A pawpaw (Asimina triloba) regional variety trial (PRVT) was established at the U.S. Department of Agriculture, Agricultural Research Service, National Clonal Germplasm Repository (NCGR), Corvallis, Ore., in Fall 1995. This orchard was a replicated planting of 28 commercially available varieties or advanced selections from the PawPaw Foundation (PPF; Frankfort, Ky.), with eight replicate trees of each selection grafted onto seedling rootstocks and planted in a randomized block design. Two years after planting, 32 trees had either failed to establish or had died after an initial healthy start. By July 1999, 25% of grafted trees had died due to a vascular wilt-like disease, and 2 years later mortality exceeded 50%. Grafted selections with the lowest symptom severity include 1-7-2, 2-54, 7-90, 8-58, 9-58, `Mitchell', `PA-Golden #1', `Taylor' and `Wilson'. Seedling guard trees were unaffected until July 2000, when six guard trees of 76 died and 10 more were declining. By July 2001, 14 guard trees were dead. No fungi were consistently isolated from declining trees. A number of bacteria were isolated from infected trees, but no specific pathogen has been confirmed as the causal agent. Polymerase chain reaction (PCR) tests for phytoplasmas and for the bacterium Xylella fastidiosa were also negative. Research is ongoing to determine if a bacterial pathogen was the cause of the pawpaw decline in the Oregon PRVT.
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.
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.
monitors biological variables such as counts of aerobic bacteria and fungi to indicate contamination or ineffective treatment [ American Public Health Association (APHA), 1995 ; Maier et al., 2009 ]. The reduction of bacterial counts from water samples
-harvesting areas. Pate and Nummer (2013) found that specialty crop production workers had high number of bacteria present on their hands and at multiple farms workers tested positive for Salmonella , Staphylococcus aureus , coliforms and generic E. coli . Our
commodities. This exposure may increase the risk of contamination by various spore-forming bacteria that are plant or human pathogens, such as Bacillus cereus , a common cause of food-borne illness. Research on the efficacy of sanitizer treatments in
and Tahardi, 1993 ), P-deficient soils ( Sanni, 1981 ), or both ( Fisher and Jayachandran, 2008 ; Janos, 1977 ). A number of the commercial products being marketed for palms also contain beneficial fungi other than AM fungi, beneficial bacteria, humic
concentrations found in hydroponic systems ( Rakocy, 1997 ). The aquaponic nitrogen (N) cycle ( Fig. 1 ) is of particular interest. Fish produce ammonia (NH 3 ), some of which ionizes in water to form ammonium (NH 4 + ). Nitrifying bacteria in biofilters convert
, 2017 ). Finally, the use of soil microorganisms (bacteria or fungus) as biostimulants for increasing the nutrient and water-use efficiency of horticultural crops has also been successfully tested ( Acikgoz et al., 2016 ; López-Bucio, et al., 2015
atmospheric nitrogen (N) by way of a relationship with Frankia bacteria, which colonize root nodules ( Stibolt, 1978 ). Such N-fixing species can survive, and often thrive, in N-poor soils in the wild ( Paschke, 1997 ); in cultivated situations, they can add