Plasmopara viticola

Downy mildew is one of the most destructive vine diseases known. It occurs especially in regions that are warm and wet during the vegetative growth stage of the vine. When control is poor and/or weather conditions are favourable, the disease may cause crop loss due to total or partial destruction of grape bunches, and also due to the secondary influence of foliage loss. While crop losses may range from 10 to 20% if poor control is exercised, favourable weather conditions, especially during flowering, may even cause total crop loss (Magarey, Wachtel & Emmett, 1994).
Due to recent epidemics in the Western Cape, Orange River and other summer rainfall regions, downy mildew has become one of the most feared diseases in vineyards. The purpose of this article is to reintroduce producers to this disease and to concentrate on control strategies under extraordinary weather conditions.
Symptoms
The fungus responsible for the disease attacks only the green tissue of the plant. Symptoms are seen mainly on the leaves, but also on bunches and sometimes on shoots.
Leaves. Depending on the incubation period and age of the leaf, initial lesions on younger leaves are either round, yellow and greasy (oil spots)
, or more angular, yellow to reddish brown, adjacent to leaf veins on older, adult leaves. The characteristic white, downy fungal growth is noticeable on the undersides of infected leaves following warm, wet nights . During warm weather conditions the leaf lesions dry out and become reddish brown, surrounded by yellow discolouration. Defoliation may occur if the infection is heavy.
Bunches. Infected young bunches initially turn brown and oily. White, downy fungal growth develops on the affected parts of the bunch after warm, wet nights. Berries are susceptible until the pea berry stage is reached. Infected berries become brownish purple, shrink and fall off. Although berries become resistant after the pea berry stage, bunch stems (rachi and laterals) are still susceptible. The latter parts turn brown and the berries attached to them become shrivelled and brown (raisin-like), dry out and may even fall off . These symptoms are sometimes confused with sunburn.Young, green shoots may also be attacked. The shoots turn brown and oily, whereafter fungal growth may appear on the infected parts. Infected shoots later become necrotic and die.
Causal organism
The organism that causes this destructive disease is Plasmopara viticola. Like familiar pathogens, Phytophthora and Pythium, this fungus also belongs to the Oömycete group of fungi. Plasmopara viticola is an obligate parasite, which means that the fungus can only survive on living tissue and can therefore not survive as a saprophyte. It forms intercellular hyphae with haustoria in the plant cells for the absorption of nutrients. Asexual reproduction occurs through the formation of sporangia on branched sporangiophores. Each sporangium gives rise to 10 zoospores, each containing two flagelli. Sexual reproduction involves the coupling of a male anteridium with a female oogonium, which leads to the formation of thick-walled oospores (Lafon & Clerjeau, 1994).
Disease cycle
Downy mildew occurs all over the world and the disease cycle has been well documented by researchers from inter alia Australia (Magarey et al., 1994), France (Lafon & Clerjeau, 1994) and South Africa (Marais, 1981).
Overwintering. Plasmopara viticola overwinters as oospores, mainly in infected residues (leaves, bunches or shoots). These structures can survive for 3-5 years. In regions with mild winters, fungal strands can also hibernate in buds or remaining leaves.
Primary infection. A very specific set of environmental parameters is essential for the oospores to germinate. At least 10mm rain should fall and the temperature should be at least 10°C for a period of 24h [10:10:24]. Hereafter zoospores are released in the soil. The next requirement is rain or water splash in order to distribute the spores to the green, susceptible parts of the vine. Using their flagelli, zoospores swim to the vicinity of the stomata where they encyst. These cysts germinate and the germ tube penetrates the stoma. The downy mildew fungus can only infect through stomata and consequently only those sections of the plant with functional stomata are susceptible. For infection to be successful, susceptible plant tissue has to remain wet for 2-3h. Oil spots or lesions occur 5-15 days after infection, depending on the weather conditions (20-25°C) and the age of the tissue. Primary infection levels are usually very low and only a few leaves in a vineyard row will have oil spot symptoms.
Secondary infection. Sporulation only occurs after a warm, wet or very humid night. Sporangiophores with sporangia grow from the stomata below the oil spots or lesions. The sporangia are distributed by rain and/or wind and once again require 2-3h of free water for germination and penetration. The potential of the sporangia to form zoospores decreases with age, especially when conditions are unfavourable. However, this is not the case with the fungal strands inside the plant tissue and old lesions or oil spots may again produce sporangia.
During favourable conditions (repeated rainfall or heavy dew in humid areas) the disease can spread extremely rapidly. According to Australian sources, 20-50 oil spots in a vineyard may increase to more than 100 000 oil spots in one night and in certain instances total crop loss has been caused in one night (Magarey et al., 1994).
Chemical control
Successful control of downy mildew depends on controlling the primary infection. Since the disease spreads incredibly fast during the secondary cycle, it is very difficult to control the disease during this explosive phase. The disease usually becomes epidemic during wet or rainy conditions, when control measures are taken too late.
The critical control period is early in the season, which is when active vegetative growth occurs. Infection before pea berry stage may cause total crop loss. From the time when the first shoots reach 10cm in length until the pea berry stage, vines must be monitored for the occurrence of oil spots. Epidemics during this period may result in total crop loss, therefore an attempt should be made to prevent or limit the primary infection. Two strategies for chemical control may be followed (Magarey et al., 1994):
Post-infection strategy. This strategy entails no chemical control until an infection period [10:10:24] occurs. A systemic product (Table 1) must be sprayed immediately afterwards, before the oil spots occur. If oil spots do occur, a systemic product is again sprayed after a warm, wet night in order to reduce sporulation and also to protect the plant against secondary infection. This strategy involves a high measure of risk, since repeated rainshowers, soil that is too wet, insufficient spraying equipment or other factors may prevent the producer from spraying his vineyards before the disease is established. In this scenario the disease may progress to the epidemic secondary cycle, which is very difficult to control. This strategy is only recommended for regions with a scant history of downy mildew and predominantly unfavourable conditions for the development of the disease.
Preventive strategy. The preventive strategy is the more conservative and less risky choice. All green parts of the vine must be protected with registered contact fungicides (Table 1). If wet conditions are forecast or experienced, vineyards should be sprayed with systemic products before, or otherwise directly after the primary infection period. During wet conditions vineyards must be sprayed at 14 day intervals and weekly during the critical period around flowering. Later in the season, when active growth is less, intervals may be extended to 3 weeks.
Important considerations are that new growth is not protected since the previous application of a contact fungicide. Rain and overhead irrigation also reduce the residue levels on the plant and spraying should be repeated if heavy precipitation is experienced shortly after applications. A good coverage of all the green parts of the plant is essential.
Cultivation practices
Successfol control of downy mildew may depend on the integration of both these chemical strategies. Additional viticultural practices may reduce the intensity of infection and facilitate disease control. Vineyard location, row direction, canopy management, weed control, irrigation and fertilisation should be properly managed to avoid excessive vigour. Humidity in the vineyard should be kept as low as possible to assist rapid evaporation of rain, dew or irrigation drops. The downy mildew fungus can only infect in free water and these actions will therefore limit the number of successful infections.

