If you grow cucumbers, you have to expect that diseases and pests can spread from time to time. Here you can find out how to recognize and combat them.
Anyone who grows vegetable in a garden is bound to run into a few aphids on their cucumbers. With mildew, gray mold and stem rot, however, the gardener’s fun is quickly spoiled. Unfortunately, cucumber plants in particular often suffer from fungi and infections. Some of these can be avoided, others can’t. But it’s important to recognize the pests and plant diseases that threaten your plants to prevent their transmission and spread to other crops. Here are the most common cucumber diseases and pests, and what measures you can take in advance.
Cucumbers are originally from subtropical regions, so they need a lot of heat. If the summer turns out cool, then the plants may well become susceptible to various diseases. Therefore, many amateur gardeners prefer to grow them in a warmer greenhouse. But not only the climate plays a decisive role, there are also various other causes that can cause diseases and pests.
The most common diseases of cucumbers
Powdery mildew is a fungal disease, unfortunately, widespread in gardens, even cucumbers in the greenhouse are not spared from it, responsible for this are two different pathogens. First, the damage develops on the surface of the leaves, the fungi literally suck the nutrients out of them. The white spots become larger and merge visibly with each other until the entire leaf surface is covered with a white coating. The leaves dry out, turn brown and then die.
If the infestation is very severe, the fungus can also be found on the stems. Wind and drafts cause these fungal spores to spread. The pathogens can also survive on plant debris. The big difference from other types of fungus is that powdery mildew thrives especially well in dry and warm weather.
It is better not to fight the disease with chemical means, because this will also make the cucumbers inedible. You can try to fight it with home remedies. Add 100 ml of milk to 900 ml of water and spray the plants with it. Garlic cloves can also be a way to fight powdery mildew. To do this, chop three cloves of garlic and pour them over 500 ml of boiling water. After cooling, fish out the garlic cloves and spray the cucumber plants with the broth.
To minimize the risk of powdery mildew, it is best to buy only resistant varieties.
Downy mildew mainly attacks cucumbers or pickling cucumbers grown in the garden or greenhouse. You can recognize it by light green to yellow-brown spots on the leaves of the plants, which show up especially in the summer months. If you turn the leaves over, you will see a grayish-purple coating on the underside.
Downy mildew can spread extremely quickly, gradually the affected plants turn yellow and die. To prevent downy mildew from spreading to all plants, you should destroy infested plant debris immediately.
To prevent the disease, you should look for resistant varieties when buying. In the greenhouse, you should avoid high humidity and therefore ventilate regularly.
Gray mold is also caused by a fungus that can survive even on fallen foliage. The flight of spores can cause the mold to spread, especially if the humidity is quite high for a long time or if dew forms on the plants at night. You will then see a gray, velvety coating on the leaves, stems and at the base of the fruit.
Combat and Prevention
To prevent the fungus, you should only water the soil, never the leaves and stems of the cucumber plants. In greenhouses, it is important that you ventilate regularly. Dispose of infested plant parts immediately in the residual waste.
Avoid watering the plants in the evening. This prevents the moisture from escaping properly during the night. You should also avoid using plant strengthening agents, as they have a significant impact on the taste of the cucumber. Rather, regularly provide a portion of fertilizer to strengthen the plant. When planting cucumbers be sure to leave enough space between each plant.
Sclerotinia wilt / Sclerotinia sclerotiorum
There are several wilt diseases that can occur in cucumbers. One of them is sclerotinia wilt. This predominantly affects cucumbers in greenhouses. It also affects peppers, tomatoes and lettuce. Infection can occur through spores. The fungus shows up as a white, woolly coating on the stems of the cucumber plants, which then begin to rot.
Although the cucumbers are sufficiently supplied with water, wilting occurs. Later, the fruits are also covered with a downy coating. Shortly after, you will then see small, black globules in it.
Combat and Prevention
Sclerotinia wilt is not so easy to fight, so it is important to prevent it from happening in the first place. Prevent the fungus by avoiding high humidity in particular, as this is where it spreads especially quickly. Make sure that the leaves are not wet for a long time. So never water in the evening and also always water only the soil.
Other wilting in cucumber plants
There are several fungi that can cause plants to wilt, even though you water them adequately. Once the plant dies above ground, the roots also begin to rot. You can see the fungal growth on the stems, and the leaves also turn increasingly brown and die. The fungi can also survive in the soil for many years and cling to plant debris, leading to new infections.
To prevent wilting, you can graft young cucumber plants onto black-seeded squash to strengthen them. As soon as a plant is affected by wilt, you should remove the entire plant and dispose of it in the residual waste.
Bacterial wilt (Erwinia tracheiphila)
Bacterial wilt is bacterial pathogens that attack the young cucumber plants. Not only cucumbers are affected by it, but also many other vegetables, because the pathogens are very widespread in the soil. As soon as injuries are present, for example due to feeding or the flowers falling off, the bacteria can penetrate the cucumbers, which are decomposed from the inside, even though the skin is still intact.
Combat and Prevention
You cannot fight bacterial wilt chemically. Only the destruction of the affected plant ensures that the pathogens do not spread to other plants. Again, it is enormously important that you always water the cucumbers from below, never the leaves.
Bacterial leaf spots
Leaf spot disease is also a bacterial infection in which transmission can occur even through the seed. Especially in summer temperatures and humid weather, the pathogens can spread. The pathogens also survive in the soil and then penetrate the cucumber plants through injuries. You can recognize the disease by yellowish spots on the surface of the leaves. If the humidity is too high, drops of mucus form on the underside of the leaves.
