By: Amy Grant
Sweet potatoes are susceptible to a number of diseases, among these is bacterial soft rot of sweet potato. Sweet potato soft rot is caused by the bacterium Erwinia chrysanthemi. Rotting may occur either when growing in the garden or during storage. Also referred to as sweet potato bacterial stem and root rot, bacterial sweet potato rot is favored by high temperatures combined with high humidity. The following article contains information on identifying the symptoms of sweet potato soft rot and how to control the disease.
Symptoms of Sweet Potato Bacterial Stem and Root Rot
As the name suggests, the bacterium, E. chrysanthemi, results in rotting of both the tuber and root system of sweet potatoes. While rotting may occur during growing, the infection is more common in stored sweet potatoes.
In the garden, foliage symptoms appear as black, necrotic, water soaked lesions. Stems are also afflicted with dark brown to black lesions along with dark streaks apparent in the vascular tissue. As the disease progresses, the stem becomes watery and collapses which causes the tips of the vines to wilt. On occasion, the entire plant dies, but more commonly, one or two vines collapse.
Lesions or rotting in the root is more commonly found during storage. Roots afflicted with bacterial soft rot of sweet potato become light brown in color and watery accompanied by lesions with a characteristic dark brown margin. During storage, some roots may appear untouched by the disease until they are cut into wherein decay becomes evident. The infected roots are streaked with black and become soft, moist and rotten.
Bacterial Sweet Potato Rot Control
Sweet potato rot is introduced through wounds, so minimizing wounding of the roots will help to curtail the incidence of the disease. Handle the sweet potatoes carefully as they are being harvested and stored, and work around them gently when weeding or the like. Wounding may be caused by mechanical means but also by insect feeding, so controlling insects will also help to control the spread of the disease.
Also, some varieties of sweet potato are more susceptible to the disease. For example, ‘Beauregard’ is very susceptible to root rot. Use cultivars with a tolerance to bacterial sweet potato rot and select only certified disease-free propagating materials. For transplanting, only use vines that have been cut above the soil’s surface.
Lastly, immediately remove and destroy any infected roots found during storage to prevent the spread of sweet potato rot.
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Sweet Potato Vine (Ipomoea Batatas) - Growing, Care Tips & Pictures
The sweet potato plant or sweet potato vine has been cultivated for around 2,000 years for its sweet tasting tubers. It is said that Columbus brought the plant to Europe from the New World on his expedition (Missouri Botanical Garden). Though the variety of sweet potato plant that produces the edible tubers can be planted for its trailing vines, more recently, different variations have been cultivated specifically for their ornamental foliage. The ornamental sweet potato vine is not grown for edible consumption.
It does produce sweet potato tubers, and though these won’t cause any harm if eaten, they tend to be bland and bitter. It is also worth noting that it is ill-advised to eat any element of a sweet potato vine that has not been specifically cultivated for food purposes, without knowing what pesticides may have been used on it that could be harmful when ingested.
Instead of being grown for the tubers, the sweet potato vine is grown for its colorful foliage, which has many uses both inside and outside. It makes an excellent container plant, with its trailing vines falling over the edges of pots in an effortless and attractive manner. It works well as an ornamental houseplant, as well as for yearlong ground coverage in outdoor flower beds. Being very resistant to drought conditions, this plant is popular in the South, where, among other things, it looks great in hanging baskets.
In some instances, lavender flowers can bloom on the vines, but they are generally considered uninteresting in comparison to the foliage of the sweet potato vines. Flowering is uncommon, happening in most instances on older varieties.
How to Grow
How to Grow
- Cultivate around the plants to prevent weeds, and to prevent side roots from developing. Weeds compete with plants for water, space and nutrients. Cultivate carefully so as not to bruise or cut the young tubers forming just below the soil. Once plants get started their growth will tend to smother out many weeds and grasses.
- Provide sweet potato plants with about ¾ inches of water weekly when they are young, and water them more as the plants mature. Do not water during the 2 weeks before harvest.
- Many gardeners prefer not to fertilize because they feel not fertilizing improves the flavor, while others feel that fertilizing increases yields. If you choose to fertilize, side dress with a balanced fertilizer about six weeks after planting. Do not use excessive nitrogen.
- Monitor for pests and diseases. Check with your local Cooperative Extension Service for pest controls recommended for your area.
The ‘Covington’ cultivar currently claims more than 80% of the acreage planted in Georgia, with additional acreage devoted to ‘Beauregard’ and other varieties like ‘Evangeline.’ All of these varieties have typical orange skin and deep orange flesh. ‘Murasaki’ is a purple-skinned variety with a white flesh that is grown for some specialty markets, and ‘O’Henry’ is a brown- or tan-skinned variety with white flesh. Ongoing variety trials are being conducted to identify any new varieties that may be of interest to Georgia growers.