PHYTOPATHOLOGY

Phytopathology (plant pathology) is the scientific study of plant diseases caused by pathogens (infectious diseases) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. Not included are insects, mites, vertebrate or other pests that affect plant health by consumption of plant tissues. Plant Pathology also involves the study of the identification, etiology, disease cycle, economic impact, epidemiology, pathosystem genetics and management of plant diseases.
The "Disease Triangle" is a central concept of plant pathology for infectious diseases[1] . It is based on the principle that disease is the result of an interaction between a host, a pathogen, and environment condition.

VINEYARD

New viticultural techniques have made possible the development of wine industries in various New World countries such as Canada. Today there is increasing interest in developing organic, ecologically sensitive and sustainable vineyards. Biodynamics has become increasingly popular in viticulture. The use of drip irrigation in recent years has expanded vineyards into areas otherwise unplantable. A consequence of irrigation is consistency of yields and a virtual irrelevance of vintage year. Other recent practices include spraying water on vines to protect them from sub-freezing temperatures (aspersion), new grafting techniques, soil slotting, new trellising methods, new canopy management techniques such as minimal pruning, and mechanical harvesting.
The implementation of mechanical harvesting is often stimulated by changes in labor laws, labor shortages, and bureaucratic complications making it difficult and expensive to hire labor for short periods of time, the need to reduce production costs, the need to harvest quickly, and the ability to harvest at night. However, retarding the utilization of machine harvesting are such things as very small ownership parcels, incompatible widths between rows of grape vines, steep terrain, and traditional views rejecting such harvesting.

