As I look out my window this morning, all I see is grey and white – a clouded sky, snow banks, and leafless trees. Much as I love winter, I also love poring over seed catalogs at this time of year, dreaming and planning the color and bounty to come. For the farmer and market gardener, seed is a big investment – not only do you have to consider which varieties are best for you and your markets, but seed quality can have a huge impact on the quality and productivity of your crops. This is especially true when it comes to potatoes. So today I want to discuss some considerations when it comes to selecting seed potatoes.

Why it’s worth buying certified seed potatoes

Botanically speaking, potatoes are modified underground stems – storage structures which, in nature, lie dormant until the next growing season. This means a potato tuber is essentially a cutting from the parent plant. If the parent plant is diseased, the tuber is likely to carry the disease into the next crop. Accumulation of pathogens from year to year will eventually cause crop failure, which used to be called “run out”, as this 1922 agricultural bulletin shows.

Certified seed potatoes are inspected and tested to ensure that they have a low incidence of tuber-borne diseases, and that the variety is correctly identified. Seed potato certification programs focus on viruses, for which the tolerance level is 5% (Wisconsin seed rule; virus thresholds for seed potatoes produced in some other states are higher) and on a small set of highly destructive pathogens for which there is zero tolerance (e.g. spindle tuber viroid and bacterial ring rot). In the US, seed potato certification is regulated at the state level, and rules vary from state to state. Generally, seed potato crops are inspected twice in the field and once at harvest. Ideally a “winter test” is also performed – a sample of harvested tubers are grown, inspected and tested for disease. Since some pathogens – especially aphid-borne viruses – can infect the crop late in the season when the plants will not show disease symptoms, winter testing is the gold standard for certification.

Certified seed potatoes destined for export out-of-state must also be graded for external and internal tuber diseases and blemishes which could cause problems in the future crop or in handling the seed potatoes (US Seed Potato Grades). This includes defects such as hollow heart, freezing injury, or tuber damage, and diseases such as common scab, late blight or soft rot. Only the highest grade – U.S. No. 1 or “blue tag” – can be exported internationally.

Seed potato certification has been very successful in limiting the impact of tuber-borne diseases in potato. For example, up until the mid-1970s, 10% or more of seed potato lots were infected with bacterial ring rot, which can cause devastating losses. Since zero tolerance standards were introduced for bacterial ring rot in certified seed potatoes, incidence has fallen to nearly zero in US seed potato lots entered for certification.

Organic certification

Organic growers in the US are required to plant organically produced seed and planting stock, unless it is commercially unavailable (NOP rule §205.204). It’s important to note that organic certification and seed certification are two separate processes, so that seed potatoes that are certified organic are not necessarily certified for low disease incidence. Seed potatoes that are both certified organic and certified for use as seed are increasingly available, and are sometimes referred to as “double certified”. If you’re in any doubt, ask the seed producer to clarify the certification status of their seed lots.

Prevention is better than cure

Using certified seed potatoes will keep many pathogens out of your future potato crops. It’s also worth asking your seed supplier about diseases that are not strictly regulated by seed certification, such as common scab (Streptomyces scabies), black scurf (Rhizoctonia solani), and Verticillium wilt. This is especially the case if you will be planting potatoes in fields without a history of potato production. These pathogens can survive in the soil for long periods, and the longer you can avoid introducing them, the better! Also ask about the incidence of diseases such as early blight (Alternaria solani), late blight (Phytophthera infestans) and blackleg or soft rot (Pectobacterium species) in the parent crop, since infected seed potatoes can be a source of inoculum in your crop.

Seed is an investment, and a few careful questions when ordering can protect you from losses later in the season. Next week I’ll talk a little about storing and handling seed potatoes, as well as varieties that we have available for trials this year. Till then – stay warm!

 

 

This article was posted in Blog Posts, Uncategorized and tagged certified seed potatoes, organic seed potatoes, tuber diseases.