Agronomy

Description

A cool-season crop, potatoes form under the ground. Despite growing underground, potatoes aren’t roots. Instead, they’re stems that can otherwise be called stolons. Potatoes, which are classified as a nightshade crop, originated from South America in the Andes Mountains. By the 1500s, they had made their way to Europe. Ultimately, the Irish discovered the potato’s value as a food crop. This connection to Ireland has caused potatoes to become known as “Irish potatoes” and be distinguished easily from sweet potatoes. In 1719, potatoes entered the U.S. as a shipment from Ireland.

Site Selection

Potatoes perform best when the selected growing site meets their needs. The crop prefers well-drained soil that is deep and loose. With loose soil, potatoes can develop a good shape and color, and they can have a smooth surface. Plus, the loose soil helps with setting a good number of potatoes. In soils that don’t drain well, potato seed pieces may decay, and the crop is less likely to yield high-quality potatoes. Selected sites should also have good water-holding capacity, low wind erosion risk and proper aeration.

Crop rotation is important for potato production. Roughly three to four years should lapse between growing a solanaceous crop, such as eggplant, pepper, tomato and tobacco, and potatoes. Rotating crops can help to reduce disease incidence and pest pressure. If converting sod or grass into a potato production area, then the site should undergo a grub and wireworm treatment. Sites known to have persistent perennial weed problems also don’t work well for potato production.

Fertility

A soil test can suggest fertilizer needs for a potato growing site. Specific fertilizer recommendations will stem from the soil test results and other factors such as crop intended use; potato variety grown; growing season duration; and nutrients provided by other sources, such as manure. Phosphorus and potassium are often provided in one application at planting, unless potassium needs are more than 200 pounds per acre. In that case, the potassium application is typically divided.

As another option, operations may split fertilizer treatments. At planting, the fertilizer may be applied as a band that stretches the row, and a sidedress application may follow during cultivation or hilling. Alternatively, operations may broadcast portions of the recommended fertilizer before planting and use banded applications at planting to supply the remaining nutrition. For nitrogen specifically, some growers break the requirement into smaller applications and provide a portion at planting, emergence and hilling. Regardless of the method used to apply fertilizer, the applications shouldn’t directly reach potato seed pieces.

Avoid applying too much fertilizer. At excessive levels, nitrogen may cause potatoes to not mature on schedule. Excessive nitrogen may also cause potato quality and grade to decline, compromise yields, extend the maturity time and encourage foliar growth that’s too vigorous. Potassium levels that are too high may trigger magnesium uptake issues and cause potatoes to feel less firm.

Soil pH is another important soil characteristic to consider when growing potatoes. Typically, the pH level should range from 5.5 to 6.0. If a selected cultivar is prone to developing potato scab, however, then lowering the pH to 4.8 to 5.5 may reduce the scab risk. Applying too much lime or compost may make potato crops vulnerable to scab.

Cultivar Selection

Several factors should influence potato cultivar selection. Cultivars should both satisfy marketplace demands and fit with the selected growing environment. Available cultivars vary based on attributes such as skin and flesh color, skin texture, eye depth, shape and size. Five categories — fingerling, gold, red, russet and specialty — can group potatoes based on their intended use, flesh or skin color, sugar levels and other characteristics.

· Thin, finger-like potatoes called fingerlings are relatively novel, and they work well if they’re roasted or boiled.

· Yellow skin and cream or yellow flesh characterize gold potatoes, which can be baked, boiled or mashed.

· As the name implies, red potatoes have red skin. Their flesh color ranges from white to cream to reddish-pink, and with their high sugar levels and low specific gravity, they’re well-suited for salads and boiling, not baking.

· With low sugar levels, russet potatoes work well in chipping and frying applications.

• Specialty potatoes are available in a wide range of hues, such as purple, blue and rose, and other attributes.

To minimize the risk of disease-related damage to a potato crop, growers may choose cultivars that offer a disease resistance benefit. Specific cultivars have demonstrated resistance or tolerance to diseases such as scab, early blight and blackleg.

The table outlines potato cultivars often grown in the Midwest, and it shares details about their maturity timing and other characteristics.

Recommended Potato Cultivars

Planting

Ideally, operations will plant potatoes when soil temperatures range from 45 degrees F to 70 degrees F, and soil should feel moist. Cold, wet or dry soils should be avoided because decay of potato seed pieces is more likely.

