Viticultural Diseases Flashcards
Powdery Mildew (Oidium)
Fungal
Native to North America, the Uncinula necator fungus has spread worldwide, and thrives even in humid yet dry conditions-rainfall is actually a detriment to the survival of its spores. The fungus during its anamorph stage, is known as Oidium tuckerii. Powdery mildew affects all green parts of the plant, marking grapes, leaves and shoots with its dusty white mildew growth. It prefers densely shaded canopies and overcast weather, and greatly inhibits bunch development and ripening. If Infected prior to flowering, yields will be reduced; if infected after fruit set, berries will struggle to achieve veraison and reach full size. Fruit affected by powdery mildew is universally avoided in the winemaking process, as it creates off-flavors in the wine. Powdery mildew, first recorded in England in 1847, spread quickly throuhout the Vitis vinifera vineyards of Europe but was soon controlled by applications of sulfur and other fungacides.
Downy Mildew (Peronospora)
Fungal
Another fungal disease that emigrated to Europe on North American vine cuttings, downy mildew spread rampantly through France and the rest of Europe in the early 1880’s. Plasmopara viticola, the agent of downy mildew, attacks the green portions of the vine, causing leaves to drop off the vine and limiting the vine’s ability to photosynthesize. The infection is first visible as an oil spot on vine leaves. As spores germinate a white, cottony growth develops on the underside of the leaves. The fungus survives the winter on fallen leaves in the soil, and its spores reach the vine again with the help of rain splatter in the spring. Arid regions prohibit its growth. The blue-stainging Bordeaux Mixture, a spray of copper sulfate, water and lime, was developed by 1885 to prevent outbreaks of downy mildew
Eutypa Dieback
Fungal
Also called dead arm, the disease is caused by the Eutypa lata fungus. Spores are carried by rain and enter the vine through pruning wounds. Common in Mediterranean climates, the disease is difficult to control as it affects a wide number of plants. Infected vines experience stunted shoot growth as the fungus releases toxins, and eventually an infected cane may die - the dead arm. The disease has a drastic effect on yield, but does not devalue the quality of the crop. In fact, Australia’s
d’Arenberg ascribes a beneficial effect on quality to the dead arm, and markets its icon Shiraz under the disease’s nickname. A separate fungus, Phomopsis viticola, manifests as a similar disease.
Esca (Black Measles)
Fungal
One of the earliest known fungal grapevine diseases, Esca thrives in warmer climates but exists worldwide, and there is no known control or cure. Unlike other fungal diseases, Esca is the result of a complex of fungi, rather than a single organism. On young vines, the disease will weaken growth, affect berry development and discolor leaves; in hot weather an affected young vine may suddenly die. In older vines, the disease affects the wood, causing the interior of the trunk and arms to soften and rot from the inside - a condition that led ancient Romans to use Esca-infected tree trunks for firewood, as its spongy interior quickly caught fire. Mature, Esca-infected vines will rarely live past 30 years of age. The disease is exacerbated by rainfall and can be spread by wind or on the pruning shears of careless vineyard workers.
Black Rot
Fungal
Native to North America, Black Rot spread to Europe with the importation of phylloxera-resistant rootstocks in the late 1800s. The disease is caused by the Guignardia bidwelli fungus, originating as a black spot on the vines’s shoots, leaves, and berries. Although yield reductions can be disastrous if unchecked, the disease can be controlled through fungicide sprays.
Bunch Rot
Fungal
Bunch Rot is a grouping of similar diseases caused by a number of fungi species. In general, bunch rots reduce crop yields and may adversely affect the character of the wine, imbuing it with moldy off-flavors. One of the most common forms of bunch rot is Botrytis bunch rot. Known in its malevolent form as grey rot, the Botrytis cinera fungus will break down the skin of berries and allow other yeasts and bacteria to rot the grapes. It spreads quickly throughout vineyards. However, if the fungus invades healthy white grapes under favorable conditions, it will instead result in the noble rot, a precondition for some of the world’s greatest sweet wines. Botrytis bunch rot requires warm weather and humidity of at least 90% to germinate.
Pierce’s Disease
Bacterial
Caused by the bacterium Xylella fastidiosa and most commonly transmitted by the glassy-winged sharpshooter—a leafhopping insect found near citrus orchards and oleander plants—Pierce’s Disease is a scourge, rendering vines incapable of producing chlorophyll and killing it within one to five years. The disease is common in the southern United States and Mexico but is steadily moving northward in California, with sightings of the glassy-winged sharpshooter and outbreaks of the disease provoking major alarm in both Sonoma and Napa counties. There is neither a cure nor a chemical control for the disease, and authorities in other countries are maintaining strict quarantines to prevent its incursion.
Crown Gall (Black Knot)
Bacterial
The Agrobacterium tumefaciens bacterium causes the Crown Gall disease in a wide variety of plant species. When affected, a vine develops tumors (galls) on its trunk, which girdle and essentially strangle the vine, withering or killing outright the portions of the vine above. The bacteria thrive in colder climates, and systemically live inside the grapevine. During winter freezes, when the vine’s trunk may be ruptured, the bacteria invade the outer trunk, rapidly multiplying and fomenting the onset of disease. The disease is spread through the propagation of bacteria-infected budwood.
Bacterial Blight
Bacterial
Caused by the Xanthomonas ampelina bacterium, Bacterial Blight often kills young grapevine shoots. They develop dark brown streaks in early spring, and eventually wither and die. Spread by rain and compromised pruning tools, the disease can be controlled by hot water treatments and copper sprays, such as the Bordeaux Mixture.
Leafroll Virus
Viral
Leafroll Virus, a condition caused by a complex of at least nine different viruses, may be responsible for as much as 60% of the world’s grape production losses. Although affected vines display radiant shades of red and gold in the autumn, such beautiful colors, combined with a characteristic downward curling of the leaves, signal the virus’s malevolent side: reduced yields and delayed ripening. Leafroll Virus, spread through propagation of infected vines or by an insect vector like the mealy bug, is currently incurable but it will not kill the vine; thus, infected vines are not always removed.
Fanleaf Degeneration
Viral
Fanleaf Degeneration, a nepovirus spread by soil nematodes feeding on infected roots, severely curtails yields and affected vineyards must be removed. A complex of similar diseases, Fanleaf Degeneration deforms shoot growth, and leads to poor fruit set and shot (seedless) berries. The leaves on an infected vine are malformed, resembling fans in appearance, and may form yellow bands around the veins. The productive lifespan of the vine and its winter durability are diminished.
Flavescence Doree
Phytoplasma
A form of grapevine yellows, Flavescence Dorée first appeared in Armagnac in 1949. Leafhopper insects and propagation of infected vines spread the disease, which will initially delay budbreak and slow shoot growth, eventually causing bunches to fall off the vine and berries to shrivel. The disease will discolor leaves, cause pustules and cracks to form, and may kill young vines. No cure exists, although insecticides may be used to control leafhopper insect populations and retard its spread.
