Einfluss von Saatzeit und Strohmulch auf Blattlausbesiedelung und Viruserkrankungen in Fababohnen (Vicia faba L.)

We investigated the combined effects ofstraw mulch and sowing date on aphid infestation and virus incidence in faba beans in organically managed small scale field experiments. Straw mulch was applied in early and late sown treatments shortly before commencing vector activity in May. Aphids were monitored by yellow water traps. In addition, crop colonisation over time was assessed for the main vectors Acyrthosiphon pisum and Megoura l'iciae. PEMV (pea enation mosaic virus) and BYMV (bean yellow mosaic virus) were most abundant in three consecutive years, constituting 22-54% and 9-69% of the detected virus total, respectively. However, relationships between Acyrthosiphon pisum activity and virus loads were less consistent within years. Significantly decreased A. pisum colonisation over time by mulching was prevalent but restricted to one or few dates within the entire colonisation period. Factorial analysis ofresulting virus loads revealed significant reduction by early sowing but not by mulching and interaction within these factors was not significant or poor. lt is suggested that relatively weak mulching effects towards visually guided aphids were assigned to rapid canopy closure in combination with the tall crop stature. In late sowings however, the ratio of soil background to vegetative cover was more permissive to interfere with vector alighting. lrrespective from mulch application, exposure of young plants lead to significantly enhanced virus loads. Straw mulching in late spring had a neutral effect on yield and did therefore not compensate for yield depression in late sown treatments. Thus, the perspectives of a mulching approach to control virus diseases in faba beans are apparently lower than anticipated.