Prepared: August, 1999
Revised: June, 2003
Sweet potatoes are grown almost exclusively in the eastern part of Texas, primarily in or near Van Zandt County. The Texas Department of Criminal Justice produces sweet potatoes in an area near Huntsville where the crop is limited to institutional consumption. In addition, there are a significant number of truck farmers in Texas who grow and sell sweet potatoes locally and through roadside stands.
It is estimated that an applicator operating a ground rig will take approximately 10-15 minutes for each acre and for each pesticide application. This includes mixing, loading, and application, but not transit time to get to and from the fields. Time per acre will change as acreage gets smaller with fewer acres serviced per loading operation. Applicators generally wear rubber gloves, long sleeve shirts and hats. When mixing and loading sprayers or nurse tanks, mask are often added.
Approximately 60 percent of sweet potatoes fields are treated with a soil applied granular insecticide at planting and 100 percent are treated with a herbicide. Post-emergence insecticide use varies greatly from year to year but can be as high as 100 percent. Over 90 percent of the post-plant treatments are ground applied. Insecticides used on small fields of sweet potatoes produced for roadside stands and farmer's markets are applied with a wide range of equipment. Smaller operations will use less chemicals because of a lesser need for optimum production.
The availability of herbicides has reduced the need for hand hoeing or roguing. Hand hoeing does take place on very small production plots grown for roadside markets but this is probably also quite limited. For such small plots the total time spent hoeing or roguing is estimated at 1-4 hours depending on weed stand.
Contract fields of sweet potatoes are dug by machines and handlers box the roots. Small plot acreage may be dug by hand. This operation may involve the grower, his family, and other unskilled laborers. Gloves made of soft leather or heavy cotton are usually worn when hand harvesting.
Foliage feeding insects are generally an'occasional' problem in Texas sweet potatoes. Damage is to plant's stems and leaves where pest include immature cutworms, corn earworms, hornworms, loopers and armyworms. Once the sweet potatoes have been transplanted to the field about 5% of the acreage will be affected by foliage feeders compared to 75 percent of the seed bed acreage. Because seed beds are the source of material for field transplants most of controls for leaf feeders are during this crop stage.
Leaf Hoppers: potato leafhopper Empoasca spp.; beet leafhopper, Circulifer tenellus
Leafhoppers are a group of small green-brown insects approximately 3-4 mm long that have piercing-sucking mouthparts. Immatures resemble adults except they have no wings. Both adult and nymphs puncture the underside of leaves and suck out plant juices where subsequent injury causes stunting and leaf curl. The hoppers inject a toxin that causes "hopper burn" on leaves which may ultimately drop from the host.
Corn Earworm: Heliothis zea
The corn earworm damages by feeding on leaves, buds, flowers or all of the above ground parts to the growing sweet potato. The pest is most susceptible to control when the larvae are small. Some work has been done on killing the adult but because of its mobility this method of control is generally difficult.
The corn earworm adult is a small moth that is characterized by light grayish-brown front wings marked with dark-gray to olive-green irregular lines along with dark areas near the tips and a dark spot usually near the wings center. Hind wings are a lighter tan with bands near the edges and fully extended, the wings (front over back) will span Ż inch. As an immature the worm varies from light green or pink to brown or nearly black. In warm weather this insect can complete its life cycle in about four weeks.
Hornworms: Manduca sexta (Linnaeus), Manduca quinquemaculata (Haworth)
There are two very large worms often described as a hornworm, the tomato hornworm and the tobacco hornworm. Both have a horn like structure on the end of their larval body. Often reaching lengths of nearly 5 inches, these green worms are capable of extensive damage as a feeding immature. The adult is the Sphinx moth; a grayish-colored insect with a wing spread of 4 to 5 inches. Damage usually occurs through the late summer months but this will generally happen once every five or six years.
Loopers: cabbage looper, Trichoplusia ni (Hubner), soybean looper Pseudoplusia includens (Walker)
Loopers are the immature stages of moths in the insect family Noctuidae. Often referred to as measuring worms because of the way the worms draws up the middle of its body as it moves, these pest are a problem when present in large numbers. Feeding is on foliage, often initiated on the interior lower portion of plants and proceeding out as the worms mature. Early instars feed on the lower leaf surfaces. Older larvae characteristically leave the larger leaf veins. Most damage is during the last two larval instars often during late season or prior to harvest.
