Symptomology and yield impact of pea necrotic yellow dwarf virus (PNYDV) in faba bean (Vicia faba L. minor)

We surveyed 33 symptomatic faba bean sites in central Germany towards the end of the growing season 2016 to analyse the suspected virus spectrum. All sites displayed plants with characteristic symptoms and had distinct funnel-shaped patches with a severely affected centre. The central core consisted of stunted, prematurely senescent plants. Symptomatic foci were scattered at random over a largely symptomless field. At two exemplary investigation sites we combined ground based yield assessments with remote sensing techniques to describe disease-loss relationships. Based on low altitude true-colour aerial imaging data, symptomatic patches were categorised into: (i) severely affected blackish core region, (ii) yellowish symptomatic periphery, and (iii) a corresponding non-symptomatic reference patch. Serological tests revealed PNYDV (Pea necrotic yellow dwarf virus) together with PEMV (Pea enation mosaic virus) as dominant and equally abundant viruses. However, because PNYDV was significantly more restricted to the focal core than PEMV, we perceived PNYDV to be the causal agent for this apparently new symptom pattern in faba bean. As both viruses are vectored persistently by leguminous aphids, the observed symptom gradient within individual foci mirrored the epidemiological development over time, starting from an initial infection point and expanding towards the periphery via secondary virus spread to successively maturing and less susceptible plants. For each investigation site the segmented symptomatic surface of category (i) was 0.8 and 0.4%, for (ii) 20.5 and 6.4%, respectively. Combining the relative yield level for each symptom category with its respective surface, the overall yield gap at the field scale was extrapolated to 4.1 and 9.2% for grain yield and for 3.9 and 1.2% for crude protein. In the symptomatic core category, TKWs (thousand kernel weights) were halved due to enhanced proportions of shriveled grain. Because PNYDV-related yield decline was determined by the number, relative surface and disease intensity of individual foci, remote sensing techniques can offer valuable options for monitoring, loss assessment and agricultural decision-making.