Certification Methods:

 

Certification inspectors typically inspect potato fields at least twice during the growing season. The first inspection tends to concentrate on viruses, off-types, blackleg (bacterial infection) and other diseases which show symptoms early. The second inspection covers the preceding as well as Bacterial Ring Rot, Late Blight and other late-occurring problems. Because of its devastating effects, there is an absolute zero tolerance for BRR in all certification programs. The zero tolerance is also in effect for other problems such as certain nematodes, tuber moth and powdery scab.

Certification inspectors typically inspect fields from one corner diagonally across the rows because some problems (BRR, for example) may be expressed row by row. The inspector assumes that he is viewing a certain number of plants in each row. By counting the number of problem plants and rows, he/she can fairly accurately estimate the number of problem plants. If the percentages exceed certification tolerances, the field is rejected.

Because field inspection is by necessity a visual process, some plants may be questionable and call for laboratory verification by ELISA, Latex Agglutination, Electron Microscopy or other methods. Most Limited-Generation schemes require laboratory testing for PVX through the first one or two field generations.

The second field inspection typically occurs well before plants have died. Therefore, the potential for undetected late-season infection is high. For this reason virtually all certification programs require some form of winter grow-out testing. For winter tests, a percentage of the tubers (usually 400 or so in Oregon) for each seed lot are planted and the resultant plants are visually examined for disease symptoms and herbicide injury. As in field inspections, questionable plants and all plants of varieties which do not express PVY symptoms are lab tested.

Because of all these constraints, seed potato production is a “white knuckle ride” and growers never know whether their crop will be certified until late in the winter in most instances.

Breeding and eye-indexing:

Because breeding clones (selections) must be kept healthy for further increase and testing, breeders go to extraordinary means to prevent disease buildup. These means include electron microscopy and a number of sophisticated serological tests including Enzyme-Linked Immunosorbent Antibody, Latex Agglutination and numerous others. 

One traditional tool used by all breeders, and perhaps the most effective, is “eye-indexing”.Eye indexing typically begins with single- or five-hill selections and continues throughout the remaining 10 or 12 years leading to release. Eye indexing involves removing an eye from a tuber using a melon scoop, numbering the eye and the mother tuber identically, and then planting the eye to produce a plant for symptom evaluation. Grow-out plants are evaluated for viruses and other problems when 8-12 inches tall. Those with obvious problems are noted and the corresponding tubers are discarded. Only tubers of healthy appearing plants are kept for replanting and increase. Eye-indexing is an essential component of most breeding programs because of the large numbers of progeny involved and severe virus pressure.

 

For more information on seed production, see http://cropandsoil.oregonstate.edu/fpsp/index.html; http://www.oscs.orst.edu/potato/index.shtml; and http://oregonstate.edu/potatoes/potliv.html#Seed

 

6. Variety Selection  

 

 

 

(see http://oregonstate.edu/potatoes/variety.htm for additional information and images of varieties)

 

Potato breeding typically proceeds as follows: Male x female crosses ® berries & seeds ® seedling transplants ® plants (clones) in pots ® tubers ® single-hill field trials (2% saved) ® 5 hill trials (5% saved), etc. ®etc. for 12 more years until the clone is named and released. These fairly typical steps are illustrated in the Oregon flowchart below.

 

Transferring pollen from one plant to another produces seeds (TPS) which can be grown into plants producing tubers.

Tubers can be propagated ad infinitum by division into seedpieces.

 

 

Considerations for selecting varieties

Many varieties and types are available to commercial potato growers; potato tubers come in many color combinations! Varietal selection at the commercial level is ultimately determined by the profit motive. Commercial growers will only use varieties they expect to be marketable and profitable. Home gardeners have more leeway.

 

The following are important varietal selection criteria if profits are to be maximized:

 

1.Intended use – frozen processing, chips, starch, fresh market (table) use, roadside marketing, etc.;

2.Color, shape, season of maturity, storage dormancy, yield, grade-out;

3.Resistance to diseases, pests and disorders; and 

4.Adaptability to local growing conditions

5.Express symptoms when pathogens are present

 

 

Common processing varieties include: 

Specific Gravity	% Starch	Texture	Common Usage
<1.060 (v. low)	<16.2	V. Soggy	Salads, pany fry, can, boil
1.060-1.069 (low)	16.8-18.1	Soggy	Pan fry, salads, boil, can
1.070-1.079 (medium)	18.2-20.2	Waxy	Boil, chips, fries, may slough when boiled
1.080-1.089 (high)	20.3-22.3	Mealy, dry	Bake, chips, fries, some varieties slough when boiled
>1.089 (v. high)	>22.3	Very mealy, dry	Bake, fries, chips; tendency to produce brittle chips and slough when boiled

Starch, why is it important to processors of fries, chips, flakes and granules?

