Chilling injury and sweet potatoes

It is commonly known that sweet potatoes cannot tolerate a heavy spring or fall frost, and this fact influences the timing of both planting and harvest in northern growing regions. However, as we head into the cooler temperatures of fall, it is important to remember that sweet potatoes are also susceptible to chilling injury when roots are exposed to temperatures of 10-12°C or less.

Chilling injury can be defined as damage to fruits and vegetables exposed to temperatures above their freezing point but below some minimum temperature (usually 5-15°C). It is common in plants originating from tropical or subtropical regions. Chilling injury differs from injury due to freezing, where damage is due to ice crystals forming in the tissues and is immediately obvious. With chilling injury, exposure to lower temperatures can damage cell membranes or affect the normal metabolism of plant tissues, resulting in a cascade of other reactions. In contrast to freezing damage, chilled roots often appear fine when dug, but may decay quickly during curing. Sometimes, symptoms may not even become evident until several weeks after roots are placed into storage.

Symptoms of chilling injury in sweet potatoes can be difficult to diagnose and include surface pitting, loss of dry matter, internal breakdown and discolouration of tissue when exposed to air (Figure 1 and 2). Chilling greatly increases susceptibility of roots to decay in storage due to a variety of fungi (Figure 3). Chilling can also negatively affect the colour, texture, taste and smell of sweet potatoes, and the core of the root may stay hard after cooking.

The key thing to keep in mind is that, when we are talking about chilling injury to unharvested roots, the important temperature is that of the soil around the roots and not the ambient air temperature. Soil temperatures at the depth of a sweet potato root are usually different than that of the air (typically being higher during the fall months). There are also many factors that can influence soil temperature such as wind, soil moisture, crop canopy, rain, sunlight and height of the hill. This is why it is a good idea to take soil temperatures directly, rather than relying solely on air temperatures when determining if there is potential for chilling injury to roots. At the Simcoe Research Station, soil temperatures averaged 21°C between September 12 and 15, and at a depth of 5 cm the lowest temperature recorded was 13.8°C, so chilling temperatures are not far away. In southwestern Ontario, the risk of injury from exposure to chilling temperatures increases greatly in late September and early October.

Furthermore, chilling injury is a function of both the temperature and the length of exposure, so exposure to one or two hours at 4°C may cause the same amount of damage as several hours at 7°C. The effects are also cumulative - one brief period of exposure to soil temperatures below 10°C may not result in any significant injury, while several days where soil temperatures dip below 10°C for short periods could be damaging.

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, and never leave harvested roots in the field overnight.

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.