Historic, archived document

Do not assume content reflects current scientific knowledge, policies, or practices.

I

Farmers' Bulletin No. 1059

U. S. DEPARTMENT OF AGRICULTURE

Contents

Page

Field diseases and their control 3

Stem rot (wilt, blue stem,

yellow blight) 3

Black rot (black shank, black

root) 6

Foot rot (die off) 8

Scurf (soil stain, rust, Jersey

mark) 9

Root rot (Texas root rot) 10

Mottle necrosis 12

Soil rot (pox, ground rot) 13

Phyllosticta leaf blight 14

Septoria leaf spot 15

White rust (leaf mold) 15

Root knot 16

General control measures for field

diseases 17

Seed disinfection 17

Hotbed sanitation 18

Vine or sprout (bed) cuttings- 18

Page

Storage rots and their control 19

Soft rot (ring rot, collar rot) . 19

Black rot 20

Internal cork 21

Surface rot 22

Dry rot 23

Java black rot 24

Charcoal rot 25

Control measures for storage rots. 25 Digging and handling sweet- potatoes 25

Management of the storage

house 25

List of causal agents of sweetpo-

tato diseases 26

Washington, D. C.

Revised March 1955

For sale by the Superintendent of Documents. IT. S. Government Printing Office Washington 25, D. C. - Price 15 cents

2

Sweetpotato Diseases

Prepared by Horticultural Crops Research Branch, Agricultural Research Service, and the Biological Sciences Branch, Agricultural Marketing Service 1

The sweetpotato is one of the principal food crops of the South. Field diseases and storage rots 2 each reduced yields by about iy2 percent for the period 1940-52. Field diseases reduce yield, affect the quality, and cause roughness, poor color, and poor shape. Suc- cessful storage depends partly on the control of field diseases, some of which are also destructive storage diseases.

Black rot, both a field and a stor- age disease that gives sweetpotatoes

a bitter taste when cooked, probably causes as much loss as all the other diseases combined. If black rot alone could be eradicated or effec- tively controlled, the losses in stor- age would be greatly reduced.

Decay caused by soft and ring rots and other storage diseases re- duces the quantity of sweetpotatoes for market and shortens the period they can be marketed. Displaying diseased sweetpotatoes also has a bad influence on the market.

Field Diseases and Their Control

Stem Rot (Wilt, Blue Stem, Yellow Blight)

Description

In the field the first indication of stem rot is a slight change in the appearance of the youngest leaves. These become dull, then yellow between the veins, and pucker somewhat. Then the vines wilt and eventually the entire plant collapses and dies (fig. 1). The stems of diseased plants darken in- side and sometimes split open at about the ground level. This dis- coloration of the stems sometimes extends 3 to 5 feet from the hill and this is a sure sign of stem rot. The fungus causing stem rot may also invade the fleshy roots and cause a

blackened ring about a quarter of an inch below the surface (fig. 2). Sprouts from such sweetpotatoes are likely to be diseased.

In the plant bed the symptoms of stem rot are similar to those in the field. Diseased plants can gener- ally be detected by the faint pur- plish tint that shows through the white part of the stem and by the yellow color of the leaves.

Distribution, Prevalence, and Loss

Stem rot probably occurs in every State where sweetpotatoes are grown. It is an important field dis- ease and the most difficult to control.

1 Revised by Harold T. Cook, principal pathologist, Biological Sciences Branch, Agricultural Marketing Service. The original edition was written by L. L. Harter.

2 Scientific names of causal organisms (fungi and nematodes) are listed on p. 26.

3

Figure 1. — Stem rot symptoms of a sweetpotato plant.

The disease is severe in New Jer- sey, Delaware, Iowa, in parts of Kansas, and in southern Illinois. From 10 to 50 percent of the crop may be destroyed in those States each year, and it has killed 95 per- cent of the plants in some fields. Al- though the losses in Maryland, Vir-

Fif/nre 2. — A section through a sweet- potato, showing the blackened ring caused by the stem rot fungus.

ginia, and Alabama are consider- able each year, they are relatively less than in other States.

Means of Distribution

The stem rot fungus can overwin- ter in the soil on the remains of dead sweetpotato vines and in the roots in storage. Therefore, the distri- bution of the disease from one field to another in the same locality may be brought about by (1) insects, (2) farm animals, (3) farm imple- ments, (4) drainage water, (5) wind, and (6) discarded diseased roots dumped on the fields, either before or after being fed to stock.

