<i>Trichoderma</i> Ear Rot on Maize – When Friends Turn Evil

Abstract Maize ( Zea mays ) is one of the world’s most important staple crops and is grown across a wide range of agroecological zones. Trichoderma species are considered beneficial fungi, applied as biological control agents and plant growth promoter in sustainable agriculture. However, recent discoveries from maize fields in Europe challenge this long-standing perception. Increasingly, Trichoderma isolates have been recovered from infected maize cobs and stalks in Germany, France, Italy, and Austria—accompanied by yield loss and significant tissue degradation. This case study explores the emergence of Trichoderma as a pathogen of maize, focusing on the symptoms and damage, the occurrence in Europe, and pathogenicity of commercial biofungicides containing Trichoderma as active ingredient. It also raises important questions: How can Trichoderma -induced damage be distinguished from other diseases like Aspergillus ? Could biocontrol strains pose a hidden risk? And what tools do we have to protect maize from this unexpected threat? This article provides an up-to-date overview of a developing plant health issue—where a former friend might turn evil. Information © The Author 2025.