Downy mildew of cucumber: causes and treatment

Regardless of growing conditions and care, cucumbers sometimes suffer from various diseases. Downy mildew accounts for more than half of all cases. It was first encountered in the United States in the 18th century and later spread throughout the world. For a long time, no cure was discovered, but modern technologies have reduced the incidence of the disease and contributed to its prevention.

Description of the disease

Downy mildew, or peronosporosis, on cucumbers is a disease that attacks foliage, stems, and eventually the fruit. Once the disease has infected one plant, it spreads very quickly to other plants, which is one of the main dangers.

The infection persists even on fallen leaves in the soil and can affect new cucumber plants within five years if timely disinfection is not carried out. Disease control measures must be strict and timely enough to prevent the disease from spreading and infecting other plants.

Cucumbers most often suffer from late blight in early June, but at this time, the disease's progression is not yet as noticeable. The peak occurs in July, when the first harvest arrives. However, the disease often progresses to such a level that saving the vegetable is impossible. Downy mildew on cucumbers is caused by various types of fungi:

• pseudofungi-oomycetes;
• other fungi from the family Peronosporaceae.

Cucumber downy mildew pathogens can survive low temperatures, which increases the incidence of the disease the following year.

Reasons

Among cucumbers, downy mildew spreads mainly due to the influence of external factors:

1. Significant temperature fluctuations. If the difference between night and day is more than 12 degrees, condensation forms in the soil—a breeding ground for bacteria and viruses.
2. Daily cold dew in the mornings and overwatering. Excess moisture is a perfect breeding ground for viruses, so it's important to monitor this.
3. Proximity between different crops. Treating downy mildew is pointless if neighboring plants have already contracted the virus. It is recommended to plant different varieties and species of crops at least 30 cm apart.
4. Regular watering with cold water. Cucumbers are heat-loving plants. Cold water can be used for watering once a week. It stimulates the cucumber's immune system but creates a favorable environment for fungus.
5. Poor ventilation of the greenhouse or other place where cucumbers are grown.
6. Aphid and other pest infestations. Many insects and parasites transmit viruses and bacteria. Treatment for downy mildew will likely be ineffective unless the underlying problem is addressed.
7. Weather. Rainy, cold, and cloudy summers increase the risk of fruit suffering from downy mildew.

The occurrence of one or several causes of downy mildew development requires the organization of preventive measures to protect the remaining crop and cure the affected one.

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Signs

Downy mildew develops slowly. The incubation period for greenhouse cucumbers lasts up to two weeks. During this period, the disease is already developing, but there are no external signs, so treatments are not yet available. If downy mildew control products are not applied promptly, the disease can reach its peak and destroy the entire crop.

1. Yellow-green spots appear on the leaves and stems, differing in color from the main mass of the planting.
2. A black or grey coating has appeared under the leaf.
3. If you look at the grey coating through a magnifying glass, you can see small black spots with maturing pores.
4. One or more leaves on the bush lose shape, become deformed and dry out.
5. The spots on the stems are increasing at a rapid rate.
6. The leaves are falling.
7. The flowers turn from yellow to brown, then black, and later fall off.
8. The fruits grow tasteless.

Therefore, if cucumbers have acquired a strange taste or have completely lost it, or the color of the pulp has changed (turned pale), this indicates the presence of various diseases in the vegetative or root system of the bush.

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Treatment of the disease

There's no guaranteed cure for downy mildew, but don't give up on your plant right away. Cucumber downy mildew treatment is difficult, but doable.

Regardless of the chosen treatment method—folk remedies or special preparations—the first step is to destroy the damaged foliage. Since the disease has an incubation period, it doesn't manifest itself immediately. Even if some leaves are already affected, while others aren't, that doesn't mean those areas are undamaged. It's possible that within a few days, the disease will appear on other parts of the plant. Therefore, it's important to inspect the plant daily for new damage.

After the damage has been removed, the entire plant is treated with fungicides:

1. This is a chemical or biological substance that suppresses the development and appearance of fungi throughout the entire vegetative and root system of a plant.
2. Biological fungicides include those that decompose under the influence of living organisms and water.
3. Chemical types include compounds with high content of copper, sulfur, mercury, iron, manganese, and potassium.

The advantages of organic compounds are that they have no impact on the environment, are safe for humans and animals, and are easy to prepare and use. The disadvantage of organic fungicides is their short-term effectiveness. Natural fungicides remain active in the soil for up to two weeks, which is very short, considering the disease's incubation period is exactly the same.

Incorrect use can be dangerous to human health. Therefore, to ensure proper use, we recommend carefully reading the instructions and watching the video and photo tutorials.

The most effective chemical fungicides:

• Planriz;
• Acrobat MC;
• Efal;
• Ridomil Gold;
• Strobe;
• Gliocladin;
• Quadris.

Chemical agents are effective at all stages of the disease, but do not guarantee complete recovery of the plant.

Biological agents

Biological products are specialized substances produced using living organisms to combat downy mildew. They are used during two main stages of plant development: pre-sowing seed soaking and plant spraying during the growing season. The following are most effective during pre-sowing soaking:

• Fitosporin-M;
• Alirin-B;
• Gamair.

Directions for use:

1. Fitosporin-M: apply one and a half liters of water and 10 grams of the product to the seeds and leave in the shade for several hours.
2. Alirin-B and Gamair: dilute 50 grams of each in 10 liters of water. Spray the seeds with the mixture and leave in the shade for two hours.

For spraying plants during the growing season:

1. Vitaplan. Apply to the entire plant once every three weeks. 30 grams per 10 liters of water.
2. Fitosporin-M and Gamair. Apply to the entire plant once a month using a spray bottle. Apply 20 grams per 10 liters of water.

Proper use of biological medications sometimes leads to complete relief from the disease, but more often, it only alleviates some of the symptoms. Therefore, repeated treatment is necessary.

Folk remedies

Folk remedies are only suitable for situations where the disease has not yet reached its advanced stages, but is still in its infancy. They are environmentally friendly and therefore easily tolerated by both humans and animals.

Popular folk remedies:

1. Baking soda. For treatment, make a baking soda solution. Dissolve 20 grams of baking soda and 7 ml of liquid soap in 2.5 liters of water. Mix thoroughly until smooth. Spray the tops of the plants every week. Baking soda disinfects the cucumbers and normalizes their condition.
2. Milk. Pour several liters of milk into a large bucket of water and leave it in the sun for several hours. Pour the resulting mixture under the roots. Reapply every two weeks. The advantage of whey as a remedy for downy mildew is that it's not only an effective treatment for existing downy mildew, but also a good preventative measure.
3. Onion peels. This safe remedy is especially effective in the early stages of the disease. Dissolve half a kilogram of onion peels in 10 liters of water. Red sweet onion peels are best, but regular onion peels will also work. Let the resulting infusion steep for several days. Spray the entire surface of the cucumber plant once a week.

Prevention

Preventing downy mildew is easier than treating it. It's recommended to plant varieties that are already immune to viral diseases. Immediately after cucumber germination, treat the crops with special fungicides. These leave an invisible coating on the leaves and stems, preventing fungal growth.

You can use silk, a plant growth regulator, to reduce the risk of damage.

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Some cucumber varieties easily cope with downy mildew thanks to their inherent genetic immunity. This immunity often emerges during breeding. These varieties include:

1. Murashka. A genetically strong plant, resistant to downy mildew, black rot, and root rot.
2. Darling.
3. Octopus. A hybrid suitable for open soil and easily adaptable to downy mildew.

However, it's important to note that no variety is 100% disease-resistant. The best thing a gardener can do is ensure timely prevention and care if an infestation occurs.