S. B. Sterrett VA Tech, Eastern Shore Agricultural Research and Extension Center
Selection of good quality seed is essential for Virginia growers. We often plant under less than ideal growing conditions in cold, wet soils. Seed need to be disease-free, physiologically young, handled gently and stored with care. Select disease-free seed lots. Commercial lots of seed must be inspected upon arrival by the Virginia Department of Agriculture and Consumer Services Inspection Service. Avoid planting seed pieces with scab lesions as seed-borne scab can contaminate fields without a prior history of scab.
Since there is no effective control for scab, sanitation is the best defense, storage of potato seed tubers before and after cutting can affect plant vigor, yield and size distribution. Quality control during seed cutting can also impact yield and tuber size distribution. Chronological age of the seed (days since harvest) may be less critical than physiological age (internal age resulting from biochemical changes within the tuber).
Physiological aging can be hastened by factors that cause increased or fluctuating rates of respiration. Stressful growing conditions prior to the harvest of the seed crop, including low moisture, high temperatures, inadequate fertility, disease pressure, frost damage or excessive bruising during harvesting, can accelerate the aging process. Fluctuating storage temperature should be avoided as aging can be rapidly hastened. Tubers held consistently at 38-40° F will be younger than those held at higher temperatures because the rate of respiration is minimal at the lower temperature. Jenkins et al (1993) found that physiological aging was hastened more by increased temperature (64° F) at the end of the storage period compared to exposure to warm temperatures earlier in the storage period. Hence, optimum early growth could be achieved by manipulating the storage temperature prior to planting. As seen in Table 1, characteristics of physiologically young seed differ from old seed.
Table 1. Response of physiologically young and old seed.
| Characteristic | Young Seed | Old Seed |
|---|---|---|
| Emergence | Slow | Rapid |
| Tuber set | Reduced | Increased |
| Tuber Bulking Period | Longer | Shorter |
| Tuber Size at Harvest | Larger | Smaller |
| Adapted from Bohl et al, 1995 | ||
Physiological age can be determined by collecting a sample of each lot, place in warm location (>60°F) and observe the sprouts (Johnson, 2004). Dormant seed will not sprout. The dormant or resting period has not been satisfied. This is usually only a problem for early plantings in the Deep South. Young seed is characterized by presence of one to few sprouts, emerging from the apical bud end of the tuber. Reduced stem number is often associated with a small tuber set and a large tuber size. Sprouting of eyes at the stem end will be delayed or may not occur. Yukon Gold is known to have strong apical dominance, with poor stand associated with late sprouting of seed pieces originating at the stem end. Middle-aged seed will have multiple sprouts, often originating from every eye. Seed of this age will more likely produce plants with multiple stems and larger tubers set per hill. Old seed will have branched sprouts that appear hairy. Sprouts are weak; resulting plants will lack vigor.
The physiological age of seed grown without stress and stored under ideal conditions will most likely be younger than those grown or stored under stressful conditions that hasten physiological aging. Without the pre-sprout test, the physiological age may not be easily estimated unless buyer knows about the growing and storage conditions of a given seed lot. Obviously, the presence of visible sprouts prior to cutting is an indication of older seed.
Cutting seed tubers results in increased respiration during the healing process, thereby increasing physiological aging in much the same way as bruising. Smooth cuts from sharp knives results in lower respiration and more rapid healing than cuts from dull blades.
As living organisms, seed tubers require oxygen. Adequate air movement through the pile during storage is essential. After cutting seed, seed should be stored in a manner that allows adequate air circulation. Too little will delay wound healing and promote seed piece decay. Too much air circulation will promote dehydration, reducing seed piece weight.
Seed tuber and seed piece size can also affect yield and size distribution. Seed tubers should weight between 2 and 10 oz (Bohl et al. 1995). Since tubers have approximately the same number of eyes, regardless of size, seed pieces cut from large tubers are more likely to be "blind" (no viable eyes) (Nielson et al (1989). This results in missing hills or skips in the field. The ideal seed piece size is 1.5 to 2 ounces (Johnson, 2005, Niemira et al, 2005). Larger seed pieces are likely to have multiple eyes, resulting in an increased number of stems per hill. Until the sprouts generate a new root system, they are dependant upon the nutrients and energy stored in the seed piece. With undersized seed pieces, establishment of the resulting plants may be delayed or unsuccessful.
The advantages of starting with disease-free, physiologically young seed are numerous. Physiologically young seed can be aged to improve sprouting through the processes of seed cutting, wound healing, and storage prior to planting. However, the process of physiological aging can not be reversed. Care in seed lot selection, handling, and management is essential for optimizing your return on investment. The cost of production and harvesting will be the same regardless of initial seed quality.
Literature Cited:
Bohl, W.H., P. Nolte, G.E. Kleinkopf, and M.K. Thornton. 1995. Potato seed management: seed size and age. Univ. of ID Coop. Ext. Sys. CIS 1031. http://info.ag.uidaho.edu/Resources/PDFs/CIS1031.pdf (2/16/05)
Jenkins, P.D., T.C. Gillison, and A.S. Al-Saidi. 1993. Temperature accumulation and physiological ageing of seed potato tubers. Annals of Applied Biology 122:345-356.
Johnson, S.B. 2004. Selecting, cutting and handling potato seed. Potato Facts. http://www/umext.maine.edu/onlinepubs/2412/htm (2/16/05)
Nielson, M., W.M. Iritani and L.D. Weller. 1989. Potato Seed Productivity: Factors influencing eye number per seed piece and subsequent performance. American Potato J. 151-160.
Niemira, B.A., W.W. Kirk, R. Schafer, J.M. Stein, and R.S. Shaw. 2005. Effect of whole seed piece weight, planting density, and a soil stimulant on yield in commercial seed potato and chip potato fields. http://www.potato.msu.edu/acrobat/publications%20for%20online/cuttreat/myconate.pdf (2/15/04)