Tuber size is an increasingly important aspect of potato quality. Buyers all along the potato market chain from those purchasing seed to end users (fresh market and processing) are requiring a uniform and consistent tuber size profile. Some businesses have implemented clauses in contracts with sizable economic incentives rewarding growers who meet specific tuber-size specifications. These same contracts usually penalize growers who produce a potato crop with too many small or large tubers. Tuber size profile is closely related to the number of tubers set per plant.
One reason obtaining a consistent tuber size profile can be so difficult is that the number of tubers produced by potato plants varies widely among years, fields and even among plants within a field. Throw in the fact that potato cultivars also have a wide range of tuber set characteristics, and it becomes clear that getting uniform size cannot be easily accomplished.
While much of the year-to-year variation in tuber set can be traced to environmental conditions, it is likely that most of the field-to-field and plant-to-plant variation can be attributed to management practices. To increase the uniformity of tuber set requires growers to focus on the following management practices: 1) seed size and physiological age, 2) early-season plant growth, 3) disease management, 4) nitrogen management and 5) plant spacing.
The number of stems produced by a potato plant is directly related to the number of tubers that plant will set. Because each stem tends to produce a certain number of tubers, the higher the number of stems, the more tubers that will be produced by each plant. Having more tubers per plant can be advantageous for cultivars like Shepody that set few tubers with many of them oversized. However, the opposite is more often the case for cultivars such as Russet Burbank, which tends to produce a high number of undersized tubers. Both seed piece size and physiological age influence the number of stems produced by each plant. Every eye on a seed piece has the potential to produce at least one stem. Consequently, the more eyes per seed piece, the more potential stems. Additionally, as seed ages, the number of stems produced by each eye increases.
The key to producing a more uniform tuber set is getting a consistent number of stems among plants by reducing variability due to wide ranges in seed piece size or physiological age.
Recent research has shown that stolon production and subsequent tuber set occurs over a relatively short period of time after plants emerge. Anything that slows plant growth during the first 21 days after emergence was shown to reduce the number of tubers set plant. Plant growth during this time frame may be negatively impacted by soil compaction, deep planting and low soil moisture.
Rhizoctonia stem and stolon canker often infects plants during the early part of the growing season-delaying emergence by girdling sprouts and pinching off stolons while setting tubers. Rhizoctonia does not infect all plants uniformly, thus causing an increase of the variability in tuber set among plants. Seed piece treatments and in-furrow fungicides that control Rhizoctonia may likely improve the uniformity of tuber size profile under conditions where this disease is widespread.
Indeterminate potato cultivars, such as Russet Burbank, tend to set fewer tubers per plant when high levels of nitrogen are applied at planting. The exact mechanism for this reduction in tuber set in not known, but may be partly accounted for by the delay in the timing of tuber initiation caused by high levels of soil available N.
Another management practice that has a large influence on the number of tubers per acre is in-row plant spacing. As seed pieces are spaced closer together, tuber numbers per plant typically decreases. However, because the seed pieces are spaced closer together, the resulting total plant population per acre increases, and the overall tuber number per acre will also increase.
From previous studies we know that potato planter performance is typically less than desirable. A lot of factors such as wide variability of seed piece sizes and shapes, incorrect planting speed, planter design and poor equipment maintenance result in highly variable seed piece spacing, which can affect tuber set.