Wetness Duration Research Articles

Growth and Competitiveness of Common Lambsquarters (Chenopodium album) after Foliar Application ofAscochyta caulinaas a Mycoherbicide

Control of common lambsquarters by the use ofAscochyta caldinaas a postemergence mycoherbicide was studied in corn and sugar beet, in 1992 or 1993. The weed was planted at determined positions in the crops. Plots were treated with suspensions ofA. caulinaspores, and wetness duration's were varied to create different levels of disease development. Application ofA. caulinaresulted in necrosis development on, and mortality of common lambsquarters. Average severities of leaf necrosis 1 wk after treatment ranged from 0.01 to 0.75. Average proportions of dead plants 3 wk after treatment ranged from 0.00 to 0.65. Necrosis development and mortality were affected by wetness duration in two experiments. Sublethally diseased plants showed reduced growth. Maximum dry matter was affected by crop and by necrosis development. Numbers of fruits per plant showed a positive, almost linear relationship with plant dry matter weight. Seed weight was less affected by necrosis than number of fruits per plant. Competitiveness of common lambsquarters was reduced after infection byA. caulina.Crop dry matter weight showed a positive relationship with the level of common lambsquarters control. In corn, yield reduction by competition was prevented by application of A.caulina, but not in sugar beet.

A site-specific sensor for measuring leaf wetness duration within turfgrass canopies

Leaf wetness is a key factor in the initiation and development of many plant diseases. Measurement of leaf wetness duration in turfgrass requires sensors that do not disrupt the canopy environment and are representative of leaf surfaces. Therefore, a small (5 mm head width) sensor was developed that can be inserted into grass blades with only minor modification of the canopy and can capture the spatial variability of leaf wetness. The sensor detected wetness onset and depletion in a tall fescue turf canopy in the field with an average error of less than 20 min when compared with a tactile method. The sensor indicated leaf wetness over 1 h before moisture was visible and detected wetness depletion 1 h after moisture was no longer visible, suggesting that the sensor is sensitive to microscopic water films. When the sensor was compared with a leaf latent heat flux. model in tall fescue canopies, no significant differences between the two methods were found in detecting wetting and drying episodes and in measuring leaf wetness durations. However, the sensor was sufficiently sensitive to record 1 h later wetness depletion ( P < 0.01) and 1 h longer periods of leaf wetness ( P < 0.01) in a high plant density canopy as compared with a low-density canopy, whereas no significant differences in moisture conditions between the two canopies was found using the leaf latent heat flux model.

Effect of Wetness Duration and Temperature on Infection of Geranium byBotrytis cinerea

Duration and temperature of postinoculation wetness periods were investigated in relation to infection of flowers and leaves of geranium by Botrytis cinerea. Infection was assessed indirectly by estimating sporulation incidence of the pathogen in inoculated tissues. Sporulation incidence in whole flowers inoculated with conidia increased sharply when wetness duration at 15, 21, 25, and 30 degrees C was increased from 8 to 24 h, 4 to 12 h, 4 to 12 h, and 4 to 6 h, respectively. The pathogen did not sporulate after shorter wetness periods at these temperatures or after 24 h wetness at 5 and 10 degrees C. No sporulation was observed in sepals, petals, and stamens when the wet period at 25 degrees C was less than or equal to 4 h, or in pistils and pedicels when wetness lasted less than or equal to 6 h, but sporulation increased to 100, 100, 100, 73, and 60% in the respective organs as the wet period was increased to 24, 12, 12, 24, and 24 h. Following conidial inoculations of the foliage, B. cinerea sporulated in 1-week-old leaves only when the postinoculation wetness period at 15, 21, 25, and 30 degrees C was greater than or equal to 6, greater than or equal to 4, greater than or equal to 6, and greater than or equal to 6 h, respectively, and in 10-week-old leaves when wetness at the respective temperatures lasted greater than or equal to 6, greater than or equal to 6, greater than or equal to 8, and greater than or equal to 28 h. Sporulation was observed in 4-week-old leaves after greater than or equal to 12, grater than or equal to 8, and greater than or equal to 6 h wetness at 15, 21, and 25 degrees C, respectively. The pathogen did not sporulate in leaves of any age group when postinoculation wet periods were 0 to 24 h at 5 or 10 degrees C, or in 4-week-old leaves at 30 degrees C. Botrytis cinerea sporulated more frequently in 1- and 10-week-old leaves than in 4-week-old leaves when wetness lasted for 8 to 24 h at 25 degrees C. The pathogen infected leaves more efficiently when conidia were applied to geranium petals that after 24 h were positioned on the leaves, than when conidia were applied directly to the leaves. Logistic regression models were developed that adequately described effects of postinoculation wetness duration and temperature on sporulation incidence and, by inference. infection incidence, in flowers and leaves.

