Fungal Diseases Flashcards
What is Powdery Mildew (Oidium)?
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 throughout the Vitis vinifera vineyards of Europe but was soon controlled by applications of sulfur and other fungicides.
What is Downy Mildew (Peronospora)?
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 1880s. 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-staining Bordeaux Mixture, a spray of copper sulfate, water and lime, was developed by 1885 to prevent outbreaks of downy mildew.
What is Eutypa Dieback?
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. This 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.
What is Esca (Black Measles)?
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.
What is Black Rot?
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 vine’s shoots, leaves, and berries. Although yield reductions can be disastrous if unchecked, the disease can be controlled through fungicide sprays.
What is Bunch Rot?
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 cinerea 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.
