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By: Bonnie L. Grant, Certified Urban Agriculturist
Speckled leaves with purplish borders may be slightly pretty but can be the sign of a serious disease of sweet potatoes. All varieties are affected by sweet potato feathery mottle virus. The disease is often referred to shorthand as SPFMV, but also as russet crack of sweet potatoes and internal cork. These names illustrate the type of damage to the economically valuable tubers. The disease is transmitted by tiny insect vectors and can be difficult to diagnose and control.
Signs of Sweet Potato Feathery Mottle Virus
Aphids are common enough pests on many varieties of plants, both ornamental and edible. These sucking insects transmit viruses into plant leaves through their saliva. One of these diseases causes sweet potatoes with internal cork. This is an economically devastating disease that reduces plant vigor and yield. Also known as sweet potato internal cork, it causes tubers that are inedible but often the damage isn’t evident until you cut open the sweet potato.
The virus has few above ground symptoms. Some varieties exhibit marked mottling and chlorosis. The chlorosis is in a feather pattern, usually showing up at the midrib. It may or may not be bordered by purple. Other species get yellow spots on leaves, again either with or without purple detail.
The tubers will develop dark necrotic lesions. Russet crack of sweet potatoes is primarily in Jersey-type tubers. Sweet potato internal cork affects several varieties, especially Puerto Rico varieties. When combined with sweet potato chlorotic stunt virus, the two become one disease called sweet potato virus.
Prevention of Sweet Potato Feathery Mottle Virus
SPFMV affects plants around the world. In fact, wherever sweet potatoes and some other members of the Solanaceous family are grown, the disease can appear. The crop losses may be 20 to 100 percent in severely affected tuber crops. Good cultural care and sanitation can reduce the effects of the disease and, in some cases, plants will rebound and crop losses will be minimal.
Stressed plants are more prone to the disease, so it is important to reduce stressors such as low moisture, nutrients, crowding and weed competitors. There are several strains of SPFMV, some of which cause very little damage, as in the case of the common strain, but russet and sweet potatoes with internal cork are considered very important diseases with heavy economic loss.
Pest control is the number one way to prevent and manage sweet potato feathery mottle virus. Since aphids are the vector, using approved organic sprays and dusts to keep their population in check is most affective. Controlling aphids on nearby plants and limiting the planting of certain flowering plants that are magnetic to aphids, as well as wild plants in the Ipomoea genus, will also reduce the pest’s population.
The last season’s plant matter can also harbor the disease, even in foliage that has no mottling or chlorosis. Avoid using diseased tubers as seed. There are numerous resistant varieties available in all the regions in which the plant is grown as well as certified virus free seed.
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Symptoms depend on the sweetpotato variety, its age, and the conditions under which it is growing they also depend on the severity of the SPFMV strain, and whether it is alone or present in the plant with other viruses.
On its own, there may be yellow spots (Photo 1), or purple ring spots (Photo 2). Feathery patterns may appear along the veins, and these too may be yellow or purple (Photos 3&4). In many cases, there are no symptoms, or they are faint and short-lived, with the new leaves looking healthy.
On the roots, the symptoms, depend on the strain and the variety. Strain O does not produce symptoms, whereas strain RC causes external cracking in rings around the storage roots (Photo 5), and internal cork – small brown to black corky spots, seen after harvest and during storage.
When SPFMV occurs in plants together with other viruses, e.g., Sweetpotato chlorotic stunt virus , its concentration becomes very high and severe yellowing and stunting occurs. Symptoms of this kind occurs in Africa and South America.
Spread of SPFMV is by aphids (Photo 6) in a non-persistent way this means that the aphids pick up the virus very quickly as they feed on the sap of an infected plant, and then pass it to a healthy plant when they feed next. But the ability to infect is lost quickly. The virus does not multiply inside the aphids. The virus is also spread in planting material, either in vegetative cuttings or in storage roots. It is not seedborne.
Pink rot infections start at the stolon end and result in rotten and discolored periderm with a clear delineation between healthy and diseased tissue. When exposed to air, tuber flesh turns pink and then brown-black. Pythium spp. that cause leak infections invade tubers through harvest wounds and continue to develop in transit and storage. Infections result in internal watery, gray or brown rot with well-defined red-brown lines delineating healthy and diseased tissue.
For more detailed information on this pathogen, please see our full Solanaceous, Late Blight article.
Late blight affects potato foliage and tubers. Foliar symptoms start with brown to black, water soaked lesions on leaves and stems which produce visible white sporulation at the lesion margins under humid conditions. Whole plants and fields may collapse rapidly. Tuber infection is initiated by sporangia from foliage being washed down into the soil and usually begins in wounds, eyes, or lenticels. Lesions are copper brown, red or purplish and white sporulation may occur on tuber surfaces in storage or cull piles. Infected tubers are susceptible to infection by soft rot bacteria which can turn entire bins of potatoes in storage into a smelly, rotten mass.