The Diseases Bacteria Cause

Wilson L. Smith, Jr., B. A. Friedman.

Diseases caused by bacteria are responsible for a large share of the spoilage of fresh fruits and vegetables in storage or during marketing.

Losses from infection by the soft rot group of bacteria are especially important. They attack nearly all vegetables and on each may cause a serious decay within a few hours. They do not cause decay of tree fruits.

Of lesser importance are bacteria that cause leaf, stem, and fruit spotting and internal discoloration and decay. Both tree fruits and vegetables are affected by bacterial diseases of this type. Though the infected areas are usually limited in size and depth and may be trimmed out without serious waste, the spots make the produce unattractive, form places for invasion by other organisms, and cause serious inconvenience and added expense in preparing the produce for use.

The bacteria that cause postharvest diseases of vegetables and fruits belong to five of the six genera of bacteria that cause plant diseases. In the soft rot group are species of the genera Pseudomonas, Erwinia, and Bacillus. Members of the genera Pseudomonas, Xanthomonas, Erwinia, and Corynebacterium are responsible for the necrotic lesions on leaves, stems, and fruits, and internal discolorations and decay.

As the two groups differ widely in the symptoms they produce and the Plants they attack, we discuss them separately.

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 lettuce, endive, and spinach; stem or stalk types, like asparagus and celery; and the leafy tops of root crops like carrots and radishes. In the second group are potatoes, carrots, radishes, parsnips, beets, and turnips.

The soft rots are well known because of the losses in white potatoes during transit and marketing. Often 1 to 2 percent of the sacks in a car or truck contain potatoes that are affected with bacterial soft rot. Losses of 5 to 10 percent are not uncommon. The appearance of one "wet sack" in a load indicates soft rot infection to the inspector or purchasing agent, and a reduction in the price of the entire load often follows. A 7-year study of railroad car inspection reports for New York City disclosed that losses due to decay of vegetables was 3.8 percent. Bacterial soft rot alone was responsible for 38 percent of the decay and was found on all of the 31 vegetables inspected, except corn and sweetpotatoes.

The first complete account of any bacterial soft rot and its causal organism was one concerning carrots by L. R. Jones in 1901. He named the causal organism Bacterium carotovorum. Later the name was changed to Erwinia carotovora. Since then soft rots of many of the vegetables have been attributed wrongly to E. carotovora, although other species of bacteria were to blame. At least two other species of the genus Erwinia E. atroseptica and E. aroideae cause soft rot of vegetables.

A number of species of the genus Pseudomonas have been described as the cause of soft rot. Among them are P. solanacearum on potatoes, P. marginalia, and P. viridilividum on lettuce and endive, and P. alliicola and P. cepacia on onions. Three species of the genus Bacillus B. polymyxa, B. subtilis, and B. megatherium cause extensive soft rot of potatoes. Each of the species has been reported to infect many different vegetables, and soft rot of vegetables after harvest may be caused by bacteria other than Erwinia carotovora. Sometimes it may be caused by a combination of several species.

Soft rot starts on leaves, stems, and seed pods as small, water-soaked, translucent spots, which later may become muddy green or greasy. Rapid softening and disintegration of the diseased tissue follows. Within 20 to 48 hours the entire structure may become a wet, slimy mass.

The first symptom of soft rot on root crops is a water-soaked appearance of the affected tissue. The diseased parts later disintegrate into a mushy mass of disorganized cells, which slough off, while the rest of the root remains firm. The bacteria may invade the plant at the crown and the decay may extend deep into the root through the innermost cells while the outer tissues remain apparently healthy.

The first symptoms on tubers often are a dark or black discoloration of the surface and a somewhat blistered appearance of the skin. The affected tissues are usually cream-colored, soft, and not watery. They are separated by a distinct boundary from the sound tissues so that, if pressed, the mushy tissue squirts from the tuber. Often the outer surface of the tubers appears healthy while the inner part is a mass of rotting cells. Upon exposure to air, the infected tissues may turn tan, gray, or dark brown. Infection may occur first at the lenticels, which at first are water-soaked and swollen. The tissue underneath is generally firm. Unless the potatoes are exposed to high temperatures, the infected area often dries up. Infected tubers have little smell until the infected tissues collapse. Then a foul odor may develop because of bacteria that live on the decomposing tissue.

THE COMPLEX NATURE of soft rot is indicated by the following summary of the symptoms of soft rot caused by different bacteria on several kinds of vegetables.

Potato Erwinia carotovora: Rot at lenticels or at injuries. Internal rot usually cream or light-tan color.