1.Higher starch means less water in tuber tissues;

2.Water is displaced by fat/oil during frying, so high water/low starch potatoes soak up more oil;

3.Oily fries, chips are soggy and unappetizing and often dark in color;

4.Oil is expensive;

5. High starch favors high product yield; for example, 

20% dry matter potatoes (high starch) will theoretically produce more pounds of chips/ton of raw product than 15% (low) starch potatoes while also requiring less oil.

Specific gravity, as shown in the table above, is a direct indicator of dry matter content of potatoes and, indirectly, starch content. The relationships are not exact and can be influenced by a number of factors including water and tuber temperatures, wetness of the tubers, etc. The specific gravity of pure water is 1.000, so potatoes are heavier than water and will sink unless the water is amended with starch, sugar or other materials to increase the specific gravity of the solution. Specific gravities are often determined by weighing a sample of potatoes in water and then in air, according to the following formula:

 

Specific Gravity = weight in air/weight in air-weight in water

A potato hydrometer produced by gluing a long, sealed, indexed tube to a float from which a basket is hung is also used for determining specific gravity. The hydrometer requires that an exact weight of potatoes be placed in the basket so that specific gravity can be read directly from the index markings on the tube. The weight in air/weight in water method is typically preferred.

Sugar, why is it important to processors?

 

1.High reducing (6-carbon or hexose) sugars cause dark-colored fried products, especially chips and French fries. Reducing sugars reduce copper in some obscure chemical reaction.In potato, reducing sugars are predominantly the 6-carbon sugars glucose and fructose, which result from the breakdown of starch from tuber reserves or the breakdown of sucrose transported from the plant leaves to the tubers. Sugars, the plant’s energy currency, are stored in tubers as starch. Starch is basically a long-chain polymer of sugar molecules. The relationship between photosynthesis and tuber carbohydrate can be crudely described as follows:

·Photosynthesis > glucose/fructose in leaves > sucrose in phloem transport system > glucose/fructose at action site(s) for respiration and/or storage as starch (a long, endless polymer of sugars) in tubers; starch is therefore a reserve energy currency.

 

Principle: for processing, we want high starch and low sugars almost always.

 

 

 

Fresh market varieties can be any of many shapes and colors

1.Round reds (red skin, white flesh) – Norland, Pontiac, Redsen, Winema, Mazama, Modoc, Bison, Viking, Red LaSoda

2.Long russets – Russet Norkotah, Century Russet, Klamath Russet, Russet Burbank, Russet Legend; Gem Russet,Klamath Russet

3.Round whites – Superior, Katahdin, Kennebec, Sebago

4.Specialty/gourmet types—red skin/yellow flesh (Desiree); purple skin, flesh (All-blue); yellow flesh (Yukon Gold, Bintje); fingerlings (LaRatte, Yellow Banana, Yellow Finn, Peanut) and so on. See http://oregonstate.edu/potatoes/variety.htm#OtherVarieties for additional listings, descriptions and pictures.

 

 

7. Planting Site Selection and Soil Preparation

·Soil type – As with most crops, potatoes appreciate a well-drained, coarse-textured soil--but in a very big way! A loose, well-drained loamy or sandy soil:

1.Favors a long season (early planting/late harvest);

2.Is more workable and resists crusting from rain or irrigation;

3.Maximizes yield and quality (yield is somewhat proportional to the length of the growing season; for example yields in the short-season Willamette Valley and Central Oregon average about 20 tons/acre while the Hermiston area produces 25-30;

4.Improves aeration which reduces seed piece and tuber decay; and

Note: Erwinia softrot bacteria prefer anaerobic (oxygen-free) conditions associated with wet, crusted or poorly drained soils; under wet conditions, bacteria easily move in free water and invade the tuber via wounds (from fungi such late blight or Fusarium sp., pinkroot,insects, etc.) and lenticels (substomatal cavities) in tuber skins. Once inside the tuber, bacteria multiply rapidly and decay seedpieces and/or tubers. Free moisture and bacteria are a lethal combination for potatoes either in the field or in storage.

A lenticel is a substomatal cavity (remember, the tuber is a modified herbaceous stem). Stomates are ruptured as the tuber skin expands during growth so that guard cells, etc., are displaced and only the cavity truly remains. Lenticels become whitish and very prominent in the presence of excess moisture. Enlarged, infected lenticels are a good indicator of wet conditions.

