Downy Mildew
(Pseudoperonospora cubensis)
Downy mildew is a common disease in cucumber crops. Infection occurs at temperatures between 16°-22°C and with high air humidity. When nighttime temperatures are around 15°C and daytime temperatures reach 25°C, the incubation period ranges from 4 to 12 days. Symptoms appear as small, round yellow spots that give the leaves a mosaic-like appearance. As the disease progresses, the spots enlarge and turn yellow, becoming angular due to the leaf veins restricting their spread. The yellow areas eventually turn brown, necrotize, and dry out, reducing the leaf’s assimilation surface and thereby decreasing the amount of assimilates needed for proper fruit growth and development, resulting in smaller and deformed fruits.
Preventive measures include:
- Proper crop rotation (avoiding planting cucumbers or other crops from the Cucurbitaceae family in the same area as a previous crop),
- Using relatively resistant hybrids,
- Growing on trellises (which improves ventilation),
- Using drip irrigation systems,
- Applying preventive copper-based fungicides.
These measures can reduce the risk of infection by this pathogen, though they rarely completely prevent it. In case of a downy mildew outbreak, systemic fungicides containing active ingredients such as fosetyl, metalaxyl, propamocarb, and dimethomorph are used. When using systemic fungicides, it is important to observe the pre-harvest interval, as cucumbers (especially pickling cucumbers) are harvested every day or every other day. The shortest pre-harvest interval, 4 days, is for the active ingredient fosetyl.
Powdery Mildew
(Erysiphe cichoracearum)
Powdery mildew is another significant disease affecting cucumbers. The fungus can begin developing at a relative humidity of 20%, but the most favorable conditions for the disease are air temperatures up to 30°C and relative humidity between 65-85%. The disease is recognized by the appearance of a powdery coating on the upper leaf surface, and in severe cases, symptoms can also be seen on the underside. Affected leaves turn yellow, necrotize, and dry out.
Preventive measures include planting or sowing resistant hybrids and applying sulfur-based products, with caution when used in protected environments as high temperatures can cause burns. Curative treatments involve fungicides with active ingredients like myclobutanil and dinocap.
Other diseases that affect cucumber cultivation include wilting (Fusarium species), seedling diseases (Pythium spp., Phytophthora spp., Sclerotinia sclerotiorum, Rhizoctonia solani), root rot (Phomopsis sclerotiodes), white mold (Sclerotinia sclerotiorum), black stem rot (Didymella bryoniae), anthracnose (Colletotrichum lagenarium), fruit scab (Cladosporium cucumerium), and cucumber mosaic virus (Cucumber mosaic virus).