Erwinia atroseptica: Rot usually starting at stem end but sometimes at injuries, black and sunken, sometimes dry. May progress through heart of tuber. Internal rot brown to black-brown color.

Bacillus polymyxa, B. subtilis, and B. megatherium: Usually starts at injuries and extends to heart of tuber. Internal rot dark brown to black, or grayish cast.

Pseudomonas solanacearum: Causes depression at point of stem attachment. Gray-brown discoloration at the surface and moist brown discoloration of water-conducting system. Entire inner tissue of potato may become soft and brown.

Lettuce, Chicory, Escarole, Endive Erwinia carotovora: Inner leaves of head at first have a greasy water-soaked appearance. Later infected areas turn dark brown and become slimy.

Pseudomonas viridilividium: Outer leaves spotted or darkened. Center of head at first firm but later soft rot develops.

Pseudomonas marginalia: Starts as greasy water-soaked spot that later turns greenish to reddish brown. The infected tissues are soft and slimy and rapidly disintegrate into a foul odorous mass.

Onions Erwinia carotovora: Affected tissue glossy or water-soaked, later mushy. Generally starts at neck, often confined to central scales. Foul odor.

Pseudomonas alliicola: Inner scales water-soaked and soft. Not unlike frost injury. Bulb appears sound from outside.

Pseudomonas cepacia: Outer scales yellow and slimy. Inner scales not affected. Upper portion of bulb shrinks and skin slips off.

Tomato Erwinia aroideae: Skin water-soaked, light-colored, greasy translucent, blistered with mass of gas and decomposing cells. Decay progresses rapidly.

Erwinia carotovora: Rot on ripe fruit brownish, often limited to slowly spreading circular lesion.

Erwinia atroseptica: Lesions on ripe fruit dark and water-soaked, and spreading medium rapidly. Medium gas formation and only slight blistering. May be firm.

Celery Erwinia carotovora: Affected lesions water-soaked and softened. Infected areas turn brown and mushy, but epidermis remains intact. Decay may affect crown, leaf stalks, and leaflets.

Melons Erwinia aroideae: Decay usually on under side of fruit. Skin shrunken but usually nearly intact. Bacterial ooze through skin. Internal infection forms irregular funnel-shaped decayed section extending into the cavity. Entire inner portion becomes soft and outer tissues may collapse.

THE SOFT ROT bacteria grow and cause infection over a wide temperature range. Erwinia carotovora and E. atroseptica grow at temperatures from about 35 to about 89 F. E. aroideae grows at temperatures up to 105 . The Optimum temperatures for growth and infection by the three organisms are 77 , 78 , and 95 , respectively. The soft rot bacteria of the genus Pseudomonas grow at temperatures from about 41 to about 102 , with an optimum around 86 . Growth of soft rot bacteria in the genus Bacillus is scant or absent below 55 and greatest at slightly above 89 . One species within this group, B. subtilis, will grow at 122 and B. polymyxa and B. megatherium at temperatures above 105 . At the lower temperatures, therefore, infection by bacterial species in the genera Erwinia and Pseudomonas is most probable. At higher temperatures soft rot would more likely be caused by species of Bacillus. Although soft rot may occur over a wide temperature range the highest percentage is apt to develop within the range of 69 to 89 , where most of the bacterial species that cause soft rot grow well.

Organisms that cause soft rot live for long periods in the soil and may infect plants before they are removed from the field. Contaminated water in large washing vats resulting from washing infected produce is also a source of infection. Infection of many vegetables may occur in trimming, as cutting knives transmit the bacteria from diseased to healthy plants.

Standing water in the field when the plants are approaching maturity, injury from exposure to sun and wind on hot, dry days, and mechanical injuries during harvesting, grading, and packing may form places for bacterial invasion and favor development of soft rot.

Soft rot often is not evident at the time of storage or when the produce is shipped to market, but in order to delay the possible development of the disease it is customary to use low temperatures during storage or transit. During short storage periods or in transit, temperatures below 50 will delay the appearance of soft rot symptoms. For longer storage periods, temperatures approaching 32 are recommended for leafy vegetables and root crops and 40 for potatoes. It is highly important to keep moisture from condensing on the produce.

Standing moisture facilitates invasion by soft rot bacteria. If the temperatures fluctuate above 32 , infection and decay follow rapidly. Low temperatures do not prevent bacterial infection, but they delay development of decay and are the common (and possibly the best) way to preserve fresh produce. Soft rot often develops rapidly, however, after the produce has been removed from refrigerated storage. Such produce should be consumed or processed as soon as possible after removal from cold storage.