THE DISEASES BACTERIA CAUSE 817 Bacteria that produce soft rot are most common on vegetables that have either a succulent, tender type of growth or fleshy storage tissues. Among the first are such leafy vegetables as The Diseases lettuce, endive, and spinach; stem or stalk types, like asparagus and celery; and the leafy tops of root crops like Bacteria carrots and radishes. In the second group are potatoes, carrots, radishes, parsnips, beets, and turnips. Cause The soft rots are well known because of the losses in white potatoes during transit and marketing. Often i to 2 Wilson L. Smith, Jr,, B. A. Friedman percent of the sacks in a car or truck contain potatoes that are affected with Diseases caused by bacteria are re- bacterial soft rot. Losses of 5 to 10 sponsible for a large share of the spoil- percent are not uncommon. The age of fresh fruits and vegetables in appearance of one '*wet sack" in a load storage or during marketing. indicates soft rot infection to the in- Losses from infection by the soft rot spector or purchasing agent, and a group of bacteria are especially impor- reduction in the price of the entire load tant. They attack nearly all vegetables often follows. A 7-year study of rail- and on each may cause a serious decay road car inspection reports for New within a few hours. They do not cause York City disclosed that losses due to decay of tree fruits. decay of vegetables was 3.8 percent. Of lesser importance are bacteria Bacterial soft rot alone was responsible that cause leaf, stem, and fruit spotting for 38 percent of the decay and was and internal discoloration and decay. found on all of the 31 vegetables in- Both tree fruits and vegetables are spected, except corn and sweetpotatoes. affected by bacterial diseases of this The first complete account of any type. Though the infected areas are bacterial soft rot and its causal organ- usually limited in size and depth and ism was one concerning carrots by may be trimmed out without serious L. R. Jones in 1901. He named the waste, the spots make the produce un- causal organism Bacterium carotovorum. attractive, form places for invasion by Later the name was changed to Erwinia other organisms, and cause serious in- carotovora. Since then soft rots of many convenience and added expense in of the vegetables have been attributed preparing the produce for use. wrongly to E. carotovora, although The bacteria that cause postharvest other species of bacteria were to diseases of vegetables and fruits belong blame. At least two other species of to five of the six genera of bacteria the genus Erwinia—E. atroseptica and that cause plant diseases. In the soft E. aroideae—cause soft rot of vegetables. rot group are species of the genera A number of species of the genus Pseudomonas, Erwinia, and Bacillus. Pseudomonas have been described as Members of the genera Pseudomonas, the cause of soft rot. Among them Xanthomonas, Erwinia, and Corynehac- are P. solanacearum on potatoes, P, terium are responsible for the necrotic marginalis, and P. viridilividum on let- lesions on leaves, stems, and fruits, and tuce and endive, and P. alliicola and internal discolorations and decay. P. cepacia on onions. Three species As the two groups difier widely in of the genus Bacillus—B, polymjyxa, B. the symptoms they produce and the subtilis, and B. megatherium—cause ex- plants they attack, we discuss them tensive soft rot of potatoes. Each of separately. the species has been reported to infect 8i8 YEARBOOK OF AGRICULTURE 1953 many different vegetables, and soft Potato rot of vegetables after harvest may be caused by bacteria other than Erwinia Erwinia carotovora: Rot at lenticels carotovora. Sometimes it may be caused or at injuries. Internal rot usually by a combination of several species. cream or light-tan color. Soft rot starts on leaves, stems, and Erwinia atraséptica: Rot usually start- seed pods as small, water-soaked, ing at stem end but sometimes at in- translucent spots, which later may juries, black and sunken, sometimes become muddy green or greasy. dry. May progress through heart of Rapid softening and disintegration of tuber. Internal rot brown to black- the diseased tissue follows. Within 20 brown color. to 48 hours the entire structure may Bacillus polymyxa^ B, subtilis^ and become a wet, slimy mass. B, megatherium: Usually starts at in- The first symptom of soft rot on root juries and extends to heart of tuber. crops is a water-soaked appearance of Internal rot dark brown to black, or the affected tissue. The diseased parts grayish cast. later disintegrate into a mushy mass Pseudomonas solanacearum: Causes de- of disorganized cells, which slough pression at point of stem attachment. off, while the rest of the root remains Gray-brown discoloration at the sur- firm. The bacteria may invade the face and moist brown discoloration of plant at the crown and the decay may water-conducting system. Entire in- extend deep into the root through the ner tissue of potato may become soft innermost cells while the outer tissues and brown. remain apparently healthy. The first symptoms on tubers often Lettuce, Chicory, Escarole, Endive are a dark or black discoloration of the surface and a somewhat blistered Erwinia carotovora: Inner leaves of appearance of the skin. The affected head at first have a greasy water- tissues are usually cream-colored, soft, soaked appearance. Later infected areas and not watery. They are separated turn dark brown and become slimy. by a distinct boundary from the sound Pseudomonas viridilividium: Outer tissues so that, if pressed, the mushy leaves spotted or darkened. Center of tissue squirts from the tuber. Often head at first firm but later soft rot the outer surface of the tubers appears develops. healthy while the inner part is a mass Pseudomonas marginalis: Starts as of rotting cells. Upon exposure to greasy water-soaked spot that later air, the infected tissues may turn tan, turns greenish to reddish brown. The