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The Why, When, and How of Potato Seed Evaluation

Thomas A. Zitter and Rosemary Loria of the Department of Plant Pathology at Cornell University offer their expertise for the Why, When, and How of Potato Seed Evaluation. They recommend:

Why. Most of the tuber defects described, when present on seed, can cause substantial reductions in yield or quality in the subsequent crop under the right environmental conditions. However, some tuber problems are much more likely to cause significant losses than others and, consequently, serve to answer the question of why potato seed evaluation is important. Diseases, such as ring rot, late blight, and leafroll (net necrosis), that are carried on or in the seed and have the potential to spread very quickly through the crop are considered to be very important. Tubers infected with such diseases must be properly discarded as soon as they are detected. Other diseases, such as Rhizoctonia black scurf and Pythium leak, that do not have significant secondary spread and for which other sources of inoculum are usually more important are considered to be less serious. Thomas However, remember that the absolute losses resulting from specific tuber problems will depend upon environmental conditions and disease management practices. The most important aspect of disease management in potato production is the use of certified seed potatoes. There are two basic classes of seed potatoes–foundation and certified. Requirements for the production of foundation class are much more rigid. Another important aspect of disease management in potato tubers is the control of disease spread in storage. Many diseases can increase significantly during the storage period, and management of the environmental conditions in the potato storage is often a critical component of control.

When. Although most of the disorders discussed can be detected just before planting, many can also be identified in seed production fields or in storage facilities. Early detection of seed disorders provides the seed grower with additional management and storage options. It provides the seed purchaser with knowledge of the condition of the seed acquired. Shipping point inspection, the final step in certification, cannot be overemphasized. It is paid for by the grower and serves as protection for both grower and buyer. It should be kept in mind that certification is not complete until the seed has been graded in conformity to certified seed grades and identified with official tags and has passed inspection by the State–Federal Inspection Service. If a problem should arise as to seed quality once the tubers are delivered to their final destination, the buyer may request a second inspection, at his or her expense, to determine if the seed lot is in compliance with the stated grades. Without inspections and shipping tags, the old adage still applies, "Let the buyer beware." Figure 1 illustrates a generalized picture of potato production in the Northeast and indicates the stages during which specific diseases and disorders affect potato production and storage and when they are most apt to be detected.

How. The 20 disorders described have been grouped into three categories, based on the location of tuber symptoms:
External. Four of the diseases and defects are diagnosed entirely from symptoms that occur on the exterior of the tuber. The position of the symptoms on the surface of the tubers, relative to the stem attachment, bud end, and lateral eyes, is often important in diagnosis.

Internal: Five of these disorders usually produce no diagnostic symptoms on the exterior of the tuber, and diagnosis must be based solely on internal symptoms. Specific tissues in the interior of the tuber must often be examined for symptoms. Therefore, either cross–sectional or longitudinal cuts must be made through the tuber to observe symptoms.

External/Internal: Eleven tuber disorders discussed here usually require examination of both external and internal tuber symptoms for diagnosis. Although the symptoms are noted externally, it is best that tubers be cut to determine to what depth the lesion has progressed. This is particularly helpful for early and late blight.
More–detailed information on symptomology is provided in the descriptions of the individual diseases and defects. Because knowledge of the location of structures and tissues in and on the tuber is important for identification of tuber diseases and defects, this terminology has been illustrated on the cover. The following terms are used in disease descriptions: periderm (common scab, powdery scab, silver scurf, mechanical injury and cracking, black spot); lenticel (bacterial soft rot, pink rot); stolon (blackleg); stem end (blackleg; Fusarium dry rot, leak, pink rot, ring rot, black spot, Fusarium wilt, net necrosis); pith (blackleg); phloem (freezing and chilling injury, net necrosis); vascular ring (ring rot, Fusarium wilt, net necrosis, Verticillium wilt); cortex (late blight, root knot nematode).

Many excellent references are available on potato disease control; but because some control recommendations change with time, only minimal information on this subject is included. Information on control, primarily with cultural practices, is included in the description of the individual disorders. 

Taken from: (Zitter, T.A., Loria, R. "Vegetable Crops", Cornell University Vegetable MD Online)
 

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