How to combat white bloom on grape leaves: causes and treatment

The appearance of a white or gray coating on grapes indicates a fungal infection. Similar symptoms indicate diseases such as mildew and oidium. There are many causes of fungal growth on the stems, vines, and leaves of grapevines.

Most often, grapes get sick due to improper care.

Reasons for occurrence

The fungus is a microscopic organism spread by wind, precipitation, and weeds. It can survive for a long time in soil spores, maintaining a static state that prevents infection of fruit and berry crops. However, with the advent of favorable conditions, the fungal spores become active and begin to multiply rapidly. Neglect of grapevine pruning is the primary trigger for disease development. In dense plantings, where vines are tangled and lack sufficient light and fresh air, excessive humidity and stuffiness are often observed—favorable conditions for all types of fungi. Furthermore, the fungus can spread to grapevines through contaminated soil that is rarely disinfected and fertilized, or through weed debris that is not promptly removed.

It's also important to remember that weakened plants with weak immune systems are primarily susceptible to infection. This can be caused by a nutrient deficiency or, conversely, an excess. This is especially true for nitrogen—if there's too much of this element in the soil, the grapevine quickly becomes depressed.

Influence of weather conditions:

• mildew develops in warm and damp weather;
• Powdery mildew is not dependent on humidity, but it thrives on temperature. Temperatures between 5 and 30 degrees Celsius are necessary for infection development.
• gray mold is activated in conditions of low temperatures and high humidity;
• Alternaria thrives in hot and humid conditions.

Signs of disease

Don't assume that a white coating on grape leaves is just the beginning of the disease. In later stages, green leaves turn brown and eventually die completely. Once the fungus has consumed all the foliage, it will spread to the shoots and fruit. Ignoring the disease can lead to the death of the grapevine within 30 days. Therefore, white, oily, matte spots on the vines and leaf blades should immediately raise concern, as they indicate a rapidly developing infection that can lead to the death of the entire plant. A diagnosis can be confirmed by carefully inspecting all areas of the grapevine. Sporulation varies depending on the disease.

Oidium

Powdery mildew sporulation appears as fuzzy white spots that can appear not only on leaves but also on stems, flowers, and fruits. As the fungal spores age, the spots darken, and the infected areas become rough and dry. The leaf blade loses moisture and wrinkles, but does not fall off, remaining on the plant, continuing to infect still-healthy areas. If black spots begin to appear on the leaves, this indicates the imminent death of the plant. If the fungus spreads to the fruit, they quickly wilt, making the berries uneatable. Yields drop by 40-50%.

The disease's danger lies in its invulnerability to ambient temperatures. The fungus survives even at sub-zero temperatures, and can overwinter in grape plant tissues before reactivating in the spring.

Mildew

Downy mildew resembles powdery mildew only on the underside of the leaf—that's where the spores leave a powdery coating. On the underside, the spots, varying in size, have an oily texture and are a dirty yellow. Gradually, the affected areas merge into a single, large ulcer. Over time, the color changes to dark brown and even black. The leaf curls and falls off. Downy mildew also affects flowers and berries—they dry out completely, turning blue and then brown. The fungus first attacks the upper part of the vine, gradually moving downward, invading new areas. In most cases, by the time the disease is discovered, the bush is already so severely affected that it cannot be treated.

Alternaria

This fungal disease attacks every organ of the grapevine. Alternaria can be identified by the silvery tint of the leaves. Gray spots contain necrosis caused by the pathogens—the parasites Alternaria alternata and Alternaria tenussimum. They penetrate the tissues of weakened plants. Without chemical treatment, the leaves and stems become covered with light yellow spots, then rusty and brown. The shoots and leaves then wither. The fungus then spreads from the leaves to the trunk and berries. The berries shrivel and develop a metallic sheen. If hot, humid weather persists, the vine can die within five days.

If, during harvesting, several infected berries end up in a container with harvested bunches, they will infect all the fruit during storage, ultimately spoiling all the grapes.

Emergency help for plants

If you discover mycelial growth on your grape leaves, the first thing you should do is treat them with a potassium permanganate solution. Spray every two weeks, generously soaking all parts of the plant with the beneficial solution. Potassium permanganate won't completely cure the problem, but it will significantly slow down the spore reproduction.

Along with the potassium permanganate solution, specialized systemic medications are administered. Their active ingredients will penetrate plant fibers and begin combating the pathogen through direct contact.

If possible, a few days in advance you can prepare a solution of humus, which you can also use to treat the plantings if powdery mildew has been noticed.

Methods of combating plaque

When a white coating appears, regardless of the disease causing it, it is recommended to begin systematically spraying with sulfur-containing products. This is because sulfur absorbed by the fungi is converted into hydrogen sulfide, which is detrimental to the mycelium. The sooner treatment is carried out, the faster the infection can be stopped.

