Downy mildew on cucumbers can affect plants of any age, but occurs late in the season in many growing regions. Infection is usually confined to the leaves, but the resulting reduction in photosynthetic area causes plant stunting, yield loss, and sunburn of fruit after defoliation.
Table of Contents
- What does downy mildew look like
- What causes the fungal infection
- Conditions for the development of the fungal disease
- When does the infection occur
- What can help against downy mildew on cucumbers
What does downy mildew look like
The first symptoms are small chlorotic spots on the top of the leaves, which often develop first on the older leaves. The lesions expand and take on an angular shape as they are bordered by the main leaf veins. The infected tissue eventually becomes necrotic (dead), resulting in leaf death and defoliation. Gray to purple mold develops on the undersides of the leaves.
What causes the fungal infection
How does powdery mildew develop on cucumbers? Downy mildew on cucumbers is caused by Pseudoperonospora cubensis, a member of the water mold (Oomycota) group of fungus-like organisms. The causal agent of downy mildew is an obligate parasite , which requires live cucumber plants to grow and survive. In most areas, it does not overwinter on plant debris, but on live cucurbits in areas with mild winter climates or on plants growing in greenhouses. The pathogen is spread by windblown sporangia that can travel up to 950 km in 48 hours by air currents.
Conditions for the development of the fungal disease
Downy mildew often kills plants late in the season and prefers warm, moist sites and dense foliage that keeps out air and light. Moisture is the main factor that promotes its spread. Like most fungal diseases, downy mildew on cucumbers spreads most rapidly after rain showers. Spread is hindered when plants are widely spaced and air circulates well.
When does the infection occur
Infection and disease development occur most rapidly during periods of moderate temperatures (15-20° C) and cool nights. Disease development is slowed by high temperatures (>35° C), while periods of fog, rain, and frequent dew favor infection. Symptoms usually appear three to twelve days after infection, depending on temperature and humidity, with new sporangia forming as early as four days.
What can help against downy mildew on cucumbers.
You can control downy mildew on cucumbers by following some simple strategies. Adequate control of downy mildew requires an integrated approach to reducing periods of leaf wetness , avoiding the disease by planting early in the season, using disease-resistant varieties, and applying remedies. Because the disease does not overwinter in many growing regions but is introduced from areas to the south, the incidence of infection is not affected by crop rotation or postharvest sanitation. Minimizing periods of leaf wetness can reduce the incidence and severity of downy mildew. Increasing air circulation in plants by selecting sites with good air circulation, increasing plant spacing, or adding trellises can help leaves dry more quickly. Avoiding overhead irrigation can help keep leaves dry and reduce disease rates.
The use of disease resistance can be very effective in controlling infection, and today most commercial varieties have some level of downy mildew resistance. Even partial resistance can delay disease onset, slow spread within a field, result in less severe symptoms, and reduce yield losses. The cultivars ‘Dasher II’, ‘Indy’ and ‘Thunder’ are all resistant to downy mildew, as well as other common cucumber diseases such as angular leaf spot and powdery mildew.
Prevention of the fungal disease
To prevent downy mildew: plant cucumbers as early as possible. This disease is a greater threat to summer crops than spring crops. Plants planted as early as possible after the last predicted frost may not be affected with fungal infections.
Home remedies for downy mildew on cucumbers: Baking soda
Baking soda is an excellent remedy for fungal diseases.
Dissolve 1 tablespoon of baking soda in 1 liter of water (preferably rain or mineral water to soften the water). In dry weather, spray this mixture on all leaves, both top and bottom.
Use organic treatment
It is possible to use very effective organic treatments against downy mildew.
Bordeaux mixture is an effective preventive treatment.
The ingredients for the preparation of 10 liters of spray are:
100 g of copper sulfate
350 g hydrated lime
10 L of water
- A wooden, earthen or enameled bucket or other container that holds 10-12 liters is necessary because copper sulfate corrodes metal vessels. Measure 10 liters of water into the container.
- Then pour off about a quarter into an enameled pitcher and stir in the 100 grams of copper sulfate. While the copper sulfate is dissolving, shake the 350 g of hydrated lime into the water remaining in the container and stir well.
- When the copper sulfate has completely dissolved, slowly pour the blue solution from the jug into the milk of lime in the bucket, stirring well each time. The characteristic sky-blue color of the Bordeaux mixture will appear, and the spray will be ready to use immediately.
Dip a piece of blue litmus paper, available at the drugstore, into the mixture. It should remain blue; if it turns pink, more lime must be added. The Bordeaux mixture should always be used on the day it is made. It is best poured into the sprayer through a fine gauze strainer or a piece of muslin to prevent clogging of the nozzle.
- Apply regularly at the beginning of spring.
- Treat throughout the growing season.
- Treat regularly, on average every two weeks and after each rainfall.