What are these spots on my cucumbers? Is there anything that I should do to remedy the situation?
— Monica Jefferson, Highwood
Cucumber vines grow fast, do not need a lot of care and thrive in hot weather with adequate water. Give them full sun and plenty of space to grow, and train them on a trellis or fence. Cucumbers perform best with regular, deep watering once a week or so and with more frequency if the weather is very hot for a period of days. Inadequate or inconsistent moisture can cause oddly shaped or poor-tasting fruit. If possible, water your cucumbers with a soaker hose or with drip irrigation to keep the foliage dry. This helps prevent leaf diseases that can affect the plant. Shallow watering or watering less frequently will tend to reduce fruit yields. Apply a mulch to help conserve soil moisture and keep weeds down. Plant a bush-type cucumber if your garden space is limited.
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It is difficult to say exactly which fungal leaf spot you have, but it is likely alternaria leaf blight caused by the fungus Alternaria cucumerina. In general, fungal leaf spots are promoted by moisture (rain and overhead watering) and humidity. Your problem appears to be minor at this point, but continue to monitor the plants.
Alternaria leaf blight often infects the mature leaves near the crown of the plant first. The spots start small — brown, often with a yellow halo — and grow into irregular shapes up to three-quarters of an inch in diameter. The spots will occasionally develop a targetlike pattern of rings.
If the infection continues to develop and become more severe, infected leaves will turn brown, curl upward, wither and die. This leaf spot does not infect the fruit but can reduce yield and quality some with reduced plant vigor. Some of the fruit may suffer sunscald due to leaf loss.
Alternaria leaf blight can be carried long distances on wind currents and can be spread within your garden by splashing water, so try to water only at the base of the plants. Wet, rainy weather favors this disease, and the damage can be severe with extended warm, wet conditions. Generally this disease is not a big concern. At this point, fungicide applications are not warranted.
This fungus, like many other diseases, overwinters in plant debris, so it is important to remove these plants from the garden at the end of the season. Most home compost piles do not get hot enough to kill disease organisms, so it is best to dispose of these plants off-site.
You should also rotate locations for your cucumbers, so that two to three years go by before planting any more cucumbers or other members of the squash family in that same location.
Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe.
If you have plant or gardening questions, contact the Chicago Botanic Garden’s Plant Information Service at 847-835-0972.
ctc-realestate@tribpub.com
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