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Sweet potato chilling injury

It's fall, temperatures are dropping and Ontario's sweet potato harvest is underway. At this time of year, growers are often concerned about the potential impact of heavy frosts on roots still in the ground. However, it is important to remember that temperatures above freezing can also negatively affect sweet potatoes.

Sweet potatoes are highly susceptible to chilling injury, which can be defined as damage to fruits and vegetables when exposed to temperatures above their freezing point but below some minimum temperature. In the case of sweet potatoes, that's below10-12°C. Chilling injury is relatively common in plants originating from tropical or subtropical regions, but the effects differ from that of freeze damage. When plants are frozen, damage is due to ice crystals forming in the tissues and is immediately obvious. With chilling injury, exposure to low temperatures can damage cell membranes or affect the normal metabolism of plant tissues, resulting in a cascade of other reactions. Unlike freezing damage, chilling injury is not always immediately visible. A sweet potato root with chilling injury may appear fine when dug, but may decay quickly during curing. In other cases, symptoms of chilling injury may not even become evident until several weeks after roots are placed into storage.

Symptoms of chilling injury in sweet potatoes can be hard to diagnose, but can include surface pitting, loss of dry matter, internal breakdown, discolouration or changes to the texture and taste of the root (Figure 1 and 2). Chilling greatly increases susceptibility of roots to decay organisms, leading to higher incidences of storage rots (Figure 3). Chilling can also lead to hardcore, in which areas of the root remain hard even after cooking.

Chilling injury is more commonly observed post harvest, if storage temperatures become too low, but can be a problem in the field later in the harvest season. Remember that it is the soil temperatures around the root, rather than the ambient air temperature, that is important, so it is a good idea to monitor soil temperatures directly when determining if there is potential for chilling injury to roots. In southwestern Ontario, the risk of injury from exposure to chilling temperatures increases greatly as we move from late September into October.

Chilling injury is a function of both the temperature and the length of exposure, so one or two hours at 4°C may cause the same amount of damage as several hours at 8°C. The effects are also cumulative - one brief period of exposure to soil temperatures below 10°C may not result in any damage, while several days where soil temperatures dip below 10°C for short periods could cause extensive injury.

All of these factors combine to make avoiding chilling injury a challenge. Monitor soil temperatures frequently, preferably early in the morning when they are likely to be lowest, and try to finish harvest before soil temperatures are consistently dropping below 10°C. If you are forced to harvest later in the season, consider separating these roots from earlier harvested roots, as these will be more likely to develop fungal rots in storage. Ensure harvested roots are moved out of the field as quickly as possible if air temperatures during harvesting operations are dropping below 12°C.

Figure 1 - Tissue decay in sweet potato exposed to chilling temperatures in the field. This root appeared fine at harvest, with decay developing during curing.

Figure 2 - Breakdown extending into the interior of the root. Other symptoms include pitting, dry matter loss and impacts on taste and quality.

Figure 3 - Chilling greatly increases susceptibility of sweet potatoes to colonization by various fungi in storage.