Winemaking
This page, which regularly changes, provides factual information on matters relating to viticulture and winemaking. The current topic is fungal diseases of the vine. My Blog will have articles with a more 'chatty' approach
Botrytis cinerea: also known as Botryotinia fuckeliana. This fungal infection, is usually very damaging but may be welcomed depending on circumstances. Botrytis cinerea belongs to the Sclerotiniaceae family of the Ascomycetes class (which has over 1,600 genera and 64,000 species, including yeasts, truffles and Penicillium).
In the form of grey rot Botrytis cinerea is most undesirable. It thrives in wet, humid conditions on vigorously growing vines. It can affect buds and young shoots, turning them brown. Later in the season, it can form grey mould patches on the leaves and infect flowers. It may then become dormant until the grapes are developed. White grapes will turn brown and black grapes may take on red tones. The grapes are covered in a grey or grey/brown mould and berries can split. Grey rot will result in off-flavours in wine, partially owing to the modified chemical composition of the grapes. However, its effect on black grapes is greater than white because it causes loss of colour, tannins and flavour. Outbreaks at harvest time (often induced by rain) can wreck a vintage - there will be a substantial loss in yield and a serious impact upon quality. The image on left below shows Botryris affected grapes. The one on the right is of grapefruit spray being used as a method of prevention
In its benevolent form, Botrytis cinerea occurs as 'noble rot' on some varieties of white grapes when certain climatic conditions occur. The ideal conditions are damp, misty early autumn mornings giving way to very warm, sunny afternoons. Some of the great sweet white wines of the world are produced from affected grapes.
Powdery mildew, also known as Oidium tuckeri (Uncinula necator). this fungal disease reached the vineyards of Europe from North America between 1845 and 1854. It causes dull, pale grey spores to grow on leaves of the vine, and it can overwinter inside the buds. Eventually the grapes will split and shrivel. In the case of severe infection, the entire harvest will be lost. The disease thrives in mild, cloudy conditions and especially where there is a dense, shady, canopy of leaves. It may be treated by dusting the vines with sulfur powder, a contact method that works as both a preventative and an eradicant. Alternatively, systemic treatments are available.
Downy mildew, also known as Peronospera (Plasmopara viticola). this is another fungal disease that came to Europe from North America and was first noticed on vines in the Bordeaux region in 1878. It had probably been carried on vines being imported as grafting stock to combat Phylloxera. The disease thrives in warm, wet weather, common to many North European wine regions. At least 10 mm of rain in a 24 hour period is necessary for there to be an outbreak. Dense white growth forms on the underside of young leaves, which may shrivel and drop off. When attacking the grapes, it causes the berries to shrivel up and turn leathery. Affected grapes show either grey or brown rot. It may be treated by contact or systemic methods: a common contact treatment is spraying the vines with 'Bordeaux mixture' - copper sulfate, lime and water. Vineyards thus treated often show a blue hue and the sprays even give a blue tint to pebbles or gravel on the soil surface. Some strains of Downy mildew have developed resistance to some systemic treatments.
Black Rot (Guignardia bidwellii). Yet another disease that was carried to Europe from the United States, being first noticed in France's Hérault Département in 1885. Infected grape berries shrivel and dry up, having turned blue-black in colour. Bordeaux mixture is an effective treatment.
Black spot, also known as anthracnose (Elsinoe ampelina). this is a cryptogamic fungal disease that originated in Europe. It needs warm, humid conditions to thrive. It can attack young foliage and manifests itself on grape berries as small black dots. Eventually the berry splits. Copper treatments are also effective for anthracnose.
Eutypa dieback (Eutypa lata). This fungal disease usually attacks older vines, entering through pruning wounds and results in vine arms dying. The only real treatment is cutting back infected vines as soon as the disease is noticed.
Esca, (also known as Black Measles on account of the spotting on the grapes of diseased vines). Esca is probably the most serious threat in the wine world today. Infected vines initially suffer loss of foliage and stunted growth, but vine death can occur very quickly. Fungal species including Phaeomoniella chlamydospora, Phaeoacremonium aleophilum, Phaeoacremonium inflatipes and Phaeoacremonium chlamydosporum are known to be involved and generally infect the vines via pruning cuts. It is reported that in 2019 the disease affected 13% of French vines. Although the disease may be treated with sodium arsenite the chemical is banned in the EU and elsewhere, as it is carcinogenic.
Phomopsis (Phomopsis viticola). This is a particular problem in New Zealand, but found in other cool regions. This disease spreads in wet springs and manifests as black spots surrounded by yellow rings on leaves. Black cracks appear at the base of shoots, which can then break off. Maintaining good air circulation around the vines by utilising open canopies is perhaps the best means of prevention.