130 Modern Food Microbiology Table 6–7 Some Bacteria that Cause Field and Storage Spoilage of Vegetables and Fruits (See Text for Others) Organisms Corynebacterium michiganenese C nebraskense C sepedonicum Curtobacterium flaccumfaciens (formerly Corynebacterium) Pseudomonas agarici and P tolaasii P corrupata Pseudomonas cichorii group Pseudomonas marginalis group P morsprunorum group (formerly P phaseolicola) P syringae pv syringae Formerly P glycinea Formerly P lachrymans Formerly P pisi P tomato group Xanthomonas campestris pv campestris X oryzae pv oryzae pv oryzicola Rathyibacter spp Janthinobacterium agaricidamnosum Streptomyces spp Xanthomonas axonopodis pv citri Xylella fastidiosa Ralstonia spp Erwinia amylovora Acidovorax valerianellae Spoilage Condition/Products Vascular wilt, canker; leaf and fruit spot on tomatoes, others Leaf spot, leaf blight, and wilt of corn Tuber rot of white potatoes Bacterial wilt of beans Drippy gill of mushrooms Tomato pith necrosis Bacterial zonate spot of cabbage and lettuce Soft rot of vegetables, side slime of lettuce Halo blight of beans Bacterial canker of stone fruit trees Disease of soybeans Angular leaf spot of cucumbers Bacterial blight of pears Bacterial speck of tomatoes Black rot of cabbage and cauliflower Bacterial blight of rice Bacterial leaf streak of rice Gumming diseases of plants Soft rot of mushrooms Potato scab Citrus canker Pierce’s disease of grapes Wilt of tomatoes Fire blight of apple/pear trees Black water-soaked spots of lamb’s lettuce been transferred to new genera including Acidovorax, Burkholderia, and Hydrogenophaga Changes have been made to the genus Xanthomonas, and it appears that this process will continue with the emphasis more on molecular genetic methods than the traditional phenotypic approaches The genus Pantoea is closely related to Erwinia, and it along with Citrobacter and Klebsiella are probably more important in the storage spoilage of vegetables than is now apparent The bacterial genera most often associated with field and storage spoilage of vegetables are Pseudomonas, Pectobacterium, Erwinia, Xanthomonas, and some of the specific spoilage conditions are listed in Table 6–7 Soft rots occur in plants of a number of species and those of carrots are well known A “soft” rot refers to the mushy consistency of the plant or vegetable in contrast to some other spoilage conditions where the product remains firm The bacteria most commonly associated with the soft rotting of carrots are Pectobacterium spp., especially P carotovorum subsp carotovorum and P carotovorum subsp odoriferum After the spoilage process is initiated, a number of soil-dwelling bacteria are involved, Vegetable and Fruit Products 131 and they include Pseudomonas spp as well as Bacillus, Paenibacillus, and Clostridium For fruits and vegetables like tomatoes and potatoes, P carotovorum subsp carotovorum causes soft rot by entering surface wounds For root vegetables such as carrots, the roots are protected for some Plant roots are protected from invading microorganisms by their possession of hydrogen peroxide and superoxide, and invading microorganisms produce catalase and superoxide dismutase to overcome this defense The Pseudomonas syringae group as well as erwiniae produce these enzymes The cementing substance of the vegetable body induces the formation of pectinases, which act by hydrolyzing pectin, thereby producing the mushy consistency In potatoes, tissue maceration has been shown to be caused by an endopolygalacturonate transeliminase of Pectobacterium origin.28 P chrysanthemi produces two pectin methyesterases, at least seven pectate lyases, a polygalacturonase, and a pectin lyase.19 Once pectin is destroyed, oligogalacturonidases are formed and utilized by the varied microbiota Because of the early and relatively rapid growth of bacteria, molds, which tend to be crowded out, are of less consequence in the spoilage of vegetables that are susceptible to bacterial agents Once the outer plant barrier has been destroyed by these pectinase producers, nonpectinase producers no doubt enter the plant tissues and help bring about fermentation of the simple carbohydrates that are present The quantities of simple nitrogenous compounds present, the vitamins (especially the B-complex group), and minerals are adequate to sustain the growth of the invading organisms