One expert offers advice, encouragement and options when Mother Nature is less than kind at harvest time.
At harvest time, growers may feel like they’re stuck between a rock and a hard place. Sure, under optimum conditions a producer with a good building can put potatoes into storage at the right temperature and humidity and that environment, with the proper settings, will manage those potatoes. But that doesn’t always happen, does it?
“Growers have to deal with what Mother Nature gives them every year,” says Michael Lewis, potato expert, researcher and owner of Agri-World Consulting of Fruitland, Idaho. “One year can be good and the next year can be disastrous in several ways—cold, frost, hot, dry …”
Nature, more often than not, will test a grower’s mettle with excessive rain, heat or drought and early frost. So what’s a producer to do to avoid harvesting under less than ideal circumstances, or, if that’s not possible, to reduce the chance of spoilage of problem lots in storage?
“There is no true formula for the conditions that growers deal with—there’s options. And grower’s deal with options every year,” says Lewis. “The goal is a happy medium—to pick the correct option or options,” he says.
Optimum conditions under which to harvest potatoes are when the soil moisture is approximately 60 to 70 per cent available moisture and pulp temperatures between 7.2°C and 12.7°C. Before the potatoes come out of the ground, Lewis says for best results try to moisten the soil to about 65 per cent AS M. Also, if growers vine kill before harvest, he warns against letting the hills dry out. “Growers have a propensity to kill the vines and then not apply any moisture or irrigation until a few days before harvest. Then they put on a small amount of water to ‘mellow the soil.’ If a grower lets the top three to four inches of soil dry out, the two to three tubers sitting in that area can dehydrate.” Those potatoes (20–30 per cent of the crop) will be dehydrated going into storage and will be prone to blackspot bruising and pressure flattening and add to the shrink potential, he says.
Lewis also doesn’t advise harvesting when the soil is too wet, “Don’t harvest right after a rain or a significant amount of precipitation. You have to wait.” He suggests, if possible, to pick areas with lighter, sandier soils to harvest, and to stay out of the wet, heavier-type soils, allowing them time to dry.
When harvesting, because bruising significantly affects the quality of the crop, make sure soil is carried up to the top of the secondary chains of the harvester, says Lewis. Not only should the harvester be kept full of potatoes but it needs to be kept full of soil as well. Take extra care not to put excessive amounts of soil, debris or clods into storage. Any tuber damage, wounds or bruises sustained during harvest greatly increases the potential for disease during storage.
Less Than Ideal Circumstances
For growers with bigger farms that goal of the “happy medium” can be elusive: “These guys are under the gun. They’ve got about a six-week window to harvest large acreages—often harvesting too early and too late … It takes time to harvest, even with multiple harvesters. They’re always fighting different issues and conditions,” says Lewis.
Whether a grower operates a large 1,000 hectare farm or a 40 hectare farm, Mother Nature often refuses to cooperate, and, sometimes, potatoes must be harvested under less than ideal circumstances.
As stated earlier, the optimum potato pulp temperatures for harvesting potatoes is 7.2°C to 12.7°C with 18.3°C as the absolute maximum. When potatoes are harvested under cool conditions, for example, at pulp temperatures nearing 7°C, they are more prone to shatter and blackspot bruising. At the other end of the scale, when approaching pulp temperatures around 15.5°C and above, potatoes become hydrated and flaccid in the field and are more susceptible to blackspot bruising.
Sometimes the larger operations will go up to 15.5°C to 18.3°C pulp temperatures because of the pressure they’re under to get the harvest completed and due to the environmental conditions, says Lewis. At and above 15.5°C pulp temperatures, the incidence of disease during harvest—soft rot (Erwinia carotovora), pink rot (Phytophtora erythroseptica), late blight (Phytophthora infestans) and pythium leak (Pythium ultimum)—increases dramatically. At harvest, pulp temperatures of 12.7°C and below drastically reduce the incidence of these diseases.
Some growers will sprinkle or wet the ground to keep it moist and to cool pulp emperatures before harvest. Evaporation keeps the soil cooler—as long as the soil isn’t too wet. Other growers will harvest at night when the pulp temperatures are lower, says Lewis. “Also, keep in mind, harvesting when the tubers are turgid or too cold—below 7.2°C—can cause shatter bruise and increase the incidence of dry rot (Fusarium sambucinum) in storage.
Management of Frosted Potatoes
When temperatures drop and a true frost freezes tubers, damaged tubers are difficult to recognize coming in out of the field and going into storage. One option a grower has is to put off harvesting for a few days after the frost, sometimes those potatoes will blacken or start breaking down and the crew can then see them and pick them out, says Lewis.
Frost damages the integrity of the cell walls of the tuber. Thus, when the tuber thaws, the liquid inside of the cells comes out of the lenticels. In 24–48 hours after the cells are damaged the tubers will begin to fall apart. Potatoes normally do not freeze until temperatures reach -2.22°C and below. A light frost may only nip tubers in the top part of the hill (and these are difficult to sort out) but a hard, extended frost can take out the top two to four tubers.
