The use of adjuvants to improve spray deposition and Botrytis cinerea control on Chardonnay grapevine leaves

Spray adjuvants have the potential to improve deposition by effecting uniform distribution of the active ingredient on plant surfaces. In order to study whether such a qualitative improvement of spray deposition would lead to improved disease control, a laboratory experiment was conducted on artificially inoculated grapevine (cv. Chardonnay) leaves. Prior to inoculation with Botrytis cinerea conidia in a spore settling tower, leaves were sprayed to pre-runoff with 1mL of a mixture of fenhexamid, a fluorescent pigment, and one of 15 selected commercial adjuvants to manipulate the deposition quality of a specific quantity of spray. Following an incubation period of 24h at high relative humidity, leaf discs were plated onto Petri dishes with paraquat-amended water agar and rated for development of B. cinerea from isolated leaf discs 11d later. Spray deposition on leaves was assessed with a spray assessment protocol using fluorometry, photomicrography and digital image analyses. B. cinerea incidences on the upper and lower surfaces of water-sprayed leaves averaged 90.4% and 95.8%, respectively. Despite full spray cover of leaves, applications with fenhexamid alone did not completely prevent infection and resulted in 34.6% and 40.8% B. cinerea incidences on the upper and lower surfaces of leaves, respectively. Through the addition of certain adjuvants, B. cinerea incidences were significantly reduced (incidences of 2.9-17.1% and 10.0-30.8%, respectively), while some adjuvants did not differ from the fungicide only treatment, even though they might have improved spray deposition. In a histopathology study using epifluorescence microscopy, distinct differences were observed in conidium mortality (20.5% vs. 31.2%), germination (60.4% vs. 51.4%) and germ tube lengths (27.8@mm vs. 19.7@mm) between Hydrosilicote and Solitaire in combination with fenhexamid, even though both adjuvants effected similar quantitative and qualitative spray deposition. The study clearly demonstrated the potential of adjuvants to improve the bio-efficacy of a fungicide directly through improved deposition on grapevine leaf surfaces, although bio-efficacy might be influenced by adjuvant mode of action in some cases.