Dry Summer / Wet Winter Cycle & Disease Pressure

In a dry summer, wet winter cycle, most rain falls outside the growing season. Mediterranean climates follow this pattern. Croatian Istria has a Mediterranean climate where rainfall reaches 800mm per year, but it rains mainly during winter, spring and late autumn, so drought is the biggest challenge and fungal diseases are less of a worry. In South Africa, viticulture mainly takes place at a latitude of 27-34° south in an area with a Mediterranean climate, with rain falling mainly between May and August. True Mediterranean climates are confined to a relatively narrow zone around the Mediterranean Sea and even narrower zones in other similar regions worldwide, and Mediterranean climate types make up only roughly 15 percent of the surface area of wine regions globally. Downy mildew is favoured by warm wet weather and botrytis by high humidity or prolonged rain, and all three diseases survive winter in the vineyard itself: downy mildew in soil, powdery mildew in buds and on bark, and botrytis in canes and ground litter.

• Istria: rainfall reaches 800mm per year, falling mainly in winter, spring and late autumn; warm, dry summers make drought the biggest challenge
• Western Cape: rain falls mainly between May and August and diminishes in a northerly and northwesterly direction, making irrigation essential in these areas
• Bordeaux is not Mediterranean: its maritime temperate climate is substantially more humid, with much greater rainfall during the growing season, than the Napa Valley's
• Napa Valley: a dry Mediterranean climate, a type that covers only 2% of the Earth's surface

Downy mildew is a major disease of grapevines caused by Plasmopara viticola. Its resting bodies, oospores, originate in infected leaf, shoot or berry tissue, and the fungus can survive 3-5 years in vineyard soil. Primary infection follows 10:10:24 conditions, when the temperature remains 10°C or more and there is at least 10 mm of precipitation over a 24 hour period; oospores then produce zoospores that are dispersed by rain-splash onto the canopy and enter the leaf through stomata. The pathogen requires free moisture for most of its life cycle. Powdery mildew, caused by Erysiphe necator (originally named Uncinula necator), carries over in infected buds, which produce flag shoots in spring, and as chasmothecia (formerly known as cleistothecia) that survive winter on the bark around the vine crown and cordon and on mummified bunches. When chasmothecia are wetted by 2.5 mm or more of rain or irrigation and the temperature is greater than 10°C, they eject ascospores; conidia then germinate within 24 hours in the absence of free water when relative humidity is greater than 40%. Botrytis cinerea survives winter as mycelia in cane tissue or as sclerotia attached to the vine or on decaying matter in ground litter. It is a weather driven pathogen, promoted by high humidity or prolonged rain in conjunction with cool or mild temperatures. On the vine side, under regulated deficit irrigation (RDI), water stress from veraison to harvest has generally increased berry skin red colour and phenolic concentration.

• Downy mildew: favourable conditions occur in warm wet weather, especially in late spring and early summer; vines are at greatest risk from 3-6 weeks after budburst until berries are pea-sized
• Powdery mildew: conidia germinate without free water above 40% relative humidity; mild (20-30°C), cloudy weather favours spread, while temperatures greater than 35°C decrease fungal growth on exposed leaf surfaces
• Botrytis: spores need water to germinate, from rain, fog, mist or dew, and once germinating, temperatures of 18-21°C give the fastest spread; Pinot Noir and Riesling are among the varieties most at risk
• Water stress under RDI: stress from berry set to veraison results in smaller berries at harvest, and stress from veraison to harvest has generally increased berry skin red colour and phenolic concentration

Too much rain close to the harvest can leave grapes tasting watery and dilute, extreme drought can cause the vine to shut down, and in overly damp and humid years grapes can suffer from rot. Rain during bloom can reduce or retard flowering, rain during berry growth can enhance the likelihood of fungal diseases, and rain during maturation can dilute the berries. Grapevine viability seems to be limited in some hot climates by rainfall amounts less than 20 inches (500 mm), although regular irrigation can overcome this. Disease pressure also shapes picking and sorting: the threat of botrytis may dictate that grapes are harvested before they have reached the parameters required for a particular end use, and sorting in poor vintages separates healthy ripe grapes from those that are unripe or affected by disease. The Margaret River wine association links the tempering influence of the Indian and Southern Oceans with very little heat stress through the long growing season, so that natural acidity is preserved alongside optimal physiological and sugar ripeness.

