Barley yellow dwarf epidemiology

Barley yellow dwarf (BYD) disease is induced by viruses that are vectored by aphids. The viruses infect grasses and cause severe damage on oats, barley and wheat worldwide. This thesis focuses on the following aspects of the BYD disease epidemiology: occurrence and genetic variability of viruses causing BYD disease, aphid population parameters and host plant damage caused by virus infection. The virus species Barley yellow dwarf virus-PAV (BYDV-PAV), Barley yellow dwarf virus-MAV (BYDV-MAV) and Cereal yellow dwarf virus-RPV (CYDV-RPV) were found in Latvia. The three year incidence of BYDV/CYDV ranged from 9 to 15% in symptomatic leaf samples of spring cereals and from 2 to 19% in random samples of pasture grasses. Sequence analyses of partial coat protein encoding region revealed close genetic relationships among all isolates of BYDV-MAV. The isolates from Sweden and Latvia are the first published BYDV-MAV sequences from Europe. Swedish and Latvian isolates of BYDV-PAV were found in two host-specific groups. A distinct variant of BYDV-PAV was discovered in Latvia and proposed to belong to a new species. Monitoring of aphid flight activity over eight years indicated cereal aphid Rhopalosiphum padi (L.) predomination over Sitobion avenae (Fab.) and Metopolophium dirhodum (Walk.). Positive linear correlations were observed between suction trap catches and population size of R.padi in a field, and between spring and summer migrations of R.padi, but not for their summer/autumn and autumn/next spring migrations. Aphid density in a field was well predicted from proportion of tillers infested and the Nachman model. A specific BYDV-PAV isolate inoculated to oats at four different growth stages decreased the grain biomass and plant height, especially when inoculated in early growth stages, but increased the number of tillers and panicles per plant. The infection did not affect 1000-kernel weight and grain volume weight. In conclusion, the results of this thesis add new knowledge and contribute to understanding the parts of the complex system of BYD disease