Cucurbits are green-leaved plants that need green pigments to make food – photosynthesis. Any discoloration will impact yield.
The good news is that brown spots on cucumber leaves eliminate most garden pests as the culprits.
The bad news is that it is almost always a bacterial or fungal infection.
Diseases are easier to prevent than they are to treat.
Despite that, if you do find brown spots on the leaves of cucumbers, most diseases are limited to the foliage and will not spread to the fruit.
Brown spots on cucumber leaves
Alternaria infections cause brown spots that spread across cucumber leaves. Anthracnose causes brown spots that drop leaving holes in the leaf. Bacterial leaf spot starts as small brown spots on the underside of leaves.
Angular leaf spot disease causes light tan brown spots between the leaf veins.
Alternaria Leaf Spot
Alternaria leaf spot is a fungal foliage disease on cucumber plants. Very few varieties are resistant to it.
On other cucurbits, such as cantaloupe and watermelon, it can spread to the fruit, but with cucumber plants, it only affects the leaves.
The symptoms start as small brown spots on mature leaves, usually with a yellow halo (ring) around the edge.
At first, the spots are only one to two millimeters in diameter. As the disease matures though, those spots enlarge and can reach a half-inch per spot.
As the disease spreads and spots enlarge, it can cause all aged leaves to brown resulting in defoliation. On new leaf growth, the initial signs of spread are brown leaf tips.
The lesions are the largest battle on mature leaves.
When this becomes more prevalent is in mid-summer when night temperatures are higher. High humidity and warm climates are when the Alternaria cucumerina fungus spreads fastest.
Longer periods of morning dew and night temperatures over 60-Fahrenheit are when cucumbers are at the highest risk.
Whilst cucumbers are considered among the best vegetables to plant in spring, that only applies to vining varieties. Planting bush varieties too early can leave the leaves in contact with moist soil for too long.
The infection becomes prevalent when the leaves are left wet for over ten hours. If you have a period of sustained rain, expect cucumbers to be more susceptible to Alternaria leaf spot.
Once the Alternaria fungus infects plants, it needs to be treated with a fungicide application.
Options to control Alternaria include liquid copper foliar sprays and sulfur-based fungicide dust applied to the upper and underside of leaves.
Both options prevent fungal spores from germinating, therefore, stopping the spread.
Repeat applications will be needed, usually every 7 to 10 days throughout the warmer weather.
Anthracnose causes similar damage to cucumber leaves as the Alternaria fungus does, only the spots dry up and fall off leaving holes in their wake.
The cause is a different fungus – Colletotrichum orbiculare. This also needs moisture to thrive, but it also needs warmer temperatures in the 75-Fahrenheit to 85-Fahrenheit range.
The holes in the leaves are how to tell the difference between Alternaria leaf spot and an anthracnose infection.
The brown spots start small and can spread to similar sizes (half an inch in diameter) however, when the necrotic spots dry, the spot falls out of the leaf leaving a hole.
Leaves infected with anthracnose can have multiple necrotic spots.
Once it spreads, the leaves on cucumber plants are left with a shot-hole appearance, as though something has been eating holes out of it.
The leaves will feel dry and brittle, and the holes will have brown brittle edges.
The wetter the leaves get and the longer they stay wet is when this is likely to become a problem.
The lower section of the plant where rainwater trickles down the stem and foliage is where early symptoms are most noticeable. More so in bush variety cucumber plants that lack the aeration to dry the leaves faster after rainfall.
On vining varieties of cucumbers, anthracnose usually becomes a problem later in the season when the canopy is closed.
Regular pruning to increase air circulation can prevent the leaves from remaining wet for too long.
To limit the spread and prevent the fungal spores from germinating, the same treatment methods apply of using a copper spray or sulfur-based fungicide repeatedly.
Bacterial leaf spot
Bacterial leaf spot is, as the name implies, a bacterial infection. These spread easier than a fungal infection and can reach the fruit.
Bacterial leaf spot is only the first stage of infection. It appears first on the underside of leaves. The spots are small, roughly a half-centimeter in size making them difficult to detect.
Regularly inspecting for leaf damage is how to identify this type of brown spot. The underside of leaves damaged by mites causes yellow spots on cucumber leaves.
When you notice similar-sized holes that are brown spots on the underside of leaves, those are typically bacterial leaf spots. If the leaf is not cut off, the bacteria can spread to the stem then spread to all parts of the plant, including the fruit.
When you prune away the infected parts of the plant, tools need to be sterilized with rubbing alcohol that is at least 70% isopropyl to prevent it from spreading to other plants by transference.
Angular leaf spot
Angular leaf spot looks similar to an Anthracnose infection. This is caused by the Pseudomonas syringae pv. Lachrymans bacteria.
It starts as water lesions on the underside of leaves, but it does not travel beyond the leaf veins. The spots will be concentrated between leaf veins.
In the early stages, the spots are yellow, gradually turning a tan brown shade. Like with anthracnose, the middle of the spot dries and drops off the leaf causing the same shot-hole appearance as anthracnose will.
Once that happens, look at the edge around the hole.
Anthracnose leaves the edges of holes brown. When the damage is caused by the Pseudomonas syringae pv. Lachrymans bacteria, there will be a white milky crust around the outer edge of the holes.
Copper fungicides help to control the spread of angular leaf spot, but once it is identified, the soil should not be used for any cucurbit plants for at least two years.
Preferably three years as the pathogens can overwinter in the soil.
Frequently Asked Questions about brown spots on cucumber leaves
Do insects cause brown spots on cucumber leaves?
The four-lined plant bug can leave brown to black sunken holes around a sixth of an inch in diameter. All other insects such as mites will cause yellow spots on the leaves. Beetles, being larger, cause more cosmetic damage, and risk infecting plants with bacterial diseases.
What is the safest way to prevent infections on cucumbers?
Keeping the leaves dry is the safest way to prevent any disease from infecting plants. Moisture is the largest threat. Avoid working on your plants (pruning or harvesting) when the leaves are wet. Do your maintenance in the mid-afternoon when all the foliage is dry and likely to stay dry.
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