Bacterial Spot of Peach as Influenced by Water Congestion, Leaf Wetness Duration, and Temperature

Development of bacterial spot on Suwanee peach leaves after inoculation with Xanthomonas campestris pv. pruni was studied at 24 and 30°C and after-inoculation wetness periods of 0, 6, 18, 24, and 48 h. Water congestion following leaf wetness resulting from exposure to 100% relative humidity for at least 36 h and growing plants in sandy soil were necessary for symptom development. Symptom development was slower at 24 than at 30°C. Postinoculation wetness periods greater than 18 h were necessary for extensive symptom development at 24 but not at 30°C. However, some symptom development was evident at either temperature with no leaf wetness after inoculation if leaves were water-congested. Symptoms did not develop when trees were grown in a sandy loam-vermiculite soil mixture.

Development and Field Validation of a Brown Patch Warning Model for Perennial Ryegrass Turf

Microclimate in perennial ryegrass (Lolium perenne 'Caravelle') was monitored for 3 years to identify environmental conditions associated with brown patch (Rhizoctonia solani) outbreaks and to develop a weather-based disease-warning model. The micrometeorological parameters measured were ambient air temperature, relative humidity, leaf wetness duration, precipitation, soil temperature, soil moisture, and solar radiation. Brown patch outbreaks were confirmed by the visual presence of foliar R. solani mycelium. An environmental favorability index (E) was developed to relate a combination of environmental conditions with brown patch outbreaks. The initial index (E 6 ) was based on relative humidity (RH ≥ 95% for ≥8 h; mean RH ≥ 75%), leaf wetness duration (≥6 h) or precipitation (≥12 mm), and minimum air (≥16°C) and soil (≥16°C) temperatures. Further analyses, however, revealed that an equally effective E was provided by a two-variable regression model (E 2 ). The E 2 model is E = -21.5 + 0.15RH + 1.4T- 0.033T 2 , in which RH is the mean relative humidity and T is the minimum daily air temperature. For both E 6 and E 2 models, a threshold value (i.e., E ≥ 6) constituted a brown patch warning. Brown patch outbreaks were predicted by both models with 85% accuracy over a 3-year period. All major infection events were predicted. In 1993, the warning model was used to field evaluate fungicide performance in perennial ryegrass and colonial bentgrass (Agrostis tenuis 'Bardot'). There were equivalent levels of blighting between the warning model and a 14-day calendar-based spray schedule, but the warning schedule provided a 29% reduction in fungicide applications.

The Microclimate in Tall Fescue Turf as Affected by Canopy Density and Its Influence on Brown Patch Disease

