Soil-borne viruses transmitted by Polymyxa graminis can lead to major yield losses in wheat crops worldwide. This work describes the identification of a soil-borne virus infecting wheat plants in Paraguay. Based on electron microscopy observations of virus particles in symptomatic leaf and molecular assays the virus was identified as wheat stripe mosaic virus (WhSMV). To our knowledge, this is the first report of WhSMV in Paraguay.
Tomato chlorosis virus (ToCV; genus Crinivirus, family Closteroviridae) was identified in tomato crops in the state of São Paulo, Brazil in 2006. Management strategies to control external sources of inoculum are necessary, because chemical control of the whitefly vector Bemisia tabaci MEAM1 has not efficiently prevented virus infections and no commercial tomato cultivars or hybrids are resistant to this crinivirus. We first evaluated the natural infection rate of some known wild and cultivated ToCV-susceptible hosts and their attractiveness for B. tabaci MEAM1 oviposition. Physalis angulata was the most susceptible to natural infection in all six exposures in 2018 and 2019. No plants of Capsicum annuum (cv. Dahra) or Chenopodium album became infected. Solanum melongena (cv. Napoli) had only two infected plants of 60 exposed. C. annuum and C. album were the least preferred, and Nicotiana tabacum and S. melongena were the most preferred for whitefly oviposition. In addition, from 2016 to 2019, we surveyed different tomato crops and the surrounding vegetation to identify ToCV in weeds and cultivated plants in the region of Sumaré, São Paulo state. Only Solanum americanum, vila vila (S. sisymbriifolium) and C. album were found naturally infected, with incidences of 18%, 20% and 1.4%, respectively. Finally, we estimated the ToCV titer (isolates ToCV-FL, USA and ToCV-SP, Brazil) by RT-qPCR in different ToCV-susceptible host plants and evaluated the relationship between virus acquisition and transmission by B. tabaci MEAM1. The results clearly showed significant differences in ToCV concentrations in the tissues of ToCV-susceptible host plants, which appeared to be influenced by the virus isolate. The concentration of the virus in plant tissues, in turn, directly influenced the ToCV-B. tabaci MEAM1 relationship and subsequent transmission to tomato plants. To minimize or prevent the damage from the tomato yellowing disease through management of external sources of ToCV, it is necessary to correctly identify the potentially important ToCV-susceptible hosts in the vicinity of new plantings.
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