Identification and distribution of thrips - vectored tospoviruses And management of iris yellow spot virus in onion

Abstract

Tospoviruses are globally distributed on wide range of host plants, inflicting yield losses of up to $1 billion on vegetables. Tomato spotted wilt virus (TSWV) was first reported in Nakuru, Kenya on tomatoes with economic losses as high as hundered percent. However, very little information exists on diversity of other tospoviruses in Eastern Africa. Therefore, this study was carried out to identify, characterize and determine the occurrence and distribution of tospoviruses on onions and tomatoes in Kenya. Further seasonality surveys were undertaken for three seasons in Loitokitok, Naivasha and Mwea where onions and tomatoes are widely grown. Presence of virus in the plants with symptoms was confirmed in the fields using tospoviruses immunostrips (Agdia ®). Identification. of tospoviruses was based on double antibody sandwich-enzyme linked immunosorbent assays (DAS-ELISA) and Reverse Transcriptase polymerase chain reaction (RTPCR) method. Thrips associated with the two crops, nearby weeds and other reservoir plants were assessed for their potential in harbouring thrips and as reservoirs for iris yellow spot virus (lYSV). The vector competence of various thrips species infesting onions was evaluated for their potential to transmit IYSV on onions. Management options such as evaluation for host plant resistance, intercropping with spider plant and use of entomopathogenic fungus (Metarhizium anisopliae ICIPE 69) were evaluated for their potential to manage vector thrips and IYSV incidence. Apart from TSWV, that was previously known to occur in Kenya, outcomes of this study resulted in the first reports of two other tospoviruses; IYSV and Tomato yellow ring virus (TYRV) in onions and tomatoes, respectively. Host range studies for IYSV indicated that apart from the various alliaceaous crops such as onion, chives, shallots and garlic; crops like snap pea, cabbage and weeds such as Chenopodium album, Datura stramonium and Amaranthus retroflexus infested with T. tabaci also tested positive fOT the virus through ELISA assays. The host status needs to be confirmed further at molecular level. These crops and weeds were abundant in the landscape in mid altitude regions in Loitokitok and could be reservoirs for IYSV. The incidence of IYSV was highest in the cool and dry season between August and September in both regions and range9 between 56.5 to 71.0 % and it was lowest between 29.9 and 32.2% in November and December in both regions. Incidence of Iris yellow spot virus positively correlated with number of thrips infesting onions in Loitokitok (R = 0.659; P < 0.0001) and Naivasha (R = 0.623; P < 0.0001). In Loitokitok, Iris yellow spot virus incidence positively correlated with IYSV severity (R = 0.52; P < 0.000 I). Among the thrips species infesting onions in Kenya, this study confirmed that only 111 Thrips tabaci was the virus able to replicate and be transmitted compared to the other non-vector thrips such as Frankljniella occidentalis and dark and pale forms of Frankliniella schultzei. Monitoring the expression of non-structural proteins as compared to the accumulation of nucleocapsid - structural proteins were found to be effective in monitoring virus replication within the thrips. Screening different onion varieties for resistance; indicated two cultivars Bombay red and Texas grano were resistant to Thrips tabaci infestation resulting in less IYSV incidence. Texas grano also produced the highest yield. Among the management options evaluated for control of Thrips tabaci and spread of IYSV, weekly application of Metarhizium anisopliae proved to be effective for control of onion thrips leading to reduction in IYSV incidence.