Potato producers should exclusively plant certified seed potatoes. Each seed piece should weigh roughly 1.5 ounces to 2 ounces, or if small whole seed potatoes are available, then they may serve as seed pieces. The seed pieces each should have one or two eyes, or buds. Operations that choose to plant seed pieces first must prepare those pieces by cutting down a seed potato to the seed piece size. Although it’s not a mandatory practice, allowing the cut wounds on a seed piece to heal, or suberize, is an option. It may help to prevent seed pieces from decaying. Another strategy to discourage decay is sanitizing production equipment well. Before planting, potato pieces used as seed should be stored at 60 degrees F for seven days to 10 days. This storage period will prime potato seed pieces for sprouting and emergence.

When designing the potato growing site, operations often space rows at 34 inches to 36 inches. Within-row spacing will vary depending on the potato cultivar, soil characteristics, moisture, fertility and harvested potato size. Generally, potato seed pieces are planted at every 7 inches to 12 inches. Finding the right in-row spacing may help to reduce incidence of several defects that occur when potatoes grow to be too large: hollow heart, knobs and growth cracks. Within a row, tighter spacing can reduce potato size. With respect to planting depth, a 3- to 5-inch depth is typical.

Some potato operations may choose to build raised beds and lay plastic mulch and drip tape for irrigation. With a plasticulture system, operations may raise good quality potatoes, control weeds and forgo hilling and cultivating needs. Laying plastic mulch, however, can cause soil temperatures to elevate. Mid- and late-season cultivars grown in a plasticulture system may not grow as large, given the warm temperatures.

Mulching potatoes with plastic tends to be a more viable option for small-scale operations relative to large-scale operations. Plasticulture demands specific equipment, and it leads to planting potatoes more slowly. These facts make it a more common option for small-scale operations. Additionally, removing the plastic mulch for harvest is more complicated for large operations than small ones.

Cultural Management

Hilling refers to adding soil cover to potato rows. Potato plants will require hilling before they begin blooming. Following hilling, potato pieces should have roughly a 6-inch depth below the soil surface; the specific depth will vary by cultivar. Typically, hilling will accompany early potato field cultivation. With hilling, the ridged soil that covers rows helps to reduce the likelihood that potatoes start to green. Plus, hilling may eradicate some weeds.

Water Management

Moisture access affects a potato crop’s yield and quality. It’s particularly critical during the period when potatoes are forming. During potato formation, too little moisture — or high temperatures — may limit yield potential. Inconsistent moisture availability — dry conditions followed by rain or irrigation — may cause potatoes to develop secondary growth or crack. Other defects associated with moisture availability include internal necrosis and hollow heart. For operations that choose to irrigate their potato crops, they should consistently apply light moisture during the potato’s development process.

Weed Control

Like in other crops, weed pressure is potentially harmful to a potato crop because it may reduce the water, nutrients and light available to potato plants. Plus, weeds may harbor insects and diseases that could infect the potato crop. Operations growing potatoes may address weed pressure by applying herbicides, cultivating the production area, laying plastic mulch, rotating crops on a regular basis or applying heat through flaming. If operations choose chemical weed control, then they have preplant and post-emergence herbicide options available.

Weed control through cultivation should occur on an as-needed basis. Too much cultivation may create compaction and limit yields. Cultivation can, however, effectively reduce weed pressure if the cultivation operation is planned early. The crop canopy should still be open, and weeds shouldn’t be too large. No cultivation should occur after the potato crop has fully bloomed. Keeping cultivation to being relatively shallow is also important in order protect the crop’s root system. When cultivating for hilling purposes, weeds may also be removed.

Insects and Diseases

Insects that may target potato crops are Colorado potato beetles, flea beetles, aphids, leafhoppers, wireworms and corn borers. Cutworms and blister beetles are other pests that may cause harm. To prevent significant insect damage, operations should adopt a regular scouting schedule.

Of the insects that may damage potato crops, the Colorado potato beetle presents a significant concern. Beetle larvae can defoliate potato plants. This damage can have yield-limiting effects, particularly if it coincides with potatoes sizing. To address Colorado potato beetle pressure, operations may treat the production site with a systemic soil insecticide or foliar applications. Note that the Colorado potato beetle has developed some insecticide resistance. For the most effective control, vary the types of insecticides applied.

Diseases may infect potatoes while they’re growing or while they’re being stored. Those that operations commonly face include early blight, late blight, common scab, powdery scab, blackleg, leaf roll virus, mosaic virus, rhizoctonia, verticillium wilt, fusarium dry rot and bacterial soft rot. Others include black dot, black and silver scurf, ring rot, purple-top wilt and white mold. Root knot and lesion nematodes may also cause harm. From a disease perspective, several strategies may reduce a potato crop’s risk of becoming infected. For example, operations may select disease-resistant cultivars, rotate crops on a regular schedule and apply fungicides appropriately.

