Just like humans, grapevines and vineyards can get sick with grapevine fungal diseases.
Grapevines suffer a range of maladies, including fungal diseases spread by spores. Powdery mildew, downy mildew, grey rot, eutypa, phomopsis, and esca can decrease yields and even kill the vine. Here’s a quick run-down of the most common vineyard fungal diseases, their symptoms, and management options.
Powdery Mildew
Powdery mildew, caused by the fungus Eriysipe necator, also goes by the name Oidium tuckeri (Oidium for short) after the garden William Tucker who first described the disease in the mid-1800s.
Powdery mildew is a fungal disease specific to grapevines. American species are more resistant than the European Vitis vinifera, and some V. vinifera varieties tend to be more susceptible than others, for example, Chardonnay and Cabernet Sauvignon struggle with powdery mildew.
Powdery mildew’s growth and spread are determined by temperature, not humidity. The fungal spores overwinter in buds and canes, then emerge in the spring, damaging shoots, inflorescences (new flowers), and grapes, ultimately reducing yield or the total tonnage of grapes harvested from a vineyard.
Powdery mildew thrives in dense, shady conditions. Management options include pruning the vine to reduce shade and applying sulfur sprays and fungicides.
Downy Mildew
Downy mildew, or Peronospora, lives inside the vine’s tissue. The fungus originated from North America and has made its way to most wine regions around the world. Downy mildew attacks green tissue and defoliates the vine, reducing photosynthesis and, ultimately, yield.
Similar to powdery mildew, downy mildew requires warm temperatures to spread. Unlike powdery mildew, downy mildew also requires humidity.
Spring and summer rains are high-risk periods for this fungal disease.
Downy mildew management options include a copper salt spray, called Bordeaux mixture, but this offers limited protection and wears off if there’s more than 20 mm of rain. Copper can build up to toxic levels in the vineyard soil, however, so Bordeaux mixture has some serious drawbacks. Viticulturalists use fungicides, vineyard design with ample drainage, and canopy management to increase airflow, all with the goal of reducing downy mildew pressure.
Grey Rot
Grey rot, caused by Botrytis cinerea, affects the grape berries. If you’ve ever purchased grapes from the grocery store and seen grey fungus growing on the berries, this is grey rot. The fungus reduces yield and grape quality. Grape bunches affected with grey rot show reduced color, flavor, aroma, and body. Grey rot is ubiquitous in grape growing regions around the world.
Growing periods of rainfall and high humidity experience greater disease pressure from grey rot.
Thin skinned varieties are most at risk from grey rot, including Semillon and Pinot Noir.
Who Knew? Semillon actually needs grey rot to make rich, luscious dessert wines in certain growing regions. Here’s a post on everything you need to know about Semillon wines.
Similar to other fungal diseases, vineyard management techniques to combat grey rot include canopy management to increase airflow and fungicides, though repeated applications can reduce effectiveness as the fungus can develop resistance.
Vineyards can also opt to plant varieties with thick skins in growing regions at risk for grey rot, for example, Petit Verdot.
Eutypa (a.k.a. Dead Arm)
Caused by fungal spores that enter the vine through pruning cuts, Eutypa (you-type-ah) reduces yields and will slowly kill a vine over a 10 year period. Viticulturalists can prune back the dead wood and re-train new canes.
Wood from infected vines must be burned to prevent further spread through the vineyard.
Eutypa is widespread in parts of South Australia, south-western France, and California.
You can usually pick out a vine suffering from Eutypa if the cordons, or arms, of the vine look uneven in size or position, or if there’s a branch coming off the lower part of the trunk to the trellis support system. The viticulturist is re-training a new cordon to replace the infected wood.
Phomopsis
Phomopsis, another fungal disease, spreads during cool, wet springs with humid conditions and moderate temperatures. Phomopsis causes canes to die off and shoots and leaves form brown cracks. Applying fungicides after budburst and then again during wet weather conditions reduce disease pressure. Like Eutypa, the infected vine material needs to be burned. Improving airflow through canopy management can help reduce spread.
Esca
Also known as measles, Esca (ess-ka) is a fungal disease common to warm, dry climates (e.g., California and Southern Spain). The disease enters the vine through pruning wounds and infects the leaves. Esca attacks grapevine heartwood.
Symptoms include shriveled, spotted berries and leaf interveinal chlorosis and necrosis causing tiger-like stripes.
This reduces the vine’s photosynthetic capacity, and thereby yield.
Esca will kill a vine in a few years.
There are no known remedies for esca, but viticulturalists continue to experiment with pruning techniques.
Check out this video describing one experimental technique.
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