La downy mildew is a trophic disease - i.e. affecting nutrition - caused by a parasite of the family Downy mildew which takes energy resources away from the host vine. If sufficient control measures are not or cannot be employed, as is the case for example in the organic management of the affected vineyard, the entire harvest may be reduced or even lost. This will be analysed in the case study of Podere Casanova, an organic winery in Montepulciano (Tuscany) that lost the entire 2023 harvest to this pest. This year, as much as the damage caused by downy mildew has been widely discussed and the government itself has thrown a drop of aid into the sea of damage suffered by winegrowers, the 2023 harvest of the real organic vineyards placed in some areas was impossible with a loss of 100% that needs to be mentioned.

Vine downy mildew: disease and symptoms

Understanding this vine disease and its damage starts with knowing how to recognise it in its various growth stages. Downy mildew attacks all the green parts of the vine, causing oily spots on the upper page of the leaf and white down on the lower page of the leaf as well as on the shoots and bunches.

Leaf attacked by downy mildew

The visible symptoms of downy mildew infection on the leaf differ depending on the age of the leaf.

Young leaves

Yellow spots (called oil spots, they can be red in some black berry varieties) appear on the upper page of the leaf and are ~1 cm in diameter with a light brown halo that fades when the leaf reaches full maturity. At this time, the spot tends to grow up to 5 cm. If the night climate is hot and humid, a white, cottony-looking down develops on the underside, exactly at the yellow spot.

downy mildew early stage

Mature leaves

As the spot ages, its centre dries out and takes on a dark brown colour, while the yellow ring remains around it. In the dry part the Peronospera is dead, while in the yellow part it is still alive and active to the extent that it can generate new spores.

If the night climate is warm and humid on the underside of the leaf, exactly at the yellow ring, white down develops, further proof that the pest is still alive.

Late stage grapevine downy mildew

The leaf in the advanced stage of the disease

Between late summer and early autumn, when this vine disease has advanced on the mature leaf, it is characterised by a mosaic appearance, i.e. small brown spots of a fairly regular shape in the areas of the veins. Partial or total defoliation may occur in the hardest affected plants or where treatment must be limited out of respect for the type of vineyard management.

Downy mildew final stage

Shoots and petioles attacked by downy mildew

Oily brown spots appear on the young shoots and leaf petioles, which can even lead to the death of the stem with consequent detachment. Just as with the leaf, if the climatic conditions are favourable, these spots can become covered with white down.

Downy mildew

Inflorescences, bunches and berries attacked by downy mildew

This vine disease affects the inflorescences, bunches and young berries with oily brown spots. As with other parts of the plant, if the nights are warm and humid, they become covered with white down. Within a short time, the spots on the inflorescences and young bunches expand until they cover the entire surface and then wither away.

Downy mildew cluster damage

The berries in formation that are affected by downy mildew harden and turn a purplish colour. They then turn dark brown, wither and fall off the bunch.

Downy mildew berry damage

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Downy mildew: types of infection and vineyard management

Since downy mildew spreads both from the soil to the plant and from one part of the plant to another, the management of the vineyard and thus the types of possible pre-infection and post-infection interventions become decisive in safeguarding the harvest. It is therefore not surprising that a winery as virtuous in its treatments as Podere Casanova, in the dramatically affected area of Montepulciano, lost its entire 2023 harvest. Before analysing the specific situation, the types of infection, possible interventions and the incidence of organic on these are given.

The downy mildew cycle

Peronospera is an obligate pest and this means that it necessarily needs a host to live and grow. However, when it is in the form of a spore (or rather oospora), it reduces its functions to mere survival while waiting for favourable conditions to develop again. The dramatic thing is that these spores fall from the infected plants with the fall of leaves and bunches of grapes and insinuate themselves into the soil where they can survive for years and years (certainly up to 5, some studies suggest 10) of both adverse weather and treatment. Downy mildew is there, latent, ready to develop when conditions return good if not optimal.

Primary infection: from soil to vine

In agronomy, there is the 10:10:24 rule, i.e. at least 10 mm of rain with a temperature of at least 10 °C for a period of at least 24 hours is necessary for the formation of downy mildew. Fortunately, this condition is necessary, but not sufficient. If the oospores germinate and generate zoospores capable of 'swimming' in free water, these are deposited on the underside of the leaf where they develop hyphae. At this point, between the fifth and fifteenth day after infection, depending on the temperature, which speeds up or slows down the incubation time in a directly proportional manner, the hyphae turn into oil spots.