Australian vineyard in the Riverina.
New World vineyard plantings have been increasing almost as fast as European vineyards are being uprooted. Between 1990 and 2003, U.S. vineyards increased from 292,000 acres to 954,000, Australian vineyards went from 146,000 to 356,000 acres, Chilean vineyards grew from 161,500 to 415,000 acres.
There are also changes in the kinds of grapes grown. For example, in Chile, thousands of acres of low-quality grapes have been replaced with such grapes as Chardonnay and Cabernet Sauvignon. This is often in response to changing consumer demand but sometimes results from vine pull schemes designed to promote vineyard change. Alternatively, the development of T budding now permits the grafting of a different grape variety onto existing rootstock in the vineyard. This makes it possible to change varieties within a period of about two years.
Which grapes are grown in vineyards, as well as how they are grown, often reflects legislation, which in turn may serve to reinforce tradition. For example, laws may restrict which varieties can be planted, whether vineyards can be irrigated, when grapes can be harvested, and so on.
Changes in laws can also influence which grapes are planted. For example, during Prohibition in the U.S. (1920-1933), vineyards in California expanded seven-fold to meet the increasing demand. However, they were largely planted in varieties with tough skins that could be transported across the country to home wine-makers. The resulting wine was of low quality.
Leading wine critic Robert M. Parker, Jr. has had a significant influence on viticulture around the world. His taste preferences have led many growers in Bordeaux, for example, to practice "green harvesting," in which whole grape clusters are removed and discarded during the growing season in order to reduce yields. Also, because of Parker's influence, many growers now strip sections of leaves away from vines to permit more direct sunlight to reach the grapes.
There is a continuing relationship between the history of alcohol and the history of vineyards that has existed from the earliest known times

OIDIUM...Uncinula necator

An oidium (plural: oidia) is an asexually produced fungal spore that (in contrast to conidia) is presumed not to constitute the main reproductive preoccupation of the fungus at that time. The hypha breaks up into component cells/ small pieces and develop into spores. Oidia can't survive in unfavourable conditions. Uncinula necator is a fungus that causes powdery mildew on grape. It is a common parasite of grape (Vitis vinifera). The fungus originated in North America. European wine varieties of Vitis vinifera are more or less susceptible to this fungus. Uncinula necator infects leaves and young berries, and can cause large damage if untreated. This parasite does not need drops of water to infect the plant leaves; high atmospheric humidity is enough. Its anamorph is called Oidium tuckeri.
This mildew can be treated with sulfur or fungicides, especially Strobilurines. Uncinula can develop resistance to organic fungicides, so it is important to use different (rotate) fungicides during one season (for example, using two or three different types of organic fungicide and sulfur). The parasite can not develop resistance to sulfur, but efficacy of this fungicide is not as high as the organic ones. But sulfur can be used as a prevention for this plant disease.

Fungicides are chemical compounds used to prevent the spread of fungi or plants in gardens and crops, which can cause serious damage resulting in loss of yield and thus profit. Though oomycetes are not fungi, they use the same mechanisms to infect plants[1] and therefore in phytopathology chemicals used to control oomycetes are also referred to as fungides. Fungicides are also used to fight fungal infections.
Fungicides can either be contact or systemic. A contact fungicide kills fungi when sprayed on its surface; a systemic fungicide has to be absorbed by the plant.

Two-Faced FungusBotrytis cinerea is a fungal disease that can blight many species of plants, including flowers, fruits, and vegetables. Depending upon weather conditions, Botrytis can take one of two forms in grapes, one as destroyer, the other as enhancer.
As "grey rot" it appears and grows during lengthy periods of humidity early in the season. Settling in on immature grapes, it multiplies rapidly. The bunches appear to be covered with a grey powder and eventually darken and drop. Yields are greatly reduced and wine made from this fruit taste moldy and oxidizes easily. In some climates, grey rot is a severe problem with most grape varieties.
In certain white grape varieties, such as Semillon, Sauvignon Blanc, Riesling, and Furmint, an infection of Botrytis can be so beneficial, even critical to dessert wines like French Sauternes, German Tröckenbeerenauslese, or Hungarian Tokaj, that the mold becomes known as "Noble Rot." Weather conditions must be right for this to occur. Ideally, a short period of humidity or rain in mid to late season, when the grapes are more ripe than green, will be followed by a sustained period of cool, dry weather, where daytime temperature hovers near 60° F.
Under these somewhat rare conditions, the Botrytis fungi penetrate the grape skins with mycelia to feed and take water from the grapes, which shrivel. Overall acidity decreases. Gums form, along with glutinic and citric acids, and the grape sugars become very concentrated.
This intense sweetness partially inhibits yeast and fermentation can be very slow, lasting for months. High concentrations of glycerol developed during these extended fermentations and the resulting wines can be exceptionally smooth and extremely long-lived, cellaring well for decades.

