Diagnosis and prevention of black leg in cucumber seedlings in greenhouses

Fusarium wilt (Fusarium oxysporum f. sp.

Rust bloom, slimy rot (Rhizoctonia solani) p. 201

Leaf diseases

Powdery mildew (Sphaerotheca fuliginea and Leveillula p. 201

Damping-off develops on seedlings sown in non-steamed soil or compost. Symptoms of the disease usually appear a few days after seed germination. The seedling stems do not rise above the soil surface and often droop. The rootlets initially look healthy, but quickly turn brown and rot. Affected cotyledon leaves are deformed, and small brown spots often appear on them.

When sowing seeds in infected soil, seedlings become diseased shortly after germination and rot before emerging to the soil surface. The pathogens of such pre-emergence rot and damping-off are various species of Pythium. Similar symptoms are also caused by Rhizoctonia solani. The main reservoirs of these pathogens are soil and plant residues, and in some cases, water.

Control. Compost for transplants should be free of root rot pathogens. Peat-based composts usually do not contain infection, but substrates with the addition of soil must be steamed.

Fig. 8.1. Damping-off of cucumber seedlings (causative agent Pythium ultimum). Constrictions of the stems at the soil surface are visible. Seedling trays, pots, and racks are sterilized with steam or chemicals; in the latter case, one should wait a certain time after treatment for the containers to clear of vapors.

From seedling boxes where damping-off has appeared, all plants should be removed, even if some of them look healthy. In greenhouses with constant outbreaks of damping-off, it is necessary to carry out a thorough inspection and identify all possible sources of infection.

Against Pythium species, etridiazole is effective when incorporated into the compost or applied by irrigation after pricking out. In controlling R. solani, the best results are provided by quintozene, also introduced into the compost four days before sowing seeds or pricking out seedlings.