Host range and genetic diversity of two polerovirus species associated with cotton bunchy top disease

Abstract

Cotton bunchy top disease causes sporadic but serious losses in cotton (Gossypium hirsutum) in Australia but little has been reported about the diversity, distribution, host range and detection of the causal agent, cotton bunchy top virus (CBTV). We have obtained the complete coding sequence of two poleroviruses from symptomatic cotton to cover all seven putative open reading frames. These species are called CBTV-1 and CBTV-2 and all seven predicted gene products differ by 25% to 49% amino acid identity, indicating they are distinct polerovirus species. A multiplex PCR for CBTV-1 and CBTV-2 was used to screen more than 700 plant samples from 36 species to identify 16 new field and experimental host species. Serological detection by tissue blot immune assay was successful when the two viruses were transmitted to chickpea (Cicer arietinum), but was unreliable for detecting the viruses in cotton. Volunteer and ratoon cotton were identified as common reservoirs of CBTV nearby to cotton cropping areas. Other species that may be regionally important reservoirs include Malva parviflora, Euphorbia hirta and Gossypium sturtianum. Both CBTV species were common and widespread in all major cotton production regions and the genetic diversity within each species was low for the genome region spanning from the 3’ end of open reading frame (ORF) 2 and complete ORF 3. From a total of 257 cotton plants displaying symptoms typical of cotton bunchy top, 256 (99.6%) were positive for CBTV-2 and 93 of these (36%) were mixed infections of CBTV-2 and CBTV-1. As a single infection, CBTV-1 was non-symptomatic, indicating that CBTV-2 is the causal agent of typical cotton bunchy top disease.