Study on estimation model for grape downy mildew prediction based on airborne sporangium concentration of Plasmopara viticola in field

Abstract:【Objective】Grape downy mildew is a typical airborne disease that can damage all green tis- sues on Vitis plants, including leaves, inflorescences, berries, tendrils and young canes. Sporangium is the important carrier of short distance transmission of the disease. The formation, maturation, germina- tion and release of sporangium are affected by meteorological factors such as temperature, relative hu- midity, rainfall, illumination and wind speed. In order to clarify the effects of airborne sporangium con- centration of Plasmopara viticola and meteorological factors (air temperature, relative humidity, rainfall and wind speed) on disease infection of grape downy mildew in Shenyang, a prediction model of grape downy mildew driven by airborne sporangium concentration of P. viticola was established to guide the effective control of the disease.【Methods】Shenyang, one of the production areas of table grapes in China, was chosen as the experimental base. Experiments were conducted during four growing seasons from 2016 to 2019. Four test plots were arranged from north to south, and the test cultivar was Centen- nial Seedless (Vitis vinifera L.), which was highly susceptible to grape downy mildew. Each experimen- tal plot was 15 m long, and 5 m wide. Vine spacing was 0.5 m within rows and 0.6 m between rows. The plots were planted on April 30, 2016; April 28, 2017; May 5, 2018 and April 29, 2019. Each plot  was irrigated and managed normally, and the cropping system was not treated with any fungicide in or- der to facilitate the onset of the disease epidemic. TRM-ZS1 meteorological ecological environment de- tector was installed in the orchard for regular recording meteorological data. The concentration of air- borne sporangia of P. viticola was monitored daily with Burkard spore trap. The incidence and degree of grape downy mildew was investigated by using the five-point sampling method and 15 fixed grape seed- lings were selected from each plot every seven days. Bivariate correlation method in SPSS 19.0 was used for correlation analysis, and Spearman correlation coefficient was used to obtain the correlation analysis results among disease index of grape downy mildew, airborne sporangium concentration of P. viticola and meteorological factors. Eight curve models were selected to fit the data of disease index and the accumulated sporangium concentration of P. viticola during 2016 to 2019. The coefficient of de- termination (R2), F value (F), significance probability (p) and estimated standard deviation (Std E) were taken as the criteria of model selection to select the best prediction model for grape downy mildew.【Re- sults】The disease progress curve of grape downy mildew was usually sigmoid, the exponential phase was from early July to late July, the logistic phase was from late July to late August, and the decline phase was from late August to mid-late September. Rainfall had an important effect on occurrence and prevalence of grape downy mildew. The variation curve of airborne sporangium concentration of P. viti- cola in the field showed a wavy change, which increased with the disease index in general. The airborne sporangium concentration decreased gradually as the disease stopped growing. In 2016, the date of first sporangia trapping in the field was June 20, and the airborne sporangium concentration increased slowly before July 1, then increased sharply, and reached the peak of 75 sporangium per m3 on September 8. In 2017, the date of first sporangia trapping was June 26, the airborne sporangium concentration in- creased rapidly after July 11, and reached the maximum of 52 sporangium per m3 on August 22. The date of first sporangia trapping was June 29, 2018, then the airborne sporangia concentration increased in a multi-peak curve, and reached the maximum of 57 sporangium per m3 on September 8. The date of first sporangia trapping in the field in 2019 was June 26, the airborne sporangium concentration fluctuat- ed at a low level before July 12, and reached the peak of 51 sporangium per m3 on September 1. Correla- tion analysis showed that airborne sporangium concentration of P. viticola was significantly positively correlated with average relative humidity before 7 days (r>0.224, p<0.030) and accumulated rainfall be- fore 7 days (r>0.209, p<0.040), but had a significant negative correlation with daily rainfall, indicating that the above three meteorological factors were the main meteorological factors affecting the airborne dispersion of P. viticola. There was no significant correlation between the sporangium concentration ofP. viticola and the average daily air temperature, relative humidity and wind speed, indicating that the daily meteorological factors were not the key factors affecting the airborne dispersion of P. viticola. The relationship between disease index and the accumulated sporangium concentration was a power func- tion by nonlinear regression analysis, and the best prediction model was based on the accumulated spo- rangium concentration before the day of disease measuring and the accumulated sporangium concentra- tion before the current week of disease measuring.【Conclusion】Based on the analysis of 4-year plot ex- periment in the field, the accumulation of sporangium concentration of P. viticola can be used to predict disease progress of grape downy mildew in the field.