Several types of tuber malformations are caused by either environmental or cultural stress.
Drought, wide fluctuation in soil and air temperature, and defoliation resulting from hail or pests often cause the development of malformed tubers.
Knobby Tubers
Water stress can trigger the loss of apical dominance in some varieties, notably Russet Burbank. The abnormal growth of lateral buds results in knobby tubers.
Rhizoctonia cankers on the stems increase the incidence of knobby tubers.
Pointed Stem End and Pointed Bud End
Tubers showing the pointed stem end defect. The pointed stem end defect is the result of growth disruption during the early stages of tuber development.
The pointed bud end defect develops when tuber growth is restricted late in the season.
Dumbbells
Dumbbell tubers have a constriction between the stem end and the bud end. Growth disruption at the midbulking stage causes irregular radial growth of tubers.