The primary infection is insidious in that it is very difficult to find: every 50 metres of rows, a maximum of 3 oil spots appear. Yet this is precisely the time to intervene in order to have the best chance of success.

Secondary infection: from leaf to other parts of the plant (leaves, bunches...)

Secondary infections can occur at any time during the growing season and this means that an oil spot can remain dormant even for an entire summer. This type of infection is called secondary because it cannot occur as long as there are no active oil spots on the plants. For secondary infection to occur, sporulation, i.e. the production of spores, must first occur, which requires a humidity of 98% or higher and a temperature of at least 13 °C, then the foliage must be wet for about 3 hours.

Organic vineyard management: what it means and how it affects downy mildew infection

Primary infection is necessary for the disease to really develop through secondary infection. This produces spores that can be spread by wind and rain to other parts of the plant or to neighbouring plants causing, where the climate permits, a disastrous epidemic. 

Avoiding dense shady foliage, which can encourage the appearance and spread of downy mildew in rainy weather, is very important. The measures possible for an organically managed vineyard are mostly of a cultivation nature and include, among others:

  • limit the planting density;
  • Arrange the trellises and prune to open the foliage;
  • lateral thinning of shoots;
  • defoliation.

With regard to the fungicides allowed in an organic vineyard, these are copper- and sulphur-based. In particular, as reported in the Tuscan Region's Manual for Organic Viticulture in point 9.2 on pages 80-82, with regard to downy mildew defence in organic vineyards, it is suggested that copper be used at a maximum rate of 6 kg per hectare per year. This limitation stems from the fact that copper has the important side effect of accumulating in the soil with a consequent decrease in microbial and biological activity in the soil leading to a "increased organic matter, decreased mineralisation and reduced availability of plant nutrients'. [p. 81].

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Case study [2023]: Podere Casanova's organic vineyard in Montepulciano

Podere Casanova is located in Montepulciano, Tuscany, and is the organic winery of Isidoro Rebatto and Susanna Ponzin, a beautiful couple of entrepreneurs from the Veneto region who by choice of life and profession have always been committed to the preservation of the landscape and the fight against the waste of resources. Their main work is the recovery of disused and reactivated hydroelectric power stations for the production of clean energy. For the winery, they have followed the same principle: they have recovered what was a bankrupt ruin and have expanded and pampered it until it has become a dream relais capable of producing wines of great quality.

I asked them for a kind of logbook to show me the climatic data and the treatments carried out in order to support this article dedicated to downy mildew as an important vine disease that deserves to be studied in depth after a year like 2023. 

Podere casanova downy mildew treatments May 2023
Podere casanova downy mildew treatments June 2023
Podere casanova treatments July 2023 downy mildew

After 3 July 2023 Podere Casanova Montepulciano decided to discontinue the treatments and renounce the 2023 harvest because the copper used on that date amounted to 1.8 kg per hectare. Considering that, by company decision, a maximum of 50% of copper permitted in organic farming for the Region of Tuscany, i.e. 6 kg/cu/ha, going ahead would certainly have meant exceeding both its own limit and the regulatory limit. Only one treatment was carried out on 25 July 2023 for the new rooted cuttings. It is also important specify that in 2021, 0.83 kg of copper per hectare was used, and in 2022, just 0.37 kg of copper per hectare.

Downy mildew 2023 podere casanova

Peronospera and organics: a brief reflection

The choice to run a vineyard in compliance with organic certification is a noble one, but in particularly rainy and hot years, respecting these limits inevitably leads to having to give up a large part of the harvest. Constant monitoring remains the most powerful weapon available, but as we have seen, primary infection is difficult to find due to the very low number of oil spots present. This makes downy mildew an insidious disease, a great risk for agriculture that must always remind us that the work of the fields, for millennia and for millennia to come, has been and will always be at the mercy of the climate. Climate change then only worsens this situation, and paradoxically it is precisely the wineries that are most virtuous with regard to the environment, those that do few treatments and in any case are always in full compliance with legal limits, that are the first victims.

I close with a question: is it worth paying a few euros more for a wine, for example a Vino Nobile di Montepulciano DOCG that is just as good if it is organic, compared to one produced with grapes from a traditional vineyard management? This higher 'business risk', which nevertheless arises with the noble aim of protecting the environment for present and future generations, whose should it be? All of the company's or also the consumer's?

A sommelier should ask himself these questions and thus make his choices, whether amateur or professional, and prefer, where possible and always with equal quality, organic wines or in any case wines obtained from responsible agriculture, even if not certified.

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