The nitrite level of carrot juice at 20°C increased rapidly for several hours when bacterial levels were above 106 cells per ml of juice. When juice was held at 20°C, nitrite cone began to decline after 16 hours, although there was no decrease in bacterial population. No increase in nitrite occurred when bacterial growth was prevented by holding the juice at 5°C or by adding potassium dehydroacetate. However, nitrite was produced at 5°C when the bacterial level was raised to 108 cells per ml.
Forty-three accessions from 11 strawberry species were screened in the greenhouse for resistance to three strains of Xanthomonas fragariae Kennedy and King. Among the accessions tested, Pen-5 of Fragaria pentaphylla Losink expressed either no symptoms or a hypersensitive reaction, while accessions Pen-2 and Pen-4 developed either no symptoms or restricted water-soaked lesions. Two accessions of F. moschata Duch were characterized by reduced translucency at the inoculation site in the course of symptom development. These accessions, representing three resistance types, were classified as highly resistant, resistant, and moderately resistant, respectively, based on mean separation of disease severity ratings. The classifications proved to be consistent with the results from measurements of bacterial populations on inoculated leaves of those genotypes. The study suggests that species of F. pentaphylla and F. moschata harbour diversified sources of resistance. Resistant genotypes were not detected in F. nilgerrensis Schlect, F. daltoniana J. Gay, F. nubicola Lindl, F. gracilis Losinsk, F. iinumae Makino, F. vesca L., F. viridis Duch, or F. orientalis Losinsk.
Mulch (black plastic, wheat straw, or bare ground) and irrigation (drip or overhead sprinkler) treatments were evaluated for their effect on center rot of onion (Allium cepa L.), caused by the bacterium Pantoea ananatis, over the course of two seasons. Irrigation type had no effect on center rot incidence or severity in either year. In contrast, center rot development was delayed by 7 to 14 days on onions grown in straw mulch or bare ground compared to those in black plastic. Straw mulch resulted in later harvest dates and was associated with reduced levels of center rot. In contrast, black plastic increased disease incidence and hastened the onset of the epidemic. The spatial distribution of disease incidence in both years indicated the presence of a primary disease gradient. At harvest, infected plants were segregated by treatment and by duration of infection [based on disease ratings taken from the time of first symptom expression (beginning at 110 to 120 days after transplanting and then every 5 to 10 days until harvest)]. Early-vs. late-infected plants had no significant effect on yield (bulb weight). However, symptom expression in terms of the number of days after planting was significantly correlated with a disease severity index. Amount of rot in bulbs from plants displaying their first symptoms only 1 to 2 days before harvest (late-season infection) was not significant from rot levels in control bulbs at harvest. However, at 4 weeks after harvest, onions from plants with late-season infections exhibited significantly more rot in storage compared to the control.
This study examined the efficacy of chlorine treatments of flume water for eliminating Salmonella spp. from inoculated wounds and intact surfaces of tomatoes (Lycopersicon esculentum). Water in a scale-model flume was chlorinated to 150 mg·L-1 of free chlorine at pH 6.5 and maintained at a temperature of 25 or 35 °C, depending on the test. Viable Salmonella were recovered from all of the inoculation sites (intact fruit surface, punctures, shaves, and stem scars) even after treatment with chlorinated water for up to 120 seconds at either 25 or 35 °C. Generally, the highest Salmonella recovery came from puncture wounds and the lowest from the intact surfaces. After 120 seconds at 25 °C, 4.9 to 5.8 log10 units were recovered from the wounds. Populations recovered after the 30-second treatment at 35 °C ranged from 4.1 log10 cfu/mL for intact surfaces to 6.0 log10 cfu/mL in the puncture wounds. At 60- and 120-second treatment times, all wounds had higher mean populations than tomatoes with intact surfaces. Although greater Salmonella survival was associated with shorter exposure to the chlorine, water chlorination cannot completely eliminate contamination of tomato fruit by Salmonella, even on intact surfaces. Stem scars, in this study, were not readily disinfected with sodium hypochlorite.
Controlling bacterial and fungal contamination in plant tissue cultures is a serious problem. Antibiotics are currently used but are not always effective, can alter plant growth, and are costly, and resistant strains can result with extensive use. Plant preservative mixture (PPM) contains a mixture of two isothiazolones—methylchloroisothiazolinone and methylisothiazolinone, which are a class of broad-spectrum, widely used industrial biocides. The isothiazolones used in PPM are reported by the manufacturer to be nonphytotoxic at concentrations suitable for the prophylactic control of microbial contaminants in plant tissue cultures. Our results indicate that PPM can be routinely added to tissue culture medium to control air- and waterborne bacterial and fungal contaminants effectively.