Armyworms: beet armyworm, Spodoptera exigua; yellowstriped armyworm, Spodoptera praefica, armyworm, Pseudaletia unipuncta; fall armyworm, Spodoptera frugiperda)
Army worms generally will be found feeding in large numbers, usually at night. Preferring leaf margins, but occasionally eating the entire plant, these pest are often most numerous in late summer and early fall. A full grown armyworm may reach 1- 1 Ż inches in length.
Sweet potato growers generally expect to have damage from soil insects if no soil insecticide is applied at planting. Soil inhabiting insects feed on sweet potato roots. About 85 per cent of the Texas acreage is impacted with yield losses running up to 75 percent. Quality is impacted as well. The soil insect pest complex includes flea beetle larvae, wireworms, and the white grub of May/June beetles . Since 1999, the sweet potato weevil has infested several fields in some parts of Texas. In 1999, an eradication plan was initiated and the insect was all but eliminated from Texas sweet potatoes. The treatment was with Penncap-M, Imidan 70WSB, and Thiodan/Phaser 50 WP.
Cutworms: granulate-Feltia subterranea (Fabricius); variegated-Peridroma saucia (Hubner)
There are a number of cutworm species that can be soil insect pest, all in the insect family Noctuidae. The more common includes the black cutworm, granulated cutworm and the variegated cutworm. These are dull gray, brown, or black, striped or spotted stout, soft-bodied and smooth, worms that may grow up to 1 1/4 inches long. Most will curl up tightly when disturbed. Cutworm larvae hide curled up in the soil by day and at night cut off young plants near the ground, feed roots and foliage.
Flea Beetle Larvae: Flea beetles that are pests of sweet potatoes in Texas are often referred to as the Systena complex. This group, which includes the palestriped and red-headed beetles, damage as foliage feeding adults and as root feeding immatures. Foliage damage is generally not an economic problem but on the potato root, larval feeding causes a "scroll" looking surface blemish that is often referred to as "writing". Extensive root damage will ultimately cause plant stunting, a downgrade in quality and yield reductions.
Wireworms: southern potato wireworm, Conoderus falli, tobacco wireworm, Conoderus vespertinus)
Wireworms are the immature damaging stage of click beetles. This pest has a one year life cycle with immatures feeding inside roots leaving surface holes and sites for disease invasion. The hard-bodied larvae are creamy white to yellowish-orange and have a brown head and tail.
White Grubs: may beetle, Cyclocephala spp. and June beetle, Cotinis nitida
The immature stage of the June beetle and May beetle feed extensively below the soil on the roots of many different kinds of plants. In sweet potatoes root damage is from these grubs that are from eggs generally deposited in late spring. White grub damage may be more prevalent in areas that have been rotated from sod.
|Pesticide||% Acres Trt.||Type of Appl.||Typical Rates||Timing||# of Appl.|
|50||ground||1 lb ai/ acre||Apply when 40 percent of plants are damaged.||1|
|75||ground||Used at planting only.||1|
|90||ground||Apply for sweet potato weevil control.||2|
|5||ground||Used very little on an as needed basis for worms.||1|
|10||ground||1 lb ai/A||Apply when 40 per cent of foliage is damaged.||1|
|5||ground||Used for soil insect control.||1|
Annual grasses are a problem in sweet potatoes because of competition for resources such as space, nutrition and moisture. All of the state's acreage is affected. Annual grasses complete their life cycle in one year. Summer annuals germinate in the spring, produce seed and die in the fall. Winter annuals germinate in the fall, produce seed and die in the spring or early summer. Pre-transplant and post emergence applications of herbicides can prevent harmful weed population build-up. A heavy weed infestation can also slow or hamper harvest.
There is no major regional differences in weed problems in Texas sweet potatoes. Cultivation is a major part of sweet potato weed management, where the practices include preseason field preparation, bed construction and post emergence tillage prior to vine elongation. Post harvest tillage may help reduce the next season's weed problems. Biological control of weeds is probably not an important factor in Texas sweet potato production.
|Pesticide||% Acres Trt.||Type of Appl.||Typical Rates||Timing||# of Appl.|
|5||ground||.75 lb ai/acre||Prior to planting.||1|
|75||Ground||.75-1 lb ai / acre||Command is applied over the top of newly transplanted sweet potato slips.||1|
|2||ground||.2 - .3 lb ai / acre||Post emergence.||1|
Broadleaf weeds are a constant threat to productive sweet potatoes where damage is by competing with the crop for space, nutrients and moisture. All of the state's acreage is impacted. The summer annual, winter annual and perennial plant groups all contain broadleaf plants that are at times considered weeds. The best time for control is when the unwanted plants are small or prior to germination. Tillage is a standard preplant and post emergence broadleaf weed control practice. Natural biological control of weeds may be an important factor in Texas sweet potato production but this is generally undocumented. Post harvest control of weeds can help reduce the following seasons problems.