Microenvironment was monitored within tall fescue (Festuca arundinacea) experiment plots in which the severity of brown patch disease, caused by Rhizoctonia solani AG-1-IA, was found to correlate with canopy density. Canopy density was varied either by planting cultivars that produced different densities, or by planting a single cultivar at three seeding rates. Air and foliage temperatures within the canopy differed by only approximately 1°C between low- and high-density canopies during 1993 and 1994 in both studies. In a wet year, 1993, leaf wetness and relative humidity did not differ significantly between low- and high-density canopies. In 1994, which was more typical in regards to weather, a denser canopy promoted leaf wetness and high relative humidity in both studies. Leaf wetness duration averaged over 10 days was 0.8 h longer in the high-density cultivar Arriba than in the low-density cultivar Fawn. In addition, the period of relative humidity above 90% was 2.3 h longer in Arriba than in Fawn. Canopies of tall fescue with different plant densities were inoculated in the laboratory with R. solani and placed under uniformly high humidity and temperature. Hyphae grew between leaf blades separated by up to 8 mm. Interblade hyphal growth occurred more frequently in high-density canopies because of the closer proximity of leaf blades, and as a result, mycelia and necrosis spread more rapidly from inoculation sites in high-density canopies. It was concluded that microenvironmental conditions and the physical proximity of leaf blades in high-density turfs can be more favorable for brown patch disease.

The relationship of leaf wetness duration and disease progress of glume blotch, caused byStagonospora nodorum, in winter wheat to standard weather data

Almost 50% of the variation in leaf wetness duration can be explained by maximum and minimum temperatures, rainfall and hours with relative humidity above 90% on a daily basis. All of these parameters can be estimated from a standard weather station. If variables related to wind are added the level of explanation increases to 69–76%. Leaf wetness duration explained up to 42% of the rate of disease increase (RDI) forS. nodorum. Leaf wetness duration was accumulated over a 5-day ‘window’ period and correlated with rate of disease increase after a 7-day ‘lag’ period. Standard weather variables could explain 20–34% of the disease increase. The relevance of these statistical models to disease prediction is discussed.

Early development of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) in relation to temperature and leaf wetness.

In controlled environment experiments to study early development of light leaf spot, lesions developed with leaf wetness durations of 16 to 48 h after inoculation of oilseed rape with conidial suspensions of Pyrenopeziza brassicae at 12 or 18°C, but not with leaf wetness durations of 0 to 13h. The incubation period was 21 to 22 days at 12°C and 14 to 18 days at 18°C for leaf wetness durations of 16 to 48 h. The latent period was 21 to 23 days at 12°C and 18 to 19 days at 18°C, and the total number of lesions increased with increasing leaf wetness duration at both temperatures. In field experiments, light leaf spot always developed on oilseed rape with a leaf wetness duration of 48 h after inoculation in both 1990/1991 and 1991/1992, but the percentage leaf area affected was less on plants placed in an oilseed rape crop than on those placed in a glasshouse. Plants moved to an oilseed rape crop immediately after inoculation nearly always developed light leaf spot symptoms when they were inoculated between 19 October 1990 and 1 March 1991 or between 27 September 1991 and 14 February 1992, but plants inoculated between 31 August and 16 October 1990 or on 20 September 1991, when estimated leaf wetness duration was less than 16 h for several days after they were placed in crops, did not develop symptoms. The latent period of light leaf spot on plants transferred to the oilseed rape crop was 15 to 40 days, and there was an approximately linear relationship between 1 (latent period) and mean temperature during this period. The accumulated temperature during the latent period ranged from c. 150 to 250 day‐degrees. The severity of lesions on these plants increased with increasing temperature from 5 to 15°C.

A Wetness Sensor for Detecting Condensation on Tomato Plants in Greenhouses

A wetness sensor, based on the electrical conductivity of a polymer, has been designed and constructed from 0·08 mm-thick flexible copper-coated polyamide film using photographic and etching procedures. The flexible sensors made good thermal contact with the surface of different shapes and sizes of tomato fruits. The space between grid elements was 0·25 mm. The performance of sensors was determined with bare grid elements and elements coated with acrylic latex and vinyl acetate based latex, in different concentrations. Sensors without a coating gave a good and repeatable response. Sensors coated with latex paints had non-reproducible responses, and overestimated the wetness period due to the slow release of moisture absorbed in the paint. The uncoated sensors were used to monitor condensation on tomato fruits in a series of experiments. The experimental results showed that the total wetness period, indicated by the impedance changes of the sensors, agreed well with that deduced from measurements of fruit surface and dew point temperatures. The present design has a suitably quick response to condensation on plant surfaces and this is attributed to the smaller intergrid gap than that used by previous workers and to the absence of a coating material. The greater accuracy in determination of wetness duration will help in the forecasting of microclimate conditions and prediction of disease epidemiology for greenhouse crops.

