Plenty of vegetable crop diseases can be blamed on microbes. Whether vectored by insects, proliferating in damp soils, blowing in on the wind or taking advantage of drought conditions to attack weakened plants,
pathogenic microbes can do a lot of damage. But some plant diseases aren’t caused by pathogens. Instead, they are caused by environmental factors which inhibit plant health. They aren’t contagious, and don’t spread, but can cause unsellable produce and reduced yields, even after harvest.
Root crops in particular are often stored long-term. Some detrimental conditions present in the crop might not be noticeable at harvest, and continue to cause damage during storage conditions. Other abiotic diseases may develop due to environmental conditions during storage. Abiotic disease often causes increased susceptibility to pathogens too. Identifying and preventing these conditions from taking root should be a priority.
Here’s a roundup of some common abiotic root crop concerns, and what to do to prevent them.
Blackheart is characterized by oxygen deficiency in the core of the root crop. Oxygen deprivation can occur in the field or when the root crop is being stored or transported. This causes internal damage to the flesh, with areas of necrotic tissue which are black in color. These areas do not have an odor and are firm.
Common reasons for oxygen depletion in the field are boron deficiency, whether due to lack of boron in soils or to an elevated soil pH, which makes boron unavailable to the plant. Rutabagas, turnips and beets are root crops with high boron requirements, while potatoes, carrots, parsnips and onions have moderate boron needs.
Compacted soils can also cause oxygen depletion. Excessive soil moisture can prevent enough oxygen from reaching tubers. During periods of high temperatures, which cause an increase in plant respiration rates, blackheart damage is more rapid.
Root crops can also develop blackheart during storage. Wet tubers, poorly ventilated storage environments or prolonged low temperature storage are all causative factors. If seed potatoes have blackheart, loss of tuber starch can cause low germination when planted, but the condition is not transmittable from seed.
In the field, beets with blackheart may have irregularly shaped leaves or older leaves with a burnt appearance. It is not readily detectable externally in potatoes.
Root splitting is common in many root crops including carrots, parsnips, beets and radishes. This occurs when soil moisture levels fluctuate, particularly early on in development.
Hollow heart is characterized by roots with hollow cavities. This can lead to soft rots if pathogens are present. The cavities are thought to be caused by rapid growth after periods of cool temperatures combined with periods of moisture stress. Potassium levels have been thought to play a role.
Internal heat necrosis is an abiotic condition found in potatoes. Some cultivars, such as Atlantic, Yukon Gold and Russet Burbank, are more susceptible than others. There are no symptoms seen on plants in the field. Externally, tubers may occasionally have blackened eyes or soft spots.
Internally, there will be brown necrotic spots, which increase over time. The crop’s flesh remains firm with no odor. High day and nighttime temperatures, low soil moisture and high soil temperatures are contributing factors. Low acid soils increase incidence. Affected tubers can have low calcium levels, so maintaining soil calcium at adequate levels may lower risk. Tubers closer to the soil surface are more susceptible. Tubers should be harvested soon after vine death. Seed potatoes may develop spindly sprouts when heat necrosis has occurred.
Planting in hills, maintaining vine cover and planting tubers closer together in the row can help decrease risk. Do not harvest when daytime temperatures are high, and do not store tubers in high temperature environments.
In onions and garlic, basal plate splitting and basal plate blow-out occur due to uneven cycles of watering. The continual drying out of soils, combined with periods of over-irrigation, cause the disorder.
Preharvest abiotic stressors can cause problems during storage of root vegetables. Root crops which suffered from periods of moisture stress can desiccate more readily during storage. This is due to cell damage that occurred when water was limited during root crop growth. The damaged cell walls become leaky, and the crop will lose water during storage more rapidly than normal.
Root crops are often stored during the winter, and any injury sustained during harvesting or packing, such as cuts and bruises, will allow pathogens to enter the plant, can significantly reduce storage shelf life. Root crops are susceptible to storage chilling injury too, and maintaining the proper storage temperature and humidity levels will prevent degradation.
Some biotic diseases can continue to survive post-harvest. If infected produce is stored, the pathogens can continue their lifecycle post-harvest, resulting in visible crop damage.
Abiotic diseases – those caused by environmental factors and not pathogens – can occur whenever plants are stressed. Planting at the proper time, providing consistent watering, testing for soil fertility and planting into healthy soils, along with proper harvest and post-harvest handling, will reduce plant stress and increase the quality of root crops. Because abiotic stressors often allow biotic diseases to gain a foothold, managing these environmental factors can prevent pathogenic crop damage too.