Cause The fungus, Plenodomus libanotidis (syn. Leptosphaeria libanotis, Phoma rostrupii), occurs in Europe and may have been observed in Oregon in 2015. The fungus, which can cause leaf spots and root rot, is seedborne. During the spring time, windblown ascospores are released from sexual fruiting bodies (pseudothecia), which developed the previous season on infected plant debris remaining on the soil surface. Asexual fruiting bodies known as pycnidia develop in the center of leaf spots as well as in lesions on petioles, stems, and storage roots. Pycnidiospores (conidia) produced by pycnidia can be moved by water splash, or equipment, tools, or workers moving through infected fields when wet. Temperatures between 57°F and 65°F with high humidity are optimum for disease.
Symptoms Infected seed result in damping-off or seedling blight symptoms. Leaf spots appear as light-brown lesions that may contain concentric rings of small, dark pycnidia. Wilting and dark-brown rot of the root tissue develop upon infection of the storage root.
- Plant carrot seeds known to be free of the fungus.
- Hot water seed treatment will control seedborne disease.
- Plow under infected crop residues after harvest.