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Certified Seed Purchase – 10 Questions to Ask

1) Can you provide a North American Certified Seed Potato Health Certificate for this seed lot?

The North American Certified Seed Potato Health Certificate provides a certification number and the results of all required inspections and laboratory tests completed for each certified seed potato lot produced in Canada and the United States. This certificate allows you to determine incidence of tuber-borne diseases in seed lots prior to purchase (see #2, below).

In addition, this form describes where the seed lot originated, the names of all farms that the seed lot was grown on, and how many years that seed lot has been grown in the field. Should you have a problem with the seed potatoes, the health certificate will help seed certification agencies identify the source of the problem and it may help you more quickly decide how to manage the pest or disease that is causing the problem.

Contact information for seed potato certification agencies and links to certification regulations for each state are hosted on the Potato Association of America website here Seed Certification Agency Contacts and Certification Regulations.

2) What are the post-harvest test results for this seed lot? What was the lot tested for?

Nearly all seed potato lots in North America are subject to post-harvest testing, but the methods used, tests conducted, and use of the test results differ by state. For example, some states grow seed lots out in southern states, such as Hawaii, and perform visual and/or lab inspections on these plants for evidence of virus infection, herbicide damage, and variety mixture. Other states use only laboratory testing for a subset of viruses, bacterial ring rot, and/or soft rot bacterial pathogens. States that rely on laboratory testing only will not be able to provide post-harvest data on herbicide injury, variety mixture, or emerging pathogens that are not routinely tested for.

In addition, the thresholds for certification vary by state. For example, states east of the Mississippi generally only certify seed if PVY incidence levels are at or below 5%. In contrast, PVY thresholds for certification are variable in states west of the Mississippi. In some states, post-harvest PVY incidence levels are provided for information only and there is no threshold value. This means that certified seed potato lots can have incidences of up to 100% PVY.

Ask for information about the post-harvest test sample. Usually a 400 tuber sample is planted or tested. If emergence is considerably less than 100% then the estimate of the disease incidence will not be as accurate. Also, the disease estimate currently reported by certification agencies provides a measure (%) of diseased plants/tubers in the SAMPLE that is observed or analyzed and not in the ENTIRE SEED LOT. This estimate does not take into consideration variation in sampling and therefore can be misleading. For example, if the estimate of PVY incidence on a 400 tuber sample is reported as 2% it means the estimate for the 400 tuber SAMPLE is 2%. The disease estimate for the ENTIRE SEED LOT is actually 3.58%, and with a risk tolerance set at 95%, the incidence will actually range between 0.87% and 3.90%. If the sample size is reduced (i.e. is less than 400) the disease estimate will be less precise. Excel tools for calculating the % disease estimate and range for the entire seed lot are provided on this website under % Disease Estimation and Sample Size Tools. Using the information provided on the health certificate you can use these tools to calculate the more accurate estimate of % disease as well as the range (also called the confidence interval). You can also request that the certification agency switch to using these tools to estimate disease reported on the health certificate.

3) Was there bacterial ring rot found on your farm in the past year?

Bacterial ring rot is a serious disease that can be spread through seed potatoes and farmers should not purchase or plant seed potatoes that have bacterial ring rot. This disease, caused by Clavibacter michiganensis subsp. sepedonicus, is a zero-tolerance disease on seed potato farms. There are no treatments that can effectively control this disease once it has infected a plant. The pathogen that causes bacterial ring rot spreads easily on potato planting and harvesting equipment, especially if farmers cut seed potatoes prior to planting and it can cause losses of 80% or more. Bacterial ring rot causes foliar wilt, chlorosis and necrosis. Tubers show discoloration and necrosis of vascular tissue and central pith.

4) Have there been reports of late blight in your area this season?

