Practising better spud management
Identifying diseases and engaging good management techniques will lead to better potatoes post-harvest.
May 5, 2020 By Ronda Payne
When the storage doors open and farmers look at their cured potatoes, they are hoping for high-quality spuds that will garner a fair price. Unfortunately, potatoes can be sneaky. Some don’t reveal problems until harvest, or worse, when they are already in storage. Potato farmers should follow beneficial management practices and keep a keen eye on what’s happening from field preparation to packaging to minimize issues.
Marjo Dessureault with E.S. Cropconsult has been working with B.C. potato growers to assess conditions, diseases and best practices to help farmers get the most from their crops. Her first piece of advice is to know the diseases.
“If you can recognize the disease, you’ll be better prepared to take action,” she says.
Determining which disease is present allows for better management and application of appropriate controls. However, treatments in potatoes vary and there are no silver bullets. Growers across the country can find detailed information in B.C.’s potato production guide.
Dessureault works with Trevor Harris, a sixth-generation farmer and co-owner of Pacific Potato in Delta, B.C. She conducts field monitoring at the site which grows seed potatoes, and knows the site well.
“She’s got information I don’t have,” Harris says. “She knows our operations well. If new stuff comes out, you gotta have someone to call.”
Hot, wet conditions give pink rot the boost it’s looking for. The disease is identified in potatoes by its tan to brown skin lesions and the tuber’s rubbery feel. If cut into, the flesh will turn pink, then progress to brown and black.
“It’s a fungal disease that infects the plant and tubers,” Dessureault says. “It can spread in storage. Pink rot, you can see at harvest, but often it’s minor until they go into storage.” Pink rot thrives in poorly drained soils and wet environments that deny oxygen. These wet environments make it easier for pink rot to penetrate the roots and the tuber via the above ground roots (stolon). “Often the infection happens at the end of the season,” she notes.
Like pink rot, soft rot likes hot, wet conditions. This disease can be present in the soil, in volunteers or even in irrigation water. “This is a bacteria disease associated with wet and hot conditions at harvest and spreads at storage,” she says.
Soft rot is characterized by sunken lesions which will expand into one large lesion and ultimately a rotten potato. Currently, there are no seed piece treatments, in-furrow treatments or growing-season foliar treatments for soft rot.
“Soft rot will happen in the field when it’s wet,” she says. “You can see that disease when you harvest. If you see it after harvest, it is because of damage [during harvest].”
Fusarium Dry Rot
One of the ugliest potato diseases is Fusarium dry rot with its dark, hard spots, dark interior and mycelium growth. It is associated most often with visible damage at harvest which will spread in storage.
“If you plant in cold soil, then Fusarium will likely be an issue and cause seed piece decay,” Dessureault notes. “Some of the seed will fail to emerge. Fusarium dry rot usually shows up about a month after harvest.”
Hot and wet conditions at harvest are associated with the spongy result of Pythium. These potatoes have grey to brown water-soaked lesions, a dark flesh ring, and when squeezed, the potato will release water, known as Pythium leak. For potatoes, planting in cold soils is unadvisable and it’s also best to avoid warm, wet soils when Pythium is present because it will lead to seed piece decay. Infection occurs during harvest and in storage and while potatoes may appear normal, the soft texture reveals the disease’s presence. “Pink rot, we don’t really get,” Harris says. “Pythium is one we have to be careful of.”
“Hopefully you’ve disinfected your truck, your equipment and [tools],” Dessureault says. “About seven to 14 days prior to planting, think about warming the seed to 10 to 13 C.”
She also recommends cutting seed with a sharp knife to promote better healing and reduce the chance of pathogens entering. Give the seed time to heal if it’s not being planted right away. Healing should be done at 10 to 15 C at 90 to 95 per cent relative humidity for a minimum of five days in piles of less than six feet.
Plant after the five days or cool to 5 to 8 C until close to planting then re-warm the seed, but not in the sun. Ensure there is a tarp over planting trucks. Plant seed when soil temperatures reach a minimum of 7 C. Fresh cut seed would ideally be planted at soil temperatures of 13 to 16 C.
