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A Technical Guide for Wine Producers
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The critical importance of preventive botrytis control in red wine cultivars
INFLUENCE OF BOTRYTIS ON GRAPE COMPOSITION
The biggest changes in grape composition as a result of Botrytis infection occur with regard to the sugars and acids, which are drastically reduced as Botrytis infection increases. However, this reduction in the sugar and acid concentrations of infected grape must is concealed by the shrinking/shrivelling of berries. An increase in the pH of grape must may be observed. This may be explained by a lower tartaric acid:malic acid ratio which results in a lower titratable acid. Other acids, such as glucon acid, citric acid and musein acid are formed, but their influence on the pH of the must is minimal. Musein acid causes instability during ageing of wine. The presence of glucon acid in must or wine is an indication of Botrytis activity, although it may also be formed by other micro-organisms. Botrytis uses nitrogen for its own metabolism and therefore reduces the levels of available nitrogen for yeast metabolism. Vitamins B6 and B10 which play an important role during fermentation, are also exhausted as a result of Botrytis.
The biggest disadvantage of Botrytis is the formation of laccase which catalyses phenolic oxidation. Laccase is a soluble enzyme excreted by Botrytis in grapes, and is always present in Botrytis infected grapes. Laccase is formed in the berry at a very early stage and augments as Botrytis infection increases. Certain cultivars contain better substrates for the formation of laccase. So for example Sauvignon blanc will form considerably more laccase at the same degree of infection as Muscadel. Laccase is soluble in must and resistant to the inhibiting effect of SO2 and alcohol and is also difficult to remove with bentonite. Laccase is very stable and may remain active in wine for months, while oxidising phenolic compounds such as tannins to quinones. The wine consequently begins to brown as a result of this increased rate of oxidation. Later this kind of wine is broken down completely by oxidation, with the result that bitter tastes and off-odours develop. The active ingedrient (Pirimetaniel), which occurs in certain fungicides, inhibits the release of laccase by Botrytis and therefore has an anti-laccase activity. The ability of SO2 to bind with other compounds also increases as Botrytis spreads. Therefore more SO2 is required in infected than in healthy must/wine to maintain the same levels of free sulphur. Botrytis cinerea also produces large quantities of glycerol. If grapes are infected by bacteria before the harvest, the glycerol will be metabolised and high levels of acetous acid and glucon acid will develop. Polysaccharides are also formed by Botrytis and may give problems with the filtration and fining of wine. From the above it is clear that Botrytis can have a drastic effect on the chemical composition of grapes and that the handling of such grapes requires special attention. Cellar practices affected by Botrytis infection include; SO2 levels, yeast strain selection, fermentation temperatures, degree of skin contact, nitrogen supplements, filtration practices etcetera. The very best option is rather to refrain from delivering Botrytis infected grapes to the cellar. Such bunches should preferably be cut out in the vneyard.
SYMPTOMS
Visually the best known characteristic of Botrytis is slip skin. The term slip skin refers to a condition where the epidermis of a berry slips off easily from the interior pulp when rubbing lightly over the berry. Such infected berries later form cracks on which dark grey downy mould spores (Fig 1) grow and multiply, and subsequently spread to adjacent bunches and/or berries. Infected vine leaves usually have a V – shaped area of brown, dead tissue surrounded by yellow plant tissue before changing to green healthy plant tissue. Young vine shoots may also be infected and are characterised by the browning of the internal tissue. Green bunches and stems may also be infected under certain conditions. If the bunch stem is infected with Botrytis, the section below the infection point will wilt and later drop off.