The disease is spread from one lo- cality to another primarily by the exchange or sale of seed sweetpota- toes and plants.

Cause

The fungus that causes stem rot can live for several years on decayed

4

vegetation in the soil until it again comes in contact with the sweetpo- tato.

Infection takes place both in the plant and in the field. The fungus in diseased seed stock planted in the plant bed grows into the plants. Such infected plants die soon after they are set in the field. Healthy plants may become infected after they are set in the field when the fungus in the soil grows into the roots.

The mycelium (threadlike web) of the fungus develops rapidly and grows up through the water-carry- ing vessels of the stem. After the vines die and turn black, the fungus lives on the decaying vegetation. Numerous fruiting bodies, or spores, develop on the dead vines. Being very small, the spores are readily carried by the wind, insects, and other agencies to other fields.

Control

Fertilizers and Fungicides. — Be- cause the fungus causing stem rot invades the plant through its roots, fungicides applied on the plants or on the roots will not control the dis- ease. Applications of lime and gypsum to the soil are of no control value.

Immune and Susceptible Vari- eties.— The following varieties, none of which are entirely immune, can be grown with comparative safety in infested soil : White Yam, Southern Green, Triumph, Eed Brazil, Yellow Strasburg, Key West, and Dahomey. The follow- ing varieties are very susceptible to stem rot : Yellow Jersey, Big-Stem Jersey, Gold Skin, Nancy Hall, Porto Eico, Red Jersey, Georgia, Nancy Gold, Kansas 40, and Mary- land Golden.

Seed Selection. — The stem rot fungus overwinters in sweetpotatoes in the storage house and grows from diseased stock into the plants de- veloped from them. Slightly dis-

eased plants are hard to detect, and, in consequence, many of them are set in the field, where the fungus continues to grow. It is, therefore, imperative that you use only healthy sweetpotatoes for the production of plants.

Healthy seed stock can be selected in the fall at digging time while the sweetpotatoes are still attached to the vines. Test each hill by split- ting the stems, and select seed only from plants with stems that are not streaked inside with black. Do this before a killing frost, as a heavy frost will sometimes darken the stem. Fall selection of seed stock is necessary, for it is difficult in the spring or during the winter to tell whether the sweetpotatoes are dis- eased. After a period in storage a blackened ring a quarter of an inch below the surface often occurs even in healthy sweetpotatoes.

Store the sweetpotatoes selected for seed in a part of the house where they will not come in contact with the general stock.

Seed Disinfection and Bedding. — In the spring just before the roots are bedded, disinfect them by treat- ing in a solution of corrosive sub- limate (mercuric chloride) or of borax (p. 17). This treatment kills only the fungus spores that may be on the surface of the root and will not kill the fungus within the root.

Bed the sweetpotatoes immedi- ately after the treatment in a prop- erly prepared plant bed (p. 18).

Crop Rotation. — The stem rot fungus will live in the soil indefi- nitely, even in the absence of sweet- potatoes. For that reason, do not plant sweetpotatoes on the same ground oftener than once in 3 or 4 years. This rotation will not erad- icate the fungus, but will reduce the losses. No other crop except tobacco is known to be attacked by this fun- gus ; therefore, any crop except to- bacco commonly grown in the re- gion may be used in the rotation.

5

Black Rot

(Black Shank, Black Root)

Description

Black rot may occur on any of the underground parts of the plant. It produces dark to nearly black some- what sunken, circular spots on the surface of sweetpotatoes (fig. 3).

Figure 3. — Black circular spot on sweet- potato caused by the black rot fungus.

When the plants are young, these spots are small and nearly round; under favorable conditions they en- large and often involve nearly the whole sweetpotato. Fruiting bod- ies, or spores, of the fungus may often be found in circular areas about one- fourth to one-half inch in diameter in the center of the spots. The surface of the diseased spot has a somewhat metallic luster and the tissue just beneath is greenish. In- fection on the plants begins as small black spots on the lower part of the stem and enlarges until the whole stem is rotted off. Frequently the infection extends up the stem to the surface of the soil (fig. 4). The name "black shank" is commonly applied to this phase of the disease. If sweetpotatoes affected with black rot are used for seed, the plants com-

Figure 4. — A small sweetpotato plant showing the characteristic blackening of the underground part of the stem caused by the black rot fungus.

6

ing from them will likely have the disease.

