Jemand schon mal gesehn ?

This is just a guess based on visual alone. Boron often results in deficiency or toxicity because it is a unique micronutrient for which the threshold levels of deficiency and toxicity are very narrow. B deficiency and excess are both widespread agricultural problems for higher plants. Depending on the age and species, plants manifest a wide range of deficiency symptoms, including stunted root growth, restricted apical meristem growth, brittle leaves, reduced chlorophyll content and photosynthetic activity, disruption in ion transport, increased phenolic and lignin contents, and reduced crop yield. The prevalence of symptoms depends on the severity of the B-deficiency condition because plants show uniform deficiency symptoms on entire leaves but sometimes in the form of isolated patches. Given the immobile nature of B, it usually accumulates in mature leaves, whereas young leaves do not receive sufficient B for proper growth. Thus, the deficiency symptoms first appear on young leaves, including thick, curled, and brittle leaves with reduced leaf expansion; corky veins; interveinal chlorosis; yellow water-soaked spots on lamina; and a short internodal distance, resulting in a bushy plant appearance. In severe cases, leaf apex necrosis and leaf dieback occur. The expansion of stems and petioles leads to hollow stem disorder in broccoli and stem crack symptoms. Just a guess. Good luck.