H. Scherm

Sensitivity of simulated dew duration to meteorological variations in different climatic regions of California

A previously verified and validated model was used in combination with hourly weather data to simulate the frequency and duration of dew occurrence over one growing season (April–October) at four sites under coastal fogbelt, coastal valley, and interior valley climates in California. Simulation results indicated similar values of dew occurrence (78–93% of the days) and mean dew duration (8.7–9.3 h day−1 or 9.7–11.2 h per dew event) at the coastal stations, despite high day-to-day variability and steep climatic gradients between these sites. Dew periods at the interior valley site, where occurrence of dew was predicted on only 23% of the days, were much shorter (mean 0.9 h day−1 or 3.7 h per dew event). The data displayed moderate seasonal trends, with the longest dew periods occurring in fall at the coastal stations and in spring at the interior valley station. Simulated dew durations showed favorable agreement with measured dew durations in an independent validation study. Based on real-system weather data and simulated dew data, sensitivity analyses were performed to compare the effects of temperature, atmospheric humidity (vapor pressure), cloud cover, and wind speed on dew duration at the four test sites. The results showed simulated dew durations most sensitive to changes in humidity and cloud cover at the coastal sites and to humidity and wind speed at the interior valley site. Relative sensitivity values were generally highest during the summer months (June–August). Spatial variations (i.e. differences between locations) and seasonal variations (i.e. differences between different seasons at the same site) of relative sensitivity values had approximately the same magnitude.

Field evaluation of fungicide spray advisories against lettuce downy mildew (Bremia lactucae) based on measured or forecast morning leaf wetness.

In a previous field study in coastal California, infection of lettuce by downy mildew (Bremia lactucae) occurred primarily on days when leaves dried late in the moming (at 1000 h or later), suggesting that fungicide sprays against the disease could be scheduled according to a morning leaf wetness threshold. Seven field trials were carried out in 1993 to 1994 to test whether sprays of maneb or fosetyl Al, applied when measured or forecast morning leaf wetness ended at 1000 h or later, would lead to a reduction in the number of fungicide applications and/or improved downy mildew conaol compared with a calendar-based schedule with three sprays. Based on measurements of morning leaf wetness, the total number of sprays in the seven trials was reduced by 67% relative to the calendar-based schedule, with no difference in disease intensity. Based on forecasts of moming leaf wetness, which were generated using a physical dew simulation model with numerical weather forecasts from the National Meteorological Center as input, about 90% of the days were correctly classified as days with or without prolonged morning wet period. However, the occurrence of important wet periods due to fog drizzle was not predicted and the forecasts for the exact time of onset and end of leaf wetness were inaccurate. It is concluded that the number of fungicide applications against lettuce downy mildew can be reduced substantially with sprays scheduled according to a morning leaf wetness threshold of 1000 h and that fog drizzle should be included in leaf wetness forecasts for coastal California

On the velocity of epidemic waves in model plant disease epidemics

Abstract Previous work on the temporal and spatial spread of plant diseases has focused on models generating epidemics with constant frontal velocity (traveling epidemic waves). But the theory of turbulent diffusion, the laws of which govern the dispersal of windborne spores of most foliar plant pathogens, suggests that spread becomes more efficient as the area with diseased plants expands. This should result in epidemics with increasing frontal velocity (dispersive epidemic waves). As a first step toward an analytic treatment of dispersive waves of plant diseases this study considers two simple models based on reaction-diffusion equations with scale-dependent diffusivity. Predictions from these models are compared qualitatively to data on the spread of potato late blight ( Phytophthora infestans ) epidemics over different temporal and spatial scales. The paper concludes with a brief discussion on the merit of simple, analytical models for studying, interpreting, and testing of abstract epidemiological concepts, such as traveling and dispersive waves.

