In 2006 Premier Russet was released by the USDA-ARS and the Agricultural Experiment Stations of Idaho, Oregon and Washington, the three states that comprise the Northwest (Tri-State) Potato Variety Development Program. This variety has improved traits relative to industry standards, most notably lower levels of reducing sugars during storage, allowing it to produce acceptable fries when stored for as long as 8 months at 42 degrees Fahrenheit. This prolongs its dormancy and provides a better quality product late in the storage season.
Premier also offers something to the seed grower in the way of potato virus Y (PVY) resistance. It is classified as being very resistant to PVYO (the common or ordinary strain) and shows a delayed infection rate both visually and when tested with ELISA to the N, N:O, and NTN strains of PVY. These strains are referred to collectively as necrotic strains because they can cause necrotic or dead tissue in the tubers as well as a reduction in yield. Premier was the first russeted processing variety to have such high resistance to any PVY strain. The observed research data regarding PVYO resistance was also seen in seed and commercial production. However, the partial resistance to necrotic strains can be problematic, with a seed lot looking visually clean for PVY during the growing season and then showing a certain level of infection with the necrotic strains in winter tests.
The research presented here is a first step in gaining a better understanding of how PVY strains progress in Premier Russet compared to the standard varieties, such as Ranger Russet. Premier Russet derives it PVY resistance two generations back in its pedigree from Bannock Russet and Western Russet. Both of these “grandparent” cultivars have good resistance to PVYO.
In initial tests, Premier Russet was infected with 10 different PVYN:O isolates, five PVYNTN isolates and two PVYO isolates. The results from these tests showed that Premier Russet was only infected by one of the two O isolates and that infection was detected with ELISA only at 59 days after inoculation. The other necrotic strains were detected with ELISA from 31 to 52 days after inoculation. In comparison for all strains, Ranger Russet days to detection ranged from 20 to 29.
These results led to studies to determine how infection of different strains of PVY would progress in Premier Russet. Another question posed was, does it make a difference in the rate of PVY infection when the virus is transmitted by aphids or when it is mechanically transmitted? To answer this question, Premier Russet and Ranger Russet (in sets of 10) were infected with different PVY strains using 10 aphids per plant, and with a second set of plants being infected by rubbing PVY-infected sap onto one leaf of each plant. After inoculation, the plants were sampled at five, eight and 13 days, then at weekly intervals up to 55 days post-inoculation in order to determine when the virus could be detected. A sensitive test (RT-PCR) was used for each sampling date to detect PVY.
For Premier Russet, PVYO was detected at 27 days at 20 percent following aphid inoculation, and at 24 days at 10 percent for mechanical inoculation, with the percentages not increasing over subsequent sample dates. For the necrotic strains, infection occurred earlier at eight and 13 days, respectively, with incidence climbing rapidly to 80 to 100 percent over the next four sample dates. Again, while this is preliminary data, it does show that Premier Russet possesses a high level of resistance to PVYO. More testing is currently being done to add to these results.
For Ranger Russet, PVY was detected starting at five days post-inoculation for all the treatments except the NTN strain transmitted by aphids. The NTN strain was detected at 20 days post-inoculation. The infection rate quickly reached 80 to 100 percent for all strains.
While the PVY resistance observed in Premier Russet can help reduce current levels of PVY in the industry, another, more broad-based resistance is available to potato breeders and is now currently in use by some breeding programs. Resistance genes Rysto and Ryadg are being bred into new cultivars and offer resistance against all strains of PVY.
Currently, the cultivar Eva (round white), released in 1999, possesses the Ryadg gene and is resistant to these PVY strains. For processing russets, A02507-2LB, a promising breeding clone, may soon be released as a new variety. In three years of field trials, this line has had 0 percent PVY in the same trials where Russet Burbank had 85 percent PVY. In these trials, plants were exposed to a mixture of O, N:O and NTN strains. Another processing russet (A03921-2) is also making its way through the Western regional trial process and shows 0 percent PVY under the same trial conditions. Both of these breeding lines possess the Rysto gene for PVY resistance.
Research shows that resistance genes Rysto and Ryadg provide extreme resistance against all strains of PVY. These resistance genes bred into new cultivars will provide the level of PVY resistance that is needed by both seed and commercial growers.
Jonathan Whitworth, Rich Novy and Darren Hall are plant scientists with USDA-ARS; Shoanpius Modal is with the University of Idaho. All four are based in Aberdeen, Idaho.