Over the last 100 years, potato growers have too often seen their income reduced by the common scab disease. In Wisconsin alone, a recent survey identified common scab as the major contributor to cull piles or potato waste. A few years ago, when I got involved in potato breeding and variety development at the University of Wisconsin-Madison, it was obvious that common scab resistance needed to be a main objective of the potato breeding program. What was not obvious at all was how to do it.
A review of scientific studies revealed more than a century of trials and tribulations with little good news. No researcher had predictably decreased the disease by adding soil amendments; some advocated that irrigating so as to avoid dry periods during tuberization may work. If asked about that approach, many growers in Wisconsin would say “I don’t think so.” Streptomyces (scab pathogens) can even “breathe through” some of the tougher soil fumigants tested. A combination of soil fumigation and long rotation can reduce common scab, but does not really ensure that growers won’t have significant losses over time.
But what about new varieties?
Varieties with various levels of resistance to common scab have been released over the years. Such varieties—including Russet Burbank, Superior, Goldrush, Pike and Lamoka—have provided comfort to growers that scab would not be their biggest worry. Scab-susceptible varieties such as Russet Norkotah, Snowden and Yukon Gold are recognized risky options for some growers.
Early work published in 1963 by Dr. Florian Lauer from the University of Minnesota laid out important methodological guidance to consistently evaluate common scab. My own previous work on identifying and validating resistance to root rot disease of snap beans was a good fit to tackle soil-borne diseases.
In the June 2015 issue of the American Journal of Potato Research, a group of researchers from across North America published results of a multi-year, multi-location common scab project to evaluate strategies to select potato varieties with predictable resistance to common scab. The team was comprised of myself; Jiwan Palta and Bryan Bowen from UW-Madison’s potato breeding program; Ontario plant breeder Eugenia Banks; and Charlie Higgins of Heartland Farms.
The project analyzed progress of selection for common scab resistance from 36 breeding trials: Eighteen of the trials were conducted in standard breeding trials in a four-year rotation versus progress made from evaluations conducted in 18 permanent field trials and locations with a history for high incidence of common scab. Three important questions were answered as given below.
How different is selection for common scab under standard breeding trial vs. permanent breeding trials?
Simply put, if the researchers had relied on standard breeding trials conducted at the Central Wisconsin Hancock Research Station under a four-year rotation, no progress would have been made: They could not differentiate susceptible varieties Atlantic or Snowden from resistant Superior’ and ‘Pike. Differences due to soil and other environmental factors equaled differences in varieties, making genetic differences very difficult to assess. On the other hand, in trials conducted in permanent fields, genetic difference was more than four times the magnitude of random variance. In plain English: The difference between susceptible and resistant varieties from permanent trials (without rotation) was easily and predictably detected.
How is the stability of varieties challenged in fields with high amounts of pathogens, and what may be the cause of variety instability?
Analyzing data only for permanent fields, the research team was able to identify varieties that were resistant to common scab and stable through time and locations. Some varieties moderately resistant to common scab, such as MegaChip, Tundra and Nicolet, performed well on average. However, when scab got really bad in permanent fields, important damage was observed.
Also, some varieties that did consistently well in Wisconsin locations did consistently poorer in Ontario. Why? Recent publications by Leslie Wanner indicate that there is abundant variability for the Streptomyces bacteria that causes common scab. In some locations, different pathogenic Streptomyces bacteria may defeat some potato varieties considered resistant somewhere else. Unfortunately, Wanner’s untimely death prevented her from digging even deeper into the subject.
Can we characterize new varieties for common scab so that people can understand the risks of adopting them?
Although the stability concept has fascinated researchers for many decades, what really matters to growers is what additional risks of choosing a new variety exist. Well, at the end of the APJR article, the researchers address just that. The article statistically and graphically shows readers how new varieties fare against standard varieties potato growers know very well. The research indicates that a new wave of Wisconsin varieties set to be released, including the yellow-fleshed Oneida Gold, chip clones W5955-1, W6609-3 and several russet varieties represent sure choices in the fight against common scab. It also shows that new high-yielding varieties Accumulator and Pinnacle need to be grown in fields rid of common scab pathogens to achieve their full potential. The overall conclusion is that evaluating common scab in permanent fields may lead to the development of varieties with consistent common scab resistance.