Phytoplasma diseases were known long before the discovery of their agent. Since the early 1930s in the former Soviet Union the infectious nature of the disease known under the name of “stolbur of tomato” has been recognized. Attempts were made to find vectors of the disease (I.K. Korachewski, 1934; V.L. Ryzhkov et al., 1934). In 1945 it was found that planthopper Hyalesthes obsoletus can be the disease vector (K. Sukhov et al., 1946). Searching for an infectious agent of plant diseases with symptoms of dwarfism, yellowing, damaged leaves and generative organs, which was unable to grow on artificial nutrient media was unsuccessfully conducted for several decades in our country and abroad. The discovery was made only in 1967 by Japanese researchers (Y. Doi et al., 1967). The causative agent of the disease, the unknown earlier phytopathogen from Mollicutes class, was similar to mycoplasma pathogens of animals. Later it was found that the life cycle of phytoplasmas is associated with the phloem cells of the plants, in which they multiply, and with vector insects of Hemiptera order which are feeding plant juice and able not only to support the phytoplasma reproduction in their body, but even transmit phytoplasma to the offspring. Phytoplasmas, like other Mollicutes, have no cell wall; they have a minimal genome known to cellular organisms, which causes their obligate parasitism. In the ex-USSR, phytoplasma disease was extensively studied by electron microscopy, immunological methods, and phytoplasma were successfully cultured on artificial nutrient media. Three decades later Italian researchers managed to culture the infective agent on artificial media and confirmed its belonging to phytoplasma by DNA sequencing (А. Bertaccini et al., 2010; N. Contaldo et al., 2012, 2013). In 1990s, a great step forward was made due to molecular methods of phytoplasma diagnosis and study. Phytoplasmas’ taxonomy was developed based on the conservative 16S ribosomal RNA gene and further elaborated with the involvement of other genes having both highly and less conserved sequences (I.-M. Lee et al., 1993; B. Schneider et al., 1993, 1997; I.-M. Lee et al. 1998, 2010; М. Martini et al., 2007). The next important step was a discovery of virulence factors of phytoplasma affecting host plants and making them more attractive to insect vectors naturally involved in the spread of phytoplasmas. In recent years, a lot of genomic data has been obtained for various phytoplasmas; attention is paid to elucidate phytoplasma metabolism which is important to understand the host—pathogen—vector interactions (К. Oshima et al., 2004; Х. Bai et al., 2006; А. Hoshi et al., 2009; А. Sugio et al., 2011; А. MacLean et al., 2011; К. Sugawara et al., 2013; Z. Orlovskis et al., 2016). In Russia, molecular methods have allowed the researchers to reveal the phytoplasma nature of a group of diseases with unclear etiology that gives the key to control of these widespread and harmful diseases. Prevention is the primary means of controlling phytoplasma diseases, including the use of healthy planting material, resistant varieties, methods aimed at spatial isolation from sources of infection, weed eradication, and the use of biopreparation and bioagents capable of producing tetracycline antibiotics.

Twenty oat cultivars and lines with different levels of tolerance to barley yellow dwarf virus were evaluated in the field under artificial infection bay aphis Rhopalosiphum padi L. with a PAV-like BYDV in drought year (2002) and normal-moisture growing seasons (2003 and 2004). Tolerance of cultivars FF 64-74, Maris Elf and Blaze were best and most stable from year to year. Illinois lines IL 2901, IL 86-5698, IL 85-1538, IL 86-6404, IL 86-5262, IL 86-1158 were excellent in tolerance and productivity in non-drought years. These cultivars and lines may be used in Russian breeding program.