The Three Major Diseases of Wine Grapes | Downy Mildew: Causes and Control Measures

There is one disease that grape growers particularly fear during rainy years: downy mildew.

Whether dealing with wine grapes or any other cultivated plants, the battle against diseases is unavoidable. Vineyards encounter various diseases, but the most prominent among them are downy mildew, powdery mildew, and gray mold (Botrytis).

These diseases are not particularly exotic ailments. While they may not occur every year, they appear every few years like influenza—common diseases that growers know well.

However, when symptoms worsen, the damage can be devastating. In some years, harvests can be completely lost. When downy mildew spread throughout Germany in 2016, severely affected vineyards had already lost all hope of harvest by summer. Some wineries, resigned to their fate, went on extended vacation well before the autumn harvest season.

It wasn't that these growers had failed to take preventive measures. Rather, the relentless rain had rendered all their efforts futile.

A common disease, yet one capable of causing catastrophic damage—this is downy mildew. Let us examine this disease in detail.

Understanding Downy Mildew

Downy mildew is called "downy mildew" in English and "Falscher Mehltau" in German. It is a highly destructive fungal disease.

This disease affects not only grapes but also various vegetables, ornamental plants, and fruit trees. It is caused by fungi belonging to the Peronosporaceae family, with wine grapes specifically infected by Plasmopara viticola, a fungus native to North America.

Instead of using the disease names "downy mildew" or "Falscher Mehltau," it is often referred to by its pathogen name, "Peronospora."

Since its introduction to Europe in 1878, it has been recognized as one of the most dangerous grape diseases. European grape varieties (Vitis vinifera) used primarily for winemaking lack resistance to this disease. Except for resistant varieties known as "Piwi," all varieties face infection risk, though to varying degrees.

Symptoms of Downy Mildew

The most characteristic symptom of this disease is the appearance of yellowish, oil-stain-like spots on leaf surfaces.

Infected areas gradually dry out from the center and begin to turn brown and wither. At the boundary between dead areas and healthy uninfected tissue, ring-shaped oil-stain-like infected zones remain, serving as launch points for continued spore dispersal. When symptoms become severe, entire trees may defoliate. Since leaves suffer significant damage, photosynthesis becomes insufficient, leading to secondary damage.

While downy mildew symptoms are visible on leaf surfaces, the actual infection site is on the undersides of leaves.

Downy mildew preferentially infects young leaves with soft cell walls, and damage spreads more easily in such tissue. However, the disease also infects shoots, flower clusters, and fruit. Infected grape clusters become purple, desiccated, mummy-like structures. When infection becomes severe, it leads to serious yield reductions.

In recent years, major outbreaks of this disease have caused harvest yields to drop to one-third or less of normal levels, making it absolutely essential to implement control measures in wine grape cultivation.

Ecology and Infection Mechanisms of the Pathogen Peronospora

The ecology of Peronospora, the causative agent of downy mildew, is complex.

While most organisms typically have one method of reproduction, Peronospora is known to have two different reproductive strategies. This makes the infection mechanisms of downy mildew more complicated.

Downy mildew infection mechanisms include primary and secondary infections, each related to rainfall, temperature, humidity, and grape growth conditions. The emphasis on preventing rain splash in disease management is primarily aimed at preventing primary infection.

The optimal activity temperature for Peronospora is in the low 20s Celsius, and spores cannot germinate above 30°C. Therefore, infection risks are highest from early spring through early summer and in autumn, while conditions typically calm down during summer. Even when infection occurs before summer, the situation may stabilize if hot, dry conditions persist afterward.

Downy Mildew Control Measures

Prevention is the most important aspect of downy mildew control.

Once Peronospora infects grapevines, it penetrates into plant tissues and spreads infection from within. Because infection spread occurs inside the grape plant, control becomes extremely difficult.

Preventive measures include physical methods such as preventing rain splash, and chemical control through fungicide applications. Physical prevention includes removing infected grape leaves and shoots from vineyards to eliminate the pathogen, improving canopy ventilation, and preventing weak, excessive growth.

Downy Mildew Control Agent: Copper

The most effective agent against downy mildew, both historically and currently, remains copper.

Bordeaux mixture, commonly heard about in grape cultivation settings, represents the quintessential copper-containing fungicide used against downy mildew. Bordeaux mixture is a solution combining copper sulfate and slaked lime.

However, the use of Bordeaux mixture has been declining recently. Some countries and regions have restricted its use. This is because the slaked lime contained in this fungicide accumulates in soil, raising soil pH and calcium concentrations, which has become a concern.

Consequently, the use of other copper-containing agents has increased, but copper itself is a heavy metal, and its accumulation in soil raises environmental concerns. Against this background, many inorganic agents, organic agents, and biological control methods that do not contain copper have been developed and commercialized in recent years. However, nothing has yet surpassed copper's effectiveness, and even wineries certified by organic organizations within the EU are permitted to apply up to 3 kg/ha of copper annually.

Control Agents Other Than Copper

In Germany, the use of potassium bicarbonate, known as one type of bicarbonate or carbonate (general term for salts formed by replacing one of the two hydrogen atoms in carbonic acid with metals), was once tested for downy mildew control. However, this agent showed no effectiveness against downy mildew and is currently used as a control agent for powdery mildew.

Additionally, some bicarbonates are prohibited under EU ecological regulations, and these agents are known to have high phytotoxicity. When used on stressed or weak vines, crystalline formations on leaf surfaces can "burn" and partially kill leaves.

It would be counterproductive to kill leaves with agents meant to prevent downy mildew from killing leaves. Moreover, this symptom occurs on "leaves" themselves regardless of the presence or absence of target fungi, making it a side effect unrelated to control efficacy. Furthermore, since bicarbonates involve applying alkaline metal components, they do not contribute to reducing environmental impact, leading many wineries to exercise caution in their use.

Recently, phosphonate-based agents have gained attention for showing superior effectiveness to copper depending on application timing.

The Increasingly Difficult Battle Against Downy Mildew

In recent years, Europe has experienced rising average winter temperatures, leading to more frequent cases where Peronospora germination conditions are met earlier than traditionally expected. Moreover, due to climate change effects, daily average temperatures are rising, and thunderstorm-like rains and increased humidity are creating more hot, humid nights. These conditions significantly elevate downy mildew infection and spread risks.

Downy mildew control is fundamentally centered on preventive measures, requiring advance preparation. However, changes in rainfall timing and amounts make such responses difficult. Rain not only disrupts control schedules, but even when fungicides are finally applied during brief breaks in rainfall, sudden downpours can immediately wash away all applications.

While copper use is effective for downy mildew control, usage amounts are strictly regulated. When rain washes away applications, re-spraying becomes necessary, but this becomes impossible once copper usage limits are reached.

These conditions work particularly against wineries with organic certification, as their copper usage limits are set especially low.

Faced with particularly severe downy mildew outbreaks, some wineries have surrendered their organic certification to apply additional copper treatments. The battle against such diseases is becoming increasingly difficult year by year.