ABSTRACT
Introduction and significance If conditions favor development of this abiotic disorder, significant loss of plant stands can occur. In general, this disorder is of low to moderate concern.
Symptoms and diagnostic features Lettuce roots are sensitive to high levels of ammonium in the soil. The central core of the root first turns yellow to light brown, then becomes dark brown to red in color, though typically a brick red discoloration is seen (437,438). In severe cases, the root xylem collapses and a central cavity forms throughout the length of the root and even into the lower crown of the plant. Lateral roots may be short, with blackened tips. In some situations the external surface of the root turns yellow or light brown and develops cracks, which mimic corky root symptoms. Above ground symptoms indicate that plants have damaged root systems and consist of poor growth, stunting, yellowing of older foliage, and wilting. Plants damaged by excess ammonium have symptoms in the vascular tissue that resemble those caused by Fusarium and Verticillium wilts. In general, the hollow root cavity symptom is more characteristic of ammonium toxicity, while Verticillium wilt tends to have black, not red, vascular discoloration.