Sweetpotatoes affected by black rot have a very disagreeable taste when cooked, and their sale has a bad effect on the market.

Distribution, Prevalence, and Loss

Black rot is present in most of the States where sweetpotatoes are grown. The disease occurs on the plants or sprouts in the hotbeds, in the fields, and on the roots in stor- age houses in the winter. Heavy losses are caused by this disease in storage houses, where it develops freely under favorable conditions and renders the sweetpotatoes unfit for sale.

Means of Distribution

Black rot is spread in about the same way as stem rot. Unlike stem rot, however, black rot continues to develop in the storage house, and sweetpotatoes that appear sound when stored may become badly af- fected within a few weeks. Spread in the storage house may be brought about by rodents, by handling the roots when you are picking them over and preparing them for mar- ket, and by the roots settling in the bins. Washing the sweetpotatoes also may distribute the germs from one sweetpotato to another.

Cause

The black rot fungus overwinters on the dead vines and other decayed vegetable matter in the soil and on the sweetpotatoes in storage. No host plants other than sweetpotatoes are known. If roots affected by black rot are used as planting stock, the fungus usually grows into the plants while in the plant bed. In- fection also takes place through the roots after the plants are set in the field. Plants that become infected early soon die, and those that con- tinue to grow rarely produce any sweetpotatoes.

Control

None of the present commercial varieties are resistant to black rot.

Black rot is easily controlled by using disease-free seed sweetpota- toes, root disinfection, clean plant beds, and crop rotation.

The most practical way to obtain black-rot-free bedding stock is to grow seed sweetpotatoes from vine cuttings or sprout (bed) cuttings (p. 18 ) . Since black rot affects only the below-ground parts of the plant, these cuttings will be free of the dis- ease. They will produce disease- free sweetpotatoes that can be used for bedding the next year, if the cut- tings are planted in fields that are free of black rot. Sorting out black - rot - infected sweetpotatoes, either at harvesttime or before bed- ding in the spring, is not a satisfac- tory way to obtain bedding stock. Many small, recent infections es- cape detection, and the disease con- tinues to develop in storage or in the plant bed.

Disinfect the seed with either cor- rosive sublimate or borax to destroy black rot spores that may be on the surface. Disinfection does not kill the fungus inside of black-rot-in- fected sweetpotatoes (p. 17).

Bed the seed stock immediately after treatment in a properly pre- pared clean plant bed (p. 18) .

Plant the sprouts on new ground or on soil that has not been used for sweetpotatoes for 3 or 4 years. Ro- tation of sweetpotatoes with other crops helps control black rot, be- cause the fungus does not affect other crops and is only able to live about 2 or 3 years in the soil.

Treating the soil with sulfur, lime, gypsum, or various fertilizers has little effect on the disease. Dip- ping the plants in a solution of bor- deaux mixture or in a lime-sulfur mixture just before they are set in the field may reduce the incidence of the disease, but does not prevent it entirely and has been found to injure the plants.

7

Foot Rot

(Die Off)

Description

Foot rot appears first as small brown to black spots on the stem of the plant near the soil line. The growth of the foot rot fungus is very slow at first, but eventually it girdles the plant and extends up the stem 4 or 5 inches. Soon the plant wilts, and rather numerous round,

Figure 5. — The lower part of a sweet- potato plant killed by the foot rot fungus.

black specks, just visible to the naked eye, appear in the diseased areas (fig. 5) . These specks are the fruiting bodies of the fungus. This disease progresses rather slowly, and the plants do not die off until about midsummer or later. In most instances the affected plants bear no sweetpotatoes, although long vines may have been produced. In hills in which the sweetpotato

Figure 6. — A sweetpotato rotted by the foot rot fungus.

8

develops, the organism causing foot rot may spread from the infected stem to the roots and cause a brown and rather firm rot. Later, fruit- ing bodies develop close together on the surface in the form of pimple- like protuberances (fig. 6). Many wounds and bruises on sweetpo- tatoes in storage become infected with the foot rot fungus.

Distribution, Prevalence, and Loss

Foot rot is distributed in the same way as stem rot and black rot. It occurs in Virginia, Maryland, Ohio, South Carolina, Iowa, California, and Mississippi. Because it is not so widely distributed as black rot and stem rot, the tottJ loss from foot rot is much less. In certain parts of Virginia, Ohio, and Iowa, it causes heavier losses than either black rot or stem rot.

