There are several types of tuber deformations or malformations such as dumbbell, kidney, pointy end, and hot dog (elongated). Tubers with these defects are also referred to as “being rough.” They are due to problems associated with apical buds and longitudinal growth. Harvest quality or grade is affected and tubers are discarded as culls.
Development and Appearance
During a period of stress, longitudinal growth slows or may even stop. When favorable conditions return, tuber growth resumes, “stop and go” growth. Therefore, appearance is due to irregular longitudinal growth, often because of a constriction. Pointy stem-end or Bottleneck: Growth disruption occurred during early bulking and the constriction is at the stem end. Dumbbell and Elongated (hot dogs): Growth disruption occurred at mid-bulking. Dumbbells have a constriction in the middle. Kidney-shaped tubers tend to have a slighter mid-section constriction and tend to curve, giving the kidney-shape appearance. Elongated tubers show little lateral growth; they also tend to be gnarled and curved. Pointy bud-end: Late-season growth disruption resulting in a slight constriction at the bud end. Often dumbbells and pointy bud-ends are associated with jelly end rot or glassy end.
Knobbiness also sometimes referred to a secondary growth is due to stimulated growth of lateral buds in one or more eyes. Protruding eyes is a similar defect. The size and shape of knobs depends on the growth stage of the tuber when stress occurs. As with the deformations reviewed above, high-temperature stress is the cause for stimulating this abnormal, lateral-bud growth. Susceptibility to knobs varies with varieties. Practices to lessen knobbiness are the same as for deformations. Unlike pointy end and dumbbells, there are no internal defects or rots associated with the formation of knobs. Knobby potatoes are considered culls, lowering marketable yield.
These deformations are primarily due to high temperature stress in the field often but not necessarily exacerbated by water stress. Note, water stress or drought alone does not cause these deformation. The severity of the deformation increases with higher temperatures and longer high-temperature periods. Basically, high temperatures, above 80oF, decrease cell division and lower the supply of carbohydrates available to the tuber. Other factors that exacerbate temperature-induced deformation are excessive nitrogen application before a high temperature period, uneven nutrient or moisture supply, hail and frost.
Potato varieties vary considerably in their sensitivity to high-temperature field stress. The rule of thumb is that longer-tuber varieties are more susceptible than rounder-tuber varieties. For example, long whites such as Kennebec are more susceptible than round whites as Atlantic. Russet Burbank is one of the most sensitive of all varieties.
Since weather cannot be controlled, when planting susceptible varieties choose areas with cooler climates. Irrigate adequately during early bulking. Field capacity should be maintained higher than 80%, closer to 90%, during tuber growth (Potato Production Stages: Scheduling Key Practices, Univ. Nebr. Coop. Ext. Circ. # 95-1249). Avoid exacerbating stresses such as excessive nitrogen.