Downy mildew on cucumbers can affect plants of all ages, but in many growing regions it does not appear until late in the season. Infection is mostly limited to the leaves, but the resulting reduction in photosynthetic area leads to plant stunting, reduced yields, and sunburn of fruits after defoliation.
What does downy mildew look like?
The first symptoms are small chlorotic spots on the upper surface 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 dieback 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 fungi-like organisms. The causative agent of downy mildew is amandatory Forsaken, which needs live cucumber plants to grow and survive. In most areas it does not overwinter on plant debris, but rather on live cucurbits in areas with mild winter climates or on plants grown in greenhouses. The pathogen is spread by wind-blown sporangia, which can travel up to 950 km in 48 hours via air currents.
Conditions for the development of the fungal disease
Downy mildew often kills plants late in the season and prefers warm, moist locations and dense foliage that blocks air and light. Moisture is the main factor that favors its spread. Howmost fungal diseasesDowny mildew on cucumbers also spreads most quickly after rain showers. Spread is hindered if the plants are spaced far apart and the 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 typically 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 periodswith leaf wetness, to avoid the disease by planting early in the season, to use disease-resistant varieties, and to use medicinal products. Since the disease does not overwinter in many growing regions but is introduced from southern areas, the occurrence of the infection is not influenced by crop rotation or post-harvest hygiene. Minimizing periods of leaf wetness can reduce the occurrence and severity of downy mildew. Increasing air circulation in plants by choosing locations with good air circulation, increasing plant spacing, or adding trellises can help leaves dry more quickly. Avoiding overhead watering can help keep leaves dry and reduce disease rates.
The use of disease resistance can control infection very effectively, and today most commercial varieties have some level of resistance to downy mildew. Even partial resistance can delay disease outbreak, slow spread within a field, result in less severe symptoms, and reduce yield losses. Varieties 'Dasher II', 'Indy' and 'Thunder' are all resistant to downy mildew, as well as other common cucumber diseases such as square leaf spot and powdery mildew.
Prevention of fungal disease
To avoid downy mildew: Plant cucumbers as early as possible. This disease is a greater threat to summer crops than to spring crops. Plants planted as early as possible after the last predicted frost may not be affected by fungal infections.
Home remedies for downy mildew on cucumbers: baking powder
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 the top and bottom of the leaves.
Use organic treatment
It is possible to use very effective organic treatments for downy mildew.
The Bordeaux mixture is an effective preventative treatment.
The ingredients to make 10 liters of spray are:
100 g of copper sulfate
350g hydrated lime
10 liters of water
- A wooden, earthenware or enameled bucket or other container that holds 10-12 liters is necessary as copper sulfate corrodes metal vessels. Measure 10 liters of water into the container.
- Then pour about a quarter into an enameled jug and stir in the 100 g of copper sulfate. While the copper sulfate dissolves, 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 lime milk in the bucket, stirring well every now and then. The characteristic sky blue color of the Bordeaux mixture is created and the spray is ready for immediate use.
Dip a piece of blue litmus paper, available at the pharmacy, into the mixture. It should stay blue; if it turns pink, more lime needs to be added. The Bordeaux blend should always be used on the day it is made. It is best to pour it into the spray machine through a fine gauze sieve or a piece of muslin to prevent the nozzle from clogging.
- Apply regularly at the beginning of spring.
- Treat throughout the growing season.
- Treat regularly, on average every two weeks and after every rainfall.
