How to Pre-cut Potato Seed Tubers

Since the last growing season there have been several questions regarding suberization of cut potato seed tubers and wound healing of potato tubers going into storage. Suberization and wound healing are the same process. When a tuber is damaged, water loss and risk of infection by bacterial and fungal pathogens is greatly increased. The damage to the tuber must heal by developing protective layers of new cells, or the tuber will rot.

The purpose of the potato tuber skin is to retard moisture loss and protect the tuber from pathogen invasion. When seed tubers are planted and the cut surface is not fully healed, it makes pathogen entry easy. This could happen from poor handling of seed (breaking the skin) or incomplete suberization of cut seed. In some cases when sprouts are long and knocked off seed tuber pieces, this can make an entry point for diseases.

Cutting seed introduces an injury that must heal, or the seed piece will not survive. If cutting and planting immediately, seed tubers can heal in the soil if conditions are favorable for wound healing. The challenge is controlling the environment in the soil for the time needed for suberization. Ideal soil conditions are when soil moisture is 60-65% and a temperature of 10-15 °C. Skin will take approximately twice as long to heal when temperatures are 7 °C compared to 11 °C and even longer at cooler temperatures (Table 1). Tubers will heal faster at warmer temperatures, but there is a higher risk of pathogen development, especially bacterial soft rot.

Pre-cutting seed has advantages and drawbacks compared to fresh cut seed. Pre-cut seed has been reported to emerge earlier, have more vigorous early growth, have a higher plant population, healthier plants, and improved yield (Table 2) compared to fresh cut seed. However, pre-cutting seed is costly and can increase risk of pathogen entry and breakdown when the pre-cut seed is not properly stored and suberized. Additionally, pointed corners resulting from cutting are more susceptible to damage when handling pre-cut seed, creating new wounds.

How does tuber skin heal? This process can be simplified into two stages. First suberin, a hydrophobic waxy material, is formed within hours to prevent water loss. Establishment of the suberin layer will prevent bacterial soft rot entry in approximately 3 days. Second is the formation of the phellogen layer, or periderm. In general, cutting and suberizing in good storage conditions will take about 7 to 21 days for a wound periderm to form. This periderm or new skin is as effective as the original skin at protecting the seed piece. However, it takes roughly 14 to 21 days for sufficient periderm formation to prevent Fusarium dry rot.

Ideal conditions for suberization of cut seed are 95-99% relative humidity (no free water), seed tuber temperature of 11-15.5 °C, and abundant oxygen to prevent carbon dioxide accumulation. If the previous conditions cannot be met this greatly increases risk of cutting and suberizing seed. When free water is left on the cut surface, this will promote bacterial diseases. It is a challenge to maintain adequate air flow, temperature, and humidity if the height of cut seed piles are greater than 3 metres.

If insufficient humidity is present in the bin or soil during wound healing, a layer of dry and dead cells may cover the cut surface and could be mistaken for a suberized layer. The resulting seed will feel dry to the touch but when planted has high risk of seed piece decay as the seed is still not healed. An easy test is to apply gentle pressure across the surface with your thumb. A layer of dead cells will usually come off, while a suberized layer remains in place.

Well managed suberized cut seed can be successfully held like whole seed. The question of how long and what temperature to hold the pile are important. This will depend on planting schedule, variety, length of storage, and physiological age of seed. Cultivars that tend to have a lower stem and tuber count typically or that are physiologically young can be stored at 11-13 °C for 1 to 3 weeks. This has been demonstrated to help overcome dormancy challenges and led to more uniform sprouting and emergence. A seed treatment can be applied prior to piling and suberization, but avoid dust treatments with talc compounds that can absorb moisture and interfere with suberization. A liquid seed treatment can be used with ultra-low volumes that dry quickly so they do not interfere with wound healing.

Other cultivars that tend to have too many stems or tubers or that are physiologically old can be suberized for a shorter time, approximately 3-14 days, and then planted. If planting is delayed well-healed seed can be cooled to 7 °C to prevent excessive sprouting

When the seed is taken to the field, ideally seed temperature should match soil temperature when planting. However, this is not always possible, so it is recommended to keep seed and soil temperatures as close as possible. Ideal soil temperature for planting is 10-13 °C. Cold seed pieces promote condensation. A film of condensed moisture around the tuber from condensation or saturated soils at 10 °C for 6 hours will favor the development of soft rot (Burton and Wiggington, 1970).

In summary, plant disease free seed, handle seed potatoes as little possible to prevent damage. When the tubers are pre-cut the best healing conditions are 11-13 °C, 95-99 % relative humidity with plenty of oxygen from fresh air. The length of time to completely heal is 7-21 days but this is highly influenced by variety and healing conditions. Cutting potato seed and the healing period can be managed to influence the dormancy of the seed potatoes and can influence the number of stems and resulting tubers per acre. When you take your seed to the field to plant manage seed temperature to be at least as warm as soil temperature to avoid condensation on the seed. Ideally, seed and soil should be the same temperature at planting.

These are general principles that have been developed studying different potato varieties. Getting to know the varieties on your farm and experimenting with them will help you determine the best way to cut and suberize seed potato tubers. Keep in mind every time a new variety comes along, take time to figure out how to best suberize the seed as there are tremendous differences in varieties. 

Resource Andy Robinson, Valley Potato Grower April 2023

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