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.