All plants vary in their water requirements according to their size and growth stage as well as the length of their maturity and time of year of maximum growth. Possibly no other major crop varies in its sensitivity to water stress based on growth stage than potato. In this section, irrigation recommendations at key production periods are based on the S-shaped growth curves of roots, vines and tubers (Pavlista, 1995)(Table 3). Soil moisture requirements are related to different growth stages (van Loon, 1981). Quality effects of water deficit and excess during these stages are described (Figure 2) (Curwen, 1994 and personal communication). Table 4 summarizes the effects of low and high soil moisture during tuber bulking and maturation stages.
1. Pre-planting to Planting:
A pre-plant irrigation is often recommended for two reasons. First, soil moisture should be about 70-80% field capacity. This will bear-saturate the field, allowing some room for rains. This level amounts to around a quarter of the allowable deficit (AD) of the soil. Soil moisture should be acceptable to support the developing roots after planting and reach emergence. Another benefit from a “pre-irrigation” is the breaking down of clods and clumps for better planting.
2. Planting to Pre-emergence (Sprouting):
[early Stage I. Vegetative (van Loon, 1981)]
Soil moisture in the top foot of soil should be 65 to 80% FC. No irrigation is recommended during this production period. First, seed-pieces at a recommended size, 2 to 2.5 ounces, have sufficient water to support the sprout until emergence. Irrigating during this period would raise the soil moisture and lower soil aeration to a level that would support several pathogens, most notable bacterial soft rot or black leg (Erwinia carotovora), and stem and stolon canker (Rhizoctonia solani). Excess moisture will also decrease tuber respiration, putting the seed-piece under metabolic stress. There is also good research data indicating that the soil population of Verticillium albo-atrum will increase and cause early dying at mid-season. Note, on the other hand, that a water deficit, too-dry soil, will decrease the healing of the cut surfaces of seed-pieces, inhibit root growth and increase susceptibility to soil pathogens such as Fusarium spp. and Rhizoctonia. In short, pre-plant irrigation and seed-piece water are more than sufficient to carry the sprouting tuber.
3. Emergence to Tuber Initiation (early vine growth):
[Stage I. Vegetative]
This is the log phase of vine growth. Roots are in the second half of their growth. During this period, the vine grows very rapidly, as much as doubling the canopy every week. With rapidly increasing foliage every week, irrigation starts low, 0.5 inch, and gradually increases every week by about 0.5 inches. At tuber initiation, about three weeks after emergence depending on variety, seed health, weather, soil, and cultural practices used, about 1.5 inches of irrigation is applied. A soil moisture of 70 to 80% is preferred, less than 65% FC would be considered a deficit. Water deficiency at this point would inhibit canopy and root growth, and indirectly weed control by less ground cover. An excess would retard root branching (development) by water-logging root hairs and promote nitrogen leaching. In short, with an increase in foliage and thereby transpiration, irrigation should begin and gradually increase as the canopy grows.
4. Tuber Initiation to Full Bloom:
[late Stage I. Vegetative or Stage II. Tuberization]
In determinate varieties full bloom marks the end of vine growth, while in indeterminate varieties full bloom starts a noticeable slow-down of vine growth, some branching still occurs. The first set of tubers are being initiated and these are in a slow-growth, development stage, the lag phase of tuber growth. Irrigation becomes increasingly important and water stress becomes less tolerable. Transpiration reaches its highest rate. Optimal soil moisture is 80 to 90% FC. Irrigation increases from about 1.5 to 2.5 inch per week on sandy type soils. Water deficit would dramatically increase tuber malformations and sugar-ends. It can also weaken plants, promoting early blight. Common scab (Streptomyces scabies) attack is promoted and the longer the deficit, the greater the attack and more pronounced and enlarged the blemishes. In areas and with varieties prone to common scab, maintaining soil moisture at 90 to 95% is suggested if possible. Excessive water will increase brown center and hollow heart of larger tubers, and promote early dying of the vine. Too much loose water, swampiness, can also promote late blight, and weaken plants promoting early blight. In short, soil moisture levels must be increased and therefore irrigation is increased. Note, also that this stage of the plant often corresponds with June and July and the hottest of weather. The length of this period is also related to variety, weather and cultural practices. It may be prolonged by excessive nitrogen.
5. Full Bloom to Plant Senescence (Tuber Bulking):
[Stage III. Tuber Growth]
At this period, the canopy and roots are fully grown except for indeterminate varieties, which have considerably slowed growth. However, now, tubers are growing rapidly and are in their log phase of growth. Here, it is key to keep in mind that tubers are 76 to 82% water and this water must come from the outside, rain or irrigation. This period runs about six weeks, usually in July and August. Irrigation plus rain should be 2 to 2.5 inches per week or about 15 inches for the period. Soil moisture should be at 80 to 90% FC. This is the period when plants have their highest demand for water and are the most sensitive to a deficit. Water deficits here will reduce tuber growth but also there would be increases in tuber malformations, early dying (Verticillium and Fusarium wilts), early blight and brown spot (Alternaria solani and A. alternata), and common scab. Water excesses increases hollow heart, swollen lenticels (stomates on tuber), black leg, late blight (Phytophthora infestans), and susceptibility to soft rot, leak (Pythium spp.) and pink rot (Phytophthora erythroseptica). In heavy vined varieties, white mold (Sclerotinia spp.) may occur. Note that from tuber initiation through tuber bulking, evapotranspiration is very high and therefore daily water use by the plant. In short, too little water will create stress making plants susceptible to opportunistic diseases, promote common scab, and drastically reduce yields and increase culls. Excessive water will increase water rots of vines and tubers, and create conditions for late blight infestation.
6. Plant Senescence to Harvest:
[Stage IV. Maturity]
This period is characterized by dying of the vine; in the case of indeterminate, lower leaves are dying. Tuber growth slows and is in the flat stage. Tuber maturation is a common term used here as tubers settle to their maximum content of dry matter and minimum content of reducing sugars, glucose and sucrose. As the vine dies, tuber skin sets, hardens and adheres to the tuber core (flesh meat etc.). Irrigation declines over this two to five week period depending on variety and climate. Soil moisture may decline to 60-65% FC. Some irrigation may benefit in wireworm and white grub control, and will avoid soil clumping making harvest easier. If the field has early blight, too much watering runs the risk of washing spores of this pathogen to the tubers. Excessive irrigation will not only stimulate tuber susceptibility to water rots, soft rot, leak and pink rot by swelling lenticels but also form an oxygen-deprived environment that promotes the pathogens the cause these rots. Water-rotted tubers can create a packaging and storage nightmare. Also too much water will increase tuber susceptibility to shatter bruise due to raised tuber water content. The reverse, too little soil moisture, can increase internal black spot bruising (IBS) as well as delaying skin set. Skin russeting of russet varieties is decreased.
Figure 3 summarizes the relations of the production periods and the relative growth of roots, canopy and tubers to field capacity or soil moisture. The graphic model is based on a determinate, mid-season variety such as Atlantic grown full season in western Nebraska. Planting would be around 1 May and emergence would be about three weeks later. Tuber initiation would be typically three weeks after emergence, mid-June. Tuber bulking would be from end of June to end of August with vine desiccation from then to mid-September and harvest from early September to early October.