Examining the interaction between droplet density, leaf wettability and leaf surface properties on fungicide efficacy.

The management of gray mold, caused by the fungus Botrytis cinerea, on ornamental plants relies heavily on fungicide applications. To improve fungicide efficacy, the manipulation of nozzle type, spray volume, and pressure influence droplet size (µm) and density (droplets/cm2) on the leaf’s surface. However, leaf wettability dictates how well the application droplets adhere and spread across the surface. When leaf surfaces are waxy (hydrophobic) or hairy (tomentose), droplets fail to adhere, impacting fungicide sorption.

The goal of this research was to evaluate how the interaction of droplet density and leaf wettability impact the efficacy of chemical and biological fungicides against Botrytis cinerea. Leaf surfaces vary between species, within species, leaf age, and leaf sides (abaxial or adaxial). Hydrophobic leaf surfaces influence fungicide efficacy by reducing fungicide droplet spread compared to the wettable and hydrophilic leaf surfaces. The presence of trichomes on the leaf surface can inhibit droplets from reaching the surface.

To quantify droplet density, a fine and coarse spray of fungicide treatments was applied with a yellow fluorescent dye. After application, Begonia x hybrida ‘Dragon wing’ leaves were placed on black, blackout curtains below a blacklight. Images were analyzed by ImageJ, using an image processing method. The number of lesions, disease incidence, were counted to observe fungicide efficacy. Results show there was no interaction between the actual droplet density within treatments applied with fine and coarse sprays. However, the interaction between spray type (fine and coarse) and treatments can have a significant effect on disease incidence. Disease incidence was significantly different between the systemic and contact fungicides for fine and coarse sprays. However, the systemic fungicide treatment had the highest disease incidence compared to the contact fungicide.

To assess leaf wettability impact on fungicide efficacy, five Begonia species (B. scharffii, B. erythrophylla, B. x hybrida ‘Dragon Wing’, B. epipsila, and B. goldingiana) were used based on their observed leaf surface type. A contact angle goniometer was used to take pictures of a droplet on Begonia leaf surfaces. The quantification of the leaf surface took place by using the ImageJ program ‘Drop-Snake’ within the plugin ‘Drop Analysis’. The number of lesions, an indicator of disease incidence, were counted to observe fungicide efficacy. Results showed the contact angles were different between the Begonia species. There was a significant interaction between the Begonia species and treatments, where Begonia ‘hairy’ and ‘waxy’ leaf surfaces can influence fungicide efficacy. However, there was no significance for the interaction between Begonia species’ contact angles and treatments.

These studies advance our understanding of how droplet density and leaf surfaces influence fungicide efficacy, thus improving our ability to manage Botrytis for diverse ornamental plants.