|Botrytis cinerea, a fungus that affects grapevines in almost all viticultural areas, can lead to the nectar of noble late harvest wines or to the vinegary aromas of acid rot.
But what are the factors determining this Jekyll and Hyde character Berry infection takes place under conditions of high humidity but factors affecting its further development are less clear. According to Marais (1981), if the period of infection is followed by relatively cool temperatures for a period of approximately 10 days, noble rot will develop; while hot, dry conditions will limit the infection to individual berries, which then generally leads to secondary infections. The process appears, however, to be more complex than this. The development of Botrytis cinerea into either noble or grey rot, in relation to environmental conditions (or “terroir”) has been the subject of a study at the INRA-Unit de Recherches sur la Vigne et le Vin in Angers, France for a number of years.
Chenin blanc, a noble cultivar of Loire Valley origin, has the versatility to produce a number of different wine styles and is particularly susceptible to Botrytis cinerea (Photo 1). Of its renowned products are the natural sweet wines of the AOC-Coteaux du Layon. The history of natural sweet wines in this appellation has a commercial beginning, according to Monnier & Joly (1995). The Dutch traders in the sixteenth century desired white wines with a low acidity, full body and high alcohol. The advanced state of maturity necessary for the production of such wines is often only possible if infection by Botrytis accelerates the process (Barbeau et al, 1996) and is vintage dependant (Asselin et al, 1996). Although the Anjou region is situated at the northern most limit for viticulture, the influence of the Atlantic ocean and the specific mesoclimate of the Coteaux du Layon result in higher temperatures and lower rainfall than the surrounding areas. The Coteaux du Layon has a variety of “terroirs” with most being based on parent material of schist. Different degrees of soil development are present with widely varying soil depths and clay percentages (Photo’s 2 and 3). In this appellation, reference plots have been selected on different “terroir” units in order to monitor the development of Botrytis cinerea into either noble or grey rot in relation to the “terroir” characteristics and microclimate of the plot (Photo 4), and the climate of the vintage. Five reference plots with the same Chenin blanc clone, rootstock, plant spacing and row direction, and of the same age, have been selected on the different “terroir” units. As prescribed by the appellation, a Guyot training system with six to eight buds per metre is used. Six to eight bunches are retained in order to comply with the maximum limit of 45 hectolitres per hectare. They have found definite differences in the time of Botrytis infection as well as in its development for the different plots under the same macroclimatic conditions. They have not been able to link these differences with microclimatic elements and other aspects related to the “terroir” therefore appear to play an important role.
In order to cement a relationship, based on the predominance of Chenin blanc in the Loire and in South Africa (26 300 ha) and on the production of natural sweet wines, Victoria Carey and Abraham Vermeulen of ARC Infruitec-Nietvoorbij visited INRA-URVV in Angers for a period of two weeks. Here they visited the experimental plots, tasted the wines of the region and participated in the daily work in order to learn more of the experimental procedures used in the investigation.
It is important that “terroirs” for the production of Chenin blanc wines with a South African identity, are identified. The development of Botrytis cinerea into noble rot on certain “terroirs” may well assist in the establishment of such an identity. The better understanding of the interaction between “terroir” and Botrytis development will also allow sites to be chosen for the planting of Chenin blanc without the danger of grey rot development. This will reduce the negative impact of Botrytis cinerea and result in positive associations for the South African wine industry. So, is Botrytis cinerea a friend or a foe for the South African wine industry This question will be approached by studying the effects of soil, microclimate and mesoclimate on the infection by and development of Botrytis cinerea on Chenin blanc. A project based on the studies of INRA-URVV (Angers) is being initiated in the Paarl wine growing region with the assistance of the Paarl Vintners. It is hoped that cooperation between INRA-URVV (Angers) and ARC Infruitec-Nietvoorbij will strengthen in the future, to the benefit of both institutes and to the individual wine industries of France and South Africa.
Asselin, C, Morlat, R, Cellier, P, Bouvet, Marie-Helene, Jaquet, A & Cosneau, M, 1996. Aptitude du cpage Chenin l’laboration de vins liquoreux en relation avec certaines units terroirs de base de l’AOC Coteaux du Layon. In: Proc 1er Colloque International “Les terroirs Viticoles”, Angers, 17 – 18 July 1996, pp 329 – 339.
Barbeau, G, Carr JP, Jourjon, F & Maite, C, 1996. Cintique de developpement de Botrytis cinerea, agent de la pourriture noble dans differents terroirs des Coteaux du Layon. In: Proc 1er Colloque International “Les terroirs Viticoles”, Angers, 17 – 18 July 1996, pp 388 – 393.
Marais, PG, 1981. Wingerdsiektes en abnormaliteite. In: Burger, J & Deist, J (eds). Wingerdbou in Suid-Afrika. Maskew Miller, Cape Town, pp 384 – 432.
Monnier, J-M & Joly, P, 1995. Les vins, mets et alcools des pays de la Loire. Portraits Oenologiques. Collection Image du Vin, Silo, France.
Victoria Carey and Abraham Vermeulen, ARC Infruitec-Nietvoorbij, Private Bag X5026, Stellenbosch 7599