Cause

Infection from the foot rot fun- gus takes place primarily through the roots or underground parts of the plant. However, during wet periods, when the growth is very

luxuriant, diseased vines are some- times found some distance from the hill. Infection takes place mostly in the hotbed by spreading from diseased seed stock to the plants. Such plants when set in the field usually die early in the season.

Spores, borne in great numbers, escape from the pimplelike projec- tions of the diseased tissue and are carried by insects or other agencies to other plants, where new infec- tions may result. If a diseased plant produces sweetpotatoes, the fungus often grows down the stem and infects them. The fungus may remain dormant during the storage period, but it will develop on the sprouts in the plant bed. As in the cases of stem and black rots, therefore, diseased seed stock pro- duces diseased plants, which in turn may produce diseased sweetpotatoes in the field.

Control

For control of foot rot, follow recommendations for stem rot and black rot — seed selection, the use of clean plant beds, seed treatment, and crop rotation.

Scurf

(Soil Stain, Rust, Jersey Mark)

Description

Scurf produces a brown surface discoloration of the root (fig. 7). The discolored areas may take the form of spots of different sizes and shapes with no definite outline, or there may be a uniform rusting of the surface of the sweetpotato. Scurf is usually worse at the stem end. The skin of the sweetpotato is not broken, and the brown color is only skin deep and can be scraped off easily with the fingernail.

Distribution, Prevalence, and Loss

Scurf is commonly found almost everywhere that sweetpotatoes are

grown and on nearly all varieties.

The losses to the crop caused by scurf are perhaps small in compari- son with those caused by some of the other diseases. Nevertheless, scurfy sweetpotatoes do not com- mand so high a market price as clean ones, although they are just as good for food.

Scurf, under favorable conditions of relatively high humidity and temperature, continues to develop under storage conditions to a limited degree. It may damage the sweetpotato skin, so that when the storage house is rather dry the root loses moisture and becomes shriveled and dried.

324275°— 55 2

9

Figure 7. — A sweetpotato showing dis- coloration caused by the scurf fungus.

Cause

The scurf fungus overwinters in storage and on the decayed vines and other decayed vegetable mat- ter in, the field. If infected sweet- potatoes are used for seed, the fungus grows up on the stem of the plants and is carried on them to the field. Later, the organism in the

field grows down onto the roots of the sweetpotatoes. Scurf is most severe on heavy soils and on those containing a large quantity of or- ganic matter. It is likewise more severe during a wet season and on low, wet ground. Such soils should be avoided.

Control

You can control scurf easily by practicing crop rotation and using clean planting stock. Clean plant- ing stock is most easily obtained by planting either vine cuttings or sprout (bed) cuttings in soil that has not been planted to sweetpota- toes for 3 or 4 years (p. 18) . Since scurf affects only the underground parts, the cuttings will be free of the fungus. If you plant the cuttings in scurf- free soil, the sweetpotatoes produced will be free of that dis- ease. Bed the scurf- free sweet po- tatoes produced from the cuttings the following year, and pull the sprouts in the usual manner. Se- lection of scurf-free seed by sorting is impractical and not very effective, because many of the scurf spots are too small to detect.

Plant the disease-free bedding stock in a clean plant bed. Other- wise, the sprouts from the disease- free sweetpotatoes may become in- fected with scurf from the bedding soil.

Seed treatment with a corrosive sublimate solution containing wet- table sulfur partly reduces scurf (p. 17), but treatment with plain corrosive sublimate is not effective.

Root Rot

(Texas Root Rot)

Description

Root rot causes a firm brown rot, resulting in complete destruction of the sweetpotato (fig. 8). Above- ground, the growth is within the

stem and may be detected by the brown color produced. The causal agent produces coarse brown or gray strands of the fungus on the surface of the roots that can be de- tected easily with a hand lens.

10

Distribution, Prevalence, and Loss

Root rot occurs in Texas, New Mexico, Oklahoma, Arizona, Cali- fornia, Arkansas, and Nevada. When the disease once gets into a field a crop may be destroyed. Large fields have been seen in which not more than 10 percent of a crop was produced. The growing crop appeared normal when viewed from a distance, but when harvested nearly all of the sweetpotatoes were found destroyed.

