The invasion biology of tomato begomoviruses in Costa Rica reveals neutral synergism that may lead to increased disease pressure and economic loss.

Since the late 1980s, tomato production in Costa Rica has been affected by diseases caused by whitefly-transmitted begomoviruses. The first was tomato yellow mottle virus (ToYMoV), a locally evolved New World (NW) bipartite begomovirus associated with the tomato yellow mottle disease (ToYMoD). In the late 1990s, the invasive NW bipartite tomato leaf curl Sinaloa virus (ToLCSiV) was detected in Costa Rica and has become established and associated with ToYMoD. Finally, the invasive Old World (OW) monopartite tomato yellow leaf curl virus (TYLCV) was detected in Costa Rica in 2012 and has also become established and is causing tomato yellow leaf curl disease (TYLCD). In the present study, we investigated the invasion biology of these tomato-infecting begomoviruses in Costa Rica in terms of (i) their biological and genetic properties and (ii) disease symptoms and viral DNA accumulation in tomato plants having single and mixed infections. We first generated infectious DNA-A and DNA-B clones and agroinoculation systems for ToYMoV and ToLCSiV isolates recovered from archival ToYMoD samples collected in Costa Rica in 1990 and 2002, respectively. Tomato plants agroinoculated with the infectious clones of both viruses developed ToYMoD symptoms, completing Koch's postulates for ToYMoV, and showing that ToLCSiV also causes this disease. However, pseudorecombinants formed between the DNA components of these viruses were not infectious, which is consistent with independent evolution in different lineages and limits genetic interactions. Furthermore, ToYMoV is well-adapted to tomato, has a narrow host range and is mechanically transmissible. The DNA-A component has a recombination event in the hot spot area and induced a symptomless infection in agroinoculated Nicotiana benthamiana and tomato plants. Tomato plants co-infected with two or all three viruses developed more severe symptoms compared with plants infected with each virus alone. Symptoms induced by the NW bipartite ToYMoV and ToLCSiV appeared earlier (∼7 d post-inoculation [dpi]) than those induced by TYLCV (∼10 dpi), but TYLCD symptoms became predominant in single and mixed infections by 14 dpi. Viral DNA accumulation was quantified by qPCR and generally revealed a neutral synergistic interaction in which the viruses co-existed in mixed infections. A transient reduction in accumulation of ToYMoV and ToLCSiV was detected in mixed infections at 7 dpi, whereas TYLCV accumulation was not affected in mixed infections and was uniform among treatments and time points. Together our results suggest that this neutral synergistic interaction will lead to increased begomovirus disease severity in Costa Rica. We discuss this in terms of begomovirus invasion biology and disease management.