Epidemiology of potato viruses affecting potato crop grown under semi-arid conditions of the Sudan

Field surveys and experiments were conducted at different locations in Khartoum and River Nile states to study the epidemiology of viruses affecting potato crop grown under the semi-arid conditions of the Sudan. Field surveys were carried out for three consecutive seasons (1999/2000, 2000/2001 and 2001/2002) at three locations in Khartoum state (Elnayia, Elshehinab and Shambat) plus one main seed growing area in River Nile state (i.e. Elmisiektab) to detect and estimate the occurrence of viruses spreading in commercial seed and ware potato field and to compare virus incidence in different seed grades. In 1999/2000 and 2000/2001, significant differences were obtained between locations and seed grades as well as significant interactions between locations and seed grades in total virus incidence. The total virus incidence () in the basic seed stock increased from zero percent to 3.3and 40in the once-grown in 1999/2000 and 2000/2001, respectively. While the incidence in the certified stocks increased in season 2000/2001 to 4and 98.1in once and twice-grown locally multiplied seed. Four important virused were recorded in commercial potato fields and were identified as potato leaf roll virus (PLRV), potato virus Y (PVY). beet curly top virus (BCTV), alfalfa mosaic virus (AMV) depending on visual diagnostic symptoms and/or sero-diagnosis using dot-lmmuno-blotting assay (DIBA). The incidence of these viruses were in the range of: PLRV, 0-13.3, PVY, 0-44.67, BCTV, 0-100and AMV, 0-100. Significant differences were noticed between seed grades and locations in incidence of individual viruses but not in all seasons. The interaction between locations and seed grades was significant in 1999/2000. Significant interactions between locations and seed grades were also observed in almost all seasons under test. The individual viruses were least in certified (GO). The present thesis reports for the first time the occurrence of AMV and potato virus M (PVM) in potatoes grown in the Sudan. Replicated experimental plots were conducted at two location, Elnayia and Shambat for 2 consecutive seasons (1999/2000 and 2000/2001) to evaluate the virus status of different seed grades and to assess the role of aphid populations in the virus spread and its impact on potato yield. The differences between seed grades were significant in all parameters assessed 9i.e. emergence (), number of stems/hill, total virus incidence and total yield). The emergencevaried between 53.6and 97.2at Elnayia and 91.2 to 100at Shambat. The lowestemergencewas noticed in GO. The number of stems/hill in the different seed grades ranged between 1.79-3.31 at Elnayia and 2.13-4.63 at Shambat. In 1999/2000 the final tuber yield in the differen seed grades varied between 6.32-18.89 (tons ha***) at Elnayia and 7-12.5 (tons ha**) at Shambat. In 2000/2001 the final tuber yield was 10.4-29 (tons ha**) atShambat. The once-grown seed (G1) produced significantly the highest yield followed by once-grown of basic seed (B1) and certified seed (GO). Total virus incidence in the different seed grades grown at Elnayia was estimated at 0.30to 87.6. At Shambat, the total virus incidence was found to vary between 56to 100. The certified seed (var. Alpha) consistently scored the smallest number of stems/hill. But the degenerative conditions at the two locations seems to influence the levels of infection in the different seed grades. Aphid populations in both locations were not detected during December and January, but they were appeared in February. The aphid count / 100 leaves was appreciably high at Shambat (i.e. 6558, 5000) in the 1** and 2** seasons, respectively while it was very low in both seasons at Elnayia (0,50). The virus build-up in locally multiplied seed potato stocks was found to differ significantly at different locations. The total virus infection ()ranged between 3-57in 2000/2001 and 2001/2002. Shambatencountered significantly the highest infection level (57), whereas the lowest virus incidence was encountered at Elnayia Gezira and Elshehinab (3each). With respect to virus build-up, it ranged between 1and 55. Shambat encountered the highest virus build-up (55), whereas the lowest virus build-up was observed at Elnayia Gezira and Elshehinab (1each). At Shambat,virus infection in different 'Alpha' seed grades was significantly different. The virus infection ranged between 9and 72in 1999/2000 and 54-100in 2001/2002the lowest was in plots grown with GO. The susceptibility of 3 different potato varieties to virus infection and subsequent final tuber yield was also studied. the total virus incidence ranged between 10-30, with var. Draga displaying significantly the greatest infection level and var. Diamant the lowest incidence. The popular variety 'Alpha' showed only 13virus infection but manifested the greatest tuber yield loss (37.5) compared to 30and 27.2yield loss for Diamant and Draga, respectively.stocks increased in season 2000/2001 to 4and 98.1in once and twice-grown locally multiplied seed. Four important virused were recorded in commercial potato fields and were identified as potato leaf roll virus (PLRV), potato virus Y (PVY). beet curly top virus (BCTV), alfalfa mosaic virus (AMV) depending on visual diagnostic symptoms and/or sero-diagnosis using dot-lmmuno-blotting assay (DIBA). The incidence of these viruses were in the range of: PLRV, 0-13.3, PVY, 0-44.67, BCTV, 0-100and AMV, 0-100. Significant differences were noticed between seed grades and locations in incidence of individual viruses but not in all seasons. The interaction between locations and seed grades was significant in 1999/2000. Significant interactions between locations and seed grades were also observed in almost all seasons under test. The individual viruses were least in certified (GO). The present thesis reports for the first time the occurrence of AMV and potato virus M (PVM) in potatoes grown