Control of Aphid-transmitted viruses of Faba bean using thiamethoxam as a seed-dressing insecticide treatment in Tunisia

Abstract

Faba bean, Vicia faba L. is the most important legume grown in Tunisia. Its productivity is affected by several biotic factors, including viruses. Most plant viruses are transmitted by various aphid species and therefor vector management is one of the main options for controlling viruses. This study aimed to evaluate the effectiveness of seed treatment with Thiamethoxam in controlling aphid-transmitted viruses. On-farm experiments were conducted at Menzel Bourguiba and Mornag-INRAT Station during the 2022/23 and 2023/24 growing seasons to manage faba bean viruses and their aphid vectors. In the 1st growing season one faba bean variety (Najeh) and one planting data were used, whereas in the 2nd growing season, two faba bean varieties (Najeh and Chourouk) and two planting dates were implemented. In both seasons, faba bean seeds were treated with two doses of Cruiser® (Thiamethoxam) (1.00 and 1.2 cc/kg) compared to untreated seeds (control) were used as treatments. Numbers of aphids on faba bean plants, and virus incidence (%) were monitored based on symptoms observed (chlorosis, stunting, yellowing, reddening, curling and mosaic/mottling). At the flowering stage, random leaf samples were collected from 400-500 plants/treatment and tested serologically by tissue blot immunoassay (TBIA) using antibodies against Faba bean necrotic yellows virus (FBNYV), Chickpea chlorotic stunt virus (CpCSV) and Bean yellow mosaic virus (BYMV). Faba bean plants were hand-harvested during the second half of June, and yield (kg) was recorded. Results of the 1st growing season showed that the incidence of CpCSV and FBNYV were 52% and 6% in untreated plots, and 20% and 0% in the plots treated with Thiamethoxam at the rate of 1.2cc/kg, respectively. In the 2nd growing season, no virus infection was detected in plots with the 1st planting date and plots treated with Thiamethoxam on the 2nd planting date at both sites, whereas viruses (e.g. CpCSV, FBNYV, BLRV and BYMV) were detected at low rate (0.4-6%) in untreated plots only in the 2nd planting date. Both faba bean varieties (Chourouk and Najeh) produced 28% higher yields than the untreated control for both doses of Thiamethoxam. Low rainfall and high temperature during the 2023/2024 growing season at both sites negatively affected aphid activity and hence virus infection, but the results obtained indicated that the virus and aphid vector management could be achieved by adjusting the planting date of faba bean crops and seed treatment with Thiamethoxam.