LIFE CYCLE OF BOTRYTIS CINEREA
Botrytis hibernates as mycelium (hyphae) and sclerotium (fruiting bodies) in dormant buds, under and in the cracks of bark, on dead matter, on bunch stems, and on rotten berries. Sclerotium are hard black structures with a diameter of approximately 3 mm and constitute the most important structure involved in the survival of Botrytis. In spring, when conditions are favourable, millions of spores (conidia) are formed by mycelium and sclerotium. For these spores to germinate free water and nutrients are required, in particular sugars such as glucose and fructose. Free water is required for at least 15 to 20 hours and may be in the form of rain, mist, dew, or irrigation. Spores can germinate at all temperatures between 1 °C and 30 °C, although temperatures between 15 °C and 20 °C are more favourable. The spores may infect young leaves, shoots, grape flowers, berry stems, stomata and wounds, which in turn may cause further production of spores. Cultivars differ considerably in their sensitivity to Botrytis infection. Cultivars with compact bunches such as Chenin blanc, Riesling, and Sauvignon blanc are a lot more sensitive to Botrytis infection than for example Cabernet Sauvignon, Merlot, and Ruby Cabernet. The ability of cultivars to produce resveratrol also has a huge effect on their susceptibility to Botrytis, and cultivars that are able to produce the most resveratrol in the shortest time are usually more resistant. Usually most primary infections occur around the time of flowering, while remaining latent in the period between berry set and veraison. These infections usually remain latent as a result of the higher resistance of green grapes. Green grapes and stems have the ability to form a layer of cork (tissue) directly below the damaged area of the cell wall to protect the plant tissue from penetration by pathogens. Botrytis has the ability to overcome this protection mechanism when the sugar concentration rises above 14%, since cork formation is considerably slower in higher sugar concentrations. The quantity and tempo of resveratrol production slow down as the sugar concentration increases, which explains the increased susceptibility of riper grapes. Under favourable climatic conditions and when the sugar concentration of the grapes rises above 8°B, the latent infections may become an important source of secondary infection. These latent infections, together with wet weather conditions in certain viticultural regions, explain the high incidence of Botrytis infection in March 2000. Damaged ripe berries as a result of oidium, hail, berry burst, bird damage, insect damage, wind damage etc., are also ideal infection points for Botrytis since they are rich in nutrients and water. Under optimal conditions Botrytis may infect and destroy a berry and form spores again in a matter of three days.
PREVENTIVE CONTROL MEASURES
VITICULTURAL PRACTICES
Successful Botrytis control is largely dependent on good preventive control strategy. Chemical control on its own is not sufficient and should go hand in hand with the right viticultural practices. It is important already at the time of establishment to select the right row direction to allow prevalent summer winds free movement through the vineyard rows for the required drying effect. The correct vine spacing and rootstock selection are also important. Vine spacing is determined inter alia by the fertility of the soil, the rootstock and scion, climate, choice of irrigation system and the availability of water. If vine spacing is too narrow it will result in excessive vigour, causing dense canopies which is more difficult to protect against Botrytis infection. Furthermore, it is important to erect a sufficiently large trellis system that will be able to accommodate the vigour of the vineyard.
Chemical control of Botrytis is not recommended for winter because of the hardness of sclerotium, which makes it difficult for fungicides to penetrate. However, it is recommended that pruned shoots be removed from infected vineyards (previous growing season) and burnt, or alternatively to work them into the soil at a minimum depth of 10 cm. This will drastically reduce the innoculum for the new growing season.
Foliage management is also of cardinal importance to control Botrytis effectively. The ideal is to obtain well aerated foliage with a large effective leaf surface. Correct canopy management includes suckering, tipping, thinning out of leaves if required, and shoot positioning. Tipping is recommended rather than topping since the latter results in more lateral shoot development in the bunch zone. Shoot positioning must only take place if a vineyard has been suckered. An even leaf thinning action is recommended for sensitive cultivars, but in such a fashion that grapes are not directly exposed to sunlight.
The correct fertiliser applications and timing are also important. If too much nitrogen is applied, especially in spring, it may result in excessively vigorous and therefore dense canopies. Chemical control in dense canopies is less effective, wet canopies takes longer to dry, and berry skins are also slightly thinner in dense canopies, which makes such vineyards more susceptible to Botrytis infections. The type of irrigation system, the irrigation frequency and timing are other factors that may play a role in Botrytis control. Overhead irrigation, for example, is very conducive to Botrytis infection since the foliage remains wet for long periods, and if possible should therefore preferably be eliminated. Excessive irrigation furthermore results in dense canopies, more compact bunches and thinner berry skins, with the accompanying harmful effects. Disease control is obviously very important as well, since the wounds caused by diseases such as oidium are very conducive to Botrytis infection.
