WINETECH Technical Yearbook 2020

Incidence and spread of aster yellows

AUGUST 2020

ROLEEN CARSTENS: SAPO, Stellenbosch KEYWORDS: Aster yellows, symptoms, disease incidence, disease spread

leaves gradually become crisp and thicker than normal leaves, and roll downwards. Some leaves exhibit a general yellowing/ reddening and turn to a golden yellow in white cultivars (photo 2) or red in red cul- tivars (photo 3). Pinotage sometimes shows a sectorial discoloration of leaves (photo 4). Leaf rolling can result in a typical trian- gular shape, as recorded on Chardonnay (photo 5). Symptoms of AY infection can be localised on a few shoots or on one cordon of the vine. In other cases, the entire grapevine can be affected. These affected shoots are thin and rubbery, can have shortened internodes with a zigzag growth pattern and later show partial or total lack of lignification (photo 6). Affected shoots display tip death, followed by node- by-node dieback of the shoots (photo 7).

Symptoms associated with aster yellows (AY) vary during the season and between cultivars. AY is caused by a phytoplasma, Candidatus Phytoplasma asteris. Phytoplasmas are very primitive bacteria without cell walls, which occur in the vascular tissue of the plant. The phytoplasma causes a decline in vine growth and enormous crop losses in some cultivars. The disease is spread by an indigenous leafhopper. SYMPTOMS Symptoms appear during summer, but infected vines can be identified from spring onwards by delayed bud break and reduced growth. Usually a combination of symptoms will develop on plants. Early in the season, affected leaves have a wrinkled appearance (photo 1). These

PHOTO 1. Leaves of infected vines have a wrinkled appearance early in the season.

PHOTO 2. Leaves of infected white cultivars turn to bronze yellow and roll downwards.

WINETECH TECHNICAL YEARBOOK 2020 49