Technical Yearbook 2023

Disorders that can be confused with leafroll Mechanical damage

Mechanical damage can easily be confused with leafroll disease infection. The interruption of sap flow through injury will decrease the photosynthetic capacity of the leaves and cause the underlying natural red colour to become visible. In photo 3.1 the reddening of a section of the cordon is caused by the cut-off of the phloem of an entire section of the cordon. This is due to the cordon arms being tightly wound around the wire during development causing a ringbarking effect as the plant grows.

Visual cues aligned with leafroll disease infection include: • Red interveinal discolouration. • Veins remain green until late in the season. • Downward curling of leaf edges in some varieties e.g., PHOTO 1. Typical GLRaV-3 symptoms including interveinal discolouration, green main veins and rolling of leaf edges in cv. Pinotage. The most common misinterpretation is mistaking the onset of GLRaV-3 for the development of natural autumn colours. Autumn colours are related to the natural degrading of green chlorophyll pigmentation exposing the anthocyanin pigments once the grapevine growth cycle is completed (photo 2). Pinot noir & Cabernet franc. • Delayed leaf fall in autumn.

PHOTO 3.1. Mechanical damage of a section (left) of the cordon of a mature Cabernet Sauvignon vine due to the ringbarking effect of the cordon arm being tightly wound around the cordon wire. Mechanical damage in young vines is also frequent. Due to the smaller circumference of the main stem of young vines, an injury here is also likely to have a detrimental effect on the entire vine. In this example (photo 3.2) the swirling effect caused by the wind of this unsupported vine caused injury to the main stem and resulted in the reddening of the leaves. While wind is a regular culprit, others include remnant training string, insects, rodents and mechanical damage during manual weed control (“skoffel”).

PHOTO 3.2. Mechanical damage on a two-year-old Pinotage vine due to the ringbarking effect at the base of the vine. Note the cavity created around the vine base as a result of the swirling effect of the wind.

PHOTO 2. Young virus-free Cabernet Sauvignon vines displaying autumn colours in grapevine nursery.

92

TECHNICAL YEARBOOK 2023