**7. Postharvest bacterial diseases**

Phytopathogenic bacteria cause postharvest diseases of economically important vegetables. Different species of bacteria belonging to top ten genera viz. *Pseudomonas*; *Ralstonia*; *Agrobacterium*; *Xanthomonas*; *Xanthomonas*; *Xanthomonas*; *Erwinia*; *Xylella*; *Dickeya* (*dadantii* and *solani*); *Pectobacterium* are devastating plant pathogens [18, 19]. They are unable to penetrate directly into plant tissue; however, they enter through wounds or natural plant openings. Wounds can be caused by insects and tools during operations like pruning and picking of the produce. The bacteria only become more active and cause infection when factors are conducive. Factors conducive to infection

*Postharvest Diseases of Vegetable Crops and Their Management DOI: http://dx.doi.org/10.5772/intechopen.101852*

are high humidity, crowding, poor air circulation, plant stress caused by overwatering, under watering, or irregular watering, poor soil health, and deficient or excess nutrients. The bacteria multiply quickly when free moisture and moderate temperatures are available. The major causal agents of bacterial soft rots are various species of *Erwinia*, *Pseudomonas*, *Bacillus*, *Lactobacillus,* and *Xanthomonas. Psuedomonas syringae* pv. *syringae*, *P. syringae* pv. *pisi* and *P. syringae* pv. *phaseolicola* causes diseases in vegetables [20]. The symptoms appear as water-soaked spots on pods that become sunken and dark-brown in color with distinctive reddish-brown margins.

Biological (culture media, diagnostic hosts, bacteriophages (phage typing); biochemical (based on properties of the bacteria in culture (gram stain, bacterial cell size, flagella), metabolic fingerprinting (API/BIOLOG system), thin layer chromatography, gel electrophoresis, conductance assays, isozyme analysis); immunoassays (agglutination, gel diffusion, ELISA, dot blot assays, immunofluorescence, flow cytometry); nucleic acid (hybridization, RFLPs, PCR, ICAN, DNA arrays, multilocus sequence typing) were used for reliable and accurate detection of plant pathogens for their effective management.

### **7.1** *Xanthomonas***: blight**

*X. campestris* pv. *vesicatoria* now reclassified as *X. euvesicatoria*, the causal agent of bacterial spot of tomato. The disease is prevalent in warm, humid, and temperate regions of the world. The genus *Xanthomonas* comprises 20 different species [21] with many pathovars causing economically important diseases on different vegetable crops [22]. *Xanthomonas* is a rod-shaped, gram-negative bacterium. It produces a yellow soluble pigment, called xanthomonadin, and extracellular polysaccharide (EPS). EPS is an important pathogenicity factor of bacteria that protect from desiccation and for the attenuation of wind- and rain-borne dispersal (**Figure 7**).

#### **7.2** *Pseudomonas***: blight**

The disease is caused by pathogen, *Pseudomonas syringae* pv. *pisi*. Another bacterium, *P. syringae* pv. *syringae* has also been reported to infect vegetable pea in a temperate region. Tender pods are chocolate brown, thin, twisted, and

#### **Figure 7.**

Xanthomonas *blight (A and B) and* Pectobacterium *soft rot (C) and culture plate of* Xanthomonas *(D and E) and* Pectobacterium *sp. (A)* Xanthomonas *tomato speck, (B)* Xanthomonas *blight on kale, (C)* Pectobacterium *soft rot on cauliflower, (D)* Xanthomonas *NA culture plate, (E)* Xanthomonas *NA culture plate, (F)* Pectobacterium *NA culture plate.*


#### **Table 4.**

*Postharvest bacterial diseases/pathosystem of vegetable crops.*

shriveled. Lesions are large on older pods and they become thin, twisted, and dry. Seeds become discolored and shriveled. Dried bacterial ooze makes the pod surface glossy.

#### **7.3** *Pectobacterium***: soft rot**

*Pectobacterium carotovorum* and *Pectobacterium atrosepticum* (formerly *Erwinia carotovora* subspecies *carotovora* and subspecies *atroseptica)* causes huge losses of economically important fleshy vegetables worldwide. *P. atrosepticum* was the first genomically sequenced plant bacterial pathogen that is taxonomically related to animal pathogens. Genomic information is now available for *P. carotovorum* strains and other "former Erwinia" species now reclassified in the genus Dickeya. Geographically, *P. carotovorum* is widely distributed and causes soft rot diseases of several vegetable crops. However, *P. atrosepticum* is an economically important pathogen of blackleg disease of potato and restricted into the temperate region of the world (**Table 4**) [23].

#### **8. Postharvest disease management**

Postharvest losses in vegetables are found due to fungal and bacterial infection worldwide. New challenges are faced under trade liberalization and globalization, and serious efforts are needed to reduce these losses in vegetables.

#### **8.1 Chemical control**

Chemical fungicides are commonly used for the management of postharvest disease in vegetables. For postharvest pathogens which infect produce before harvest, the fungicides should be applied at field level during the crop season, and/or strategically applied as systemic fungicides. At the postharvest level, the fungicides are often applied to reduce infections already established in the surface tissues of produce or they may protect against infections occurring during storage and handling. Fungicides used during postharvest are actually fungistatic rather than fungicidal under normal usage. The fungicides are applied on the produce as dips, sprays, fumigants, treated wraps, and box liners or in waxes and coatings. Dip and spray methods are very common in postharvest treatments. The fungicides generally applied as a dip or spray method are benzimidazoles (e.g. benomyl and thiabendazole) against anthracnose, and triazoles (e.g. prochloraz and imazalil) and fumigants, such as sulfur dioxide, for the control of gray mold used for postharvest disease control [24, 25]. Dipping in hot water (at 50°C for 5–10 min, depending on the size of produce in combination with the fungicide) is also used for effective control of the disease. Sodium hypochlorite as a disinfectant is used to kill spores of pathogens present on the surface of the vegetable produce.
