Noble Rot is a form of the botrytis cinerea fungus that can form some of the greatest sweet dessert wines in the world. For this to happen, the exact right conditions must develop, and there are only a few places in the world where this happens, most notably in Bourdeaux (Sauternes) and Germany. It requires damp mornings and dry afternoons. When this happens, the rot attacks the ripe grapes, and eats the water inside the grapes without breaking the skin. By sucking out the water, the sugar in the grape becomes concentrated, allowing a wine to be produced that is both high in alcohol and sugar. In addition, the fungus flavors the wine in it's own special way, adding notes of honey and marmalade. This process is tricky, and requires a great deal of hand-picking. Pickers must go through the vineyard several times, each time picking only those grapes that are affected by the rot in just the right way. Some years the fungus just isn't created, and entire crops are lost. Because of the unpredictability, and extreme level of care required to produce these wines, they are always very expensive. And they are amazing!
Most diseases these days can be combated by spraying the vines protectively throughout the growing season. This is usually done by tractor, although in extreme cases helicopters or planes can be used. Diseases take the form of mildew, rot, or other long term diseases.
Mildew comes in two main forms, Powdery, and Downy. Powdery Mildew, also called Oidium, attacks all the green portions of the vines, forming powdery spores. If it reaches the buds or the grapes, the grapes do not ripen properly and eventually split open. Spraying the vines with sulphur solves this problem. Downy Mildew, called Peronspera, is similar to phylloxera, in that it came over from America to infect European vineyards. It also attacks the green portions of the vine. Downy Mildew thrives in damp conditions, and can be stopped by modern fungicide sprays.
Rot can take three forms, grey, noble, and black. All three are the same fungus, botrytis cinerea, and are created in damp, humid conditions. Grey rot affects the young berries, reducing yield quantities a great deal. Spraying can halt the process, but it must be eradicated before the grapes begin to ripen. Noble rot is actually a beneficial form of the fungus, that can create outstanding dessert wines. More on this in a separate post. And black rot is brought about by heavy rains, affecting the leaves and vines. Black rot can be prevented by spraying something called Bordeaux mixture, a solution of lime and sulphate.
Vines are also susceptible to long term diseases, and include fungal as well as bacterial diseases. Eutypa and Phomopsis are two fungal diseases that can combine to form a condition known as "Dead-Arm," which can reduce yield substantially, without affecting quality. Pierce's, a bacterial disease spread by insects called sharpshooters, can also reduce yields. All of these fungal and bacterial diseases are currently without cure, and prevention by cordoning off vineyards can often be the only way to prevent them. With these diseases, as well as other long term viral diseases that attack vines, once they have become widespread, the vineyard will have no choice but to dig up all the vines, sanitize the land, and plant entirely new cuttings.