Tuber External Growth Defects: CHAINING
Tuber Chaining also called Gemmation affects harvest yields. The small tubers formed compete with the primary tuber for nutrients and thereby lowers the weight of the marketable tuber. There is no effect on fresh market quality of the primary tuber except for reduced tuber size. For processing, the texture and starch content may not be desirable. Tuber chaining may also interfere with harvest separation of the primary tuber from stolon.
Development and Appearance
Tuber chaining refers to the initiation followed by limited growth of tubers at the nodes of a stolon after the apical tuber at the end of the stolon was initiated and has begun growing. The disorder’s name refers to the chain appearance of the series of little tubers along the stolon. The development or initiation of these is due to a break down of apical dominance of the primary tuber. This is a hormonal phenomenon associated with auxin (IAA) concentrations basipetal from the primary tuber. The sizing of the primary tuber is inhibited due to the lessened carbohydrate (sucrose) taken in because of competition with the secondary tubers. The primary tuber, in severe cases, can have a glassy or soft texture and have a low specific gravity making it undesirable for processing.
A related phenomenon is sprouting from buds on the nodes of the stolon. The sprout does not affect quality but will compete for nutrients much like tuber chaining, and is also related to loss of apical dominance.
The cause of tuber chaining is high soil temperature. Soil temperatures around the daughter tubers that are above 75oF, especially early to mid bulking or tuber growth (Potato Production Stages: Scheduling Key Practices, Univ. Nebr. Coop. Ext. Circ. # 95-1249), will promote tuber chaining. Once this disorder has begun, the return of cooling soil temperature around tubers will not overcome the disorder. In other words, the chaining and nutrient competition will continue for the rest of the season, thereby yields are lowered. On a physiological level, once apical dominance is lost by the primary tuber, it does not return. Note, low soil moisture does not cause this disorder but is usually associated with high soil temperature.
Varieties, in general, are susceptible but round tuber varieties, most white chipping and red varieties are particularly prone to tuber chaining.
Since air temperature cannot be controlled, the best practices to avoid over-heating of the soil is to plant deep, to hill and to maintain a good trapezoidal structure to the row. Seed pieces should be at least six inches below the surface; eight inches would not hurt. In severe hot weather, cooling the ground with irrigation may be necessary.