Fungal Diseases in the Vineyard Flashcards
Explain Powdery Mildew and its effect on the vine.
Powdery mildew overwinters in buds and on canes. It then attacks young, green parts of the vine, which start as dull grey patches and become black patches as they advance. Patches can damage young shoots, inflorescences and grapes, reducing yield. Grapes can also split at véraison and become targets for other infections. The growth rate is determined by temperature, with an optimum temperature around 25°C (77°F), and it thrives in shady conditions. Unlike other mildews it does not require high humidity and so can spread in relatively dry conditions, especially in dense, shady canopies.
How can you manage Powdery Mildew?
Keeping an open canopy to reduce shade and the density of leaves is now regarded as the preferred approach.
Applications of sulfur help to prevent and treat the disease. Growers will spray the vines from a couple of weeks after budburst and up to véraison. It is important to spray early in the season as the disease is easier to prevent than to contain if it gets established.
Systemic fungicides can be effective and, as they penetrate the green tissue, are not washed off by rain. However, the fungus can become resistant to some fungicides, so only a limited number of applications can be made in one year.
Explain Downy Mildew and its effect on the vine.
Downy mildew is caused by Peronospora, a water mould that lives within vine tissue, not on the surface. It attacks green parts of the plant, especially young leaves and flowers, reducing yields by defoliating the vine. Grapes can also be affected, but this is less important than the threat of defoliation.
It needs rainfall and warm temperatures (20°C / 68°F) to spread. High risk periods are warm springs and stormy but warm summers. The symptoms are yellow, circular ‘oil spots’ and then white, downy fungal growth on the underside of leaves.
How can you manage Downy Mildew?
Traditionally, sprays made from copper salts have been used to prevent the spread of downy mildew. So-called Bordeaux mixture, a combination of copper sulfate and lime, became the standard treatment from the 1880s. Protection from these copper sprays only lasts until 20 mm of rain has fallen.
Other fungicides can also be used. They should be applied from about a month after budburst to véraison.
Good drainage and an open canopy that dries quickly are helpful to avoid the fungus developing and spreading.
Explain Grey Rot and its effect on the vine.
Botrytis cinerea is a fungus. It results in loss of yield and drop of quality in the wine (colour, body and aroma/flavour) Grapes are vulnerable if there are any points of entry (e.g. grapes having rubbed against each other in tight bunches or punctured by birds/insects), leading to whole bunches being attacked. If the flowers are affected, the fungus can stay dormant in the grape and re-emerge after véraison. Varieties with tight bunches or thin skins are most at risk: Semillon, Sauvignon Blanc, Pinot Noir and many others. Common in all grape growing areas.
Spores are typically present in the vineyard and become active in periods of rainfall and high humidity.
How can you manage Grey Rot?
Selecting grape varieties that have small grapes with thick skins and therefore high levels of resistance (e.g. Petit Verdot) and protecting the grapes against other pests (which could split the skin of the grape) are the most important options.
Keeping an open canopy and removing the leaves around bunches can also reduce the spread of grey rot.
Traditional sulfur and copper sprays are ineffective, but other fungicides can be used. They should be applied at key points in the season – when flowering is nearly complete, at the end of grape formation, at bunch closure (when the grapes in a bunch get large enough so that they touch each other) and véraison. However, fungicides quickly become ineffective as the fungus develops resistance.
Attention has therefore turned to using antagonistic bacteria; for example, Bacillus subtilis and other forms of biological control.
Explain Eutypa Dieback and its effect on the vine
Eutypa dieback (also called Dead arm) is a fungal trunk disease that leads to rotten wood in vines and can affect whole vineyards. It reduces yields significantly and kills vines over a ten-year period if not tackled. Spores are spread by wind over long distances. Infection occurs through pruning wounds in moderate temperatures and especially during rain. The effects of the fungus are evident in spring with short young shoots and yellow leaves.
How can Eutypa Dieback be managed?
Eutypa dieback is difficult to control, although pruning late and applying fungicide to pruning wounds can be effective.
Affected trunks can be cut back 5–10 cm beyond the visible symptoms and treated with fungicide. Dead wood must be burnt to avoid spores spreading.
Some biological controls (e.g. Bacillus subtilis) may be effective.
If a plant is badly affected, the options are either to retrain from a sucker (a shoot that grows from the base of the vine) left on the trunk (which causes a loss of yield for two years) or removing the vine and replanting it.
Explain Phomopsis cane and leaf spot and its effect on the vine
Phomopsis is a fungal disease that causes a reduction in crops. It is particularly prevalent in years with cool and wet springs followed by humidity and moderate temperatures. Infected canes whiten and break off easily. Shoots growing from these canes develop brown cracks at their bases. Leaves are also affected. Grenache is very susceptible, while Cabernet Sauvignon is less prone to it.
How can Phomopsis cane and leaf spot be managed?
Fungicides should be applied three weeks after budburst and then again every two weeks if wet conditions continue.
Diseased wood should be removed and burnt.
It is best to prune early or late and not in rainy weather.
Explain ESCA and its effect on the vine
Esca is a complex fungal disease caused by a group of organisms particularly prevalent in warmer and drier climates; for example, in southern Europe and California. It typically enters the vine through pruning wounds. Symptoms include tiger-striping of the leaves and spotting inside the wood.
Esca reduces the yield of the plant and leads to its death within a few years.
How can ESCA be managed?
As there are no chemical controls, most attention has been paid to prevention of the disease: sourcing disease-free stock, trying new, less detrimental, pruning techniques, not pruning in the rain, removing prunings promptly from the vineyard and disinfecting pruning wounds.
