Grapevine

Climate

Like any crops, grape growth is limited by certain climatic conditions. Generally, in Northern hemisphere, grapevine matures from Equator to 50° Northern latitude: in Southern, from Equator to 45° Southern latitude (exceptions may exist). Grapevines need a reasonably long growing season (150–200 days free of frost) with relatively low humidity (about 600 mm per year) during the vegetative season, but sufficient soil moisture is necessary. If soils are well drained, water is not a problem; excess of water in soils (ponding) is a big problem. Of course, possibility of irrigation brings to nothing water supply problems. The temperatures from April to September are crucial for reaching good development of the vine and ripening of the fruits. When temperatures are below 10 °C, vines stops growing. The optimum temperature is between 25 and 30 °C. Temperatures higher than 38 °C will stop plants growth. The index of Winkler and Huglin is the sum of daily temperatures above 10°C during the growth season from April to October. Early cultivars reach maturity when tis index is from 1600 to 2000°C during the period between blooming and ripening. For late cultivars, the value is 3000°C.

Soils

Grapes can be produced on a number of different soils – chemical fertility is important, but not more important than soil structure and texture. In a very general way, clayish, fertile soils give robust red wines: sandy soils give aromatic white wines. Sandy or gravelly clay loams are most desirable; local soil attributes together with local climate characteristics, confer to wines different and often unique organoleptic characteristics as flavor, alcohol content, taste, acidity. Alkaline soils must be avoided, as well as very acidic ones, or they both must be amended, if economically suitable. Good drainage is crucial for quality production. In most countries, vines are grafted on rootstocks that are carefully selected depending on resistance to Phylloxera and characteristics of the soil.

Biology and annual cycle

Vine grape’s annual cycle is divided in two phases, vegetative and reproductive. Vegetative stages begins with “weeping” in early spring: sap is coming from the roots and it goes out through the pruning cuts. After that (temperature is between 4 and 13°C, depending on cultivars, with and average value of 10°C) buds swell and burst (March-April) ; following the burst of buds, at the beginning slowly sprouts (shoots) are formed: afterwards, shoots and foliage emerge quicker during the spring and the summer. An important phase is the lignification of shoots during August: shoots become “canes”, thus producing dormant buds for production of new sprouts during the next season. All these processes are ruled by temperature regime and presence of hormones in the plants.

The first part of reproductive stage begins roughly at the end of May with pre blooming stage. Blooming last to mid June: during blooming, usually a certain share of flowers do not continue the process and drop. Pollination begins after blooming and ends with formation of “buckshot berries”. Not each flower gives a berry: on average, the fruit set value is between 60 and 30%, depending on cultivar and climate. The berry increase their volume until veraison (August), when berries take the color which is typical for the cultivar. After veraison, ripening continues to the harvesting in Autumn. After the fall of leaves in the end of Autumn and during the winter, vine grapes are dormant and more resistant to frost.

Crop Protection

Grapevines are damaged by a long list of parasites as fungi (most formidable), insects and viruses. E.g. the charts above show the complexity of the protection means applicable against two of the most dangerous fungi disease of grapes, which also nowadays, if undetected, can completely destroy the yield. Against Plasmopara viticola, applications should begin between the stages of shoots length 10 centimetres and pea sized berries. For powdery mildew (Uncinula necator) the spraying starts immediately prior to grapevine blooming. Infections depend both on rain and temperature regime. These two examples show that preventive control of primary infection and following accurate management during the whole season are crucial for managing the diseases. Basically, fungicides and insecticides can be divided by way of action: “contact” and “systemic”. Contact chemical cover the foliage and kill the parasites, but they are flushed away by rain and do not enter the plants, thus they do not have therapeutic effect against infections. As a consequence, it is necessary to repeat the spraying after every rain, on average 1 time in 7 days during the season. Systemic chemicals enter the plants, kill the parasites and their action is longer, on average one spraying is needed in 15 days. For improving crop protection and decreasing the amount of applied chemicals, growers are adopting integrate crop protection systems: this means that they use different tools for monitoring the actual presence of insects or the probability of starting infections in order to precisely know when it is absolutely essential apply chemicals. Nevertheless, during the season, several treatments are carried out with specialized spray dryers that deliver chemicals on all parts of the plants’ canopy with great precision. Operators must be protected from chemicals: active carbon filters mounted in the air conditioning system of cabin are mandatory for operator’s safety. As a whole, spraying can amount up to the 55% of tractor activity in vineyards.

Irrigation

Traditionally in Europe grapevine has been cropped without irrigation, also because the plant is physiologically rather resilient to drought. About 600 millimetres of precipitations per year are sufficient for obtaining good quality grapes and thus quality wines, if precipitations are regularly spread during the season. As a whole, this value is well below the average precipitations’ value common in North and Central Europe: in South Europe rainfall can be less than that. More, in areas where excellent wines are produced (AKA designation of origin guaranteed and protected), irrigation is often forbidden by local production protocols, in order to avoid a worsening of the grape quality, which means an increasing of the ratio between water and sugars contents inside the berries. However, if the natural precipitations are not sufficient for viticulture or drought is severe, grapevine plant suffer and also quality of product gets worst: in these cases irrigation is justified and necessary as “rescue irrigation” or “deficit irrigation”, that means applying the minimal amount of water only during drought period or/and drought sensitive stages of plants’ growth. Most sensitive stages for grapevine are between fruit set and veraison. Drip irrigation is the most rational method for irrigation in vineyards (not the most cheap); this method does not allow for spreading of diseases and it is possible to reach an high water efficiency, meaning less losses of irrigation water and less volume of water necessary per hectare. More, in this way only the band beneath the rows is irrigated, and only this band must be tilled, thus allowing for keeping turf on interrow spaces and for precise application of fertilizer where they are needed.

Harvesting

The fruit of grapevine is a berry, the whole of berries make a cluster or bunch of grapes. Parts of berry are skin, seeds and pulp. Skin contains anthocyanins, which give color to vine. Seeds contains tannins, responsible for astringency of vine. Pulp contains water, sugars( which fermentation gives alcohol and carbon dioxide) and organic acids, which have influence on vine taste, acidity, stability and color. Of course, each cultivar differs from other; as guidelines, the berries are ripe when water content is 70-80%, sugars are from 15-24% (ratio is glucose: fructose = 1:1) and organic acids ( mostly tartaric and malic) are between 4-10%. Different quantities and ratios of these compounds explain the huge variation of flavor in vines after the process of fermentation. Content in sugars is determined by testing the juice with a Brix refractometer, and this helps in determining the stage of maturation; in conjunction with chemical tests also a berry sensory analysis is possible, based on 20 descriptors as berry color, sweetness, acidity and aroma.