Grapevine downy mildew: when and how to deal with it

Grapevine downy mildew is one of the most dangerous diseases affecting vineyards. Caused by the fungus Plasmopara viticola, it is native to North America and was accidentally introduced to Europe in the 1870s through imported American vines, which are resistant to P. viticola; by contrast, European grapevines have proven to be very susceptible to the disease.

This grapevine disease causes significant damage, including reduced yields, lower grape quality and weakened plants, leading to knock-on effects in subsequent years. It is still a significant problem, and 2023 was a critical year for Italian vineyards due to its impact. Today’s blog article is all about grapevine downy mildew: what it is, how it develops and how to prevent it spreading to your vineyard.

Symptoms of grapevine downy mildew

What are the symptoms of grapevine downy mildew? The disease attacks all green parts of the plant. Here are the tell-tale signs by which you can recognise it:

• On young leaves, translucent yellow spots called oil spots appear, followed by a characteristic whitish mould that forms on the underside of leaves.

• The grape clusters darken, become covered in mould and then shrivel.

• The most developed bunches turn brown and then dry out, without visible mould growth: in this case we are talking about larval downy mildew, which is typical of late attacks, i.e. in summer.

• On older leaves, usually at the end of the season, small polygonal spots form, confined by the leaf veins (mosaic downy mildew).

• The vine shoots turn brown and dry out, and can also become distorted or develop cracks, from which mould grows.

Plasmopara viticola: biology

At the end of summer, as leaf-fall approaches and the vines no longer provide suitable tissue for colonisation, P. viticola produces oospores (fertilised cells) from the infected parts of the plants in order to survive. The oospores allow the fungus to overwinter on fallen leaves on the ground. After their maturation period ends in late November/early December, they are ready to germinate (i.e. awaken) in spring, as soon as they are exposed to water and temperatures above 9-10°C.

Once this happens, the oospores release spores which are then spread by heavy rain and wind, infecting vine tissues (primary infections) through splashes of water. The fungus penetrates the tissues and invades them, extracting nutrients from the plant. After an incubation period, it produces sporangia (structures wherein spores are formed), which appear on the vine in the form of white mould. The germination of oospores lasts for several months, resulting in multiple waves of primary infections.

These primary infections, in turn, generate cycles of secondary infections. The white mould is actually composed of the structures within which P. viticola reproduces, generating further spores that are then dispersed by wind and rain to infect other tissues, leading to secondary infection. When the climatic conditions become unfavourable, the fungus produces oospores to survive the winter, in anticipation of initiating new infections the following spring.

The severity of a grapevine downy mildew epidemic depends not only on weather conditions favourable for P. viticola – mild temperatures and frequent rainfall in spring/summer – but also on the number and distribution of primary infections in the vineyard.

How to prevent grapevine downy mildew

The development of P. viticola is influenced by the prolonged presence of moisture on the leaves, including that from rain, fog and dew. A vineyard is more prone to downy mildew if it is situated in a humid area: that is why, before planting it, the environment should be carefully evaluated (position, exposure, presence of shallow aquifers, etc.). Even the choice of vine and rootstock is crucial because some grape varieties are more susceptible to the disease than others. It is equally important at this stage to decide on the planting distance and the training system. On the same topic, you can also learn more about how to choose a grape variety and set up the vineyard.

Moving on to the management of the vineyard, we recommend that you:

• Fertilise in a balanced manner.

• Avoid overwatering and ensure good soil drainage to prevent waterlogging.

• Prune regularly (summer and winter pruning).

• Keep control of grass growth under and between vineyard rows, for example with a brushcutter, flail mower or garden tractor.

In practice, you must do your best to prevent the vegetation from crowding itself and humidity from stagnating in the vine canopy, so as to avoid creating a habitat favourable to the initiation and cyclical progression of infections.

In the vineyard, grass cover-cropping is the practice of growing vines alongside herbaceous species, which is beneficial for biodiversity, among other things. On that subject, you can find out the specific benefits of grass cover-cropping and how to tidy the vineyard using a brushcutter. As for biodiversity, here you can discover which plants grow well with vines.

Protecting the vineyard from downy mildew

As we have said, downy mildew infections of the grapevine are linked to weather conditions. A rule of thumb that is still considered fairly effective is the rule of “three tens”, which is a set of conditions that triggers the primary infection cycle, and which you can use to predict when to initiate protective countermeasures. These three conditions, which here in Italy generally occur in March/April, are: vine shoots that are at least 10 cm long, a minimum of 10 mm of rainfall within a 24-48 hour period, and a minimum temperature of 10°C. Bear in mind, though, that infections also occur in different conditions, for example with less rainfall.

So to protect your vineyard from the fungus, it is essential to closely monitor temperature and rainfall trends, and observe the presence of symptoms among the plants. After the first outbreaks of primary infection, the incubation period of P. viticola – and therefore the progression of the disease, including the onset of secondary infections – varies depending on average temperature and relative humidity. The rule of three tens, coupled with incubation period tables, provides a useful framework for timing downy mildew treatments. We also recommend that you follow online agrometeorological bulletins, which are issued by regional authorities: they provide an overview of the main diseases in the area, as well as indications on when to carry out treatments and with which pesticides.

The selection of fungicides for downy mildew depends on both the phenological stage (growth stage) of the vines and weather conditions. These include the stages:

• from vegetative recovery to pre-flowering

• from pre-flowering to fruit set (transformation of flowers into fruit), which is a particularly delicate period because the plants are actively developing and therefore vulnerable to disease

• from fruit set to grape harvest, when downy mildew is less dangerous because it can no longer attack the grapes

In the first stage, take action in advance of potentially infectious rain events or immediately afterwards, before the incubation period of P. viticola ends. Use traditional contact fungicides, which are broad-spectrum substances that protect surface vegetation without being absorbed by the plant tissues: they can be copper-based or contain other active ingredients.

In the second stage, we recommend using cytotropic or systemic products, which should be mixed with contact products. Cytotropic fungicides are absorbed by the plants, so offer longer-lasting protection. Systemic fungicides enter the plant’s lymph flow, thus also protecting new growth. In the final stage, however, it is best to only use copper-based contact products. To apply downy mildew treatments in all stages you can use a backpack mistblower.

If you prefer not to resort to synthetic products, copper is the only active ingredient you can rely on. Adjust the concentration based on the vines’ phenological stage and repeat the treatments at intervals that maintain continuous coverage, considering both weather conditions and the growth rate of the plants. Like other fungicides, copper is prone to being washed off by rain, but it has nevertheless been a mainstay in the fight against downy mildew since the late 19th century.

Just be aware that excess copper in the soil, where it tends to accumulate, can lead to both nutritional imbalances and toxicity in plants. You can therefore bolster grapevines' defences against diseases using natural substances like resistance inducers, as well as extracts with fungicidal properties, such as sweet orange essential oil. If you only use copper-based fungicides against downy mildew, the good prevention practices we mentioned above will be even more instrumental.

Defending against parasites and vine diseases is just one of the many activities to be carried out in the vineyard: to finish off, we suggest you check out this quick review of the month-by-month jobs to be done in the vineyard.