Estimation And Application Of Horizontal Distribution Of Cucumber Leaf Wetness Duration In Solar Greenhouse

Leaf wetness duration?LWD?is one of the important input variables of plant disease model,which is related to the infection of many leaf pathogens and affects the pathogen infection and development rate.In order to estimate the horizontal distribution of cucumber leaf wetness duration and accurately predict the occurrence time and location of initial infection of cucumber downy mildew in solar greenhouse,nine sampling points were set up in two different greenhouses?Xiaotangshan greenhouse and Fangshan greenhouse?in Beijing in March and September 2019 according to the chessboard method to deploy temperature,humidity and light sensors.The fixed-point visual inspection method was used to collect the data every 1h.Before planting,one suction type sporangium catcher shall be deployed in Xiaotangshan greenhouse and Fangshan greenhouse at a height of 1m from the ground.The one investigation point of downy mildew shall be set at each sampling point.The investigation time of Fangshan greenhouse was September 9,and Xiaotangshan greenhouse started from April 1.The inoculation method was natural inoculation.The Relative humidity model?RHM?and Back propagation neural network model?BPNN?were used to quantitatively estimate and analyze the leaf wetness duration,The estimated leaf wetness duration of BPNN was used as the input of early-warning model of cucumber downy mildew,and the time of initial infection in all directions in greenhouse was predicted.The main results are as follows:?1?BPNN obtained similar results under the experimental conditions of two greenhouses,which was more accurate than RHM in estimating of leaf wetness duration,the mean absolute error were 1.81h and 1.61h,root mean squared error were 2.10 and 1.87,and coefficient of determination were 0.87 and 0.85.In sunny and cloudy conditions,the spatial distribution of leaf wetness duration was generally in the South>the Middle>the North.In the South,the average leaf wetness duration was the longest,12.17h·d^-1;from the east to the west,the spatial distribution of leaf wetness duration was generally in the East>the West>the Middle.In the Middle,the average leaf wetness duration was the shortest,4.83h·d^-1.The average leaf wetness duration in rainy days is longer than that in sunny days and cloudy days,the average leaf wetness duration in spring and autumn rainy days were 17.15 and 17.41h·d^-1.These changes and differences had an important impact on the distribution of leaf wetness duration in the horizontal direction of cucumber population in greenhouse,which was closely related to the occurrence rule of most high humidity cucumber diseases.In this paper,the method of regional analysis of the wet duration of cucumber leaves in greenhouse was proposed,which can provide a reference for simulating the spatial distribution of leaf wetness duration in greenhouse.?2?The early-warning model of cucumber downy mildew has the same prediction date with the actual observation date,the mean absolute error was about 1 day,the determination coefficient was 0.98,and the Youden index was 0.98,The results show that the prediction model can accurately predict the time of the initial infection and the emergence of the disease in all directions in the greenhouse,and can provide the basis for the management of cucumber downy mildew in greenhouse,which is of great significance to control the disease prevalence and reduce the use of pesticides.