CHEMICAL CONTROL
Preventive chemical control against Botrytis is of cardinal importance, but unfortunately Botrytis, like most other fungi, has the ability to build up resistance to fungicides. Botrytis cinerea is a eucariotic organism and may contain up to seven nuclei, while the hypha point of mycelium may obtain up to 50 nuclei. Of these just a few are required for survival. The fungus can therefore afford to store genetic information for long periods, even though not required. It is therefore possible to compensate for the weakening of certain nuclei with other nuclei that are not impaired. So for example a Botrytis cell may find another physiological route for a metabolic process if it is blocked by a fungicide. This characteristic of Botrytis consequently results in the formation of tolerant strains under field conditions. In the light of the above information it is therefore clear that over the long term, as with many other fungal diseases, an anti-resistance strategy is required for successful chemical control of Botrytis. In South Africa various chemical groups have already been registered, such as benzimidasoles, dicarboximides, hydroxy - analids and anylinopyrimidines, the latter three being more commonly used. If four chemical sprays are given in one season, it is recommended not to use the same chemical group more than twice.
SPRING
If Botrytis is diagnosed before flowering and/or if the vineyard had Botrytis problems the previous year, and a wet spring is predicted, an early spray against Botrytis should preferably be arranged at this stage. This will considerably reduce the infection levels before the sensitive flowering period. Normally a preventive contact fungicide with multiple contact areas that kill spores will be used for this period, for example Kaptan or Chlorothalonil. At this stage fungicides against which Botrytis may build up resistance should be avoided.
FLOWERING PERIOD
Grapes are extremely sensitive to Botrytis during flowering and therefore control is important around 80 % cap fall. A reliable fungicide against Botrytis is recommended at this stage, especially if the vineyard is flowering under cold, wet conditions. Translaminary extinguishing fungicides such as benzimidasoles, hydroxy-analides or anilinopirimidins are recommended because they kill new infections and penetrate young flower structures, where they destroy the so-called latent infections.
BOTRYTIS CONTROL WHEN BUNCHES START CLOSING
At this stage grapes are sprayed mainly to protect stems and kill Botrytis that may occur on unfertilised, infected flower parts in the bunch. Once the bunches close, it is very difficult to control or kill these infections. Chemical sprays with an evaporation effect are the most effective at this stage.
VERAISON
The most dangerous period for secondary Botrytis rotting starts when veraison occurs under wet or humid conditions. Under such humid conditions mycelium can develop very quickly and cause an epidemic. It is therefore important to spray the bunch zone at the time of veraison with an effective fungicide. If wet conditions occur, a fungicide that also has a slight curative action, should preferably be sprayed.
BOTRYTIS CONTROL BEFORE HARVEST
It often happens that grapes have to be pressed before optimal ripeness as a result of serious Botrytis rot. A last spray against Botrytis shortly before pressing is often given to ensure that the grapes achieve physiological ripeness while remaining healthy. This spray is particularly important if conditions before pressing are humid. It is important, however, for producers to take note of the abstention periods.
GENERAL
The high incidence of Botrytis in the 1999/2000 season caught many producers unawares, and large scale losses occurred due to smaller yields and poor wine quality. Botrytis is very hard to control at the end of the season, therefore a preventive spraying programme, as well as the correct viticultural practices, should be followed to prevent the same situation as the past season from occurring. The most important stages for chemical control are at the time of flowering and when bunches start closing. The harmful influence of Botrytis on the quality of red wine in particular is too big to take any chances.
SUPPLEMENTARY LITERATURE
BUILT, J. &DUBOS, B., 1990. Botrytis bunch rot and blight,13-15 in: Compendium of Grape Diseases. PEARSON, R.C. & GOHEEN, A.C, eds. APS Press, Minnesota.
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Botrytis cinerea is a very important fungus and the cost to wine and table grape producers amounts to millions annually. Under favourable conditions Botrytis cinerea results in noble rot in white grape cultivars. Wines made from noble rot grapes are much in demand and very expensive due to the unique character of the wine and the high risks involved. In most cases Botrytis is very harmful, however, and results in poor crops and low wine quality. Red wine made from Botrytis infected grapes usually has a substandard colour and texture, and is also often associated with off-odours.
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