Organic Broadcaster

Research identifies potato varieties that do well on Midwest organic farms

By Ruth Genger

The potato is an important staple food—globally the fourth largest food crop after maize, wheat and rice. For many of us, potatoes are also a beloved comfort food. Since potatoes regularly make the Environmental Working Group’s “Dirty Dozen” list (a list of the most pesticide-contaminated fruits and vegetables), organic potatoes are in high demand by savvy consumers.

For organic growers, variety choice is an important decision—varieties differ in productivity under organic conditions, in tolerance of environmental stresses, pests, and diseases, in storability, and in taste. However, most potato breeding efforts are focused on conventional production methods with high fertilizer and pesticide inputs, and variety recommendations rarely are specific for organic production. Increasing numbers of sources exist for organically grown seed potatoes, but it can still be difficult to find seed for heirloom varieties. The cost of shipping seed potatoes from producers in other regions to the Midwest can be considerable. There is a need for increased availability of Midwestern potato varieties suited to organic production.

I’ve been involved in ongoing research through the University of Wisconsin-Madison on organic seed potato production and variety selection since 2007. Our research team includes Doug Rouse, Shelley Jansky, and Amy Charkowski.

Our long-term research goals are to breed and select potato varieties that perform well in the Midwest under organic management, and to increase the availability of organically grown high quality seed potatoes in the Midwest. To reach these goals, we work with a network of Midwest organic farms engaged in participatory on-farm potato variety trials and trials of seed potato production. We also trial potato varieties and seed potato production methods on organically managed land at the West Madison Agricultural Research Station. In this article, I describe our previous and ongoing organic variety trials.

Since the quality of seed potatoes can have a huge effect on crop health and productivity, I have also described the seed potato production process, and some approaches we are taking to support increased organic seed potato production and organic potato breeding in the Midwest.

Trials of Commercially Available Varieties

In the two years since I last wrote for the Organic Broadcaster, our research team has continued potato variety trials with the goal of finding varieties suited to organic farms in the Upper Midwest. Our previous research (reported in the July|August 2012 Broadcaster) focused on varieties that were commercially available, with trials at six Wisconsin organic farms and on organic research station land. Trials identified several productive varieties, including Chieftain, Reddale, Red Maria, and Dark Red Norland (red varieties), Satina and Keuka Gold (yellow varieties), Langlade and Kennebec (white varieties), Red Thumb and Papa Cacho (red fingerlings), Austrian Crescent (yellow fingerling) and Adirondack Blue and Caribe (specialty varieties).

In general, we found that varieties that thrived and yielded well under organic management showed good row closure (a measure of vine vigor, and a characteristic that tends to suppress weeds) and tolerance for hopperburn (leaf damage caused by potato leafhopper).

Tuber defects, including defects due to disease, contributed to yield loss. On average, across all farms and varieties, 24 percent of the total harvest was culled due to tuber defects in 2011, and 38 percent in 2012. Increased losses in 2012 were partly due to increased tuber shape defects – these commonly occur when plants are stressed during tuber bulking, as occurred in the very dry and hot conditions of 2012. In both 2011 and 2012, tuber defect diseases contributed significantly to tuber culling. In particular, common scab (caused by Streptomyces scabies), silver scurf (caused by Helminthosporium solani) and black scurf (caused by Rhizoctonia solani) were problematic. Varieties differed for the level of damage caused by these diseases. Detailed trial results are available on our project website: organicpotato.wisc.edu.

The results of these trials gave us insight into characteristics that are most valuable in potato varieties for organic production—early vine vigor and canopy closure, leafhopper tolerance, and resistance to tuber defect diseases. Based on trial results, we began crossing varieties likely to be good parents for new potato varieties developed for organic production. Selected progeny from these crosses were entered in our 2014 trials at the West Madison Agricultural Research Station, and we will continue to evaluate them there and on organic farms.

Trials of Heirloom Varieties

More recently, we have focused on heirloom and specialty potatoes from the Seed Savers Exchange (SSE) potato collection. The SSE potato collection contains more than 500 potato varieties, including many heirlooms unavailable from any other source. This collection is an underutilized resource for organic growers as a source of parental material to breed for organic conditions.

Unfortunately, many of the SSE variety stocks are infected with potato viruses that impact their growth and productivity. We began collaborating in 2010 with SSE to eradicate potato viruses and other pathogens from selected varieties in their potato collection. The varieties we chose for curative treatment had been reported by SSE members or potato researchers to have useful traits, such as disease resistance or excellent taste. We have now cured 72 heirloom potato varieties of viral pathogens, allowing us to evaluate disease-free stocks of these varieties.

In 2013, we evaluated 24 SSE potato varieties in cooperation with 26 organic farm and garden partners. Each farm grew a subset of the 24 trial varieties, in comparison to the best-yielding commercial varieties for each market class from the 2010-2012 trials. All 24 varieties were also grown in replicated trials at the West Madison Agricultural Research Station.

Several heirloom varieties yielded as well or better than check varieties. Of these, Early Bangor (red) out-yielded Dark Red Norland at 5 of 6 on-farm trials; Purple Valley (pigmented flesh) out-yielded Adirondack Blue at six of six on-farm trials; and Australian Crawlers out-yielded Langlade at four of six on-farm trials. Yield results are included with the online version of this story. Search “potato research” at the very bottom of the page.

