In potato production, it is not unusual for one minor problem to lead to another bigger one. This is a common situation within the weed-insect-disease relationship. Weeds harbor insects, insects spread disease. When those forces work together they can pose a destructive threat to the health of the crop, the size of the yield and-in the case of seed production-the marketability of the final product.
Perhaps no case of this is more evident or more devastating than the current trio many growers are fighting: nightshade, aphids and Potato Virus Y-better known as simply PVY. In recent seasons, these three pests have combined to give many growers ravaged crops and vastly reduced yields.
HISTORY OF PVY
In the last five years, researchers have seen an increase in the occurrence of PVY in fields across the United States.
"Of the viruses that limit yield and reduce tuber quality, PVY is the most important virus across the United States today," said Neil Gudmestad, plant pathologist and crop consultant for North Dakota State University. "That's true from coast to coast."
Many researchers are attributing this spread to the fact PVY is a highly transmittable disease which can be quickly and easily passed from field to field. Gudmestad explained the transmission process.
"Though PVY is commonly transmitted through seed, it also is passed via aphids who will probe an infected plant, contract the virus on their stylets, then move on to probe a healthy plant in the same or a neighboring field," Gudmestad said. "The aphid only has to probe for a few seconds to contract the virus. They are nervous feeders and will relocate to other plants when they are disturbed, infecting the new host plant immediately."
Additionally, PVY can be spread mechanically among infected plants via humans or equipment.
PVY has evolved over the years, developing different strains with a variety of symptoms, some of which are visible and some detectable only through ELISA (Enzyme-Linked ImmunoSorbent Assay) tests conducted at university and extension labs.
The severity of the symptoms also depends on the variety of potato, but common visible indicators of PVY are mosaic, tuber necrosis, mild mottling and distorted or brittle leaves.
According to Gudmestad, the losses caused by PVY can be substantial.
"A good rule of thumb across varieties is for every 1 percent of PVY in a crop you will lose 0.4 to 0.6 percent of your yield. That means a crop with 10 percent PVY will lose 4 percent to 6 percent of its yield," Gudmestad said. "You will also lose quality because PVY reduces the size of the tubers, making them much less desirable in the marketplace."
NIGHTSHADE WEEDS: A ZERO TOLERANCE POLICY
While certified seed is helping some growers tackle the PVY issue, many researchers are also encouraging growers to curb the threat by addressing how the disease is transmitted in the first place: via aphids and their preferred host environment, nightshade.
Though nightshade weeds-such as eastern, eastern black, cutleaf and hairy nightshade-have long been yield- and quality-robbing weeds for growers, PVY has brought a new series of nightshade issues to light.
Nightshade and potatoes are in the same plant family; not only are the weeds a favored habitat for PVY-transmitting aphids, but nightshade can also act as a carrier and harbor the PVY virus itself.
For some growers, that relationship makes even one nightshade a considerable, even intolerable, threat.
Joe Thompson, seed grower in Alliance, Neb., said PVY is too great a concern to allow any nightshade to emerge within his fields.
"On our farm we have a zero tolerance policy for nightshade and PVY," Thompson said. "Nightshade is the biggest threat to potatoes because they are in the same family, and nightshade can ultimately act as potatoes do. Plus, if you have a place to harbor the enemy, the aphids, right next door or within a field, you really don't have a chance."
Pamela J.S. Hutchinson, University of Idaho potato cropping systems weed scientist, has been researching nightshade weed biology and control for years. More recently, she teamed with Juan Alvarez, University of Idaho potato and cereal grains entomologist, to examine the relationship among nightshades, aphids and PVY in potatoes. Hutchinson said nightshade weeds pose a lingering concern even for cautious growers.
"Growers need to work on alleviating nightshades from all their crops in rotation with potatoes because seeds from weeds such as hairy nightshade can remain 90 percent viable up to five years in the soil," Hutchinson said.
"If you've done everything else to protect your potato crop but have viable nightshade seeds remaining in your field the next time you plant potatoes, your crop is still at risk for infections."
Hutchinson said hairy nightshade, the nightshade dominant in Idaho and most Pacific Northwest potato production areas, is a resilient weed that germinates all season long; growers must consider season-long control to keep this aphid and virus "safe haven" out of their fields.
"With early maturing potatoes, you need to be particularly vigilant," Hutchinson said. "When the vines start natural senescence, allowing more sun to reach the soil surface, dormant hairy nightshade seeds can germinate in time to produce new viable seeds before the end of the season."
Studies conducted by Hutchinson have shown that the presence of nightshade weeds also diminishes yield. With as few as one hairy nightshade plant per meter of row, significant yield loss occurred in Russet Norkotah. This early-maturing variety, unlike Russet Burbank, has a foliage canopy that does not always completely close between the rows and may start natural senescence relatively early in the growing season, in turn allowing light to break through and stimulate hairy nightshade seed germination.
Hutchinson found the right mix of residual herbicides controlled nightshade from the start of the season until harvest. She recommended growers develop their weed control programs based on the individual field's needs and to respond early to any new weed species.
"One of the best weed management tools you have is to know the weed history of your field: understanding the weeds that are present and the weed population density," Hutchinson said.
"You want to target the weed species in your field with the right amount of the most appropriate herbicide active ingredients. A few of the new herbicides can only be used pre-emergence and work best when applied shortly after a hilling operation and before potatoes and weeds begin to emerge."
Hutchinson said the four herbicides that showed the best control of hairy nightshade were Chateau, Matrix, Outlook and Eptam. She recommended using them together in two-way tank mixtures or combined with other herbicides including Dual Magnum, ProwlH20, Sonalan, Stalwart and/or Sencor for broad-spectrum control.
Hutchinson added, three-way tank mixtures might be needed depending on which weed species are in the field and if population densities are high.
Thompson used tank mixing to control his weed populations and praised the overall control he got with a tank-mix of Chateau combined with Dual or a related generic form.
"Prior to Chateau we didn't have a chemistry that was anywhere close to as effective on nightshade as Chateau has been," Thompson said. "By having Chateau in our program we've been able to maintain our zero-percent tolerance for nightshade and PVY all season long."
APHIDS: A SEASON-LONG APPROACH
If minimizing nightshade populations is the first step in preventing the spread of PVY, the second is controlling the aphids that actually serve as the vector for the virus. Jeff Miller, independent researcher with Miller Research LLC, has conducted extensive research on potato vectors, including aphids.
University studies show that while a variety of aphids transmit PVY, the two leading species for PVY transmission are green peach aphids and potato aphids.
"Aphid control is very possible with these insecticides," Miller said. "We used seed treatments of Belay and Cruiser. The results were impressive. Our tests didn't detect a single aphid in the treated plots the entire season."
Application timing can play a major role in extending aphid control throughout the season and, in turn, reducing the spread of PVY.
One of the most efficient vectors of PVY, the green peach aphid, does not usually move into potato fields until mid- to late season. When Belay is applied at cracking, the product can be absorbed by the plant in a matter of days and provide longer residual control of green peach aphids toward the end of the growing season. This residual control is what growers are looking for in areas where virus transmission is a key factor of yield volume and quality.
"Belay was most impressive when used at cracking," Miller said. "By applying Belay at the cracking stage (emergence) you're basically starting the aphid-control clock later in the season and extending the window of protection. We liked that aspect of Belay better than in-furrow treatments."
With vigilant pest management, including a nightshade/aphid combination approach, growers can help prevent PVY from infecting their fields.
Controlling insect and weed pressure with effective tools that provide residual strength can keep fields healthy through the season, maximize yields at harvest and bring more back to the bottom line.