|Yokomi, Raymond - Ray|
|Selvaraj, Vijay Anadraj - Foreign Agricultural Service (FAS, USDA)|
|Maheshwari, Yogita - Foreign Agricultural Service (FAS, USDA)|
|Hajeri, Subhas - Central California Tristeza Eradication Agency|
Submitted to: Citrograph
Publication Type: Trade Journal
Publication Acceptance Date: 10/16/2017
Publication Date: 3/10/2018
Citation: Yokomi, R.K., Selvaraj, V., Maheshwari, Y., Hajeri, S. 2018. California mild CTV strains that break resistance in Trifoliate Orange. Citrograph. 9(2):66-69.
Interpretive Summary: Poncirus trifoliata is a popular citrus rootstock and is resistant to all strains of Citrus tristeza virus (CTV) except those with a Resistance-Breaking (RB) genotype. Using a CTV monoclonal antibody, MCA13, which discriminates between some CTV strains, three MCA13-positive CTV isolates collected from California were subjected to Next Generation Sequencing and full-length sequences of their genomes were constructed. Analysis showed these isolate were members of the RB genotype. Greenhouse evaluation showed that three California RB isolates caused mild disease in virus indexing tests. All three isolates were replicated in P. trifoliata, the hallmark of RB isolates. Pest control districts in portions of Fresno and Tulare counties and all of Kern County use MCA13 to identify potential severe strains; trees infected by MCA13-positive isolates are moved from commercial orchards to limit spread of harmful CTV strains. This report documents that California has some MCA13-reactive isolates that replicate in P. trifoliata but are mild. Further evaluations are needed to show if these isolates need to be subjected to eradication.
Technical Abstract: This is the final report of a project to characterize California isolates of Citrus tristeza virus (CTV) that replicate in Poncirus trifoliata (trifoliate orange). Next Generation Sequencing (NGS) of viral small interfering RNAs (siRNAs) and assembly of full-length sequences of mild California CTV isolates that react to the strain-discriminating monoclonal antibody, MCA13, revealed that Resistance-Breaking (RB) strains of CTV exist in California. Greenhouse indexing in a citrus host range showed these isolates were mild. These isolates replicated in leaves, bark and roots of trifoliate orange which is resistant to all other strains of CTV. Because of these characters, an effort is underway to develop an infectious recombinant RB CTV vector to express foreign genes in roots and foliage of existing citrus for the purpose of mitigating HLB disease.