28 Trakia Journal of Sciences, Vol. 17, № 1, 2019 Trakia Journal of Sciences, No 1, pp 28-33, 2019 Copyright © 2019 Trakia University Available online at: http://www.uni-sz.bg ISSN 1313-7050 (print) ISSN 1313-3551 (online) doi:10.15547/tjs.2019.01.005 Original Contribution INFLUENCE OF THE VEGETATION PERIOD ON THE SPREAD OF ECONOMICALLY IMPORTANT VIRAL DISEASES IN BURLEY TOBACCO Y. Yonchev 1 , N. Keranova 2* 1 Institute of Tobacco and Tobacco Products – Markovo, Markovo, Plovdiv, Bulgaria 2 Department of Mathematics and Informatics, Faculty of Economics, Agricultural University-Plovdiv, Bulgaria ABSTRACT The present study explores the influence of the vegetation period of Burley tobacco on the spread of viruses such as TMW, PVY-Complex, CMV / PVY-Complex, TSWV and CMV. To establish this relation, a correlation analysis is applied and the proven effects are represented by linear regression models. In 2014, the number of days from replanting has a strong positive impact on the percentage of plants infected by PVY-Complex (0.985**) as well as by TMV (0.781*). For 2015, the very strong effect was only seen on CMV / PVY-Complex (0.976**). In 2016, the duration of the period had a positive effect on the spread of CMV / PVY-Complex (0.868*), CMV (0.904 **) and TSWV (0.966**). In 2017 there is a very strong positive correlation between PVY-Complex (0.885*), CMV (0.948**) and TSWV (0.955**) on one hand and the planting period on the other. As a result of the conducted study over the entire four-year period, it has been proven that during the first two years the increase in the vegetation period leads to an increase in the incidence of PVY complex. During the second half of the analyzed period, CMV and TSWV are proved to be affected by the length of the time from the replanting. Keywords: viruses, correlation analysis, regression analysis INTRODUCTION Tobacco production is among the major branches of agriculture in Bulgaria. Tobacco is cultivated in more than 100 municipalities, and in a large part of them it is the main livelihood of the local population (1). Four varieties are cultivated in the country - Oriental tobacco - Basma and Caba Kulak and big-leaved tobacco– Flue-cured - Virginia and Air Cured- Burley, occupying an area of 9963 ha (2). Tobacco cultivation should take into account the complicated interactions between the genotype and agro-ecological conditions as well as the presence of different phytopathogens (3). Some of them are viral diseases and the damage to the crop is significant. One of the most common viruses ___________________________ *Correspondence to: Neli Keranova, Department of Mathematics and Informatics, Faculty of Economics, Agricultural University-Plovdiv, 12 “Mendeleev” Blvd., 4000, Plovdiv, Bulgaria, nelikeranova@abv.bg causing serious economic losses is the genus Potyvirus. The three potyviruses PVY (Potatovirus Y), TEV (Tobacco etch virus) and TVMV (Tobacco vein mottling virus) often occur as a tobacco viral complex (4-6). Another economically important virus is TMV (Tobacco mosaic virus) of the genus Tobamovirus. It is spread in all the countries where tobacco is grown. The illness it causes is called a simple tobacco mosaic virus. Until the 1980s, the disease was a serious problem for tobacco production (7). Kovachevski et al. (8) report a reduction in dry tobacco yields to 50% after TMV contamination. With the introduction of TMV-resistant varieties of tobacco, the economic importance of the virus decreases. CMV (Cucumber mosaic virus) is a virus also spread worldwide which causes considerable damage to tobacco production (9). In Africa it can be found in Zambia and Morocco (10). The disease it causes with tobacco is called cucumber mosaic virus.