An in vitro technique for measuring contact angles on the corneal surface and its application to evaluate corneal wetting properties of water soluble polymers

The development of an in vitro technique capable of measuring the extent and duration of corneal wetting by demulcent polymers is reported. The technique required modification of a contact angle goniometer to facilitate the measurement of contact angles of demulcent polymer solutions on freshly enucleated rabbit eyes. Contact angles of the test solutions on the corneal surface was measured under physiologically relevant conditions. Contact angle measurements were performed under conditions to ensure that the tissue was not dehydrated and the native surface characteristics of the tissue were unchanged. Additionally, experimental procedures for contact angle measurements were developed in order to provide a partial simulation of in vivo fluid dynamics that is typically observed upon topical instillation of a drop of a test polymer solution into the eye of a patient. This multistep experimental procedure was initiated by briefly dipping a freshly enucleated rabbit eye in the test polymer solution. The treated eye was then immersed upside-down in an oxygenated, lactated Ringer's solution and placed in the chamber of the goniometer. This medium was used to maintain ocular tissue viability and corneal surface integrity over the duration of the experiment. For measurement of contact angles, a well defined air bubble was slowly introduced into the chamber of the goniometer at close proximity to the enucleated eye. The air bubble was then entrapped on the corneal surface. Angle of contact of the air bubble against the corneal epithelial surface was visually measured with the goniometer. A new air bubble was entrapped at intervals of 5 min and the contact angle was measured as a function of time. The degree and duration of reduction of contact angle was observed to depend on the type and concentration of the water soluble polymer used. Most demulcents were found to rapidly desorb from the corneal surface. For these demulcent polymers the contact angle returned to pre-treatment values within 25 min of the start of the experiment. Hyaluronic acid showed the most sustained wetting of the cornea. The in vitro kinetic measurement of contact angle demonstrated the time dependent, marginally functional mucomimetic properties of demulcent polymers. The kinetics of desorption was considered to partially simulate the fluid dynamics in the eye. The validity of the inverted air bubble as well as this new technique for the kinetic measurement of contact angles was established on synthetic (polyethylene) surface prior to its utilization on freshly enucleated ocular surface. The measured contact angle and estimates of critical surface tension for polyethylene surface was in close agreement with values reported in the literature and with values obtained from conventional techniques of measurement of contact angle. The contact angle of lactated Ringer's solution on the rabbit corneal surface range from 45 to 55°. Removal of mucin from the corneal surface increased the contact angle of the cornea in the range of 55–65°. This increase in contact angle of the corneal epithelium devoid of mucin indicated decreased wetting and demonstrated its greater hydrophobicity than the mucin coated epithelium. Adsorption of demulcent polymers solutions decreased the contact angles to as low as 30° indicating enhanced wetting of the cornea. The measurement of contact angles of various demulcent polymer solutions possessing different surface tensions made it possible to estimate the critical surface tension of the rabbit corneal epithelium. The critical surface tension, representing the minimal surface tension of a formulation necessary to completely wet and spread over the corneal epithelium was estimated at 39 dyn/cm. Therefore, the reported air bubble trapping along with the kinetic technique for measurement of contact angles may be useful as an in vitro screening tool for the evaluation of new demulcent polymers. The technique may also be used for the identification of superior tear substitutes for the treatment of dry eyes.

