In a recent report, Sastry Jayanty, assistant professor and extension specialist-potato postharvest physiologist at Colorado State University’s San Luis Valley Research Center, said properly timed vine killing can improve tuber maturation and skin color, thereby improving the value of growers’ crops.

“Tuber maturity is a predominant factor influencing quality, both at harvest and throughout the storage season,” he wrote in the paper, Managing Tuber Maturity to Improve Skin Set. “There are a number of agronomic factors that may have an effect on the physiological age of the crop and therefore on tuber maturity at harvest. Bruising is probably the single most important factor that reduces the financial returns of the potato industry due to shrinkage and susceptibility to pathogens.”

Achieving proper maturity is complicated, and Dr. Sastry said timing is everything. When potato vines begin to die, bulking declines and tubers begin to mature. “Vine desiccation helps in promoting physiological maturity,” he wrote. “Physical maturity is about skin set.”

According to Dr. Sastry, physiological maturity tolls the optimum time for vine kill and harvest. “Tubers after bulking starts skin set,” he said. “Skin set generally takes about 40 days. Proper skin set provides resistance to skinning and shrinkage.”

According to industry research, over half of shrinkage occurred within the first week following harvest. “Tuber skinning and shrinkage were markedly reduced when tubers were allowed to mature,” he stated.

Vine killing allows tuber maturation to be artificially induced, allows skins to set, controls tuber size, limits the spread of disease and facilitates separation of vines from tubers. Dr. Sastry said there are three methods for vine killing: mechanical, chemical and combinations of the first two methods.

“Mechanical method of vine killing should be 14 to 21 days before harvest so that tubers are able to mature and set skin,” he wrote. “A combination of mechanical and chemical methods can increase the effectiveness of vine desiccation and, in turn, shorten the tuber maturation process.”

Dr. Sastry also noted that the yield and the specific gravity of potatoes can be affected if tubers are not mature at the time of vine kill. Prolonged intervals between vine kill and harvest can also affect yield and specific gravity. “Plants may continue to bulk after chemical desiccation whereas killing vines mechanically may terminate growth faster,” he added.

He noted that there are some important considerations to be taken into account for harvesting and storing potatoes.

According to Dr. Sastry, rapid vine kill can cause stem-end discoloration. “Excessive late fertilization interferes with vine and tuber maturation,” he added.

Increased soil moisture can increase blackspot susceptibility, while decreased soil moisture can hinder rapidity and the degree of skin set. Complete dryness of soil may lead to shrinkage and susceptibility to pressure bruising.