The causal organism lives from one season to the next in the soil on dead vegetable matter and probably on growing winter crops and weeds. It is killed by hard freezing, and this alone probably restricts the dis- ease to the Southern States. The disease may be observed occasion- ally as early as May or June, but it does not become serious until Au- gust when the vines are usually well developed and the sweetpotatoes are of considerable size. From August on the disease increases in severity, and by harvesttime in September and October, a large percentage may be destroyed. The disease may oc- cur in spots of various sizes within a field. Not all hills and not all sweetpotatoes in a hill are neces- sarily destroyed.

Cause

The root rot fungus lives from one season to the next in the soil and on seed sweetpotatoes. The or- ganism gains access to the under- ground parts of sweetpotato plants and spreads in both directions, in- vading the vines from 6 to 12 inches above ground. It may enter the end of the sweetpotato or may cause spots of varying sizes on the surface.

Control

Root rot is more severe on black, poorly drained soil and during wet seasons. The disease is very difficult to control or eradicate, because it

Figure 8. — The characteristic shriveling produced by the root rot fungus.

11

occurs on a great variety of plants, tate crops, and use disease-free It is particularly destructive on cot- sweetpotatoes for seed. Use grasses, ton and alfalfa. To control root corn, and other cereals in the rota- rot, cultivate deep and clean, aerate tion, as they are partially or com- the soil, apply stable manure, ro- pletely immune to the disease.

Mottle Necrosis

Description

Mottle necrosis, a field disease of sweetpotato, produces brownish, somewhat sunken spots, which are irregular in shape and size (fig. 9). Usually the sweetpotato remains more or less firm. Cutting the root crosswise through one of the brown, sunken surface spots reveals the most striking symptom of the

Figure 9. — A sweetpotato with a large part of the surface brown and some- what sunken, a condition characteristic of the advanced stage of mottle necrosis.

Figure 10. — A cross section through a sweetpotato, showing the characteristic mottling of mottle necrosis.

disease : irregularly shaped patches of chocolate-brown dead tissue which appear to have no connection with one another and give the cross section a marbled appearance (fig. 10) . The entire sweetpotato may be involved even though there is but a small spot of diseased tissue on the surface.

Distribution, Prevalence, and Loss

Mottle necrosis occurs in most States where sweetpotatoes are grown. It is not so prevalent, how- ever, in the South as in the northern part of the sweetpotato-growing area. The loss varies from year to year, depending upon soil and weather conditions and on the va- rieties grown. The entire loss throughout the country is relatively small. However, in certain isolated districts where such varieties as the Yellow Jersey are grown, losses as high as 40 percent of the crop some- times occur during seasons espe- cially favorable to the disease.

12

Cause

Mottle necrosis may be caused by either of two fungi. These fungi probably enter through the small fibrous roots and spread through all parts of the sweetpotato.

Control

No method for the control of mottle necrosis has been worked out. The disease is most severe during

seasons of abundant rainfall and in soils that are fairly light and sandy, although some infection may occur in fairly heavy soils. Very suscepti- ble varieties are Triumph, Yellow Jersey, Big-Stem Jersey, and Georgia. Occasionally other va- rieties may be slightly infected. Do not plant susceptible varieties in soils where mottle necrosis has occurred in the preceding 3 or 4 years.

Soil Rot (Pox, Ground Ret)

Description

Soil rot produces symptoms very different from those of other sweet- potato diseases. In a heavily in- fested soil the plants are dwarfed and often produce only one or two short vines. The leaves are small, thin, and pale green. The above- ground symptoms are the result of injury to the root caused by the dis- ease. Any of the underground parts of the plant may be attacked. Many of the lateral feeding, or fibrous, roots are destroyed, and those that remain are often more or less mal- formed. Nearly black flecks, or spots, of varying sizes and appear- ance, occur on the feeding roots and underground part of the stem. The decayed spots may occur on only one side of the root or may girdle it, thereby cutting off the food supply. In the early stages of soil rot the diseased spots seem to be covered by the skin of the sweetpotato, which later breaks, leaving conspicuous holes or pits. On the swollen roots these pits often attain a diameter of one-half inch or more and have a jagged margin (fig. 11). The en- larged root is sometimes girdled; the sweetpotato continues to enlarge on each side of the point of infec- tion and becomes dumbbell-shaped.

Figure 11. — A sweetpotato showing typi- cal soil rot pits.

13

Distribution, Prevalence, and Loss

Soil rot occurs in California, more or less generally in the South- ern States, and in practically all Northern States where sweetpota- toes are grown. It has become a limiting factor in sweetpotato pro- duction in Louisiana. The disease does not occur generally throughout a State, but it is more or less local- ized. It may be bad in one field or locality and absent in another only a few miles away.