Phenological Analysis of Brown Rot Blossom Blight of Sweet Cherry Caused by Monilinia laxa

The influence of temperature and wetness duration on infection incidence of sweet cherry blossoms by Monilinia laxa was determined in controlled environment studies. Potted sweet cherry trees (cv. Star) were inoculated in full bloom (5 × 10 3 conidia per milliliter), exposed to 0-24 h of wetness duration and incubated at temperatures between 5 and 20 C. The disease incidence increased with longer wetness duration and temperature within the range tested. A nonlinear model was used to describe the infection incidence as a function of temperature and wetness duration. Analytical tools were developed to determine a posteriori the infection date of blossoms observed in the field, based on the severity of the symptoms of the disease and the climatic conditions. A temperature-dependent physiological time was introduced to account for the influence of temperature on incubation period. An infected blossom passes through several macroscopically distinct stages of symptom severity. The characteristic change of the relative frequencies of the observed symptoms was used as a measure of the time that had elapsed since infection. The developed methods facilitate the analysis of field data in which the influence of climate, phenological stage of the host, and inoculum density on infection incidence can be elucidated

Development and Evaluation of an Infection Model forStemphylium vesicariumon Pear Based on Temperature and Wetness Duration

Nonwounded fruits of pear cultivar Passe Crassane and plants of cultivar Conference were inoculated with conidia of Stemphylium vesicarium at temperatures ranging from 5 to 30°C and wetness durations from 0 to 24 h. Plants and fruit were maintained for expression of symptoms of brown spot at 20°C and 80 % relative humidity. Polynomial regression models using the log 10 transformation of disease severity as the dependent variable were developed for fruit and leaf infection, and described well the effects of temperature (T) and duration of wetness (W). The coefficients of determination for the fruit and leaf infection models were 0.88 and 0.92, respectively. Optimal conditions for infection were > 24 h of continuous wetness at 22.6°C for fruits and 21.1°C for leaves. The leaf model was evaluated in 40 field trials in Spain and Italy under a wide range of wetness duration periods and temperature conditions, and was validated in relation to its capacity to predict infection periods. The coefficient of determination for the regression of observed disease severity on the predicted disease severity was 0.83 (P = 0.0001) and the slope was not different from I (P = 0.44), but the intercept was -0.08, indicating that the model overestimated infection risk. However, the leaf model under-predicted disease when high relative humidity periods with low wetness duration occurs. The leaf and fruit models also were relatively accurate in identifying likely infection periods in the field. During 2 years of a study in a commercial orchard, the leaf model accurately predicted six, and indicated broadly the other four, of ten infection periods

Effects of Temperature and Wetness Duration on Sporulation ofBotrytis cinereaon Strawberry Leaf Residues

Disks cut from dead strawberry leaves (autoclaved or air-dried) were inoculated with a conidial suspension of Botrytis cinerea (10(6) conidia per ml) and incubated at various temperatures (T) ranging from 5 to 30 degrees C. Sporulation (conidia per cm2) on leaf disks was determined after exposure to wetness durations (W) of 3 to 11 days. Optimum temperature for sporulation was between 17 and 18 degrees C at all wetness durations. Sporulation levels of 10(5) to 10(7) conidia per cm2 were observed between 15 and 22 degrees C, after 7 days of continuous wetness. As temperature increased or decreased from the optimum, sporulation decreased for the same wetness durations. Very little sporulation was observed at 25 degrees C and no sporulation was observed at 30 degrees C. Logarithmic polynomial models best described the effect of T and W on sporulation of B. cinerea on dead strawberry leaf tissue. Coefficients of determination for data from all repetitions of the experiments were at least 0.81. The latent period of B. cinerea on dead leaf tissue was longest at the lowest temperature (6 to 7 days at 5 degrees C) and decreased to <3 days as temperature increased to the optimum (15 to 22 degrees C). Interrupted wet and dry periods of 5. 12, and 24 h directly affected sporulation. Total hours of wetness and the duration of individual wet periods had the greatest effect on the amount of inoculum produced at 20 degrees C.

