Aster Yellows: Determination of its Vectors and DNA Sequence

Project Number
UP KK 02

Project title
Aster yellows: Determination of its vectors and DNA sequence

Project leader
Kruger, K

University of Pretoria. Faculty of Natural and Agricultural Sciences. Department of Zoology and Entomology.

Team members
Kruger, K
Pietersen, G
Joubert, J
Stiller, M
Smit, N
Venter, F

Project description
To assist with the development of control strategies of Aster yellows phytoplasma (AY) in grapevine in South Africa, the project aimed to determine (i) if Austroagallia spp., Cicadulina spp. and Toya sp. (Hemiptera: Auchenorrhyncha) are vectors of AY in grapevine, (ii) the genome sequence of AY and (iii) the influence of olfactory and visual stimuli on the behaviour of the cicadellid vector Mgenia fuscovaria. In addition, the weekly monitoring of leafhoppers in an AY-infected vineyard in Vredendal that commenced in 2009 continued.
Controlled transmission experiments with field-collected Austroagallia spp., Cicadulina spp. and Toya sp. from AY-infected vineyards were carried out with Catharanthus rosea (periwinkle), Nicotiana benthamiana and Vitis vinifera (grapevine) as recipient plants. None of the recipient plants exposed to the insects tested positive for AY. Transmission assays were further carried out with Cicadulina spp., Toya spp. and Exitianus sp. with an artificial sucrose feeding medium. One Exitianus sp. adult tested positive for AY but did not transmit the phytoplasma to the artificial feeding medium nor did the species transmit AY to N. benthamiana or periwinkle.
The partial genome of AY has been determined from samples of AY-infected grapevine, periwinkle and M. fuscovaria using the Illumina Genome Analyzer II platform (ARC-OVI) followed by assembly of the genome with the aid of CLC Workbench Genome. Results obtained confirm a good identity with Aster yellows reported elsewhere for the sequences analysed. In order to determine if adult M. fuscovaria are preferentially attracted to AY-infected compared with AYfree grapevine branches (cv. Colombard), a dual-choice cage experiment with field collected adult leafhoppers was carried out. Adults of M. fuscovaria preferred AY-infected to uninfected grapevine branches. The volatile profiles of AY-infected compared with AY-free branches of cv. Colombard and cv. Chenin blanc collected during summer differed qualitatively as well as quantitatively for several
compounds, although fewer differences occurred in the volatile profiles of cv. Chenin blanc. Differences in volatile profiles between AY-infected and uninfected plants in autumn were less pronounced than in summer. Olfactometer tests to determine the influence of chemical cues on host plant selection by adult M. fuscovaria showed no difference in attraction to AY-infected compared with AY-free branches or purified air in summer. In autumn, adult M. fuscovaria preferred purified air compared to AY-infected cv. Colombard and odours of AY-infected branches of cv. Chenin blanc to purified air. The influence of colour as a visual stimulus on M. fuscovaria was determined in a field trial. More adults were trapped on yellow followed by lime green than on red, blue and dark grey sticky colour cards in the field. Yellow is suitable for monitoring M. fuscovaria. The results on visual and olfactory cues suggest an interaction between these cues in plant choice by M. fuscovaria. Nymphs and adults of M. fuscovaria overwintering on herbaceous plants in vineyards tested positive for AY, highlighting the importance of weed management.

Kruger, K, Douglas-Smit, N, Pietersen, G, Joubert, J, Stiller, M and Carstens, R. 2012. Transmission of aster yellows phytoplasma by Mgenia fuscovaria (Hemiptera: Cicadellidae). Presentation at the 9th Winetech Grapevine Virus Workshop. 29 May, Stellenbosch, South Africa.

Kruger, K. 2013. Leafhopper transmission of aster yellows phytoplasma in South Africa. South African Society for Enology and Viticulture and Winetech Workshop: Virus and Phytoplasma Diseases. November, Stellenbosch, South Africa.

Kruger, K, Venter, F, Schroder, M L. 2015. First insights into the influence of aster yellows phytoplasmas on the behaviour of the leafhopper Mgenia fuscovaria, Phytopathogenic Mollicutes, Mnth Jan v. 5 (p. S41-S42)



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