2.4.1.a Downy Mildew

What are the symptoms of downy mildew on leaves?

Downy mildew (Plasmopara viticola) develops on all the herbaceous organs, particularly those that are actively growing (rich in water). Downy mildew causes "oil spots" that are often observed on young leaves, and are characterized by the appearance of discoloured, yellow, oily-like patches on the upper surface, followed by the formation of fairly dense white down on the underside consisting of conidiophores and conidia. The damaged tissue turns brown and dries out. The mosaic appearance of mildew is usually observed at the end of the season on older leaves. Downy mildew outbreaks are manifested by numerous small yellow to reddish-brown polyhedral spots, bordered by the veins.

Extract from Vigne Vin Occitanie

What are the symptoms of downy mildew on grapes?

• From inflorescence appearance until the end of flowering, the stems turn a brownish-red colour and develop a crook shape. The inflorescences can be totally ruined: they dry up and eventually drop off.

• The flower buds and young berries (up to the stage when berries are "the size of leadshot") are covered with white efflorescences (conidiophores), this is "grey rot" appearance.

• After fruit set, the berries turn reddish-brown to purplish, this is the "brown rot" appearance, or "berry depression".

• After véraison, the berries are no longer susceptible.

Extract from Vigne Vin Occitanie

Can these symptoms be confused with another disease?

Confusion between downy mildew and powdery mildew is possible. The "oil spots" appearance of downy mildew often observed on young leaves can also be observed on leaves affected by powdery mildew, at the beginning of the attack. These are the very first manifestations of both these diseases, which, as they evolve, have different appearances and are easier to distinguish from each other (see Oidium or powdery mildew ). Apart from this similarity, downy mildew is relatively easy to identify and no other disease has similar symptoms.

What is the biology of the fungus associated with downy mildew?

Vine downy mildew is preserved in the form of oospores present on the leaves attacked in the autumn that have fallen to the ground. These winter eggs are produced by sexual reproduction. In spring, after maturation, the eggs germinate in water at an average temperature of 11°C and release biflagellate zoospores that can move in water and cause primary contamination. After an incubation period ranging from 10 to 20 days depending on the temperature, conidiophores (fruiting bodies containing conidia) appear on the undersides of the leaves. The conidia ensure secondary contamination or transplantations when it rains. The incubation phase (the period between contamination and appearance of symptoms) is directly linked to temperature, and can be just five days in the summer. During the 2020 vintage, mildew outbreaks appeared on bunches more than a week after contamination was observed on leaves.

Extract from Vigne Vin Occitanie

What are the practices that favour downy mildew developement? What prophylactic measures should be implemented?

Several practices can promote an appearance of downy mildew and/or its development on leaves as well as on bunches. Certain prophylactic measures exist to limit the development of the fungus:

• Concerning canopy management and maintenance: if green operations (trimming, leaf thinning, removal of secondary growth, removal of lateral shoots) are not carried out in an optimal fashion, leaf density will likely be higher and the ventilation of the vines may not be adequate. This creates favourable conditions for mildew to develop within the canopy. Prophylactic solutions: The removal of certain leaves, lateral growth and crowded bunches reduces the growth of foliage, improves ventilation in the microclimate and reduces the risk of fungal contamination. The same goes for the removal of suckers on the lower part of the vine.

• Load management: it is possible that excessively crowded vegetation and/or bunches of grapes can favour the appearance of downy mildew: a heavy load does not allow as much ventilation as a moderate load and can create a climate favourable for downy mildew. Load distribution conditions can also favour the development of Plasmopara viticola, when the bunches are crowded, the air circulates poorly and the microclimate can be more humid and less open. Prophylactic solutions: the fruit load is determined during winter pruning and possibly during trimming and thinning operations later in the season, which remain corrective practices for the load.

• Grass management: grassing under the rows and between the rows maintains a humid microclimate. The evaporation of this humidity and the transpiration of the plant cover, in addition to leading to lower temperatures and greater risks of spring frosts, can encourage mildew development. Furthermore, a lack of grass control may result in the plant cover contaminating the bunches and leaves closest to the ground. Prophylactic solutions: when total or partial grass cover is implemented, it is important to limit its height in order to avoid any risk of contamination of the grapes and leaves by the plant cover. Grass cover can be useful to create competition with the vines for the water supply, moderate water stress then reduces the vines' vigour and vegetative expression.

• Choice of variety according to the plot's characteristics: some grape varieties are more sensitive than others to downy mildew, such as Grenache or Cabernet Sauvignon. Some grape varieties are also more vigorous than others and their planting in humid, poorly protected areas can favour downy mildew development on the vines. This is notably the case at Château Latour where the Merlot is more sensitive to mildew than the Cabernet Sauvignon. Prophylactic solutions: adapt the variety to the soil characteristics and the climatic conditions (choose a less vigorous grape variety and/or rootstock on plots with high humidity).

