Epidemiology of pea necrotic yellow dwarf virus (PNYDV) and possibilities for aphid control in organic faba beans.

In 2016, the pea necrotic yellow dwarf virus (PNYDV, genus Nanovirus) caused substantial yield losses in numerous faba bean and pea crops in Austria. The virus was first identified and described in green peas in Germany in 2009, and it was subsequently detected in Austria in 2010 in both green peas and faba beans. The aim of the project "Epidemiology of pea necrotic yellow dwarf virus (PNYDV) and possibilities for aphid control in organic faba beans" was to collect epidemiological data on this emerging virus, to identify its host plants and the most important aphid vectors involved in its transmission. Because nanoviruses are aphid-transmitted, aphid management is the sole option for reducing infection rates. Two aphid control strategies were tested in organic faba beans: applications of organically approved insecticides (in collaboration with biohelp, an Austrian company specializing in biological plant protection) and the use of faba bean-oat mixed cultures. Within the project, natural infections with PNYDV were confirmed in several legume species, including pea (Pisum sativum), grass pea (Lathyrus sativus), faba bean (Vicia faba), lentil (Lens culinaris), common vetch (Vicia sativa), Pannonian vetch (Vicia pannonica), chickpea (Cicer arietinum), and Vicia hirsuta. Among weed species, Vicia hirsuta was identified for the first time in Austria as a host plant of PNYDV. Soybean (Glycine max) is no host plant for the virus. Consequently, field peas, faba bean, common vetch, and Pannonian vetch should not be used as catch crops, since they serve as a "green bridge" enabling overwintering of the virus. Testing for additional legume viruses revealed frequent mixed infections, with up to five different viruses detected in individual plants. In addition to PNYDV, infections with pea enation mosaic virus (PEMV), alfalfa mosaic virus (AMV), turnip yellows virus (TuYV), pea seed-borne mosaic virus (PSbMV), and bean yellow mosaic virus (BYMV) were confirmed. The green pea aphid (Acyrthosiphon pisum), the black bean aphid (Aphis fabae) and the green peach aphid (Myzus persicae) could be confirmed as the major vectors of PNYDV. Field experiments revealed the highest aphid densities on Pisum sativum, Lathyrus sativus, Vicia sativa, V. villosa, Medicago sativa, Lens culinaris, Lotus corniculatus, Trifolium spp., and Trigonella foenum-graecum, demonstrating species-specific differences in aphid attractiveness. In field trials significantly more black bean aphids were found on the white flowering faba bean variety "GL Sunrise" than on the colored flowering variety "Alexia". Conversely, significantly fewer green pea aphids were found on "GL Sunrise" than on "Alexia". The attractiveness of the two field bean varieties to the green pea aphid and the black bean aphid is obviously different. Higher abundances of green pea aphids were associated with increased virus symptoms and reduced yield. In contrast, no relationship between black bean aphid abundance and either virus symptoms or yield was detected in 2018 and 2019. Green pea aphids are able to overwinter anholocyclic on perennial legumes and typically colonize and infect legume crops earlier in the growing season than black bean aphids. This temporal pattern suggests that green pea aphids are primarily responsible for initiating primary infections in the field, whereas black bean aphids are more likely to contribute to subsequent virus spread within the crop. The trials demonstrated that the selection of varieties can result in agronomic differences in susceptibility to viral infection. Results from field bean-oat mixed cropping trials indicate that both aphid abundance and incidence of virus-infected plants were lower in mixed cropping systems than in pure field bean stands. The benefits of mixed cultivation were particularly pronounced in years characterized by high aphid populations and elevated virus pressure. Nevertheless, several practical aspects remain unresolved, including the selection of regionally appropriate partners for mixtures, or the effects of mixed cropping systems within crop rotations.