Post Harvest Tests
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 |
|