“Pythium will really have a go at your seed if you leave it in the sun,” she says. Avoid planting in poorly drained soils. “In general, cold and wet soils are not good for planting potatoes.”
Another caution is to control regrowth. “In September there’s often lots of regrowth and more rain events,” Dessureault says. “It’s a good time for late blight.”
Sharing in all the wrong ways
Unfortunately, these diseases work together to exacerbate each other, and pests help to increase disease risk. She notes that seed with Fusarium planted in cold soil will lead to soft rot, and insects can create disease access. Insects like wireworm and tuber flea beetle can lead to Pythium, Fusarium and soft rot.
“Limiting insect damage is important because they create entry points,” she says. “Pythium and Fusarium enter tubers through wounds. [These diseases] are in the soil, just waiting, waiting for a wound to enter.”
Growers should look at their tubers and fields two weeks before harvest. Take note of wet areas, swelling lenticels (the potato’s breathing pores) and decay. “It’s about thinking ahead to storage,” she notes. “When you’re doing inspections before harvest. Do you need more people? What to harvest first or last.”
When harvest comes, it’s best to do so in dry weather and to plan to harvest problematic areas (like low-lying areas) last to avoid the spread of any potential diseases. There should be no excessive irrigation prior to harvest and she says to not harvest water-logged areas.
“If you know ahead you’re going to have an issue, harvest at the lower temperature,” she says.
Ensure soil and flesh temperatures are at about 10 C. Avoid hot temperatures for harvesting. Mature tubers should have well-set skin as immature potatoes are more susceptible to injuries and therefore disease.
“We’re very careful of temperature at harvest,” Harris says. “If it’s above 60 F [15 C], we shut down. So quite often we’ll harvest in the morning and [by] early afternoon we’ll shut down.”
He advises that bruising issues must be managed at harvest as it will lead to dry rot in storage. “You control your speeds and drops, keep your chains full, elevators full,” he says. “We cool our potatoes down to 50 F [10 C] as fast as we can at harvest time. You have to know how to manage what you’re going to get.”
No matter how good pre-harvest and harvest practices are, there are still opportunities for issues in post-harvest. According to one grower study, about 60 per cent of potato damage occurred in the washing and bagging process.
Dessureault recommends that throughout the process there must never be any drop of more than six feet and that storage involves good air movement. Equipment must be sanitized between lots and storage preparation should include: removing infected tubers, excess dirt and debris, no wet potatoes, keep problematic tubers separate and ensure space of two feet from the top of the pile to the ceiling.
Wet or potentially diseased potatoes should be cured at 10 C to help reduce disease and at 8 to 9 C for severe disease. This is done at 85 to 95 per cent relative humidity for three to four weeks with continuous dry air ventilation. “You want a lower relative humidity, but not too low,” she says. “That will cause other problems.”
Healthy potatoes can be cured at 10 to 15 C at 92 to 97 per cent relative humidity for 10 to 14 days with one to two hours of humidified ventilation a day. Monitor piles for wet spots and prevent condensation and CO2 accumulation.
Harris notes that his operation will use extra fans if there is a lot of moisture, as was the case in 2019. “If you see humidity or moistures anywhere, you put the fans in there,” he says. “At the same time, you want 90 per cent humidity. If they come in wet, at the start of the season you turn the humidifiers off, then you turn it back on.”
There are post-harvest treatments available for Fusarium dry rot, pink rot and soft rot. “There’s a lot of damage that can happen at your washing and packing time,” Dessureault says. “Pay attention to the temperature of your water and your tubers. They should be about the same.”
She also cautions to look for sharp surfaces that can lead to cuts and damage, minimize drops and handling and disinfect regularly during the washing and grading process.
When potato seed goes in the ground it holds the promise of the best possible crop, but diseases like pink rot, soft rot, Fusarium and Pythium can be hard to spot until it’s too late. By understanding the best practices farmers can reduce their risk.
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