The loss caused by soil rot may range from practically nothing to almost complete failure in different fields and seasons. Losses are most severe during dry seasons and on poor soils. Soil rot is especially severe on soils that are alkaline or only slightly acid. It does not de- velop much in fairly acid to acid soils ; that is, with a soil reaction of pll 5.2 or less.

Cause

The soil rot fungus lives in the soil from one season to the next. Most of the infections probably oc- cur after the plants are set in the field, although infection may result from infested soil in the hotbed and from infected seed sweetpotatoes.

Control

No adequate control measure is known for soil rot. Some research results indicate that application of sulfur to reduce the soil reaction to pH 5.2 or less will reduce the amount of soil rot and increase the yield. Apply sulfur with consider- able care, as it may make the soil re- action unfavorable for succeeding crops. Apply the sulfur broadcast and incorporate it thoroughly into the soil 2 to 4 weeks before the plants are set out. The amount of sulfur you should use will depend on the soil type and pH. Use stable manure and green-manure crops to improve the soil and rotate crops.

Avoid introduction of the soil rot organism into new fields and new localities. Since cattle or other live- stock can carry the germs on their feet, do not allow them to roam from infested to disease-free fields. Thoroughly clean plows and other farm implements used to cultivate infested fields before the equipment is taken into clean fields. If you purchase plants from outside sources, be sure they are free of soil rot. Obtain disease-free planting stock for use on new fields by means of vine or sprout cuttings (p. 18) .

Phyllosticta Leaf Blight

Phyllosticta leaf blight causes roundish or angular brownish spots one-eighth to one-half inch in diam- eter on the upper side of the leaf ( fig. 12) . A number of black bodies about the size of a pin point and just visible to the naked eye are scat- tered within the spots. The bodies are slightly raised, round, and con- tain numerous colorless spores.

The fungus does not live on any other plant, nor does it occur on

other parts of the sweetpotato plant. It probably lives through the winter on the dead leaves. The disease oc- curs every year in practically all the Southern States, but it is less com- mon in New Jersey, Delaware, Maryland, Iowa, Kansas, and Illi- nois.

Phyllosticta leaf blight has never been serious enough to require reme- dial measures.

14

Figure 13. — A sweetpotato leaf showing white spots caused by the septoria leaf spot fungus.

Figure 12. — Circular phyllosticta leaf blight lesions on a section of a sweet- potato leaf.

Septoria Leaf Spot

Septoria leaf spot is character- ized by circular, white spots $bout one-eighth inch in diameter on the upper surface of the leaves (fig. 13) . Within these white areas are one or more black specks, just visible to the naked eye. These specks* con- tain numerous spores that, upon es- caping, may be carried by insects or other agencies to other leaves and start new infections. Like the or- ganism causing leaf blight, this fun-

gus does not live on other plants or on other parts of the sweetpotato than the leaves. It probably over- winters on the dead leaves in the field.

Septoria leaf spot is very widely distributed, having been collected in most States where sweetpotatoes are grown. This disease is not serious enough to require remedial meas- ures.

White Rust (Leaf Mold)

The first symptom of white rust is the loss of the green color in spots on the underside of the leaf (fig. 14) . Later these spots become

brown and covered with a whitish, viscid growth, which finally be- comes more or less powdery. This powdery white mass is made up of

15

Figure 14. — Sweetpotato leaf injury caused by the white rust fungus.

numerous spores. These spores fall on other leaves and under fa-

vorable conditions will cause new infections. No great harm results from the attack of this fungus, though it may sometimes produce swellings on the stems and petioles (leaf stems) and cause malforma- tions of the leaves and young shoots. White rust is widely distributed and occurs on a number of other plants, among them- the wild morning- glories.

White rust is more prevalent dur- ing wet seasons. It is frequently found on sweetpotato plants in most of the Southern States. Under favorable weather conditions it oc- curs in New Jersey and other North- ern States where sweetpotatoes are grown. This disease has never been serious enough to require remedial measures.

Root Knot

Description

Root knot is a nematode disease characterized by small galls or swellings on the fine feeder roots, stunting, and yellowish plants. The vines are seldom killed. On the sweetpotatoes this disease causes surface blemishes, pitting, and sometimes severe cracking. Inside the sweetpotato and roots, the nema- todes may cause brownish spots about one-sixteenth inch or smaller in diameter. Most of the spots oc- cur within one-fourth inch of the surface, but some spots may extend as much as 1 inch deep.