In 2014, we evaluated 60 SSE varieties in cooperation with 35 organic farm and garden partners. Again, each farm grew a subset of trial varieties, and all varieties were grown in replicated trials at the West Madison Agricultural Research Station. Data compilation from these trials is ongoing, and will be presented in a workshop titled “Organic Potatoes: From Starts to Storage” at the 2015 MOSES Organic Farming Conference.

Seed Potato Production

As we find heirloom and other rare varieties with potential for organic production, the importance of a locally responsive seed system becomes more apparent. After all, identifying good varieties doesn’t help farmers if no seed is available.

Seed production in potatoes differs from most other vegetables since potatoes are vegetatively propagated—from seed tubers, rather than true seed. Potatoes can produce fruit containing true seeds, and you can collect the seeds much like tomato seeds and start new plants (don’t eat the fruit though – it is toxic). However, since potatoes do not breed true, tubers are planted instead, ensuring that the variety identity and characteristics are maintained. Since potato tubers are swollen underground stems, tubers are essentially stem cuttings from the mother plant.

The disadvantage of vegetative propagation is that many pathogens can be carried in the cuttings—whether these are tubers (e.g. potato), rhizomes (e.g. hops) or slips (e.g. sweet potato). If the parent plant is infected with a pathogen, the tubers are likely to carry the same infection, and some infections can seriously affect yields and quality.

Viral pathogens are the most common problem in production of quality seed potatoes, but some bacterial pathogens can also be serious (e.g. Clavibacter michiganensis ssp. sepedonicus, the cause of bacterial ring rot). In commercial production of seed potatoes, tuber-borne diseases are managed by excluding pathogens at the early stages, inspecting for disease symptoms, and testing for pathogens.

The commercial seed potato system (see box) starts with plants that are tested for a comprehensive panel of pathogens, guaranteed to be disease-free, and grown in tissue culture. When transferred to a protected greenhouse environment, the plants produce small disease-free tubers—“minitubers” —which are planted in the field the following year to produce the first generation of seed potatoes.

The seed potato crop is inspected for disease symptoms during the growing season and at harvest, and a post-harvest sample is grown in the winter (in greenhouses or tropical climates) to find those diseases that cannot be detected except in the “daughter” plants. The crop must be below stringent thresholds for viral, bacterial, fungal and other diseases in order to be replanted as a seed crop (“foundation seed”) or sold as seed to a production farmer (“certified seed”).

Ideally, seed potato farms are isolated from large-scale potato production, to avoid disease spread. In isolated areas, seed crops can generally be replanted for 5-7 years before the pathogen load is too high for certification. However, disease can spread rapidly in crops grown near large-scale commercial plantings, since aphids spread viral pathogens, and other pathogens—such as late blight – are wind-spread. High pathogen load in a seed crop can result in crop failure.

Trials under Organic Management

Our early trials of organic seed potato production focused on field grown “foundation seed” planted on Wisconsin organic farms – six in 2007 and two (plus two organic research station sites) in 2008. We found that the majority of seedlots met standards for certified seed (90 percent in 2007 and 85 percent in 2008) and for foundation seed (79 percent in 2007 and 62 percent in 2008). Two sites were in the Central Sands of Wisconsin, surrounded by large-scale potato production; excluding these sites from the data increases the success rate, as would be expected.

These observations suggest that seed potato production is feasible on many organic farms in Wisconsin, especially those more isolated from large-scale potato production. More recently, we have focused on seed potato production from greenhouse-produced (and disease-free) minitubers, trialing drip-irrigated plasticulture and straw-mulched production systems that have shown good productivity on organic research station land. These systems will get a real-world check with on-farm trials in 2015 and 2016 that will include an economic analysis.

Breeding for Organic Production
New varieties of potato are found by crossing existing varieties and selecting amongst the progeny. The selection environment matters—the most likely path to varieties suited to organic production is to select them on organic farms.

In the coming year, we will be providing training on crossing potato varieties to interested farmers, and also providing them with true potato seed (TPS) from our crosses of varieties that performed well in previous organic trials. Tom Wagner, legendary independent potato breeder (tatermaterseeds.com) kindly provided us with true potato seed for 68 potato breeding populations, which are also available for trialing on Midwest organic farms. Participating farmers will evaluate the progeny of these crosses, select tubers from promising lines, and evaluate these lines in subsequent years. We are hopeful that, by initiating on-farm selection at the earliest possible point in variety development, we will find potato varieties that are adapted to organic production systems with good early vigor, tolerance to environmental and pest stresses, and robust yields—tasty, too!

The potato is a genetically complex crop. As a “highly heterozygous autotetraploid” (what a mouthful!), it contains four copies of each chromosome and has a great amount of variation between genes on different chromosomes. One result of this complexity is that crosses between different potato varieties can result in a wide range of plant and tuber types. We look forward to seeing the new combinations of shape, color and taste that may emerge from this collaboration between potatoes, farmers and researchers —and we especially look forward to sharing tthem with you!

To learn more about our ongoing potato trials, see labs.russell.wisc.edu/organic-seed-potato.

Ruth Genger is a researcher at the University of Wisconsin-Madison, Department of Plant Pathology. She has been conducting on-farm research into organic seed potato production and variety selection since 2007. She welcomes questions from farmers via email at rkgenger@wisc.edu.

From the January | February 2015 Issue

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