E. SMYRNIOUDIS, N.I. KATIS
Plant Pathology Laboratory, Faculty of Agriculture, Aristotle University of Thessaloniki, 54 006 Thessaloniki, Greece
J.A. TSITSIPIS, J.T. MARGARITOPOULOS
Laboratory of Entomology and Agricultural Zoology, Department of Crop and Animal Production, University of Thessaly, Fytokou Str. 38 446, Nea Ionia, Magnesia, Greece
R. HARRINGTON
Department of Plant and Invertebrate Ecology, IACR Rothamsted Experimental Station, Harpenden Herts AL5 2JQ, United Kingdom

The epidemiology of BYDV in Greece and its economic importance in cereal crops have not been studied in detail.  In this study, the seasonal appearance of aphid species, the life cycle category of Rhopalosiphum padi and Sitobion avenae and various factors affecting the virus epidemiology were examined.  Aphid population monitoring was done by means of suction traps in four regions of Greece (Thessaloniki, Velestino, Kopaida and Koroivos) during 1996-98.  The species R. padi, Rhopalosiphum maidis, Metopolophium dirhodum and Sitobion avenae, major pests of maize and known vectors of BYDV, were captured in all regions and years.  Higher aphid populations were observed in Thessaloniki than in other regions with an outbreak during 1997 (12433 alatae captured).  Differences as well as similarities were observed in the pattern of captures.  However, in all regions a peak in flight intensity of aphid vectors was observed during May and June.  Probably, this suggests a migration from wheat and/or barley to maize crops.  Flight activity was observed during the whole summer period suggesting a high risk of virus spread.  In addition, a significant number of alatae was captured during August and September probably due to the aphid migration to winter crops.  The life cycle category of the examined clones originated from northern Greece contributes to the former.  All clones of R. padi and S. avenae were examined were found non-holocyclic and overwintered in weeds or winter cereal crops.  This also suggests a high risk for summer maize crops.  However, BYDV incidence in maize crops was rather low.  In addition, all clones of R. padi examined were able to transmit BYDV.  However, a considerable interclonal variation in vectoring ability was observed at low temperatures.  An important factor affecting BYDV spread is aphid movement.  Drought stress and high temperatures were found to promote aphid dispersal and virus transmission.  Moreover, the predator Coccinella semptepunctata had a similar effect on aphid movement and virus transmission.  On the other hand, the parasitoid Aphidius rhopalosiphini did not affect aphid movement and virus spread.  Lastly, the effect of BYDV on growth and yield of the maize varieties Manelis, Polaris, Oulis, Maltos, Atlantis and Damon was examined.  Virus infection did not affect growth and yield of the varieties Manelis, Polaris, Oulis, and Maltos.