Post Harvest Testing

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The purpose of the post harvest test (PHT) is to preview a seed lot's disease status for the next crop year. Late season virus transmission by aphids is difficult to detect in field inspections as they seldom produce recognizable symptoms. A reliable PHT identifies if virus infections have occurred and enables the grower to determine the crop’s suitability for seed purposes.
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The PHT is performed on a random sample of seed tubers collected by the seed grower. Sample collection is discussed later in this section. The PHT is not part of the Canadian Seed Potato Certification program. It is an industry-administered program designed to provide a level of assurance of seed potato quality, which is not achievable through the official certification program. Participation by growers in the PHT is voluntary. The value of the PHT is underestimated, since the test is not considered a condition of certification. Nevertheless, it is suggested that all seed lots and fields should receive a PHT.

There are three options for PHT available to the industry.
These include:

Visual inspections of the growing plants at southern grow-out sites (California, Florida or Hawaii)
Visual inspections at the southern grow-out site combined with a laboratory test (E.L.I.S.A.) on leaf material collected at the southern site
A laboratory test (E.L.I.S.A.) of leaf tissue collected from plants produced in a growth chamber or greenhouse from tuber samples received from the grower.

The reliability of the PHT results depends upon the following:

The quality of the sample collected.
The post harvest test method. Both the grow-out and laboratory PHTs are capable of detecting viruses, but only the grow-out test can detect foreign varieties. The grow-out test is based on a visual assessment of disease symptoms. Expression of visual symptoms is subject to many external factors (i.e. growing environment and variety), which limits reliability. The laboratory test (E.L.I.S.A.) is more reliable at quantifying virus levels because it is not affected by external factors. Certain varieties such as Shepody and Russet Norkotah normally mask the field symptoms of mosaic. A laboratory PHT for PVY should be a standard practice for these cultivars.

Seed potato tubers for Post Harvest Testing (PHT) and for BRR testing must be selected at random to provide an unbiased sample that is representative of the whole field or lot. To ensure the samples submitted for testing truly represent the seed crop, it is suggested that seed growers collect a field sample of about 1000 tubers from each field. A test sample can then be randomly collected from the field sample.

The most efficient method of collecting a representative tuber sample from the crop is to collect the field sample during the harvest operation when all tubers are equally accessible. To ensure a representative sample is collected, one farm staff member should be given the training and responsibility for collecting the tubers. The sample collector should be instructed to collect a uniform number of suitably sized tubers (1 to 2 ounces (30-60 g) from every load delivered to the storage. The number of tubers collected per truck arriving at the storage will depend on the size of the field being harvested.

When the harvest of the field is complete, the field-samples should be bagged, labelled (inside and outside the bag), and placed into proper storage for safekeeping. This field sample will be later sub-sampled for the individual tests.

Four hundred (400) suitably sized tubers are required for most post harvest tests. These samples are randomly collected from the field-samples. It is suggested that the remaining portion of the field-sample be held in the storage as long as possible. This material may be useful if for testing for the presence of genetically modified organism (GMO) or BRR.

Four hundred (400) 1-2 oz (30-60 g) tubers are required for the grow-out PHT. The total weight of the sample should not exceed 50 lbs (22.7 kg). Note: Samples containing tubers in excess of 3" (7.6 cm) in diameter may be rejected for the grow-out PHT. All samples should be packed in new bags and labelled twice (inside and outside) with a tear proof label providing the following information:

The grower’s name
The Variety
The Class
The Certification Number for current production year
Grower field numbers.

Tubers submitted for the laboratory PHT or BRR test could be any size since only portions of the tubers are sent to the laboratory.

BRR Testing
The current regulatory requirement for testing for Clavibacter michiganensis subsp. sepedonicus, the causal bacteria of Ring Rot, is that all certified seed potato lots except for Pre-Elite, Elite I, and Certified classes, that are sold by a farm unit, must be tested. Contact the Canadian Food Inspection Agency (CFIA) for a list of BRR accredited laboratories. The size of the BRR test sample required is based on the number of hectares that were entered for certification.

Total size of seed potato field	Number of tubers
Larger than 1.000 hectare	400 tubers
between 0.500 and 1.0 hectare	200 tubers
between .250 and .449 hectares	100 tubers
between .025 and 0.249	50 tubers
less than 0.025 hectares	1 tuber each form 1% of the number of plants harvested (minimum of 5 tubers maximum 50)

A note of caution: A negative BRR test is not a guarantee of freedom from ring rot and diligence is required when purchasing seed, since the producing farm's history with the disease is something more important than the results of the laboratory test.
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Written by D. Lidgett

For more information about the content of this document, contact Shelley Barkley.
This information published to the web on February 2, 2005.
Last Reviewed/Revised on September 5, 2008.