How does the grape get from the vineyard to the bottle on the shelf? How does soil affect wine quality? What exactly does oak do?
Winemaking – what can go wrong
Soil Problems
Either through their geological composition, or through continuous cultivation, some soils become deficient in key nutrients. After soil analysis, this can be solved by adding chemical fertilisers. However, the trend at present is to return to more natural treatments like compost and manure. This is all part of the move by some growers towards ‘organic viticulture’.
Weather Problems
Winter frost – temperatures below -16 degrees will freeze the vines and kill them. The main advice to grape growers is don’t plant your vines in areas where this sort of frost is a regular occurrence.
Spring frost – once the buds have opened, it only needs the temperature to dip to zero degrees for the new growth to be damaged. The vines are not killed, but the loss of the embryonic flowers can drastically reduce the current year’s crop. Heaters, fans and water sprays can be used to limit the damage.
Poor weather at flowering – extremes of temperature and rain can severely disrupt the flowering. The knock-on effect is that the grower ends up with a smaller crop than normal.
Hail – although often very localised, a hail storm can bruise and batter the vine. Damaged grapes can very quickly succumb to grey rot. Firing rockets into the clouds can precipitate the water as rain rather than as ice.
Drought – where irrigation is not an option, summer drought can actually cause the grapes to stop ripening.
Rain at harvest – a year’s toil can be compromised by a couple of days rain just before the harvest. The water causes dilution and the extra humidity can trigger grey rot.
Pests
The most significant pest of the vine is Phylloxera vastatrix. Other invertebrate pests include various mites and caterpillars. Many of these are controlled with pesticides. Baboons, kangaroos, rabbits, birds and deer all damage vines by eating leaves, fruit and nibbling bark.
Diseases
Mildew – Two versions of mildew, downy and powdery, attack the green parts of the vine. The leaves and fruit are damaged, and ultimately the crop is ruined. Mildew can be prevented by a regular spraying programme.
Rot – Caused by a mould called Botrytis cinerea, the destructive ‘grey rot’ attacks the grape bunches and destroys them. However, in certain circumstances, rather than ruining the grapes, the Botrytis mould can cause ‘noble rot’. In this case, the grapes shrivel up, concentrating sugar, acid and flavour. These hand picked ‘noble rot’ grapes are used to make many of the world’s great sweet white wines (e.g. Trockenbeerenauslese and Sauternes
