Controlling Hairy Nightshade

A continuing UI Potato Research Series

Published in the April 2012 Issue Published online: Apr 09, 2012 Pamela J.S. Hutchinson, UI potato cropping systems
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Hairy nightshade plantsThere are weed scientists at other land-grant universities and ARS-USDA sites in the United States who work with potatoes; however, I'm the only one who concentrates 100 percent on potato weed management. I've been with the University of Idaho for 12 years.

The major focus of my research and extension program is on the following five areas: 1) hairy nightshade biology and control, 2) weed management in new potato varieties, 3) potato weed control strategies with new and existing tools, 4) herbicide environmental fate, and 5) nightshade weed interactions with potato viruses and insect vectors.

Hairy Nightshade

Hairy nightshade (Solanum sarrachoides Sendt.) is one of the most difficult weeds to control in Idaho and Pacific Northwest potato fields. It can emerge close to the same time as potatoes, germinate the entire growing season and produce viable seeds even if it emerges a little more than a month before potato harvest. Research conducted by my project here at the Aberdeen R&E Center has shown that as low as one hairy nightshade per meter row growing the entire season in Russet Norkotah reduced U.S. No. 1 and total tuber yields of that variety 21 and 16 percent, respectively, compared to yields from control plots kept weed-free the entire season (Table 1).

Russet Norkotah has a determinate growth habit, relatively small stature and does not always close over the furrow between potato rows. In contrast, indeterminate growth, taller, Russet Burbank, closing over rows with its full canopy, also is more competitive and it took at least two hairy nightshade plants per meter row to reduce yield in that variety significantly lower than the weed-free U.S. No. 1 and total tuber yields-10 and 9 percent, respectively (Table 1). Moreover, it wasn't until the nightshade was solid seeded at approximately 100 plants per meter row than when Russet Burbank yield reduction was numerically similar to that of Russet Norkotah, growing with one nightshade per meter row.

We've also found that the time frame-known as the critical interference period-of hairy nightshade to reduce Russet Norkotah U.S. No. 1 tuber yields by five percent or more compared to the weed-free yields is between 7 and 21 days after potato/nightshade emergence (DAE) (Figure 1). The critical period for total tuber yields was 8 to 18 DAE. If the crop is kept weed-free for that critical period, generally no yield reduction should result in that variety.

Although weeds emerging after 18 or 21 DAE would most likely not affect Russet Norkotah yields, control efforts after that time may make harvest more efficient, reduce hairy nightshade seed production and consequently reduce density of that weed in subsequent years. Last but not least, since this weed also serves as an alternate host for nematodes, diseases and insects, controlling hairy nightshade season-long helps in more ways than one.

The Potato Variety Selection project here at Aberdeen has long conducted trials determining metribuzin herbicide tolerance of new and not-yet-released cultivars; however, when new herbicides were about to be labeled in potatoes, questions arose as to how new varieties tolerated those herbicides. Therefore, my project started conducting trials to determine the tolerance of standard and newly-released varieties to Outlook (dimethenamid-p), Chateau (flumioxazin) and eventually, Reflex (fomesafen). This research was especially timely because after Outlook and Chateau were labeled in 2005 and 2006, we experienced cool, wet springs two years in a row. Fast herbicide metabolism is the major mechanism for potato safety to potato herbicides. We saw in our trials that these spring conditions slowed growth of some varieties compared to others, which most likely slowed herbicide metabolism by those slower-growing varieties, resulting in early-season crop injury.

Just before or at the same time as in our trials those two years, commercial and seed potato growers saw injury in their fields planted with some of the same varieties. I learned specific injury symptoms from my trials, which helped me identify herbicide injury in the grower fields. Also, when the trial varieties recovered once warm, sunny weather conditions prevailed, I was able to give the growers confidence that their crop also would begin recovering. Fortunately, that early-season injury did not result in any significant tuber quality and yield reductions in the trials or grower fields.

Besides these studies and studies determining how soon potatoes can be planted after herbicides are used in other crops and vice versa, we've also looked at herbicide carryover in seed potatoes, green manure effects on weeds in potatoes, how a new trap crop-sticky nightshade (Solanum sisymbriifolium)-can be used to eradicate the devastating potato cyst nematode (Globodera spp.) from potato fields, and last but not least, what effect hairy nightshade has on Potato virus Y (PVY) spread by green peach aphids (Myzus persicae Sulzer) in potatoes.

We know from previous research done by former UI entomologist Dr. Juan Alvarez that green peach aphids (GPA) spread PVY at a greater rate from hairy nightshade to potato than from potato to potato. Early results from our current research indicate that early-emerging hairy nightshade plants-taller than later emerging nightshade at the time GPA are bringing PVY to a field-seem to enhance PVY spread more than the smaller nightshade plants. So, it's possible that not only do those early-emerging nightshades reduce potato yields due to the early competition, PVY spread also is affected. We also are trying to determine if high-more-than-lower hairy nightshade densities result in greater virus spread.

We'll be conducting two more years of refined trials to get the total answer. This PVY interaction study is part of a larger multi-discipline, multi-state Specialty Crop Research Initiative (SCRI) grant project to develop comprehensive strategies for managing PVY in potato. The overall project includes other researchers and ARS-USDA scientist from Idaho, Wisconsin and New York.

I've been blessed over my 12 years here with funding from the Idaho Potato Commission, federal monies such as the SCRI grant, and great support from the crop protection product industry. I also am grateful to my hard-working project crew. Coincidentally, my husband, Tom Salaiz, is a UI turfgrass management research and extension support scientist here at Aberdeen. We both enjoy our work and I'm certainly looking forward to many more productive years with UI. PG