This bacterium is characterized by its ability to produce a potent toxin, called the wildfire toxin, in the host plants and on many nutrient media. Only 0.05 of this toxin can produce a yellow lesion on a to-bacco leaf in the a b s e n ce of bacteria. A more detailed discussion of the nature and of the m o de of action of this and other toxins will be found in Chapter 4 in the section on microbial toxins in plant disease.
Development of Disease
T h e wildfire bacterium overwinters in the soil, in dried or cured d i s e a s ed tobacco leaves, on s e ed from infected s e ed capsules, on s e e d b ed covers, and in the roots of many w e e ds and crop plants. From these sources the bacteria are carried to the leaves by rain splashes or by wind during wet weather. T h e y may also b e spread by contami-nated tools and hands during handling of the plants.
Ver y high humidity or a film of moisture on the plants must b e pres-ent for infections to occur and henc e for d e v e l o p m e nt of epidemics.
Water-soaked areas present in the leaves during long rainy periods or during rains accompanied by strong winds are excellent infection courts' fpr the bacterium and result in extensive lesions within 2-3 days. T h e bacteria enter the leaf through the large stomata and hyda-thodes and through wounds c a u s ed by insects and other factors (Fig.
63). Certain insects such as flea beetles, aphids, and white flies also act as vectors of this pathogen.
Once inside the leaf tissues the bacteria multiply intercellularly at a rapid rate. At the same time they secrete the wildfire toxin which spreads radially from the point of infection and results in the forma-tion of the chlorotic halo. This consists of a rather broad zone of cells which is free of bacteria and surrounds the bacteria-containing spot.
T h e toxin affects the cells by acting as an antimetabolite of methio-nine, i.e., it inhibits the utilization by the cells of the essential amino acid methionine, which is a structural analog of the toxin. T h e toxin also causes a marked breakdown of proteins and simultaneously an increase or decrease in the activities of several host enzymes.
In favorable weather the bacteria continue to spread intercellularly and through the toxin and enzymes they secrete c a u se the breakdown, collapse, and death of the parenchymatous cells in the leaf tissues they invade. C o l l a p s ed cells are invaded by the wildfire bacteria and also by saprophytic bacteria and fungi which aid in the disintegration of the tissues. D e a d, disintegrated areas of the leaf are loosely held together and, during humid weather, they are easily detached from the healthy tissues and fall to the ground or are carried by air currents to other plants.
Wildfire of Tobacco
Control
Several tobacco varieties resistant to wildfire bacteria are available already, and ne w ones are b e i ng introduced as fast as they are devel-oped. Wheneve r possible, therefore, only resistant varieties should b e planted.
With susceptible varieties, it is important that control practices be -gin in the s e e d b e d, since the d i s e a se often starts there. Only healthy s e ed should b e used, and if it is s u s p e c t ed of b e i ng contaminated with bacteria it should be disinfested by soaking it in a formaldehyde solu-tion for 10 minutes. T h e s e e d b ed soil should b e sterilized, preferably with steam, before planting or with a chemical, such as Vapam, My-lone, methyl bromide, in the fall. After seedlings appear, and if wild-fire has b e e n present in the area during the previous year, s e e d b e ds should b e sprayed with a neutral copper fungicide and streptomycin.
T h e streptomycin sprays should b e continued at weekly intervals un-til plants are transplanted. If isolated spots of wildfire appear, the in-fected plants plus all healthy plants in a 25-cm b a nd around them should b e destroyed by drenching with formaldehyde. Only healthy seedlings should b e transplanted into the field and they should b e planted only in fields that did not have a d i s e a s ed crop during the pre-vious year. Overfertilization, especially with nitrogen, should b e avoided, since rapidly growing, succulent plants are m u ch more sus-ceptible to the d i s e a se than those that have m a de a slow, normal growth.
S e l e c t ed References
Anderson, P. J. 1924. O v e r w i n t e r i ng of tobacco wildfire bacteria in N e w E n g l a n d. Phy-topathology 14: 132-139.
Braun, A. C. 1955. A study on the m o de of action of the wildfire toxin. Phytopathology 4 5 : 6 5 9 - 6 6 4 . b u r l ey tobacco, Nicotiana longiflora, a nd hybrids. Phytopathology 44: 186-187.
F a r k a s, G. L., a nd L. L o v r e k o v i c h. 1965. E n z y me levels in tobacco leaf tissues affected by the wildfire toxin. Phytopathology 55 : 5 1 9 - 5 2 4 .
H e g g e s t a d, Ç . Å., M. O. N e a s, a nd J. G r o s s o. 1956. C o m p a r i s on of various streptomycin dust a nd spray treatments for wildfire control in tobacco plant b e d s. Plant Disease Reptr. 40: 4 8 - 5 1 .
353
Hill, J. B. 1930. T h e z o o g l o e ae of Bacterium tabacum a nd their relation to the p r o b l em w e e ds by tobacco leafspot bacteria. Phytopathology 34: 163-174.
Wolf, F. A. 1922. Wildfire of tobacco. N. Carolina State Agr. Expt. Sta. Bull. 246:
27 p p.
Wooley, D. W., R. B. Pringle, a nd A. C. Braun. 1952. Isolation of the p h y t o p a t h o g e n ic toxin of Pseudomonas tabaci, an antagonist of methionine. J. Biol. Chem. 197:
4 0 9 - 4 1 7 .