Distribution, Prevalence, and Loss

Root knot occurs wherever sweet- potatoes are grown, but it is usually considered a minor disease of that crop. In some fields it may cause serious reduction in yield and qual- ity.

The root-knot nematodes are able to live from one season to another in the soil and in sweetpotatoes in

storage. They are spread by means of root-knot-infected seed sweet- potatoes and plants, by infected transplants of other kinds of plants, and by farm animals, farm imple- ments, drainage water, and wind.

Cause

Root knot is caused by root-knot nematodes (sometimes called nemas and eelworms) . The nematodes feed on the roots of hundreds of kinds of plants. While most com- mon in the Southern States, root- knot nematodes may occur in any part of the country where sweetpo- tatoes are grown. Soil in which in- fected plants have been grown con- tains numerous nematode larvae, which are slender microscopic worms about one- fortieth inch long. These enter the roots and develop to become males, which are slender worms about one-twentieth inch long, or females, which are pear- shaped and about one-twentieth inch long by one-thirtieth inch wide. The males leave the roots,

8 Prepared by A. L. Taylor, nematologist, Agricultural Research Service.

16

but the females remain embedded during their whole lives. Each female may produce several hun- dred eggs.

Control

For control of root knot in sweet- potatoes use plants that are free of nematodes and plant them in soil free of nematodes. Plants grown from infected seed sweetpotatoes or in plant-bed soil infested with root- knot nematodes will certainly be- come infected and carry the infec- tion to the field soil. Slice samples from the lot of sweetpotatoes that are to be used for seed and examine for the brownish spots that indi- cate the presence of root-knot nematodes. If you find nematodes, the lot should be discarded. If there is any possibility that the soil to be used for the plant bed has been in- fested with nematodes, fumigate it before use. For this purpose, fumi- gants having as the active ingredi- ent methyl bromide, ethylene di- bromide, or a mixture of dichloro- propene and dichloropropane are

satisfactory. These soil fumigants are sold under various trade names and should be used as recommended by the manufacturer.

Control root-knot nematodes in field soil by crop rotation or by soil fumigation. Since suitable rotation crops vary in different parts of the country, consult your county agent or the State agricultural experiment station. For field-soil fumigation, use either ethylene dibromide or dichloropropene - dichloropropane (D-D) fumigants as directed by the manufacturer.

Several sweetpotato varieties have been reported as being highly resistant to root-knot nematodes at various locations. However, it is now known that sweetpotatoes may be attacked by several different spe- cies of root-knot nematodes, and sweetpotato varieties that show re- sistance at one location may not be resistant at another location where a different species of nematode is present. Local experience is the best guide in selecting resistant va- rieties.

General Control Measures for Field Diseases Seed Disinfection

Disinfect sweetpotato roots just before they are bedded by dipping them for 8 to 10 minutes in a solu- tion made by dissolving 1 ounce of corrosive sublimate (mercuric chlo- ride) in 8 gallons of water. Use only wooden vessels for the disin- fecting solution.

To control scurf, add 5y2 pounds of wettable sulfur to 24 gallons of the corrosive sublimate solution. This treatment will not kill fungi within the sweetpotato, but it will destroy spores on the surface. After about 10 bushels have been treated in 24 gallons of solution, add one-half ounce of corrosive subli- mate dissolved in hot water and

make up the solution to the original volume by adding water. Repeat this process after the treatment of each 10 bushels of seed until 30 bushels are treated. Then discard the solution and prepare a fresh one.

If corrosive sublimate cannot be obtained, disinfect the seed sweet- potatoes by immersing them for 5 minutes in a 2- to 2i^>-percent solu- tion of borax. Prepare this solu- tion by dissolving 5 pounds of bo- rax in 30 gallons of water. Borax can usually be purchased at a gro- cery store. The disinfecting quality of the borax is not reduced by re- peated use and it can be used in metal vessels. However, if the vol-

324275°— 55 3

17

ume of the solution does not cover the sweetpotatoes, add more solu- tion.

Red the sweetpotatoes immedi- ately after they are treated and water them thoroughly. Otherwise chemical injury and reduced sprout- ing may result.