||% Acres Trt.
||Type of Appl.
||# of Appl.
||.75 - 1 lbs ai / acre
Bacterial Root and stem rot or bacterial soft rot in sweet potatoes is caused by the bacterium Erwinia chrysanthemi. When conditions are favorable this disease causes a soft rot in vines and stored potatoes. Invading the host through wounds this bacterium survives in crop debris or on weeds. Sources of inoculum may include contaminated wash water and harvesting equipment. The disease is favored by warm, humid weather where plants often have black lesions on stems which is generally accompanied by a dark streaking of the vascular tissues.
Rhizopus Soft Rot, often referred to as the bread mold fungus, is caused by Rhizopus stolonifer. Sometimes confused with bacterial soft rot, rhizopus lesions will have a hair-like or fuzzy appearance caused by developing mycelia. Care after harvest, including well cured potatoes, a clean packing environment, using disinfectant wash water and use of resistant potatoes, helps manage this pest.
Fusarium Rot, fusarium spp. May be called different names depending on the infection site, stem rot, root rot, end rot or a surface rot. All can cause major problems in sweet potatoes where infected plants may not survive the slip stage and diseased roots rot in storage. Lesions on stored roots are circular and commonly exhibit light and dark brown concentric rings. Use of sanitary practices and properly handling harvested roots helps control this organism complex but resistant varieties are a valuable control tool.
Soil Rot, caused by the bacterium Streptomyces ipomoea is a storage and root disease that is often called pox when found on storage roots. When fibrous roots are infected dark, necrotic lesions develop and under severe disease conditions plants die. Resistant varieties, crop rotation and soil fumigation are control options. This disease is a wide spread problem in Texas.
Russett Crack and internal cork are problems caused by the sweet potato feathery mottle virus. Because the virus is transmitted by aphids, foliar insect management is a consideration. Externally this disease is characterized by cracks running length-wise on the potato. Internally, dark corky spots develop in the potato flesh and rarely occur. However, foliar symptoms which vary from vein clearing to purple ring spots are more serious. It is believed that all sweet potatoes carry this virus but some varieties are more likely to develop symptoms than others.
Java Black Rot, Diplodia gossypina a disease that moves from both ends of an infected potato, is a serious problem in storage. As the decay progresses infected tissue turns from a yellowish color, to brown, to black. The fungus requires a damaged area for entry into roots but generally cannot establish in stored roots that are properly cured. Post harvest handling can result in new wounds and provide for fungal invasion.
Charcoal Rot, Macrophomina phaseolina like Java black rot is a storage rot of sweet potatoes. It is a soil borne fungus with a wide host range. The pathogen survives on plant debris or in the soil. Often confused with java rot this disease will have two different appearing zones within the same infected root. This is apparent as a reddish leading edge of diseased material followed by a dark to black zone of active decaying potato flesh.
||% Acres Trt.
||Type of Appl.
||# of Appl.|
|Dicloran(Botran)||5||ground||1 lbs ai/100 gallons water||Treated when processed||1|
|5||seed root treatment||4 lbs ai/100 gallons of water||At planting Bedding||1|
|Dicloran (Botran)||25||seed root treatment||at planting||1|
Most of Texas sweet potato acreage has a problem with nematodes, usually the root knot nematode. This pest causes galls on roots, blemishes on mature potatoes and often a general stunting of vines. Approximately 80 percent of the states' acreage is affected.
The life cycle of most nematodes is relatively short and is usually completed within 3-4 weeks. The root knot nematodes life stage consist of an egg, four larval stages ( J1, J2, J3, and J4) and adult. Nematcides are applied prior to planting. Rotation with non host and good field sanitation should be followed to lessen nematode problems. There are resistant sweet potato varieties,.
||% Acres Trt.
||Type of Appl.
|dichloropropene + chloropierin|
|10||ground||10.8 - 17 gallons formulation per acre||Two weeks prior to transplanting sweet potato slips.||1|
|aldicarb(temik)||20||ground||10 - 20 lbs of 15G formulation per acre||Apply Temik to soil prior to transplanting sweet potatoes.||1|
Texas Cooperative Extension
College Station, Texas
Extension Plant Pathologist Overton, Texas 75684
James V. Robinson
Overton, Texas 75684
Overton, Texas 75684
Frank Dainello. 1996. Texas Commercial Vegetable Production Guide. Texas Agricultural Extension Service Publication. 129 pp.