Bacterial Soft Rots of Vegetables Occurrence and Importance
Bacterial soft rots occur most commonly on vegetables (and some annual ornamentals) that have fleshy storage tissues, such as potatoes, carrots, radishes, onions, hyacinths, iris, or fleshy fruit, such as cucum-ber, squash, eggplant, tomato, or succulent stem, stalk or leaves, such as cabbage, celery, lettuce, spinach. T h e y are found all over the world and cause serious diseases of crops in the field, in transit, and e s p e-cially in storage, resulting in greater total loss of produce than any other bacterial disease. Nearly all fresh vegetables are subject to bac-terial soft rots and may d e v e l op a serious decay within a few hours in storage or during marketing. Bacterial soft rots cause severe economic losses by reducing quantities of produce available for sale, by reduc-ing the quality and thus the market value of the crops, and by greatly increasing expenses for preventive measures against soft rots and for preparation of partially affected produce for use.
Symptoms
T h e soft-rot symptoms produced on fruits and other fleshy organs in the field or in storage are very similar on all the hosts. At first there appears on the tissue a small water-soaked lesion which enlarges rap-idly in diameter and in depth. T h e affected area b e c o m es soft and mushy (Fig. 64). Its surface may b e c o me discolored and somewhat d e p r e s s ed or it may appear wrinkled or blistered. T h e margins of the lesions usually are well defined at first but later b e c o me obscure. T h e
Bacterial Soft Rots of Vegetables 355
F i g. 64. (A) Soft rot of potato c a u s ed by Erwinia carotovora. (B) N u m e r o us infections of potato tuber with the soft rot b a c t e r i u m. (Photos by courtesy of the D e p a r t m e nt of Plant Pathology, Cornell University.)
tissues within the affected region b e c o me o p a q ue in a short time or appear cream-colored and slimy, disintegrating into a m u s hy mass of disorganized cells. In certain fruits, tubers, etc., the outer surface may remain intact while the entire contents have c h a n g ed to a turbid liq-uid (Fig. 65). Frequently, however, cracks d e v e l op and the slimy mass exudes to the surface where, upon exposure to the air, it turns tan, gray, or dark brown. A whole fruit or tuber may b e converted into a soft, watery, colorless, d e c a y ed mass within a period of 3-5 days. If, however, the rot develops in a dry atmosphere where there is a rapid loss of water by evaporation, the infected tissues dry up quickly and may remain attached to the healthy tissues or may slough off. Infected fruits and tubers of many plants are almost odorless until the infected tissues collapse w h e r e u p on secondary bacteria, living off the decom-posing tissues, produce a foul odor. Cruciferous plants and onions, however, w h en infected by soft rot bacteria, almost always give off an offensive sulfurous odor.
Whe n root crops are affected in the field, symptoms may also
de-F i g. 65 . (A) Cross section of potato tuber exhibiting s y m p t o ms of bacterial soft rot. (B) Bacterial soft rot on potato in the field. N o te the initiation of infection through the stem end. (Photos by courtesy of the D e p a r t m e nt of Plant Pathology, Cornell University.)
velop on the lower parts of the stem, which b e c o me watery, turn black and shrivel. This also results in stunting, wilting, and death of the aboveground parts of the plant.
Infections of succulent leaves and stems are seldom important in the field. However, w h en these parts are infected in storage or in packages, rapid softening and disintegration of the d i s e a s ed tissues follows and may yield a wet, green, slimy mass within 1 or 2 days (Fig. 66).
The Pathogen: Erwinia carotovora and Other Species
Although bacteria belonging to other gener a (e.g., Pseudomonas) can cause soft rots, the most common and the most destructive soft rots are c a u s ed by bacteria in the genus Erwinia. Of these, Erwinia caroto-vora s e e ms to b e the most prevalent. I n d e e d, there is evidence that all other soft rotting Erwinias may b e specialized forms of Erwinia caro-tovora.
Bacterial Soft Rots of Vegetables 357
Erwinia carotovora is a rod-shaped bacterium, 1.5-3.0 μ in length by 0.6-0.9 μ in diameter. It occurs singly or in chains, and at tempera-tures of 27°C or higher it forms much longer rods and also filaments.
E a ch rod has two to six peritrichous flagella. T h e bacterium produces no capsule and is a facultative anaerobe. T h e soft-rot bacteria can grow and are active over a w i de range of temperatures. T h e minimum, optimum, and maximum temperatures for their growth in culture are 2°C, 2 5 ° C, and 37°C, respectively, while the respective temperatures for d i s e a se d e v e l o p m e nt are 5 ° C, 2 2 ° C, and 37°C. T h e bacteria are killed at about 50°C. Certain strains of E. carotovora b e c o me less viru-lent after repeated transfers in culture, but they rarely b e c o me aviru-lent.
Development of Disease
T h e soft-rot bacteria overwinter in infected fleshy organs in storage and in the field, in debris that contains parts of infected plants, in the soil, in the p u p ae of the seed-corn maggot (Hylemyia cilicrura), and in the p u p ae of several other insects (Fig. 67). T h e d i s e a se may first ap-pear in the field on plants grown from previously infected s e ed pieces, as is frequently the case with potato. S o me tubers, rhizomes, bulbs, etc., b e c o me infected by the bacterium, which may b e present in the
F i g. 66. Bacterial soft rot of c a b b a ge c a u s ed by Erwinia carotovora. (Photo by courtesy of U . S. D e p t. Agr.)
Fig. 67. Disease cycle of bacterial soft rot of vegetables caused by Erwinia carotovora.