• Rain near harvest: grapes might taste watery and dilute
• Controlled moisture stress through irrigation, applied at the right times during growth and ripening, can enhance quality and control yields
• Botrytis: a laccase enzyme produced by the fungus reduces flavour, colour and storage stability of wine
• Flowering: ample early rain is beneficial to initial growth, but rain during bloom can reduce or retard flowering

The Barossa Valley has a Mediterranean climate with warm-climate vineyards on the valley floor and cooler vineyards up in the surrounding hills, cool, wet winters, warm, dry summers, low humidity and 220mm of growing season rainfall. Margaret River's climate is classically Mediterranean, with warm to hot, dry summers and mild, wet winters, and most annual precipitation falls between May and September; the regional wine association bills it as having the most distinct rainfall patterns of any grape growing region in Australia. Average annual rainfall there is between 850 and 1,200 millimetres, with December, January and February each receiving a monthly average of less than 25 millimetres and an average of 275 millimetres over the October to April growing season, though the national regional profile lists 202mm of growing season rainfall. In South Africa, the Cape Doctor, a southeasterly wind that blows across the southwestern Cape during spring and summer, inhibits the development of disease in the vineyards. In California, the Napa Valley has a dry Mediterranean climate, and annual rainfall across its AVAs ranges from up to 24 inches (61 cm) in Los Carneros, the lowest in Napa Valley, to up to 60 inches in Calistoga. Some parts of Chile will have warm and dry summers almost every year, while Bordeaux experiences much greater vintage variation. Bordeaux averages 950 mm of rainfall a year, and many of the best Left Bank estates are planted on gravel mounds known as croupes, which drain better after showers and storms and retain heat for late-ripening Cabernet Sauvignon.

• Napa Valley: dry Mediterranean climate; AVA rainfall ranges from up to 24 inches (61 cm) in Los Carneros to up to 60 inches in Calistoga
• Barossa Valley: Mediterranean climate, 220mm growing season rainfall, low humidity
• Margaret River: annual rainfall between 850 and 1,200 millimetres, mostly May to September, and 275 millimetres in the October to April growing season; the national regional profile lists 202mm
• Bordeaux Left Bank: gravel mounds (croupes) give better drainage after showers and storms and retain heat, helping late-ripening Cabernet Sauvignon

Essential components of strategic downy mildew management include prediction of infection based on weather events, monitoring the vineyard for symptoms, knowing the disease cycle, and understanding how cultural practices and fungicides affect control. For powdery mildew, protectant fungicides go on blocks with a history of infection starting at 5-10cm shoot growth, with 7-10 day spray intervals under high pressure; sulphur sprays are cheap and effective and commonly used early in a spray program. For botrytis, recommended practice is to remove leaves and shoots around bunches to improve airflow and reduce humidity, manage irrigation to avoid berry damage and splitting, and use Botrytis fungicide sprays when warm and wet weather is forecast. Rootstocks are a management strategy for adverse vineyard conditions such as drought: 1103 Paulsen, 110 Richter and 99 Richter are classed as tolerant of drought and Ramsey and 140 Ruggeri as highly tolerant, while 3309C, Schwarzmann and 101-14 are susceptible. The V. riparia x V. rupestris group, which includes 101-14, Schwarzmann and 3309C, performs best on soils that dry out slowly and have moderate-high water holding capacities.

• Powdery mildew: start protectant fungicides at 5-10cm shoot growth in blocks with a history of infection, and keep 7-10 day intervals under high pressure
• Sulphur: cheap and effective as a contact fungicide early in a spray program; coverage is key, especially at 15°C and below
• Canopy: dense or shaded canopies with limited airflow increase powdery mildew risk, so keep canopies open through pruning, shoot thinning or positioning
• Rootstocks: 110 Richter is classed as drought tolerant and 140 Ruggeri as highly tolerant, while the V. riparia x V. rupestris group, including 101-14 and 3309C, does not tolerate drought conditions

One wine education body names 2005, 2009 and 2010, and more recently 2016, 2018, 2019 and 2020, as great Bordeaux vintages that are mostly the result of dry and warm weather, something that cannot be guaranteed in Bordeaux. In cooler, wetter years, less Cabernet is typically used in blends, with more Merlot taking the lead. In Bordeaux in 2023, the key flowering period went well, but subsequent bouts of hot weather and rain brought humid conditions that enabled mildew to thrive. Around 90% of Bordeaux vineyards were affected by mildew to some extent, with Merlot hit hardest in the view of Simon Blanchard of Derenoncourt Consultants, white wine grapes not really hit, and Cabernet Sauvignon, Cabernet Franc and Malbec more resistant. The French agriculture ministry forecast Bordeaux's protected designation of origin production to fall 10% to 3.72 million hectolitres, which would be the smallest volume since 2017. In Margaret River, the maritime influence brings low seasonal variation, but spring storms can be detrimental to fruit development and thus yields if flowers and new shoots are exposed.

• Bordeaux 2005, 2009, 2010, 2016, 2018, 2019 and 2020: named great vintages by one wine education body, mostly the result of dry and warm weather
• Bordeaux 2023: around 90% of vineyards affected by mildew to some extent, with Merlot most affected
• Bordeaux 2023: the French agriculture ministry forecast protected designation of origin production to fall 10% to 3.72 million hectolitres, the smallest volume since 2017
• Margaret River: spring storms can be detrimental to fruit development and yields if flowers and new shoots are exposed

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