until the vegetables have been essentially consumed or destroyed The malodors that are produced are the direct result of volatile compounds (such as NH3 , volatile acids, and the like) produced by the biota When growing in acid media, microorganisms tend to decarboxylate amino acids, leaving amines that cause an elevation of pH toward the neutral range and beyond Complex carbohydrates such as cellulose are generally the last to be degraded, and a varied biota consisting of molds and other soil organisms is usually responsible, as cellulose degradation by Erwinia spp is doubtful Aromatic constituents and porphyrins are probably not attacked until late in the spoilage process, and again by a varied biota of soil types The genus Brenneria causes diseases of trees such as bark cankers, necrosis on walnut trees, oozing of sap from acorns, etc.18 The genes of P carotovorum subsp carotovorum that are involved in potato tuber maceration have been cloned Plasmids containing cloned DNA mediated the production of endopectate lyases, exopectate lyase, endopolygalacturonase, and cellulases.39 The Escherichia coli strains that contained cloned plasmids showed that endopectate lyases with endopolygalacturonase or exopectate lyase caused maceration of potato tuber slices These enzymes, along with phosphatidase C and phospholipase A, are involved in soft rot by this organism Carrots infected with Agrobacterium tumefaciens undergo senescence at a faster rate because of increased ethylene synthesis In normal uninfected plants, ethylene synthesis is regulated by auxins, but A tumefaciens increases the synthesis of indoleacetic acid, which results in increased levels of ethylene The citrus canker pathogen, Xanthomonas anonopodis pv (pathovar) citri wreaked havoc on the Florida citrus industry in the late 1990s The canker consists of external scabs and cork-like lesions that occur on oranges, limes, grapefruits, and other citrus fruits The organism enters fruit trees through leaf stomata Once inside, they employ a type III secretion system (see Chapter 22) to induce increased cell division that leads to the brownish cankers The infected fruits produce more ethylene, which leads to senescence, increased ripening, and premature falling of fruits from trees This infection does not destroy trees The bacterium is spread from tree to tree by wind, rain, insects, and other nonspecific means The genus Xanthomonas is undergoing reclassification, thus some of the species and pathovars noted in Table 6–6 are likely to be changed Most form yellow mucoid and smooth colonies and 132 Modern Food Microbiology Figure 6–1 Tomato diseases—(A and B) nailhead spot; (C and D) bacterial spot; (E) bacterial canker; (F) bacterial spot Source: From Agriculture Handbook 28, USDA, 1968, “Fungus and Bacterial Diseases of Fresh Tomatoes.” produce the yellow-pigmented xanthomonadins The mucoid colonies are due to xanthans, which are typical of the genus.56 Bacterial canker of stone fruits is caused by P syringae pv syringae, and this pathovar has been reported to cause disease in over 180 species of plants.25 Some of the more important bacteria that cause field and storage spoilage of vegetables are presented in Table 6–7 Some genera and species listed are undergoing taxonomic changes The plant Vegetable and Fruit Products 133 Figure 6–2 Lima bean diseases—(A and B) pod blight; (C) seed spotting; (D) yeast spot Pea diseases—(E) pod spot; (F) anthracnose; (G) scab Source: From Agriculture Handbook 303, USDA, 1966, Chapter corynebacteria represent a diverse collection, many of which not belong to this genus Some have been transferred to the genus Curtobacter The plant pathogenic and field spoilage pseudomonads and xanthomonads are also diverse The appearance of some market vegetables undergoing bacterial and fungal spoilage is shown in Figures 6–1 and 6–2 134 Modern Food Microbiology Fungal Agents A synopsis of some of the common spoilage conditions of vegetables and fruits is presented in Table 6–8 Some of these spoilage conditions are initiated preharvest and others postharvest Among the former, Botrytis invades the flower of strawberries to cause gray mold rot, Colletotrichum invades the epidermis of bananas to initiate banana anthracnose, and Gloeosporium invades the lenticels of apples to initiate lenticel rot.13 The largest number of market