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Further epidemiological studies on the Sudanese isolates of onion yellow dwarf (OYDV) and its effect on production of onion crops were carried out during two consecutive seasons under field conditions. Four OYDV isolates were recovered from four different localities in Khartoum and River Nile States where the common bulbing onion (Allium cepa L.) is the major crop. These isolates were named after their respective localities [viz., Gezira Islanj (GI), Wad Ramly (WR), Wad El basal (WB) and El Damar (D) isolates] and their characteristics were studied. Five different types of symptom patterns were encountered in the different onion growing seasons surveyed and hence could not be assigned to any specific OYDV isolates. Field surveys and ʺon growingʺ tests revealed that the true disease incidence was consistently substantially higher than the observed disease incidence and they were positively correlated (r2 = 0.63). This may prove to have an important epidemiological consideration in disease forecast and disease management. The different OYDV isolates were found to reduce significantly the onion bulb yield as measured by bulb size, bulb weight and onion bulb quality (total soluble solids and dry matter content). In the field experiment, the various OYDV isolates were found to significantly reduce plant growth in terms of leaf width and leaf length. They also reduced significantly the seedstalk height and seedstalk diameter but not the number of seedstalks. The productive capacity in terms of number of days to anthesis, flowering percentage, umbel diameter, total number of florets per umbel and percent of abortive florets, were seriously affected by the different OYDV isolates. Consequently, the seed yield was drastically reduced and losses were 61.2, 65.3, 71.8 and 83.9% due to infection with WB, WR, GI-and D isolates, respectively.