• Drainage management: Some poorly-drained plots can retain water on the surface for several days and create a humid climate that is very favourable to fungus growth. Prophylactic solutions: Drainage before planting plots that are prone to water retention and stagnation an can reduce the risk of downy mildew outbreaks in the vineyard.

• Phytosanitary treatment management: downy mildew is a fungus that develops easily when the climate is hot and humid. During summer conditions, the absence of treatment before a period of rain can lead to significant contamination by mildew. Prophylactic solutions: follow the weather forecast in order to adapt the treatment periods and their effectiveness as best as possible. Forecast bulletins are also available from experts.

What measures exist to prevent downy mildew in organic agriculture?

Prevention is the best way to manage downy mildew. There are no systemic products, only contact products, which are quite susceptible to being washed off, and therefore must be regularly renewed after an accumulation of 20 to 25 mm of rain. The sole means of chemical control of downy mildew in organic agriculture is with copper salts. These are copper fungicides whose main formulations are:

• Copper sulphate (20% copper metal, neutralized with slaked lime): like Bordeaux mixture, it can be used throughout the growth cycle, it is very effective against mosaic mildew, not very phytotoxic, and has a very good efficiency/wash-off ratio.

• Copper hydroxide (50% copper metal, mixture of copper sulphate and soda): it can be used at the beginning of the season, either in the case of light rainfall, or when there is a high cryptogamic threat. Copper hydroxide can be used within 8 hours of rainfall in cases where the the vine was unprotected. It is not considered to be very phytotoxic but is more susceptible to wash-off than copper sulphate.

• Copper oxide: (up to 75% copper metal, oxidation of the CU2+ ion) is generally used during the summer when significant rainfall is expected, it is advisable to avoid its use around flowering, it is very phytotoxic.

• Copper oxychloride: (50% copper metal, a mixture of copper and hydrochloric acid), seldom used.

What parameters determine the first treatment?

The first treatment does not seem to be linked to a precise phenological stage but more to winter and spring weather conditions as well as to the detection of primary outbreaks. Two treatments are particularly critical in the fight against downy mildew:

• The one that prevents primary contamination. If the risk of contamination is high at the end of winter, the first treatment must be applied relatively early before any primary outbreaks can occur. Conversely, if the risk of primary contamination is low at the end of winter, treatments can be started later.

• And the treatment preventing secondary contaminations (cycles with n repetitions) or transplanting. This protection is less dependent on weather conditions. When primary outbreaks exist, the protection must be carried out constantly, until the end of the treatment campaign.

In all cases, it is important to avoid re-infection, i.e. secondary contamination. The risk of contamination and the first treatment's timing are determined by the weather conditions (rainfall and temperatures at the end of winter and at the beginning of spring) but also according to the ease of access to the plot and its intrinsic characteristics (flooding, vigour, etc.). Once protection has started, treatments should continue before any rain.

There are also predictive models provided by institutes, organizations and/or agricultural cooperatives that are useful tools for decision-making when implementing the first treatments.

What parameters determine treatment renewal?

Mildew protection is preventive, which means that the products used are contact products, which greatly increases the risk of wash-off of sprayed products. To ensure optimal coverage, the protection must be renewed regularly depending on rainfall. It appears that the wash-off of the various products depends more on the rainfall than on the product formulation itself. 30 to 40% of copper is washed off after 20 mm of rain. It is therefore recommended to renew the treatment after 20 to 25 mm of rain and to favour regular applications using low doses rather than heavy applications which would be washed off anyway and require another application.

Rainfall is the main factor that determines a renewal, but there are others: when the product's effective period ends (about ten days on average but this varies depending on the commercial product and its formulation), or when new unprotected herbaceous organs have grown.

The absence of re-treatment completely negates the effectiveness of the initial treatment.

Are there alternative methods to copper?

There are alternative methods used to replace copper or as a complement (with the objective of reducing doses) to fight against fungal contaminations, especially downy mildew. The first method concerns the use of preparations or natural extracts with biocide effects, for example, sweet orange essential oil, which is authorized in organic agriculture.

Another method consists of the use of certain natural defence stimulators (NDS) of plants authorized in organic agriculture (See section on Non-chemical alternatives for pest and disease control ).

Finally, the use of plant extracts in herbal teas and/or decoctions, when combined with treatments, enables dosages to be reduced and combats cryptogamic diseases by stimulating the plant's defences. Horsetail decoctions or teas, rich in silica, borage bark decoctions rich in anthraquinones or willow (Salix) teas rich in salicylic acid can be used to fight against downy mildew and/or powdery mildew thanks to their active principle (See section on Non-chemical alternatives for pest and disease control ).