Postharvest storage rots in Ontario sweet potatoes
Brian Collins, MSc Student, University of Guelph Melanie Filotas, Specialty Crops IPM Specialist/OMAF and MRA
From ONvegetables in The Grower, June 2013
Sweet potatoes are susceptible to a variety of postharvest storage rots. While a number of different bacterial and fungal pathogens can cause storage rots in sweet potatoes, over the last several years fungal pathogens in the genus Rhizopus and Fusarium have been the most common causes of postharvest storage losses in Ontario sweet potatoes.
Rhizopus soft rot, caused by the fungus Rhizopus stolonifer, is one of the most common postharvest diseases of sweet potatoes in North America. R. stolinfer is extremely common in air and soils, and enters sweet potato tissues through wounds, such as those occurring during harvest and packing. Sweet potatoes with Rhizopus soft rot develop a white, hairy fungal growth (Figure 1) which produces very large quantities of dusty black spores (Figure 2). A ring rot can also occur when the fungus infects the middle portion of the root. Infected tissue often has a pronounced, sweet odour, which often attracts fruit flies. This fungus can spread rapidly, and a soft wet decay can spread over the entire root within three days under storage conditions (13⁰C). Wet, cool soil at harvest can make sweet potatoes more susceptible to this disease. Sweet potato cultivars can vary considerably in susceptibility to Rhizopus soft rots, with the white fleshed cultivars like O’Henry often being more susceptible than orange-fleshed cultivars. However, even resistant cultivars like Beauregard can suffer losses to Rhizopus under the right conditions (e.g. cool, wet weather at harvest and injury to roots during harvest or packing).
Fusarium root and surface rots
Sweet potatoes are susceptible to several different diseases caused by Fusarium species, including Fusarium root rot, surface rot and stem canker. While Fusarium stem canker affects plants in the field, root rot and surface rot are predominately post-harvest diseases and are significant storage diseases of sweet potatoes in many sweet potato growing regions.
Fusarium root rot, caused by the fungus Fusarium solani, causes light and dark brown circular concentric lesions on the skin of sweet potatoes, which may coalesce and overlap over time. Fusarium root rot is generally dry and infected roots remain firm. Symptoms often originate from the end of the root and thus this disease has also been referred to as Fusarium end rot. Upon cross-section of severely infected roots, discolouration varies from light to dark brown (Figure 3) with lens-shaped cavities that often have white fungal mycelium growing inside. If infected sweet potatoes are stored in a humid environment, a white fungal growth can develop on the exterior of the roots near the infection site (Figure 4).
Fusarium surface rot is caused by the fungus Fusarium oxysporum and by some strains of F. solani. Symptoms appear as circular light brown, firm dry lesions. Lesions are generally superficial and often occur at points of injury to the root. Early symptoms of root rot can be easily confused with surface rot, as external symptoms are very similar. The differentiating feature is that surface rot does not colonize the inner tissue of the root and symptoms are restricted to the outer layers of tissue.
Both species of Fusarium are persistent soilborne pathogens, and require wounds in the fleshy roots of sweet potatoes for infection to occur. These wounds are most often caused by mechanical harvesting, leading to the more common storage rots. However rots can occasionally occur in the field through wounds caused by insects, nematodes or rodents, and through growth cracks. This pathogen will not spread between roots in storage unless new wounds occur. However, F. solani can spread from seed roots to sprouts, leading to Fusarium stem canker in the resulting slips. Sweet potatoes with minor F. solani infection can appear healthy but if used as parent material the pathogen can be transferred to the sprouts.
All of the fungi discussed here are widespread in the environment, and are likely present in all sweet potato fields. However since they can only enter sweet potatoes through wounds in the skin, the best way to manage these diseases is through sanitation and proper handling of roots to minimize entry points for these pathogens. Specific management techniques include:
- Careful handling of roots at harvest to minimize injuries to the skin.
- Completing harvest early to avoid chilling temperatures (prolonged exposure to 10-12⁰C or less), which can predispose roots to storage rot pathogens.
- If possible, avoid harvesting from wet soil. Extremely dry conditions can also affect disease incidence by increasing the rate of skinning, thus creating more sites for infection.
- Properly cure roots as soon as possible after harvest to heal any wounds that do occur during harvest.
- Ensure the entire storage facility has adequate air flow to maintain appropriate temperatures (13-16⁰C) and humidity levels throughout. In some cases, the storage rots have been reported to be more severe at the bottom of solid containers or along outer walls, where there is inadequate heating or air flow.
- Remember that wounding of roots at the packing stage can also lead to development of Rhizopus soft rot after sweet potatoes have left the storage facility. Gentle handling during packing and minimizing/cushioning any drops along the packing line can help reduce incidence of the disease.
- The fungicides Scholar (fludioxonil) and Bio-Save (the biofungicide Pseudomonas syringae) are registered for the control of Rhizopus soft rot on sweet potatoes in Ontario. There are no fungicides registered for control of postharvest Fusarium diseases in sweet potatoes in Canada.