Over time, the spots flow into each other and turn brown, withering and tearing of the leaves occur. As the disease progresses, the infestation can also spread to the cucumbers, which then rot.
To prevent bacterial leaf spots, ventilate regularly in the greenhouse. In the garden, make sure to rotate crops regularly, as leaf spot disease only affects cucurbits. If there was an infestation, you should not grow plants from this family in the same place for at least three years.
Cucumber Mosaic Virus
Small aphids are responsible for the transmission of cucumber mosaic virus. The transmission occurs within a very short time, because there are very many host plants. After about 10 to 14 days the first signs appear. If the temperature is below 20 °C / 68 °C, wilting occurs, and if the temperature is above 20 °C / 75 °C, the typical mosaic spots appear. However, it can also happen that these two phenomena go hand in hand.
They are mostly bright mottling, which can be very different, depending on the strain of the virus. These spots can also appear on cucumbers, and wart-like structures are also formed here. A good thing is that the virus cannot survive on dead plant parts.
Combat and Prevention
Since aphids transmit the virus, these pests must be controlled early. It is best to choose robust cucumber varieties right away when buying.
The most common pest of cucumbers
An infestation of the cucumber aphid can be recognized by the fact that the aphids attack the cucumber plants in large colonies. They suck the plant sap from the cells of the leaves and thus cause reduced growth. In addition, they excrete honeydew again, which can lead to sooty mold or even to periods of leaf wetness. As a result, other fungi can infect the plants more easily. In the course of a season, the cucumber aphid produces many generations, causing considerable damage over a long period of time, the cucumber aphid can transmit the cucumber mosaic virus.
Fleeces or nets against pests are not usually used in cucumbers. It is important to regularly look for the aphids, especially on the underside of the leaves and on the stem, and to fight them in a timely manner when they appear. It is also recommended to encourage beneficial insects. This can be done with the help of hem structures. At best, beneficial insects such as the Aphidius colemani (ichneumon wasp) are used early on for prevention.
If the cucumber plant is already infested, plant protection products based on potash soap can be used.
Leaf miner fly
Leaf miners are 1-3 mm large, black-yellow colored flies with transparent wings. The main damage is caused not by the leaf miner fly itself, but by its maggots. These eat into the leaves of the cucumber plant, leaving behind leaf miner galleries. Visible spots appear at the sucking sites. The infestation can cause growth impairment. If the infestation is particularly bad, the plant dies.
No preventive measures are available for home and allotment gardens. Infested plant parts should be disposed of immediately to prevent further infestation.
With the help of yellow panels infestation can be detected quite quickly. At best, diseased plant parts should be removed (dispose of leaves). Furthermore, the use of suitable insecticides and beneficial insects such as the ichneumon wasp (Dacnusa sibirica or Diglyphus isaea) is recommended.
The spider mites settle with their eggs and webs on the underside of the leaves and suck on them. This creates bright spots on the upper surfaces of the leaves. As the infestation increases, the leaves gradually turn yellow and eventually die. In addition, the webs of the mites are then clearly visible and even envelop the shoot tips of the infested cucumber plant.
Increased humidity can prevent or minimize infestation of spider mites. If there has already been an infestation in the previous season, it must be combated as far as possible.
In general, plant protection products based on potash soap as well as pyrethrins or rapeseed oil can be used against the spider mites. In addition, beneficial insects such as lacewing larvae, flower bugs, predatory bugs, hoverfly larvae, gall midges (Feltiella acarisuga), the Stethorus ladybug or predatory mites (for example Phytoseiulus persimilis or Amblyseius californicus) also help.
Originally from Asia, the Japanese beetle causes severe damage to cultivated and wild plants. The Japanese beetle is 8 to 12 mm long. The upper side of the wings appears metallic copper, while the head and body shimmer golden-green. The Japanese beetle is easily recognized by the white tufts of hairs, of which it has five small ones on each side of the abdomen and two larger ones at the end of the body. The Japanese beetle belongs to the leaf scarab beetles (Scarabaeidae) and is similar in appearance and biology to the June or garden foliage beetle. Therefore, there is a risk of confusion. However, unlike the June beetle, this one does not have the typical white tufts of hairs.
The cockchafer grubs live in the soil of moist meadows and feed primarily on grass and herb roots. This can cause significant damage to grassy areas or herbs. After the larvae pupate, the adult Japan beetles hatch between May and June. The main flight period of the Japanese beetles is between mid-May and mid-August. Adult beetles feed on the leaves of the plant – sometimes down to the twigs, resulting in leaf skeletons (skeletonized feeding). Infested leaves turn brown and may fall off. If a plant has flowers and/or fruits, these are also not protected from additional feeding by the beetle. Interestingly, it can be observed that they seem to eat individual plants completely bare, while neighboring plants show hardly any damage.
Mechanical cultivation of the soil should be done in the early fall. This reduces the chance of survival of larvae feeding near the soil. As part of this tillage, these larvae should be caught. Avoid watering grassy areas during the peak flight period, as this reduces the attractiveness for females to lay eggs. Furthermore, higher grass cutting heights can counteract spread and reproduction of the Japanese beetle. You can visually monitor host plants and surrounding soil. If you find an infestation, be sure to report it to the appropriate plant protection service.
Various beneficial insects, including birds, moles, shrews, roller wasps, and ground beetles, feed on the larvae in the soil. Bacteria (Paenibacillus popilliae), parasitic nematodes (including Steinernema sp, Heterorhabditis sp.), or entomopathogenic fungi (Beauveria bassiana, Metarhizium anisopliae) can also be used against the larvae. Small accumulations of the beetles can be controlled with attractant traps (pheromones) and by mechanical collection.
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