Technical Abstract: Tomato chlorosis virus (ToCV), and Tomato infectious chlorosis virus (TICV), family Closteroviridae, genus Crinivirus, cause interveinal chlorosis, leaf brittleness and limited necrotic flecking or bronzing on tomato leaves. Both viruses cause a decline in plant vigor and reduce fruit yield, and are emerging as serious production problems for field and greenhouse tomato growers in many parts of the world. The viruses have been found together in tomato, indicating that infection by one crinivirus does not prevent infection by a second. Transmission efficiency and virus persistence in the vector varies signifiantly among the four differenct whitefly vectors of ToCV; Bemisia tabaci biotypes A and B, Trialeurodes abutilonea, and T.vaporariorum. Only T. vaporariorum can transmit TICV. In order to elucidate the effects of co-infection on crinivirus accumulation and transmission efficiency, we established Physalis wrightii and Nicotiana benthamiana source plants, containing either TICV to ToCV alone, or both viruses together. Vectors were allowed to feed separately on all virus sources, as well as virus-free plants, then were transferred to young plants of both host species. Plants were tested by quantitative RT-PCR, and results indicated host-specific differences in accumulation by TICV and ToCV and alteration of accumulation patterns during co-infection compared with single infection. Vector tranmission efficiency of both viruses corresponded with virus concentration in the host in both single and mixed infections. This illustrates that crinivirus epidemiology is impacted not only by vector transmission specificity and incidence of hosts, but also by factors such as virus competitiveness in host plants.