Anti-bacterial peptide gene (shiva) was introduced into potato plants to improve the resistance to bacterial diseases. The 21 potato clones were selected in the medium containg 50 mg/L kanamycin and 13 transformants were confirmed by GUS activity assay using 4-methylumbellyferyl glucuronide (MUG) and PCR by NPTII specific primer sets. A 0.5-kb band was confirmed by PCR in the most transformants of T0, T1, and T2 generations. As a result of PCR with primer set chosen at shiva and GUS, expected 690-bp fragments were produced in the most transformants of T0, T1, and T2 generations. For southern blot analysis, potato genomic DNA digested with HindIII was separated on 0.7 % agarose gel, transferred to nylon membranes, and detected with nitroblue tetrazolium salt (NBT). As a result of southern analysis, different single bands were detected in the transformants with tPAL5 promoter and 1 to 3 bands were acquired in the transformants with CaMV35S promoter. To analysis protein level of transformants, NPTII ELISA kit were used. In several transformants, optical density (O.D.) values were 10- to 20-fold higher than non-transformants. Tubers were screened for resistance to Erwinia carotovora subsp. carotovora. The concentration of the inocula was 106 cells/mL. After inoculation, tuber slices were incubated aerobically for 48 h at 20 °C. The symptoms of soft rot in transgenic plants were considerably weakened in comparison with non-transformants.
Pierce's disease, caused by the bacterium Xylella fastidiosa Wells et al., is widespread among muscadine grapes (Vitus rotundifolia Michx). To determine whether shoot-tip culture would be effective in eliminating X. fastidiosa, shoot tips of infected grape plants were cultured on Murashige and Skoog medium amended with 9 μm benzyladenine. Shoots and callus that developed tested negative for the presence of X. fastidiosa. Shoot-tip culture appears to be a promising method of obtaining muscadine grape plants free of Pierce's disease. Chemical name used: 6-benzylaminopurine (benzyladenine).
Juice made from carrots stored at 20, 25, 30 or 35°C for 0,2,4 or 8 days contained negligible amounts of nitrite when sampled immediately. The bacterial populations were higher in juice made from carrots stored for longer periods and at the higher temperatures. Juices incubated at 35°C for 4 hours accumulated nitrite in amounts that tended to correspond to the bacterial populations in the fresh juices.
More than 700 accessions of Brassica vegetables were screened for resistance to bacterial (Erwinia) soft rot disease using a newly developed testing procedure. Dipping a needle in 2-day-old bacterial culture and pricking petioles of plants gave the most-consistent and distinguishable results in both seedling greenhouse and mature plant field tests. High humidity (100%) and warm temperature (higher than 23°C) are the two essential conditions for this test to be successful. So far, immune material has not been found. In B. rapa, less than 7% of the accessions showed some degree of resistance. High correlation was found between seedling greenhouse tests and mature plant field tests. Genetic study showed that soft rot resistance in B. rapa is a quantitative trait. The broad-sense heritability was 60% and narrow-sense heritability was 42% in the tested population. Following three cycles of recurrent selection, the resistance level in cycle three population was improved by 2.4 disease score points (1–9 scale) compared to the original parental population and the disease score of the best line in cycle 3 was 2.7 compared to a susceptible check on which the disease score was 8 under greenhouse conditions. Under field conditions, the best cycle three line scored 2.0 in comparison to the susceptible check, which scored 7. From our study, the recurrent selection works well for improving the resistance level to the soft rot disease in B. rapa.
Fewer postharvest technologies are available for use on organic than conventional fruits and vegetables. Even though biopesticides are perceived as likely candidates for postharvest use on organic produce, only some biopesticides will be approved as organic compounds for various reasons. An example is the definition of a biopesticide used by regulatory agencies such as the EPA which includes compounds that will not be considered organically acceptable. Fortunately, there are other existing or new technologies that could be acceptable on organic fruits and vegetables. Some examples are hot water immersion treatment or a hot water rinsing and brushing, new innovative controlled atmosphere techniques, alternative sprout control agents, naturally occurring volatiles and biofumigants. More research is needed on each of these technologies, both singly and in combination with each other.