A compartmental model was developed to describe the progress with time of light leaf spot (Pyrenopeziza brassicae) on leaves of winter oilseed rape (Brassica napus) during the autumn in the UK. Differential equations described the transition between the four compartments: healthy susceptible leaves, infected symptomless leaves, sporulating symptomless leaves and leaves with necrotic light leaf spot lesions, respectively. The model was fitted to data on the progress of light leaf spot on winter oilseed rape at a single site during the autumn of the 1990,1991 season. Model parameters were used to describe rates of leaf appearance, leaf death, infection by airborne ascospores (primary inoculum) and infection by splash-dispersed conidiospores (secondary inoculum). Infection was dependent on sufficient leaf wetness duration. The model also included delay terms for the latent period between infection and sporulation and the incubation period between infection and the appearance of necrotic light leaf spot lesions. This modified SEIR model formulation gave a reasonable fit to the experimental data. Sensitivity analysis showed that varying the parameter accounting for the rate of infection by ascospores affected the magnitude of the curves after the start of the epidemic, whilst including a parameter for conidiospore infection improved the fit to the data. Use of ascospore counts from different sites and different years showed variation in spore release patterns sufficient to affect model predictions. [source] Effects of temperature and wetness duration on conidial infection, latent period and asexual sporulation of Pyrenopeziza brassicae on leaves of oilseed rapePLANT PATHOLOGY, Issue 4 2000T. Gilles 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. [source] Infection of onion leaves by Alternaria porri and Stemphylium vesicarium and disease development in controlled environmentsPLANT PATHOLOGY, Issue 3 2000H. Suheri 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 (R2 > 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. [source] Quantitative resistance to symptomless growth of Leptosphaeria maculans (phoma stem canker) in Brassica napus (oilseed rape)PLANT PATHOLOGY, Issue 2 2009Y. J. Huang Quantitative resistance to Leptosphaeria maculans in Brassica napus was investigated in field and controlled environments using cultivars Darmor (with quantitative resistance) and Eurol (without quantitative resistance). In field experiments, numbers of phoma leaf spot lesions in autumn/winter and severity of stem canker the following summer were assessed in three growing seasons. There were no differences between Darmor and Eurol in number of leaf lesions in autumn/winter. However, stem cankers were less severe on Darmor than Eurol at harvest the following summer. In controlled-environment experiments, development of leaf lesions at different temperatures (5,25°C) and wetness durations (12,72 h) was investigated using ascospore inoculum; symptomless growth of L. maculans along leaf petioles towards the stem was quantified using quantitative PCR and visualized using GFP-expressing L. maculans; growth of L. maculans within stem tissues was investigated using GFP-expressing L. maculans. There were more leaf lesions on Darmor than Eurol, although there was no difference between Darmor and Eurol in L. maculans incubation period. There were no differences between Darmor and Eurol in either distance grown by L. maculans along leaf petioles towards the stem or quantity of L. maculans DNA in leaf petioles, but L. maculans colonized stem tissues less extensively on Darmor than Eurol. It was concluded that quantitative resistance to L. maculans operates during colonization of B. napus stems by the pathogen. [source] Environmental conditions influencing Sclerotinia sclerotiorum infection and disease development in lettucePLANT PATHOLOGY, Issue 4 2004C. S. Young The environmental factors that influence infection of lettuce by ascospores of Sclerotinia sclerotiorum, and subsequent disease development, were investigated in controlled environment and field conditions. When lettuce plants were inoculated with a suspension of ascospores in water or with dry ascospores and exposed to a range of wetness durations or relative humidities at different temperatures, all plants developed disease but there was no relationship between leaf wetness duration or humidity and percentage of diseased plants. Ascospores started to germinate on lettuce leaves after 2,4 h of continuous leaf wetness at optimum temperatures of 15,25°C. The rate of development of sclerotinia disease and the final percentage of plants affected after 50 days were greatest at 16,27°C, with disease symptoms first observed 7,9 days after inoculation, and maximum final disease levels of 96%. At lower temperatures, 8,11°C, disease was first observed 20,26 days after inoculation, with maximum final disease levels of 10%. Disease symptoms were always observed first at the stem base. In field-grown lettuce in Norfolk, 2000 and 2001, inoculated with ascospore suspensions, disease occurred only in lettuce planted in May and June, with a range of 20,49% of plants with disease by 8 weeks after inoculation. In naturally infected field-grown lettuce in Cheshire, 2000, disease occurred mainly in lettuce planted throughout May, with a maximum of 31% lettuce diseased within one planting, but subsequent plantings had little (, 4%) or no disease. Lack of disease in the later plantings in both Norfolk and Cheshire could not be attributed to differences in weather factors. [source] Accounting for periods of wetness in displacement of Fusarium pseudograminearum from cereal strawANNALS OF APPLIED BIOLOGY, Issue 1 2010D.P.S. Lakhesar Displacement of pathogenic Fusarium species from cereal residues by other fungi is an important mechanism for the effectiveness of fallows and crop rotations on disease management, as well as in potential biological control. The effect of fluctuating environmental conditions on the rate of displacement was assessed using two different approaches. In the first, wetness durations between 4 and 10 h were simulated by spraying water onto straw inoculated with Fusarium pseudograminearum and antagonists in a greenhouse. For a given cumulative period of wetness, displacement of F. pseudograminearum was generally higher for short (4 h) than longer (10 h) wetting durations, indicating that it was the number of wetting events, rather than their individual durations, that determined the rate of displacement. In the second approach, exponential decay models using thermal time adjusted for rainfall were fitted to published data on survival of Fusarium species in residues. Heat sums calculated from the mean temperature of days on which rain fell, or rainday-degrees (RDD), gave good fits to data from short-term experiments on displacement of F. pseudograminearum by antagonists under natural conditions. RDD and two other indices, decomposition days (DCD) and corrected degree-days (CDD), were equally satisfactory for modelling straw decomposition and mortality of Fusarium in longer term data sets. Such models could be useful for predicting the effects of environmental variation on rotations and biocontrol for Fusarium management in cereals. [source] | ||||||||||