 

5.Reduces tuber malformations caused by mechanical resistance to expansion. Rocks can be especially deforming.

6.Improves fertilizer and water use efficiency

·Soil Preparation:

1.Potatoes do not require a fine seedbed as with grains and most seeded crops, so don’t overdo it! In fact, potatoes suffer from excess preplant tillage because of potential soil crusting and associated anaerobic (low oxygen) conditions which favor seed piece decay and stand loss.

2.Never irrigate prior to emergence unless a drought situation exists! If the soil is excessively dry, irrigate well ahead of time and plant when the soil is still moist but not wet.

Example: in 1998, the OSU Seed Certification crew over-watered the greenhouse between eye-index plantings and emergence and successfully destroyed (rotted) 90% of the eye index seed pieces! In contrast, under normal preemergence irrigation in 1999, more than 90% of the plants survived.

 

What is eye-indexing? Remember the explanation from earlier?

 

8. Preparing Seed 

North American producers mostly plant seedpieces cut from large, whole tubers.

 

 

Seed tubers should always be warm (>50F) and dry when cut. Warm tubers, especially those whose "eyes" are just beginning to “peep” (that is, with buds or eyes just beginning to enlarge), are physiologically active and begin to heal and emerge rapidly after planting.

Seed pieces should ideally be cut, treated with a suitable fungicide and planted immediately after under good field conditions. Good field conditions imply warm (>45F) and moist but not wet soils. If seed can not be planted immediately because of scheduling or weather complications, special storage precautions (55-60F, high humidity, good ventilation, shallow piles) should be taken to prevent overheating and disease spread and promote rapid healing.

·Cutting causes problems!. Most U.S. seed tubers are large and require cutting; however, some growers, especially in the midwest where seed piece decay is a severe problem, prefer small whole seed tubers called single drops. Because the tubers are not cut, single-drop seed is highly resistant to decay. Most of Europe and the rest of the world use whole seed.

Cutting is a very effective way to spread most potato diseases including some viruses and most bacteria and fungi. Therefore, strict sanitation should be observed at all times, but especially between seed lots. If the seed lot has an obvious disease problem, discarding the seed is a legitimate option. Local potato extension people or other experts should be consulted if serious disease levels are suspected.

Cutters and handling equipment should be thoroughly cleaned and sanitized between lots to prevent spreading diseases from one seed lot to another. For example, if a seed lot has Bacterial Ring Rot, a highly infectious disease, it is vital that the source of the BRR be tied to a specific lot/grower. Seed growers almost invariably pay for ring rot problems provided the commercial grower can provide proof of guilt. If BRR, or other serious seed quality problems are suspected, look for an independent third party inspection and unbiased “expert” witnesses for corroboration. Florida studies from a few years ago demonstrated that a 1% BRR seed infection could result in 100% infection of cut seedpiecs due to contamination and spread throught the cutting equipment.

A seed cutter’s checklist:

1. Warm seed to 50F+ for a few days before cutting to prevent tissue tearing, wake the seed up physiologically, and get it ready for rapid healing and early emergence. Sprouts should just be “peeping” when seed is cut.

2.Seed is mechanically cut at the rate of 1 to several tons/hour/cutter.

3.Keep cutter blades sharp and clean; a sharp blade does less tissue damage.

4.Keep a close eye on the seed being cut—diseases, varietal type, mechanical condition, excess debris, freezing or heat injury, etc.

 

5. Keep a record of planting sequences and where seed lots are located in fields for insurance, legal purposes.

 

6. Always sanitize between lots to prevent disease spread between lots.

·Seed Treatment: Apply fungicides to cut seed pieces immediately after cutting while surfaces are still moist enough for powder to adhere; fungicides are also sometimes applied as liquids either as a spray or dip. Liquids may cause bacterial problems in some situations because of the water involved, but typically not if the seed is handled properly.

Common seed piece fungicides include – Captan, Dithane, Manzate, Maxim, Tops; most are applied as a dust at the rate of 0.5 – 1.0 lbs/100 lbs. of cut seed and can be very beneficial, especially under less than perfect planting/pre-emergent field conditions. Labels for these and other agrichemicals can be found at http://www.greenbook.net or http://www.cdms.net. Other materials such as straight Douglas Fir Bark or talc are sometimes used with success. Douglas Fir bark may act as a simple drying agent to prevent anaerobism and movement of bacteria, but it is thought to have fungicidal properties as well?

·Precutting???

In general growers prefer to cut, treat and plant if conditions are good (moist but not wet soil > 45F). Some growers pre cut seed up to a month early, but Pre- cutting is dangerous because of possible decay.

 

9.Planting