“It gets really tough once you start putting frosted potatoes into storage. To sort them out is one of the few choices, but not putting them into storage is the best choice—get them sold,” says Lewis. If the potatoes have to go into storage, the more air they’ve got, the more chance of drying up the frosted tubers and controlling the potential for in-storage diseases. Frosted potatoes are easily infected by bacterial soft rot.
Frosted potatoes that break down in storage release a tremendous amount of water: “A potato is approximately 80 per cent water, thus a 10 ounce potato contains eight ounces of water. You put a rotten one in and you’ve got to dry up eight ounces of water. It’s not an easy task,” says Lewis. “I’ve heard potato specialists mention five per cent frost as the maximum amount to put into storage, but five per cent frost is an exorbitant amount of water going into storage. I don’t think a grower could handle that much water even with the most modern storage.” Lewis recommends if you have to put frosted potatoes into storage, one to two per cent should be the maximum, and even that depends on the grower’s storage facility and the amount of air they have. The more air they’ve got, the more chance of controlling diseases and frost, he says.
If frosted potatoes are placed into storage, a grower must dry them up and keep soft rot at a minimum. In order to do that, the air must be continuous and the humidity must be reduced, at least for a short period of time, says Lewis. At this point, the options are not good. It all depends upon the frost percentage that’s in storage. If you shut the humidity off before wound healing is complete, you could cause additional problems,” he warns. “You’ve got to heal all the potatoes the best you can, and you’ve got to dry up the damaged ones and that’s not easy.” Rotten or frosted potatoes supply humidity and this will help with wound healing. You have to wound heal the potatoes at 10°C to 12.7°C and 95 per cent relative humidity for at least two weeks, says Lewis.
One effective way to minimize the damage caused by frosted or diseased potatoes in storage is in the method used for piling. Lewis says the stair-step method often promoted by universities is not the way to pile disease or frosted potatoes in storage. “If you put damaged potatoes into one area as you do with the stair-step method, it will heat up into a hot spot and a hot spot can take out an entire pile. Distributing disease or frosted potatoes across the face of the pile keeps from building a concentration of damaged potatoes in one area as well as increasing the air distribution for drying damaged tubers,” he says. The amount of bruising from tubers rolling down the face of the pile is minimal.
Management of Diseased Potatoes
Management of potato lots infected with disease is unique for each crop and must be evaluated for best management practices taking into consideration the specific disease and the stresses the crop has been under. For example, dry rot does not spread from tuber to tuber during storage, but those infected with dry rot are more prone to bacteria from soft rot. Because dry rot enters through wounds gained during harvest, it is imperative to take measures to minimize tuber damage during that time. Lewis suggests curing lots in storage at 12.7°C or lower for approximately two weeks to facilitate wound healing.
Like dry rot-infected lots, tubers with late blight and pink rot are more susceptible to invasion from soft rot bacteria. But both late blight and pink rot are easier to control in storage than other diseases like pythium leak and soft rot because late blight and pink rot are “hard rots”—they don’t break down as fast as the “wet rots” thus allowing a grower time to dry up the disease.
Phosphorus acid is a fungicide that works well at controlling late blight and pink rot in storage. When correctly applied on the piler as potatoes go into storage, research indicates phosphorus acid, at recommended rates, will take the spread potential of pink rot and late blight to zero. “This product helps keep disease from spreading and infecting other potatoes. This is the only product I have seen that reduces the spread of those two diseases to almost zero.” But Lewis also recommends sorting out diseased potatoes on dirt eliminators, harvesters and conveyor belts rather than letting them go into storage because at warmer pulp temperatures (12.7°C–15.5°C) and wet conditions soft rot can move into those already infected tubers. “If you can eliminate them before they go into storage, that’s the goal.” He notes phosphorous acid does not have any effect on soft rot or dry rot.
Pythium leak, another devastating tuber disease, enters lenticels in the field and wounds during harvest. Lewis advises to sort out tubers with pythium leak before storage, “Best option is—don’t harvest it, it’s a wet rot and can wreck a storage,” he says. Tubers infected with pythium leak are easily invaded by bacterial soft rot, which can rapidly infect other tubers in storage. Again, it is important to avoid cuts and scrapes during harvest and to try to not harvest above pulp temperatures of 18.3°C.
Bacterial soft rot spreads rapidly in storage under favourable environmental conditions. Soft rot takes advantage of wounds or weakness in tubers produced by cuts and bruises or by other diseases such as dry rot, pink rot, late blight and pythium leak. “Soft rot is very difficult to manage once in storage because it gives off heat and moisture and infects other tubers quickly,” says Lewis. Soft rot can rapidly get out of control in a storage facility and can create hot spots, which can rot previously healthy tubers, he says. Cooler harvest and storage temperatures suppress soft rot bacterial activity, says Lewis.
Mother Nature can be accommodating or unpredictable, fickle and, sometimes, downright cruel. And when all is said and done you have what you have, says Lewis. “In most cases, damaged potatoes will not get better, only controlled—the goal of a good harvest and storage manager is making sure problem lots don’t get worse.” A critical evaluation of each lot, a watchful eye and careful consideration of the options just may help you out from between that rock and a hard place. Kari Belanger