Development of an Infection Model for Botrytis Bunch Rot of Grapes Based on Wetness Duration and Temperature

Grape berries were dipped in conidial suspensions of Botrytis cinerea and incubated for 4, 8, 12, 16, or 20 h of wetness at temperatures ranging from 12-30 C. Berries were infected after 4 h of wetness at all temperatures tested. Incidence of berry infection increased with increasing wetness duration at all temperatures. A multiple regression model described the logit of infection as a function of the interaction of wetness duration and temperature (R 2 = 0.75). This model was incorporated into an infield environmental monitoring station and evaluated for two seasons on Thompson Seedless table grapes in the central valley of Chile. Applications of captan (3,2 kg/ha a.i.) or vinclozolin (1 kg/ha a.i.) were made according to the Botrytis model or a standard phenological spray program that consisted of four sprays (at bloom, cluster thinning, veraison, and preharvest) plus additional sprays after major rain events. Disease incidence and severity at harvest were similar whether applications were made according to a standard program (six to nine applications in 1991-1992, four to five in 1992-1993) or according to the Botrytis model (two to four applications in 1991-1992, zero to five in 1992-1993). In some vineyards, postharvest disease was significantly less when sprays were made according to the Botrytis model recommendations compared with the standard spray program

Effect of Temperature and Wetness on Infection of Black Raspberry by Aeciospores ofArthuriomyces peckianus

The optimum temperature range and leaf wetness duration for germination and infection of black raspberry by aeciospores of Arthuriomyces peckianus were determined experimentally. The germination of aeciospores on water agar increased with temperature from 12 to 28 degrees C. Maximum germination of aeciospores occurred in free water and declined as relative humidity was reduced below 100%. The effect of temperature on the ability of aeciospores to infect leaf disks of black raspberry was described by the model y = -171.688+ 21.609T-0.544T(2), where y = percent successful infection of aeciospores and T = temperature (C). The predicted optimum temperature for infection was 19.9 degrees C. The optimum duration of leaf wetness for infection was 10 h at 22 degrees C, but infection could occur in as little as 6 h at 22 degrees C.

Some remarks on dew duration on top of an orchard

Based on the energy balance approach, a simple dew duration estimation scheme for an isolated leaf, which is exposed to the sky at a short distance above a canopy, is tested against dew periods forming at the top of an apple orchard. As far as the canopy is concerned, three different boundary conditions are impressed on the model: (A) the canopy beneath the exposed leaf has the same temperature as the leaf which cools to the dew point as a result of its radiative loss; (B) the canopy is at air temperature and therefore warmer than the leaf; (C) between the canopy, the orchard air and the underside of the leaf there are no vertical gradients of temperature and humidity and no net radiation. The simulated wetness duration by model A fits the observations best, while model B under-estimates observed dew duration and model C overestimates the wetting period.

Effect ofUlocladium atrumand Other Antagonists on Sporulation ofBotrytis cinereaon Dead Lily Leaves Exposed to Field Conditions

The potential of the antagonistic fungi Aureobasidium pullulans, Chaetomium globosum, Gliocladium catenulatum, and Ulocladium atrum to suppress sporulation of Botrytis cinerea was tested in nine experiments on dead lily leaves exposed to varying microclimatic conditions in the field. U. atrum competed successfully with naturally occurring saprophytes, mainly Cladosporium spp., colonized the dead lily leaves, survived dry periods, and consistently reduced sporulation of naturally occurring B. cinerea. U. atrum reduced the area of the leaf surface covered with conidiophores of B. cinerea by 80-96% compared to the control treated only with water. Germination rates of conidia of U. atrum, determined 18 h after field application, varied between 0 and 99%, depending on duration of leaf wetness periods, which ranged from 0 to 18 h, and on temperatures during leaf wetness periods. Germ tube length, determined 5-6 days after application, increased with total leaf wetness duration unless individual wetness periods were short. C. globosum reduced sporulation of B. cinerea only in three of nine experiments. A. pullulans, G. catenulatum, and a mixture of the fungicides chlorothalonil and maneb did not suppress sporulation of B. cinerea. The differential effect of the antagonists may mainly be caused by differences in response to the microclimatic conditions. The high saprophytic competitive ability of U. atrum under various microclimatic conditions makes this fungus an attractive candidate for the development of a biological control product aimed at suppression of sporulation of Botrytis spp. on necrotic leaf tissue. To our knowledge, U. atrum has not been described as an antagonist of Botrytis spp. or other fungal plant pathogens