fruit and vegetable spoilage conditions occur after harvesting, and although the fungi most often invade bruised and damaged products, some enter specific areas For example, Thielaviopsis invades the fruit stem of pineapples to cause black rot of this fruit, and Colletotrichum invades the crown cushion of bananas to cause banana crown rot.13 Black rot of sweet potatoes is caused by Ceratocystis, neck rot of onions by Botrytis allii, and downey mildew of lettuce by Bremia spp.9 Some of the spoilage conditions listed in Table 6–8 are discussed below Gray mold rot This condition is caused by Botrytis cinerea, which produces a gray mycelium This type of spoilage is favored by high humidity and warm temperatures Among the vegetables affected are asparagus, onions, garlic, beans (green, lima, and wax), carrots, parsnips, celery, tomatoes, endives, globe artichokes, lettuce, rhubarb, cabbage, Brussels sprouts, cauliflower, broccoli, radish, rutabagas, turnips, cucumbers, pumpkin, squash, peppers, and sweet potatoes In this disease, the causal fungus grows on decayed areas in the form of a prominent gray mold It can enter fruits and vegetables through the unbroken skin or through cuts and cracks Sour rot (oospora rot, watery soft rot) This condition of vegetables is caused by Geotrichum candidum and other organisms Among the vegetables affected are asparagus, onions, garlic, beans (green, lima, and wax), carrots, parsnips, parsley, endives, globe artichokes, lettuce, cabbage, Brussels sprouts, cauliflower, broccoli, radishes, rutabagas, turnips, and tomatoes The causal fungus is widely distributed in soils and on decaying fruits and vegetables Drosophila melanogaster (fruit fly) carries spores and mycelial fragments on its body from decaying fruits and vegetables to growth cracks and wounds in healthy fruits and vegetables Because the fungus cannot enter through the unbroken skin, infections usually start in openings of one type or another.27 Rhizopus soft rot This condition is caused by Rhizopus stolonifer and other species that make vegetables soft and mushy Cottony growth of the mold with small black dots of sporangia often covers the vegetables Among those affected are beans (green, lima, and wax), carrots, sweet potatoes, potatoes, cabbage, Brussels sprouts, cauliflower, broccoli, radish, rutabagas, turnips, cucumbers, cantaloupes, pumpkins, squash, watermelons, and tomatoes This fungus is spread by D melanogaster, which lays its eggs in the growth cracks on various fruits and vegetables The fungus is widespread and is disseminated by other means also Entry usually occurs through wounds and other skin breaks Phytophthora rot This market condition, caused by Phytophthora spp., occurs largely in the field as a blight and fruit rot of market vegetables It appears to be more variable than some other market “diseases” and affects different plants in different ways Among the vegetables affected are asparagus, onions, garlic, cantaloupes, watermelons, tomatoes, eggplants, and peppers Anthracnose This plant disease is characterized by spotting of leaves, fruit, or seed pods It is caused by Colletotrichum coccodes and other species These fungi are considered weak plant pathogens They live from season to season on plant debris in the soil and on the seed of various plants such as the tomato Their spread is favored by warm, wet, weather Among the vegetables affected are beans, cucumbers, watermelons, pumpkins, squash, tomatoes, and peppers Blue mold rot is a post-harvest disease of apples and pears that is caused by Penicillium expansum while gray mold rot is caused by Botrytis cinerea In an attempt to control these fungi on apples and pears, two biocontrol agents have been tested—Candida sake and Pantoea agglomerans.32 When ... pectin is destroyed, oligogalacturonidases are formed and utilized by the varied microbiota Because of the early and relatively rapid growth of bacteria, molds, which tend to be crowded out, are of... consumed or destroyed The malodors that are produced are the direct result of volatile compounds (such as NH3 , volatile acids, and the like) produced by the biota When growing in acid media,... genes of P carotovorum subsp carotovorum that are involved in potato tuber maceration have been cloned Plasmids containing cloned DNA mediated the production of endopectate lyases, exopectate lyase,