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The onset and cessation of surface wetness on maize ears were simulated with six models, using hourly meteorological data, to examine the linkage between wetness duration and possible forecasting of fungal infections that produce mycotoxins. Two threshold models (using relative humidity and dew point temperature), one regression model (using humidity and wind speed), and three physical models based on the energy balance approach, were compared. Also, spatial and temporal variability in wetness duration was measured and simulated at three sites located at distances up to 29 km from a central weather station. The estimated wetness values were compared with observations from cylindrical wetness sensors placed near ear level in maize canopies. The results relate to potential mycotoxin warning systems and indicate that threshold models can provide reasonable estimates of ear wetness duration in this region, that a comprehensive physical model can give superior estimates, and that wetness estimates made from a central weather station data can be extended to nearby crop fields with a moderate degree of confidence. Key words: epidemiology, mycotoxins, surface wetness models

Weather variables, leaf blight, and airborne spores of Botrytis squamosa were monitored in onion field plots at the Holland–Bradford Marsh, Ontario, in 1976. Incidence of spores showed marked daily periodicities with peaks normally between 0900 and 1200 hours Eastern Standard Time (EST). Daily spore counts were low ( 1000 spores) during29 July to 16 August, and moderate (100 to 1000 spores) between 17 and 28 August. Spore production was observed only on necrotic portions of onion leaves. Circumstantial and correlative evidence indicated that spore production was promoted by persistent leaf wetness (> 13 h), high temperatures during wetness periods (14 to 20 °C), and by leaf dieback but was restricted by brief wetness periods [Formula: see text] and by cool temperatures [Formula: see text]. Linear correlation analyses of 2- to 4-day running means of data indicated that important limiting factors in spore production were wetness duration during 7 to 28 July, temperature during 29 July to 28 August, and severity of dieback during 7 July to 16 August. Lodging appeared to suppress incidence of spores after 16 August. Spore release was promoted largely by declining relative humidity (RH) and by rain but occasionally by increasing RH. Striking peaks of airborne spores occurred during rain showers. Spore counts when leaves were dry failed to correlate with wind speed, and many spores were dispersed when wind speeds were low(1 to 4 km/h).

Three artificial populations, with leaf area indices (L) of 4.4, 1.5, and 0.54 were submerged in identical tubs of 1-m3 capacity. Experiments showed that the maximum vertical temperature gradient of the water varied and was dependent on the ratio of solar radiation to wind speed, the leaf area index, and the arrangement of the leaves in the population. The time of day of the maximum temperature gradient was also dependent on L and leaf arrangement. The interception of the light energy by the leaf surface heated the water locally, while the shadow beneath caused the temperature to remain low. With fewer plants, water evaporation was slightly greater. Evaporation resulted in a marl deposit which was confined to the upper surface of the leaves.

A simulation model for the prediction of threshold concentration values of gaseous sulphur dioxide for damage to Cladina rangiferina over a 24-h period is presented. The model requires inputs of near surface relative humidities and air temperatures as well as lichen surface temperatures, and nearby airport wind speeds. An initial value of percent lichen water content must also be specified. Studies of lichen water relationships provided expressions utilized in the model for the calculation of the percent lichen water content. The model was applied to four microclimatically different days for an open lichen–spruce woodland near Inuvik, N.W.T. Boundary-layer resistance was found to be not as important as internal thallus resistance to sulphur dioxide uptake. For all days examined, threshold gaseous concentration values of sulphur dioxide for a 24-h period ranged from 20 to 30 μg∙m−3.

Samples of the lichen Cladina rangiferina were exposed to various concentrations of sulphur dioxide in a fumigation cuvette system for time periods ranging from 1 to 18 h. The cuvette system provided airstreams which, within the limits of measurement, had identical flow rates, temperatures, and relative humidities. Sulphur dioxide was added to one airstream as a treatment, while the other remained free of sulphur dioxide for control purposes. In some experiments, wind speed could be varied from 8 to 250 cm∙s−1 in a small wind tunnel placed in the fumigation cuvette. The system allowed for the calculation of sulphur dioxide uptake by the lichen sample. Potassium release was shown to increase with an increasing uptake of sulphur dioxide (time-integrated flux), but was not uniquely related to sulphur dioxide concentration alone. Extrapolation of a linear transformation of the potassium release curve to zero potassium efflux resulted in a threshold uptake value of 240 μg SO2∙g lichen−1. Internal lichen resistance and lichen boundary-layer resistance were experimentally determined for later use in an electrical analog model for the calculation of sulphur dioxide uptake.

In Lake Opinicon, Frontenac County, Ontario, the temperature gradient at midday from 10 cm below the water surface to soil, in clear weather in August, was found to be 10 Celsius degrees (C°)/m in shallow water where there was a large macrophyte biomass and only 0.2 C°/m in deeper water with fewer plants. There was a direct correlation between the log of the biomass per cubic metre and the steepness of the gradient. Reference is made to temperature gradients developed in calm weather during phytoplankton blooms in Scottish lakes.The biomass of aquatic macrophytes growing in a heated greenhouse in tubs with a capacity of 1 m3 increased from early January to the end of February. With increased biomass, the temperature gradient became larger. The size of the gradient correlated with the incoming radiation. Temperature gradients within large populations of the pondweed Potamogeton richardsonii were enhanced by low temperatures near the bottom.Diurnally produced temperature gradients with an algae bloom in the same tubs in the open in late June showed strong correlation with solar radiation, 8 C°/m with cumulative radiation of 22 MJ/m2 and 4.6 C°/m with 12 MJ/m2. The gradients produced by a population of P. richardsonii with tips 17 cm from surface were from 5.2 to 3.4 C°/m but showed less correlation with radiation when the wind velocity was above a threshold value. No gradient was produced in tubs of water without plants.

An Apple Scab Alarm was constructed that senses temperature and the duration of surface wetness, then electronically combines these data and displays the result as a single number representing the potential severity of apply scab infection. The performance of the Alarm was compared to independent estimates of potential infection severity which utilized temperature data from a thermohygrograph and surface wetness duration data from a deWit Leafwet Recorder or an electrical impedance grid. Except for two occasions when errors of about 2 h appear to have occurred in the detection of surface wetness duration by the deWit recorder, the independently calculated estimates agreed with the Alarm display within 8.6% during 18 periods of rain or dew.

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