The fungus Mycosphaerella, also known as Didymella bryoniae, can cause serious problems, particularly in cucumber cultivation. It can infect leaves, stems, flowers and fruits. The fruits can be affected on the inside (internal fruit rot) and the outside (external fruit rot).
Generally speaking, the first infections in crops grown under glass are caused by ascospores (sexual spores) which enter the greenhouse mainly via the air. For infection to take place, there needs to be high relative humidity (RH >85%). Internal fruit rot is caused by the fungus penetrating the fruit via the flower. Stems, older leaves and the outside of fruits become infected via damage (wounds). The risk of infection can be somewhat reduced by aiming for lower RH in the greenhouse.
It is thought that mildew-tolerant varieties are slightly more susceptible to Mycosphaerella, perhaps because growers of these varieties use fewer mildew products which also help to control this fungus, such as Collis, Rocket and Fungaflor.
Images: BASF and Wageningen University & Research.