CUCUMBER - FUNGUS

Severe defoliations greatly decrease yields. The youngest leaves (before having about half of final area), stems, petioles and fruits are not infected.
Occasionally, early symptoms could be confused with mosaic but in this case yellow spots are not restricted among veins and grey to black growth doesn´t occur on the lower surface of spots. Also, mosaic begins to appear on the youngest leaves.

Disease cycle: The fungus overwinters in the Mediterranean basin both on cultivated and weed Cucurbitaceous species and infection gradually spreads during summer by wind and air currents to the Northwest. Initially infected leaves serve as a source of inoculum for neighbouring leaves and plants. Within a crop, spores are spread by wind as well as by water droplets during rain or overhead irrigation. Wet and relatively warm weather is favourable for the disease to spread. Presence of water on leaves is indispensable for infection. Optimal temperature ranges between 18 and 23 °C. Under such conditions, the life cycle lasts 3 - 4 days only and unprotected plants can be almost completely destroyed within several days. This fungus tolerates well without loss of viability even at elevated temperatures above 30 °C.

Control: No control method has 100 % effectivity but in combination they can reduce the losses:

• resistant varieties;
• early sowing;
• select fields and manage the crop to promote air movement and reduce humidity levels inside the crop canopy;
• avoid excess overhead irrigation. Consider irrigating during the late morning to facilitate rapid leaf drying. If possible, use trickle/drip irrigation;
• maintain good weed control in the field. Control alternate weed hosts, e.g. wild cucumber, and volunteer cucumbers in neighbouring fence rows and field edges;
• consider washing equipment and tools before moving from one field to another;
• inspect crops for symptoms weekly;
• if possible, work in diseased fields at the end of the day;
• remove and dispose of infected plants. Do not compost them;
• frequent fungicide application if weather is favourable for disease spread. Application should be done every 5-7 days depending on disease severity;
• use of antagonistic fungi such as Trichoderma viride and Trichoderma harzianum instead of chemical fungicides (https://jppp.journals.ekb.eg/article_149526_8eaf102a24fc79b9d035ae07b0b149b2.pdf).

Causal organisms: various species of Pythium, Rhizoctonia, Fusarium, Aphanomyces, and other fungi.

Host range: All these species are rather polyphagous - with some exceptions all vegetable and ornamental species, sugar beet and some weeds can serve as host plants.

Occurrence and importance: The disease occurs wherever vegetable or sugar beet are grown but serious damage can be caused under unfavourable conditions, in cool, wet soils. For cucumbers it can be important during growing of transplants for early cultures or after direct sowing into the field if weather is cold and rainy.

Symptoms and confusable diseases: Typical symptoms are softening and darkening of hypocotyl and root tissues of plants after they emerge. The affected plant either suddenly collapses or is gradually yellowing and in this case plant and root system development is very poor. Preemergence attacks result in darkening of tissues and plants failing to emerge. Tissue can be covered by mycelial growth of different colour. Symptoms caused by all these pathogens are very similar and it is rather difficult to differentiate them from each other.

Disease cycle: These pathogens are either soilborne (Pythium, Rhizoctonia, Fusarium, Aphanomyces) or seedborne (Colletotrichum). Soil borne pathogens infect roots and base of stem only and survive in soil in the form of oospores, sclerotia, chlamydospores or as saprophytes on plant debris, seedborne pathogens can infect all aboveground parts of the plant and survive on seeds or in plant debris as mycelium or conidia. Due to such a broad range of pathogens the disease can occur under various conditions of temperature and soil humidity. Bad soil structure, acid reaction, malnutrition as well as nitrogen surplus, and low intensity of light together with high temperature are predisposing factors.

Control:

• pathogen-free certified seeds;
• treat seeds with fungicide;
• soil sterilization (for transplants only), good structure and balancing pH;
• good growing practice: proper spacing of plants, balanced fertilization, correlation between temperature and light intensity (avoid excessively high temperature if light intensity is low);
• mulch to increase soil temperature fungicide application.
• Trichoderma-supplemented compost could effectively suppress root disease causal fungal phytopathogens (https://www.pphouse.org/upload_article/97431566_9_IJEP_May_2022_Kumhar.pdf).

Host range: Powdery mildew on cucumber is a disease caused by two pathogens: Erysiphe cichoracearum and Podosphaera xanthii.

Occurrence and importance: Field cucumbers can be heavily infected in some years but losses in yield are not so important.

Symptoms and confusable diseases: Typical symptoms develop in the form of white powdery cover on both upper and lower side of leaves which can later get yellow and die. Stems and petioles can also be infected but fruits are not damaged.

Disease cycle: Powdery mildew overwinters in fruiting structures on plant debris. When temperatures warm in late spring, a secondary spore (conidia) forms and blows to leaf tissue to cause infections. Changing of warm dry periods with light rains or night dews is most favourable for disease spreading but other conditions are not limiting because mildew does not need liquid water on leaves and it can develop at very broad range of temperatures from 10 - 35 °C. After infection, powdery mildew does very well in dry conditions. Infected tissues form more spores that can also be blown in with the wind to infect plants in new locations. The cycle of spore production, dissemination, and infection occurs continually all summer long. When temperatures cool in late summer, the fungus switches from producing conidia to producing fruiting structures that contain the spores for winter survival.

Host range: All cultivated species from the genus Cucumis (cucumber, melon, pumpkin) can be infected.

Occurrence and importance: Anthracnose occurs mainly on melons and pumpkins, for cucumbers it has little importance.

Symptoms and confusable diseases: At the beginning lighter spots occur on leaves, rapidly growing, coalescing and later necrotizing. Necrotized tissues fall out. During wet periods pink conidia can be seen on spots. Similar spots are also present on petioles. On fruits wet, sunken spots are formed which grow even after harvest.

Disease cycle: The anthracnose fungus can survive in infected plant debris and seed. Spores form on infected leaves and fruit. These easily spread by splashing rain, irrigation, on hands and tools. The disease develops in warm, moist environmental conditions. Anthracnose first appears mid to late season after the canopy closes. During wet summers, the disease can be quite severe.

Control:

• Some resistant varieties of cucumber are available.
• Purchase clean seed from a reputable source. Do not save seed from infected plants.
• Rotate vegetables so three years go by before planting any member of the squash family in the same location.
• Avoid getting leaves wet with water by using drip irrigation, a soaker hose, or careful watering technique.
• Do not work in plants when wet.
• Remove and destroy infected vines at the end of the season in small gardens.
• After working in an infected area, clean and sanitize any tools you used, and clean dirt off of your shoes