Leaf symptoms appear as numerous small, water-soaked or pale green spots. These spots gradually dry and die, turning white to gray and becoming angular. Often, the lesions are surrounded by a yellowish halo. Their center may tear away, leaving ragged holes in the leaves. Most severe symptoms develops on the infected fruits, and resemble insect stings. Small (about 3 mm), gray, slightly sunken, gum-oozing spots appear first. Later, the spots enlarge and finally become distinct sunken cavities or scabs. Affected fruit are often invaded by opportunistic pathogens such as soft-rotting bacteria that produce a mushy, foul-smelling decay. On highly resistant fruits, especially on certain squashes and pumpkins, irregular, knob-like formations may develop.
The symptoms are cause by the fungus Cladosporium cucumerinum, which overwinters in plant residues, in cracks in the soil or on infected seeds. Early spring infection may come from either of these sources. The fungus begins to develop spore-producing structures and release spores. Spores are spread by insects, clothing or tools, or are blown long distances in moist air. High air humidity and moderate temperatures increase the risk of infection. Temperatures around 17 °C and alternating between 12-25 °C, together with moist weather, frequent fogs, dews or light rain are the most favorable for the development of the fungus. Symptoms can appear 3 to 5 days after the penetration of the fungus in the plant tissues has taken place.
Direct biological treatment of cucumber scab is not possible. Use fungicides based on a copper-ammonium complex that is certified organic to slow down the spread of the pathogen.
Always consider an integrated approach with preventive measures together with biological treatments if available. Use fungicides containing chlorothalonil or based on a copper-ammonium complex. Seeds can be surface-disinfected with 0.5% sodium hypochlorite for 10 min to eliminate the pathogen. Fungicides containing dithiocarbamates, maneb, mancozeb, metiram, chlorothalonil and anilazine are also effective against C. cucumerinum.