ENEMIES AT THE STAKES There are many pests and diseases that can attack and kill grape vines. Red spiders, moth grubs and various mites, bugs and beetles can all prey on the plant above ground. Most of these may be controlled with either sulfur sprays, or by newer "green" methods, such as introducing predacious insects and protective cover crops between vine rows.
Often the ends of vine rows are planted with a single rose bush. Insects, mildew and fungi seem to prefer the sweet smell of roses, which perform a "canary in a coal mine" function for grapevines, providing early warning of the need to treat for pests or diseases.
In climates with summer rainfall, molds such as oidium, mildew, white rot, grey rot (see box at left) and black rot may be prevented by regular sprayings of a solution of copper sulfate, slaked lime and water (Bordeaux mixture). Research is ongoing into biological methods of controlling these fungal problems.
New vineyards are particularly susceptible to destruction from gophers and moles. There are many methods of control and eradication, including attracting predatory raptors, trapping, poisoning, flooding and even a device that implodes burrows.
Deer, raccoons, possums and other mammals can consume a lot of fruit, damage more, and even harm the vines, especially young plants and shoots. Vineyard fencing usually serves to keep these larger animals at bay.

The grape vine is the source of all wine. Reaching the highest level of quality in wine is only possible by starting with the highest quality fruit. Maximizing fruit quality from any vineyard site can be a lengthy process, because the end results are revealed only after several seasons of comparison.
Grapes are the largest fruit crop on earth2. The grapevine prefers the temperate climate in which it evolved, with warm, dry summers and mild winters. Winters of sustained cold kill grapevines. High humidity promotes vine disease. Tropical temperatures disrupt the normal vine cycle of winter dormancy.
Grapevines are fairly adaptable plants, growing in a wide variety of soil types, from light sand to packed clay, and flourishing around the globe in the temperate bands between 20° and 50° Latitude, north or south of the Equator. They are successfully grown in Europe, the Balkans, Asia, Mediterranean and South Africa, South Australia and New Zealand, most of North America and a good portion of South America.
THE BEST PLANS...There are multiple and interlacing factors to consider when starting a vineyard, in order to ultimately achieve highest fruit quality. In selecting a site, the average length of the ripening season, the normal annual weather conditions, the soil type and chemistry, fertility and drainage, the topography, sun exposure, and likely pest problems should all be taken into account well before the first vine is planted.
That information will bear upon the decisions of vine variety, vine density, row direction and spacing, irrigation and frost protection methods, vine training system, as well as fertilization and pest control management. These in turn will affect choices in crop load, canopy management, harvesting, and pruning. At each step in establishing and maintaining wine vines, the grower must evaluate and commit to a course of inevitable compromise between highest quality and practical economy. Yet the results of even the most carefully researched and executed decisions are ultimately at the whim of Nature.

Viticulture is the science, production and study of grapes which deals with the series of events that occur in the vineyard. Grapes are grown for fresh fruit, dried fruit or for the grape juice, which can be used (amongst others) to produce wine. Duties of the Viticulturalist include: monitoring and controlling pests and diseases, fertilizing, irrigating, canopy management, monitoring fruit development and characteristics, deciding when to harvest and vine pruning during the winter months. Viticulturalists are often intimately involved with winemakers, because vineyard management and the resulting grape characteristics, provide the basis from which winemaking can begin.

A grape is the non-climacteric fruit that grows on the perennial and deciduous woody vines of the family Vitaceae. Grapes grow in clusters of 6 to 300, and can be black, blue, golden, green, purple, red, pink, brown, peach or white. They can be eaten raw or used for making jam, grape juice, jelly, wine and grape seed oil. Cultivation of grapevines occurs in vineyards, and is called viticulture. One who studies and practises growing grapes for wine is called a viticulturist.
Raisins are the dried fruit of the grapevine, and the name actually comes from the French word for "grape". Wild grapevines are often considered a nuisance weed, as they cover other plants with their usually rather aggressive growth. Many species of grapevines exist, including:
Vitis vinifera, the European winemaking grapevine. Native to virtually all of mainland Europe.
Vitis labrusca, the North American table and grape juice grapevines, sometimes used for wine. Native to the Eastern U.S. and Canada.
Vitis riparia, a wild vine of North America, sometimes used for winemaking and for jam. Native to the entire Eastern U.S. and north to Quebec.
[[Vitis
Vitis vulpina Frost grape. Native to the Midwest east to the coast up through New York.
Vitis cognitiae Ornamental Grape from East Asia, grown for its crimson autumn foliage