Response surface models for germination and infection of Bremia lactucae, the fungus causing downy mildew of lettuce

Abstract Two response surface models, a third-order polynomial and a modified Gompertz function, were evaluated for their ability to describe germination and infection of Bremia lactucae on lettuce in relation to temperature and leaf wetness duration. The models were based on data obtained from three independent experiments in which lettuce plants were inoculated with the fungus and exposed to a range of leaf wetness durations (0–24 h) at each of six fixed temperatures (5–30°C). Germination was assessed at the end of each wetness period, and infection was evaluated by measuring disease severity 10 or 14 days after inoculation. In the Gompertz model, rate and asymptote parameters could be expressed as simple functions of temperature using quadratic and cubic functions, respectively, and parameter estimates were obtained in a simple three-step procedure. Models based on the modified Gompertz function had non-significant lack-of-fit statistics, and yielded higher correlations between observed and predicted germination and infection levels than third-order polynomials. The presented methodology may be adapted to mosdel processes in other biological and ecological systems in which fast early growth or development is observed in relation to temperature (or a similar factor) and a time-like variable.

Sporulation of Bremia lactucae Affected by Temperature, Relative Humidity, and Wind in Controlled Conditions

ABSTRACT The effects of temperature (5 to 25 degrees C), relative humidity (81 to 100%), wind speed (0 to 1.0 m s(-1)), and their interactions on sporulation of Bremia lactucae on lettuce cotyledons were investigated in controlled conditions. Sporulation was affected significantly (P < 0.0001) by temperature, with an optimum at 15 degrees C, and by relative humidity (RH), with sporulation increasing markedly at RH >/= 90%. There was a significant effect of exposure time in relation to temperature (P = 0.0007) but not to RH. In separate experiments, both RH and wind speed significantly (P < 0.0001) affected the number of cotyledons with sporulation and the number of sporangia produced per cotyledon. No sporulation was observed at wind speeds of >0.5 m s(-1), regardless of RH. In still air, the number of sporangiophores produced per cotyledon increased linearly with RH from 81 to 100% (P = 0.0001, r = 0.98). Histological observations indicated that sporulation may be affected by stomatal aperture in response to RH, as more closed stomata and correspondingly fewer sporangiophores were present at lower RH. These results are important for understanding the mechanism of RH effects on sporulation and for predicting conditions conducive to downy mildew development.

Incorporation of Temperature and Solar Radiation Thresholds to Modify a Lettuce Downy Mildew Warning System

ABSTRACT The effect of temperature on infection of lettuce by Bremia lactucae was investigated in controlled environment studies and in the field. In controlled conditions, lettuce seedlings inoculated with B. lactucae were incubated at 15, 20, 25, or 30 degrees C during a 4-h wet period immediately after inoculation or at the same temperatures during an 8-h dry period after the 4-h postinoculation wet period at 15 degrees C. High temperatures during wet and dry periods reduced subsequent disease incidence. Historical data from field studies in 1991 and 1992, in which days with or without infection had been identified, were analyzed by comparing average air temperatures during 0600 to 1000 and 1000 to 1400 Pacific standard time (PST) between the two groups of days. Days without infection had significantly higher temperatures (mean 21.4 degrees C) than days with infection (20.3 degrees C) during 1000 to 1400 PST (P < 0.01) but not during 0600 to 1000 PST. Therefore, temperature thresholds of 20 and 22 degrees C for the 3-h wet period after sunrise and the subsequent 4-h postpenetration period, respectively, were added to a previously developed disease warning system that predicts infection when morning leaf wetness lasts >/=4 h from 0600 PST. No infection was assumed to occur if average temperature during these periods exceeded the thresholds. Based on nonlinear regression and receiver operating characteristic curve analysis, the leaf wetness threshold of the previous warning system was also modified to >/=3-h leaf wetness (>/=0900 PST). Furthermore, by comparing solar radiation on days with infection and without infection, we determined that high solar radiation during 0500 to 0600 PST in conjunction with leaf wetness ending between 0900 and 1000 PST was associated with downy mildew infection. Therefore, instead of starting at 0600 PST, the calculation of the 3-h morning leaf wetness period was modified to start after sunrise, defined as the hour when measured solar radiation exceeded 8 W m(-2) (or 41 mumol m(-2) s(-1) for photon flux density). The modified warning system was compared with the previously developed system using historical weather and downy mildew data collected in coastal California. The modified system was more conservative when disease potential was high and recommended fewer fungicide applications when conditions were not conducive to downy mildew development.

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. Wilcoxons 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