Sulfur-based products can be used as a preventative measure. To do this, prepare a solution of 40 grams of sulfur and 10 liters of water. At the first signs of trouble, increase the amount to 100 grams of sulfur per bucket of water. It's best to apply sulfur at temperatures above 20 degrees Celsius, otherwise it won't evaporate properly. However, at temperatures above 27 degrees Celsius, the sulfur solution can cause leaf burn. Apply the solution 3-4 times per season, at two-week intervals. Stop spraying when the berries begin to ripen.

Colloidal sulfur is considered one of the most effective inorganic fungicides. It effectively combats gray mold, powdery mildew, and mildew. It also inhibits spider mites. Colloidal sulfur vapor penetrates the fungus and destroys it from the inside, without harming the plant itself. For grapes, the application rate is 80 grams per 10 liters of water. The resulting concentrate is sufficient to irrigate 60 square meters of plantings. No more than five treatments are required per season.

If the white coating has just appeared, spraying the bushes with a 1% Bordeaux mixture solution will help stop sporulation. This solution is best used regularly, starting in early spring, as soon as the winter cover has been removed. If it's cool outside, colloidal sulfur is better.

If the grape vines are young enough and haven't grown too large, they can be treated with whey, which has proven effective against various fungi. Spraying should be done midday, as the milk protein is activated by sunlight. To combat powdery mildew, prepare a whey solution at a ratio of 1 part milk to 3 parts water. Water the vines generously, allowing the liquid to drip off the leaves. This natural fungicide is completely safe for plants, so this beneficial treatment can be repeated frequently, every 7-10 days.

Ready-made chemicals are primarily used against powdery mildew. The best ones include Acrobat, Skor, Topaz, Tilt, Topsin, Thiovit Jet, Hom, Oxyhom, and others. These products are typically applied twice, which is sufficient to suppress the spread of the disease.

Topaz, Ronilan, Topsin-M, and Rovral are designed to combat gray mold. These products have a strong chemical effect on the fungus, but often cause resistance in one or another pathogen. Therefore, these products should be alternated to prevent the fungus from acclimating to the chemicals. If flowers or fruits begin to rot, they should be treated with a product based on captan, folpet, or captafol.

If you're dealing with Alternaria wilt, copper-containing products are preferable for treating grapes. A copper-soda solution prepared from 50 grams of soda ash, 200 grams of grated soap, and 10 grams of copper sulfate diluted in water works well. Contact fungicides containing the active ingredient mancozeb (Mancodim, Ordan MC, Rapid Gold, Ridomil Gold, Ditan M-45) are also useful in the spring. If the fungus has penetrated the plants, systemic fungicides such as Skor, Quadris, Strobi, and Horus can help.

Preventive measures

The appearance of this disease-causing plaque can be prevented with preventative measures. For example, in early summer, spray with an ash solution. The solution is prepared from 1 kilogram of wood ash and a bucket of water. For better adhesion, grated soap is added to the mixture.

Iodine can help prevent powdery mildew and mildew. A solution is prepared by mixing 10 liters of water and 10 milliliters of iodine. Fitosporin-M is a popular preventative biological product. It is sprayed on plants throughout the season.

In addition to beneficial spraying, it is important to adhere to agricultural literacy:

• Destroy plant debris that harbors fungal spores. Remove all grass from the garden and burn it.
• do not plant crops that are more susceptible to fungal diseases close to grapes;
• When purchasing grape seedlings for planting, choose healthy and well-developed bushes;
• Perform sanitary pruning in early spring and fall. In summer, promptly remove any branches and shoots that are crowding the bush. Avoid overcrowding;
• Use sterile tools for pruning. Before moving on to the next bush, disinfect the pruning shears;
• water the grapes early in the morning so that the leaves have time to dry during the day;
• Feed your grapes properly – nitrogen is only needed during the first stage of growth. After that, the plant will only need phosphorus and potassium supplements.

Will the affected grapes survive?

Unfortunately, fungal diseases are difficult to treat. This is because they are most often noticed on grapevines when the infection has already almost completely taken hold. Such grapes cannot be cured, so don't even waste time on futile attempts. Remember that while you're trying fungicides, the fungus is already targeting healthy vines. Therefore, it's best to remove severely infected plants and focus all your attention on saving the remaining healthy vines.

The soil left empty after the grapevines are harvested is carefully cultivated. It is dug over, disinfected, and treated with complex mineral fertilizers. It is not recommended to plant any plants in this area for the next two years. This time will allow the soil to "rest" and rid itself of harmful microorganisms.

Conclusion

If sporulation outbreaks begin to appear throughout the grape vines, then not only general agricultural practices but also the use of strong fungicides are necessary. However, in some cases, even pesticides are ineffective in controlling the disease. The only remaining option is to remove the affected plants from the area.