gray, or dark brown. Infection may infected tissues are soft and slimy and occur first at the lenticels, which at rapidly disintegrate into a foul odorous first are water-soaked and swollen. mass. The tissue underneath is generally Onions firm. Unless the potatoes are exposed to high temperatures, the infected area Erwinia carotovora: Affected tissue often dries up. Infected tubers have glossy or water-soaked, later mushy. little smell until the infected tissues Generally starts at neck, often confined collapse. Then a foul odor may to central scales. Foul odor. develop because of bacteria that live Pseudomonas alliicola: Inner scales on the decomposing tissue. water-soaked and soft. Not unlike frost injury. Bulb appears sound from out- THE COMPLEX NATURE of soft rot is side. indicated by the following summary Pseudomonas cepacia: Outer scales yel- of the symptoms of soft rot caused by low and slimy. Inner scales not af- different bacteria on several kinds of fected. Upper portion of bulb shrinks vegetables. and skin slips off. THE DISEASES BACTERIA CAUSE 819 Tomato bacterial species in the genera Erwinia and Pseudömonas is most probable. At Erwinia aroideae: Skin water-soaked, higher temperatures soft rot would light-colored, greasy translucent, blis- more likely be caused by species of tered with mass of gas and decompos- Bacillus, Although soft rot may occur ing cells. Decay progresses rapidly. over a wide temperature range the Erwinia carotovora: Rot on ripe fruit highest percentage is apt to develop brownish, often limited to slowly within the range of 69° to 89°, where spreading circular lesion. most of the bacterial species that cause Erwinia atroseptica: Lesions on ripe soft rot grow well. fruit dark and water-soaked, and Organisms that cause soft rot live spreading medium rapidly. Medium for long periods in the soil and may gas formation and only slight blister- infect plants before they are removed ing. May be firm. from the field. Contaminated water in large washing vats resulting from wash- Celery ing infected produce is also a source of infection. Infection of many vege- Erwinia carotovora: Affected lesions tables may occur in trimming, as cut- water-soaked and softened. Infected ting knives transmit the bacteria from areas turn brown and mushy, but epi- diseased to healthy plants. dermis remains intact. Decay may af- Standing water in the field when the fect crown, leaf stalks, and leaflets. plants are approaching maturity, in- jury from exposure to sun and wind Melons on hot, dry days, and mechanical injuries during harvesting, grading, Erwinia aroideae: Decay usually on and packing may form places for bac- under side of fruit. Skin shrunken but terial invasion and favor development usually nearly intact. Bacterial ooze of soft rot. through skin. Internal infection forms Soft rot often is not evident at the irregular funnel-shaped decayed sec- time of storage or when the produce tion extending into the cavity. Entire is shipped to market, but in order to inner portion becomes soft and outer delay the possible development of the tissues may collapse. disease it is customary to use low tem- peratures during storage or transit. THE SOFT ROT bacteria grow and During short storage periods or in cause infection over a wide tempera- transit, temperatures below 50° will ture range. Erwinia carotovora and E, delay the appearance of soft rot symp- atroseptica grow at temperatures from toms. For longer storage periods, tem- about 35° to about 89° F. E. aroideae peratures approaching 32° are recom- grows at temperatures up to 105°. The mended for leafy vegetables and root optimum temperatures for growth and crops and 40° for potatoes. It is highly infection by the three organisms are important to keep moisture from con- 77°, 78°, and 95°, respectively. The densing on the produce. soft rot bacteria of the genus Pseudo- Standing moisture facilitates inva- monas grow at temperatures from about sion by soft rot bacteria. If the tem- 41° to about 102*^, with an optimum peratures fluctuate above 32°, infection around 86°. Growth of soft rot bacteria and decay follow rapidly. Low tem- in the genus Bacillus is. scant or absent peratures do not prevent bacterial below 55° and greatest at slightly infection, but they delay development above 89°. One species within this of decay and are the common (and group, B. subtilis, will grow at 122° possibly the best) way to preserve and B. polymyxa and B. megatherium at fresh produce. Soft rot often develops temperatures above 105^. At the lower rapidly, however, after the produce temperatures, therefore, infection by has been removed from refrigerated 820 YEARBOOK OF AGRICULTURE 1953 Storage. Such produce should be con- raised circular areas, about one-six- sumed or processed as soon as possible teenth to one-eighth inch in diameter. after removal from cold storage. They are surrounded by a character- Good control of bacterial soft rots istic white halo. Spots are superficial may be obtained by care in sorting and do not develop into soft rot. and packaging the produce. Spread- ing of infection is retarded by use of Beans sterilized trimming knives, eliminat- ing and disposing of diseased produce Common blight {Xanthomonas pha- before it comes in contact with healthy seoli) and halo blight {Pseudomonas material, sorting out injured or bruised phaseolicola): Cause circular to irregu- produce, and careful handling to lar-shaped, watery or greasy-appear- avoid causing mechanical injuries. ing spots on the pods. The margin of Curing potatoes at moderate tempera- the common blight spots becomes al- tures and high humidity before storing most brick red as the spots become or shipping allows the injured surfaces older. A yellowish crust of bacterial of the tubers to "cork over" ; the "cork ooze is sometimes evident on the com- layer" prevents the entrance of most mon blight spots and a grayish-white bacterial pathogens. crust on the halo blight spots.

