Second growth is a physiological potato problem induced by prolonged air temperatures above 28 C and water stress. These two factors interact to limit the tuber growth rate, thus causing second growth. Inadequate soil moisture alone does not result in the initiation of second growth.
Southern Ontario is being hit hard this summer by a long period of excessive heat and drought which is causing second growth on some varieties. Potato varieties differ in their susceptibility to second growth. Fresh market European varieties are often more susceptible because they were selected in countries where the growing season is rarely hot.
There are three distinct types of second growth:
- Heat Sprouts: Sprouts are produced from stolons or tubers and grow as new sprouts or develop into above ground leafy stems-long stolons that protrude from the hill often produce new plants.
- Tuber Chaining: A series of small tubers form on a single stolon or new tubers form on stolons of daughter tubers.
- Little Tuber Disorder: Little tubers form directly on daughter tubers or on short sprouts emerging from the tuber. They form when tubers are “physiologically old”. Prolonged high temperatures and water stress cause tubers to age.
Physiologically old seed potatoes are prone to produce little potatoes instead of sprouting normally when planted.
Cultural practices that promote the uniform growth of plants and tubers throughout the season help minimize second growth. The cultural practices are:
- Uniform spacing for the desired plant population
- Adequate amount of fertilizers. Supplemental applications of liquid Ca fertilizers help the crop tolerate heat stress.
- Uniform soil moisture sufficient to meet the water requirement at the different growth stages of the crop. This is easier said than done this season!