If the answer is yes, you should determine whether seed lots from this farm are at risk of carrying late blight. If late blight was present on the farm, ask which strain (genotype) of the pathogen was found and whether it is resistant to metalaxyl / mefenoxam. Late blight is a particular concern when seed is purchased for use on organic farms since late blight management options for organic farms are limited. Once a late blight epidemic had started in an organic potato field, crop destruction is often the only control option. In addition, late blight can spread across a community very quickly and destroy both potato and tomato crops.

5) What methods do you use to manage aphid-borne potato viruses?

In North America, there are two aphid-borne potato viruses that can cause significant losses in potato production, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV). Both viruses can cause necrosis in seed potatoes and both viruses cause significant yield losses in infected plants. PVY causes necrotic rings to form on the surface of potatoes and PLRV causes necrotic flecking inside potatoes.

Seed potato producers have several tools available to manage the spread of potato viruses in their seed crops. The most important management method is to harvest seed potatoes in late summer or early fall to avoid large late-season aphid flights that can quickly spread PVY and PLRV into seed potatoes. Most seed potato growers use systemic insecticides (neonicotinoids) and this has caused PRLV to become very rare in North American seed potatoes.

Neonicotinoid insecticides have little effect on PVY spread. Mineral oil sprays, border crops, and field isolation all can reduce spread of PVY into seed potato crops. Except for very early generation seed potato lots, it is unlikely that you will find a seed potato lot that is free of PVY.  A small incidence (5% or less) of PVY in seed lots planted on commercial farms should not cause significant yield losses. Seed potato farmers should not plant seed lots that have more than 0.5% PVY incidence. For more information on PVY please see Potato Virus Y.

6) Was this seed lot produced in a field with a history of soil-borne viruses?

Two soil-borne potato viruses, Potato Mop Top Virus (PMTV) and Tobacco Rattle Virus (TRV), can cause significant losses in potato. PMTV is spread by Spongospora subterranea, a soil-borne microbe that causes the disease powdery scab. TRV is spread by the stubby root nematode. This nematode causes little damage on its own, but 100% losses can occur if TRV is present. Both PMTV and TRV can persist in soil for many years. Seed potato farmers should avoid planting potatoes in fields infested with these viruses. Lots with PMTV or TRV are high risk lots because farmers are likely to infest previously healthy soils with these viruses if PMTV- or TRV-infected lots are planted. For more information on PMTV and TRV please see Potato Mop Top Virus and Tobacco Rattle Virus.

7) Was this lot exposed to herbicide?

Herbicide drift and carry-over from previous seasons or from equipment that has not been properly cleaned will reduce the productivity of seed potatoes. Seed potato herbicide injury can include tuber cracking and unusual root and sprout growth on the seed tubers. The tubers may also be more susceptible to decay in storage and in the field. Seed potato farmers should be both scouting for herbicide damage and taking precautions against herbicide damage in their seed potato crops. Lots with herbicide damage that will impact yield should not be certified.

8) Was any variety mixture detected?

Farmers who produce early generation seed potatoes may grow 20 or more varieties. Variety mixture can occur at many points during production. For some market classes, variety mixtures may not be a significant problem. For example, a small amount of Dark Red Norland in a Yukon Gold seed lot can be removed during grading. However, for other market classes, such as varieties used for chips or fries, mixtures may adversely affect processing and there is a low tolerance for these mixtures.

9) What are the storage conditions used for this seed lot?

Ideal storage conditions for seed are 38ºF, 95% humidity. Seed potato farmers should clean their warehouses prior to storing potatoes and should have protocols in place to insure that the seed potato lot identity is maintained throughout storage. Farmers should also monitor the carbon dioxide levels in their warehouses. If carbon dioxide levels are too high, the tubers will develop a condition called black heart. Tubers with severe black heart will not sprout vigorously and may not grow at all.

10) Have you noticed any symptoms of tuber breakdown or soft rot?

Potato pathogens that cause tuber decay will spread quickly through a seed lot during planting, especially if the seed potatoes are cut prior to planting. Try to purchase seed potato lots that have not had blackleg in previous years and that have remained healthy throughout the winter storage season.


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