THE BACTERIAL DISEASES that cause Peas spotting and wilting are most impor- tant during the growing period, but Bacterial blight {Pseudomonas pisi): sometimes they continue to develop Pod spots in the early stages are small after harvest and cause damage in and water-soaked. In more advanced storage or at the market. The wilt stages they are larger, slightly sunken, diseases cause discoloration of the greasy, or water-soaked, irregular- internal tissues and are often not dis- shaped, and have gray or grayish- covered until the vegetable is cut brown centers. when it is prepared for food. The symptoms of the more important of Cucumbers these diseases are: Bacterial spot {Pseudomonas lachry- Tomato mans): Spots on cucumbers start as minute, circular, water-soaked areas. Bacterial spot (caused by Xanthom- Later the affected tissues dry and onas vesicatoria): Spots on mature crack and the centers of the spots green fruit are brownish-black, ele- become sunken and chalky white in vated, scabby areas, one-eighth to color. A gummy exúdate is some- one-fourth inch in diameter, with times present on the spots. A break- feathered or irregular margins. Old down and soft rot frequently follow. spots are sunken, gray, or bleached and the affected skin is dry, paperlike, Cauliflower and Cabbage and ragged. Spots are superficial and do not develop into soft rot. Leaf spot {Pseudomonas maculicola): Bacterial speck {Pseudomonas tomato): Leaf spots are water-soaked at first Causes dark brown, slightly raised and then become brownish or pur- specks, one thirty-second to one-six- plish gray. They coalesce and become teenth inch in diameter, with definite elongated as they enlarge and give margins. Spots are superficial and the leaf a ragged appearance. Spots decay does not follow. on the cauliflower head are small, Bacterial canker (Coryne bacterium gray to brown, and affect both the michiganense): Spots on mature green epidermis and deeper tissues. Later fruit are light brown to brown, slightly develops into soft rot. THE DISEASES BACTERIA CAUSE 821 Black rot {Xanthomonas campestris): brown and only the shell of the po- Causes yellowing of the leaves and tato holds it together. blackening of the veins. Leaves shed Fruits and vegetables affected with from stalk. Often followed by soft bacterial spot and wilt diseases at har- rot. vest should be carefully graded and Celery sorted to eliminate all that show symp- toms of disease and then should be Bacterial blight {Pseudomonas apii): refrigerated promptly to retard devel- Causes numerous small irregular- opment on the apparently sound shaped spots on the leaflets. Spots produce. They should be used as are.yellow at first, but later are rusty promptly as possible after they are brown with a yellow border or halo. removed from the refrigerated storage.

Stone fruits WILSON L. SMITH, JR., is a pathologist in the division oj handlings transportation^ Bacterial spot {Xanthomonas pruni): and storage, Bureau of Plant Industry, Fruit spots at first are small, circular, Soils, and Agricultural Engineering, at and light brown. Later they enlarge, Beltsville, Md. He has higher degrees from darken, dry out, and crack. A viscid, the University oj Maryland and Cornell yellowish, gummy exúdate is some- University. His major research studies have times present on the lesions. concerned bacterial diseases of plants. B. A. FRIEDMAN is a pathologist at the Lemon Market Pathology Laboratory of the division of handling, transportation, and storage of Black pit (Xanthomonas citri): Fruit horticultural crops in New York City. He spots are sunken, roughly circular, obtained his doctorate in bacteriology at one-fourth to one-half inch in diam- New York University. eter, brown at first and later black. The white part of the peel beneath the pits collapses and turns light brown to reddish brown.

Potato

Ring rot (Corynebacterium sepedoni~ cum): Causes odorless decay that is confined at first to vicinity of vascular ring of the potato. Afí'ected tissues are cream-colored to pale lemon yel- low and have a soft cheesy texture. Starts at stolon attachment and pro- gresses through vascular system to the eyes. Causes characteristic cracking that extends into the vascular ring. Soft rot frequently follows. Brown rol {Pseudomonas solanacearum): May cause a slight depression at the stolon attachment. Sometimes shows as a grayish discolored patch on sur- face of potato. Causes moist brown discoloration and slight softening of the vascular ring. A gray sticky bacterial ooze often exudes from the vascular tissue. Later the interior becomes soft Blue mold rot on lemon.