Other defects known to affect potatoes include sunscald, sunburn, tipburn, hollow heart and blackheart. With hollow heart, potatoes have a hollow, or empty, space inside of them. Blackheart refers to the potato flesh developing a black color. It’s attributed to a potato’s interior having too little oxygen available due to poorly drained soil or inferior ventilation during storage. Potatoes with cracks, enlarged lenticels, heat sprouts and greening also have diminished quality.

Harvest and Storage

Potato harvest should be timed to follow the plant vines dying. This is particularly important for potatoes that will be stored. When the vines have died, the potatoes underground tend to have good skin set. They tend to not bruise as easily, and mature potatoes also have lower free sugar content.

If potato plant vines are delayed in naturally dying back, then operations may accelerate that process by applying a vine killer. Considering that cultivars today tend to keep their green vines longer, facilitating vine kill may be necessary. One option is to apply chemicals to trigger vine senescence. Alternatively, using vine beaters may also cause potato vines to die. After killing vines, waiting 10 days to 15 days to harvest may help to set potato skins and minimize bruising during harvest.

Often, operations use mechanical harvesters to harvest their potato crop. Harvest and handling should be expedited as much as possible. At overly warm and overly cool temperatures — warmer than 85 degrees F and cooler than 50 degrees F — potatoes are more susceptible to bruising. The risk of bruising also varies by soil and potato condition.

If operations harvest potatoes early, then those potatoes are considered “new” potatoes. Typically, new potatoes have diameters that measure 1.5 inches to 2.25 inches. Frequently, operations may sell new potatoes at a premium. To harvest an adequate new potato crop, operations may harvest potatoes before the crop’s vines have died. New potatoes have thin skins, so handling practices should account for that. After gingerly washing and drying new potatoes, operations should make new potatoes available at markets soon after harvest.

A 60- to 120-day dormancy period is typical for many potato cultivars. Potatoes require a storage environment with the right ventilation, temperature and relative humidity to maintain quality. During the first two weeks in storage, the potato crop should essentially be prepped for long-term storage. Cuts and bruises need to heal. In an environment that has 85 percent to 95 percent relative humidity and air movement, temperatures should start at roughly 65 degrees F. Gradually, reduce the temperature by roughly 1 degree per day. If storing potatoes for more than three months, the long-term storage temperature should be roughly 40 degrees F. The specific storage temperature may depend on the potato crop’s intended use. Throughout the storage period, retain the high relative humidity level to maintain potato firmness, discourage shrinkage and minimize bruising. To prepare stored potatoes for shipment, increase the temperature to 50 degrees F. Bruising is less likely at those warmer temperatures. Harvested potatoes exposed to light may turn a green color, so block sunlight’s reach to avoid development of the greening disorder.

Potatoes may sprout during storage. To discourage sprouting, operations may apply a sprout inhibitor. If storing potatoes for roughly three months to five months, a field-applied sprout inhibitor can be absorbed by the green vines, which route the inhibitor into potatoes growing underground. If operations plan to store potatoes for longer periods, then they may apply another inhibitor after the potatoes enter storage. The ideal application time would stretch between wound healing and dormancy break.

Sources

Bogash, Steven M., William J. Lamont Jr., Lynn F. Kime and Jayson K. Harper. 2013. Potato Production . The Pennsylvania State University. State College, PA 16801.

Brandenberger, Lynn, James Shrefler, Eric Rebek and John Damicone. Potato Production . Oklahoma Cooperative Extension. Stillwater, OK 74074.

Kaiser, Cheryl and Matt Ernst. 2011. Potatoes. University of Kentucky. Lexington, KY 40506.

Midwest Vegetable Production Guide for Commercial Growers. 2017. Potato . Purdue University. West Lafayette, IN 47907.

Nair, Ajay, Donald Lewis, Vince Lawson, Laura Jesse, Lina Rodriguez-Salamanca. 2017. Commercial Potato Production Guide . Iowa State University. Ames, IA 50011.

Purdue University. 2017. Midwest Vegetable Production Guide for Commercial Growers . Purdue University. West Lafayette, IN 47907.

Yanta, James P. and Cindy B. Tong. 2013. Commercial Postharvest Handling of Potatoes (Solanum tuberosum) . University of Minnesota Extension. Minneapolis, MN 55455.