Tuber External Growth Defects: HAIR SPROUTING
Premature sprouting of tubers late in the season or early in storage affects quality of the harvested tubers. Besides appearance, the sprouts soften the tuber by taking up its nutrients and lowering the starch content. The taste of the tuber is uneven with a sweeter taste near the sprout. This makes the tuber unacceptable for processing since frying will turn this area dark.
Development and Appearance
Sprouts appear from the eyes of the primary tuber. The hair sprout is very thin, hair-like, and is too weak to emerge or grow much. A related to hair sprouting is heat sprouting which may remain underground, or emerge and become green and leafy. Tuber sprouting is a hormonal phenomenon involving the lack of development of tuber dormancy, related to abscissic acid (ABA).
Causation
The cause of heat and hair sprouting is high soil temperature. The conditions for this disorder are similar to that for tuber chaining (see above) except that exposure to high soil temperature is toward the end of the growing season, late bulking or plateau stages (Potato Production Stages: Scheduling Key Practices, Univ. Nebr. Coop. Ext. Circ. # 95-1249). Once sprouted, the sprouts will grow under good conditions. In other words, tuber dormancy does not develop. Hair sprouting is also a symptom of infection with Colletotrichum atramentarium, related to black dot, and associated with aster yellow carried by leafhoppers and psyllid yellow carried by potato/tomato psyllids.
Susceptibility
Varieties, in general, are susceptible but varieties with short tuber dormancy periods may be more so.
Cultural Practices
Practices to minimize this disorder are the same as for tuber chaining. Since air temperature cannot be controlled, the best practices to avoid over-heating of the soil are to plant deep, to hill and to maintain a good row structure. In severe hot weather, cooling the ground with irrigation may be necessary.