Chemicals, variety, timing affect vine kill
Vine desiccation is an important decision during the growing season, potentially affecting tubers well into the storage season. Many factors impact vine kill, including chemicals, variety and timing. Knowing each can lead to a profitable harvest of healthy potatoes.
Vine kill plays a vital role before harvest. It can control weeds and tuber size, promote senescence and aid maturity - but not a lot of vine desiccation research is being performed today, because most of the chemicals available on the market are active in killing the plant.
"Research is done primarily if new products come out or if somebody wants to look at a new (chemical) use or new variety," said Mark Pavek, Washington State University potato Extension specialist. "There are not a lot of reasons research is being done, because most growers know how to use these products."
That leaves a handful of chemicals. Sulfuric acid tends to be the fastest, leaving vines wilted in a matter of hours, but there are dangers to the product because it can leave severe chemical burns on the handler and corrode the equipment if precautions are not taken before application, such as protective clothing to cover exposed skin. Many herbicides are not as caustic as sulfuric acid, but they tend to work slower, often requiring a second application to reach full mortality in many cases, said Don Drader, Syngenta agronomy service representative.
"Thorough desiccation of the plant allows for an easier harvest," Drader said. "There are some chemicals that do a different job of desiccation, and you need to look at the benefits of desiccation."
One aspect of vine kill is that it eases harvest by simultaneously killing weeds with the vines, preventing tubers from being carried over the back of the potato harvester, Drader said.
However, one of the more important benefits is timing harvest to allow better tuber size and skin thickness, as well as lowered moisture loss and disease incidence in storage. Tuber maturation is a secondary aspect of desiccation.
The potato skin thickens during the maturation process, decreasing bruising during harvest and transportation. Frost can play a role in maturation through natural vine kill, but it also interferes with harvest and can damage tubers near the surface. Varieties can mature on their own, but most mature through vine kill, which may be the most effective method.
Desiccation timing is critical because it plays a role in determining when potatoes will be ready for harvest.
"Once a potato vine starts to naturally senesce, die down, a desiccant will provide a more uniform die-down of the vines, allowing for a more uniform skin set on all of the tubers at harvest," Drader said.
There are no set rules in desiccation, but there are factors that can ease the process. One is the potato variety. Not all varieties have the same hardiness, so the same chemical can have differing effects on different plants.
"Some varieties stay green, and you either kill the vine or it freezes," Pavek said.
"Some varieties need only one application, but with a more aggressive vine we recommend two applications."
One way to increase desiccant efficacy is to roll the field before application. Rolling helps open the vine to allow greater contact with the desiccant and has the added benefit of covering cracks in the soil to protect the tubers from coming in contact with the chemical.
Using a second chemical with a different mode of action can increase efficacy and allow greater desiccation. Harvest date also plays a role. Plan ahead to apply desiccants to allow for complete skin setting for full maturation prior to harvest.
But varieties don't always need to reach full maturation before harvest, especially specialty potatoes, which are grown for a specific size.
"Early harvest goes straight to markets, so you need that knowledge up front," Pavek said. "If you have a market for a certain size of potatoes, like fingerlings, you want to stop it from growing. You may not see as many issues in potatoes that are not stored."
A desiccant should be timed to allow maximum skin setting before harvest, which can be 14 to 21 days. Scientists never recommend waiting for a frost for natural desiccation. Artificial vine kill is the safest way to ensure a quality harvest.
"A light frost will be OK, but a heavy frost could damage the tubers, and we don't know how much damage we will see," Drader said. "If a grower wants to harvest late in the fall, a desiccant is well worth taking a look at."
--By Everett Brazil III, Spudman contributor
Photos by Bill Schaefer