Post Harvest Tests

Post-harvest tests (PHT) are conducted annually by PCAN to assess seed quality and to ensure standards are met as agreed upon in the Nebraska seed potato Rules and Regulations. These tests are used to detect the amount of current season spread of virus and other diseases masked under Nebraska conditions.  Samples for post-harvest testing are to be sent to the PCAN office no later than October 15.

Requirements for sending and collecting PHT samples

  • Tubers should be selected at random from the field, harvester, conveyor or storage unit from which they have been pulled.
  • Individual seed pieces should be roughly the size of a golf ball, about 1 7/8” to 2”. This makes planting easier and reduces freight costs.
  • Do not send seed that is rotten, deteriorating, damaged or too large.
  • Samples should be properly labeled. One tag should be attached to the outside of each bag, and another tag placed inside. Grower name, harvest date, lot/field number, tuber number, crop year and variety should be included on each tag.

Minimum number of tubers according to lot size

  • 0 to 1 acre: 125
  • 1 to 10 acres: 250
  • 10 to 70 acres: 500
  • 70+ acres: 1,000
  • Nuclear lots too small to sample (2,000 pounds or less) are not required to be post-harvest tested if they are to be recertified in the state of Nebraska.

PHT Preparation Procedure

An essential part of the post-harvest test is the preparation procedure. This is done for the purpose of breaking seed dormancy, so samples can germinate and grow during the winter.  The process is two-fold.

The first stage involves samples being dipped in a gibberlic acid (GA)-water solution (Figure 2) , after being placed in a warm space for a period of time (Figure 1). 

Figure 1

Figure 2

After a period of drying, the samples are then treated with bromoethane (C2H5Br) in an enclosed container (Figure 3).

Figure 3

Winter Grow-Out

Samples are then planted in the Fall, often in a warmer location so growth is possible (Figure 4). 

Figure 4

At maturity plants are visually inspected for the presence of varietal mixtures, PVY, PLRV and other diseases (Figure 5). In addition to visual inspections, leaf samples from each plant are lab-tested for PVY and PLRV using an ELISA test. Growers are then informed of the results of these tests as a percentage based on the number of plants tested in the laboratory or evaluated in the field. 

Figure 5

Tolerances for PHT

Lots with serious virus or varietal mixtures in the test samples which exceed the field tolerances listed in Figure 6 are not recommended for further certification.  Lots of double the field tolerances for G5 will not be eligible for re-certification. If a seed lot exceeds its virus or varietal mix tolerance for its generation level, but does not exceed double the field tolerance, it is not recommended for re-certification.

Figure 6. Tolerances for various diseases and varietal mixture for PHT

Class:

Foundation

 

Certified

Disease

Generation:

Pre-nuclear

Nuclear

1

2

3

4

5

0 thru 5

Leaf roll

Mosaic

Total Virus

V.M.

0

0

0

0

0.1

0.1

0.2

0.1

0.2

0.2

0.4

0.2

0..3

0.3

0.6

0.3

0.4

0.4

0.6

0.4

0.5

0.5

0.6

0.5

0.6

0.6

0.6

0.5

1.0

2.0

3.0

0.5