An Empirical Model for Predicting the First Symptoms of Sooty Blotch and Flyspeck of Apples

Measures of leaf wetness duration were related to the appearance of symptoms of sooty blotch (caused by Peltaster fructicola, Leptodontium elatius, and other fungi) and flyspeck (caused by Zygophiala jamaicensis) on apple (Malus x domestica) during the 1987 through 1994 growing seasons. Hours of leaf wetness of 4 h duration or greater accumulated from the first rain that occurred 10 days after petal fall provided the best measure of the time of symptom appearance. Over the 8 years of the study, symptoms appeared after an average of 273 h of leaf wetting (range 209 to 310 h). No consistent temperature effects on the time of first symptom appearance were found. A threshold value of 200 to 250 h of leaf wetting is suggested to initiate fungicide applications for the control of sooty blotch and flyspeck.

Comparative Effects of Temperature and Interrupted Wet Periods on Germination, Penetration, and Infection ofPuccinia reconditaf. sp.triticiandP. striiformison Wheat Seedlings

Under optimal temperature and nonlimiting wetness duration, the infection efficiency (defined as the proportion of inoculated urediniospores causing lesions on wheat seedling leaves) was 12 times greater for Puccinia recondita f. sp. tritici than for P. striiformis. Penetration of both species, however, was similarly affected by a 1-h dry period interrupting a 24-h wet period 1, 2, 4, 6, 8, or 16 h after inoculation at several temperatures between 5 and 30 C. Appressoria from germinated P. r. tritici urediniospores prior to penetration were unable to survive the 1-h dry period. An interruption of the wet period by a dry period did not affect ungerminated urediniospores, which were able to infect leaves during a subsequent dew period. The minimal continuous dew period necessary for infection increased from 4 to 6 h at optimal temperatures (8 C for P. striiformis, 15 C for P. r. tritici) to at least 16 h at suboptimal temperatures. Infection by P. r. tritici occurred over a wide range of temperatures (5-25 C), whereas infection by P. striiformis was restricted to a narrower range (5-12 C). Percentage of infection as a function of the duration of the continuous dew period was described by a Richards's function with temperature-dependent parameters. For a dry period interrupting the 24-h dew period before the minimal continuous dew period necessary for infection, percentage of infection at specific temperatures was fitted by a negative exponential function of time of interruption. If the dry period occurred after the minimal dew duration for infection, percentage of infection was the same as with a continuous wet period

Comparative Study of Microclimate and Downy Mildew Development in Subsurface Drip- and Furrow-Irrigated Lettuce Fields in California

Microclimates and downy mildew (caused by Bremia lactucae) disease progress were monitored in neighboring lettuce fields with subsurface drip or furrow irrigation during five trials in 1992 to 1993. The trials included a total of ten irrigation events during which soil moisture, soil temperature, canopy air temperature and humidity, leaf wetness duration, wind speed, and solar radiation were recorded. Disease intensity was assessed at intervals, beginning at thinning of the crop and ending just before harvest. Wilcoxon's Signed Rank Tests were computed to compare microclimates between drip- and furrow-irrigated fields, separately for days before or after irrigation. There were no significant differences in microclimate between the two irrigation methods before irrigation. Within 3 days after irrigation, there were significantly longer overall leaf wetness periods (P ≤ 0.0025) and a trend toward higher daytime humidity (P ≤ 0.1254) and longer morning leaf wetness periods (P ≤ 0.0863) in fields with furrow irrigation. Air temperature and nighttime humidity were not consistently different between the two irrigation methods. Downy mildew developed in four of the five trials, and disease intensity was always lower under drip irrigation than under furrow irrigation. The magnitude of the differences in disease was small, however. It appears that, on most days in coastal California, mesoclimatic variations outweigh microclimatic modifications that could potentially influence disease development