Excessive amounts of boron are injurious to plants ; therefore, do not pour the unused portion of

the borax solution on land to be used for crop production. Corro- sive sublimate is very poisonous. Sweetpotatoes that have been treated with either corrosive sub- limate or borax should never be fed to animals or used as food. The chemicals and the solutions remaining after treating should be carefully disposed of or kept out of reach of children and animals.

Hotbed Sanitation

The repeated use of the same soil year after year in the plant bed is probably one of the chief means of distributing many sweetpotato dis- eases. Disease-producing organ- isms multiply in the rotting roots and manure. If the same soil is used the next year, the sweetpotatoes are at once exposed to infection. Fur- thermore, when bedding sweetpota- toes, farmers frequently throw the diseased ones to one side. These eventually become mixed with the soil, and the disease germs may be carried on the shoes and by chick- ens and other agents to the plant bed. As a result, plant beds that might otherwise produce healthy plants become badly infected.

Haul away all soil that has been used once in the plant bed and all the rubbish around the bed. Soak thoroughly the framework of the plant bed and the ground around it with a solution of 1 pint of commer- cial formalin in 30 gallons of water or with a solution of 1 pound of

copper sulfate in 25 gallons of water. Repeat this treatment after about 24 hours. Obtain the soil, or preferably sand, for the hotbed from a place where sweetpotatoes have never been grown. Rich soil is not necessary for the hotbed; in fact, some of the best results have been obtained by using pure sand. Use a grade of sand or soil that will not bake or form a crust. Clean the farm implements that were used to handle and haul away old soil and disinfect the implements with a solution of formaldehyde before new soil is handled.

The use of stable manure in the hotbed is a questionable practice unless you have taken care not to feed discarded sweetpotatoes to stock or throw them on the manure pile. Do not throw diseased sweet- potatoes in the yard where infected parts may be carried around on the feet of poultry, other farm animals, and workers.

Vine or Sprout (Bed) Cuttings

You may produce disease-free seed stock easily by planting vine cuttings or sprout (bed) cuttings in soil that has not been planted to sweetpotatoes for 3 or 4 years. Since such diseases as black rot and scurf do not occur on the above- ground parts, they are not carried on the vine or bed cuttings as they

are on sprouts that are pulled in the usual way.

Make vine cuttings by cutting the sweetpotato vines into sections so as to include at least two buds or leaves. Then insert one end, usu- ally the larger, into the ground. A disadvantage with vine cuttings is that the cuttings cannot be made

18

until the vines have grown to a con- siderable length.

You can obtain sprout or bed cut- tings almost as early as sprouts. The sprouts are allowed to grow a few inches longer than usual and then cut about 11/2 inches above the soil level instead of pulling them in the usual manner.

To obtain good results from vine or sprout cuttings, take the follow- ing precautions :

1. Make cuttings from vines that are not affected with stem rot. The

stem rot organism grows out into the vines 3 to 5 feet from the hills and its presence cannot always be detected without pinching open the vine.

2. Plant cuttings on new ground or on ground on which sweet- potatoes have not been grown for at least 3 or 4 years.

3. Disinfect the seed stock and bed it in a plant bed prepared ac- cording to directions given on page 18.

Storage Rots and Their Control

Soft Rot (Ring Rot, Collar Rot)

Soft rot, caused by the bread mold, is a very destructive disease of sweetpotatoes in storage. It may set in soon after the crop is placed in storage and continue to spread throughout the storage peri- od, depending largely on the condi- tion of the roots when stored and on the management of the house. - The decay begins usually at one end and progresses rapidly, requiring only a few days with favorable tempera- tures and humidity to destroy the entire sweetpotato. At first the soft-rot-affected sweetpotatoes are soft, watery, and stringy. After decay and the escape of moisture, they gradually become firm, hard, shrunken, and brittle. Such dry sweetpotatoes are frequently re- ferred to by the farmer as being af- fected with dry rot, which in reality is a dried-up soft rot. If the skin is broken while the sweetpotato is still soft, a moldy growth, some- times referred to as whiskers, forms on the surface (fig. 15).

The soft rot disease often spreads from one root to another by contact. The spores of the black mold pro- duced on the surface may be carried

Figure 15. — A sweetpotato showing the moldy growth, or whiskers, of the fun- gus causing soft rot.

19

Figure 16. — A sweetpotato infected with ring rot.

by flies or wind currents to other roots in the same house or may be spread to them by handling. New infections may take place if the spores light on a wounded surface and if the temperature and moisture conditions are favorable.

Ring rot differs from soft rot in