Epidemiological study of Cucumber green mottle mosaic virus in greenhouses enables reduction of disease damage in cucurbit production

Since 2007, the tobamovirus Cucumber green mottle mosaic virus (CGMMV) has become widespread in Israel, causing severe damage to trellised cucumber and melon in greenhouses and watermelon grown in open fields. To reduce disease damage below the economic threshold, this study focused on four objectives: (a) monitoring the patterns of virus distribution within commercial cucumber greenhouses; (b) studying the potential transmission of CGMMV by agrotechnical activities; (c) virus localization in plant tissues; and (d) searching for techniques that might be adapted for mitigating the disease in trellised cucurbit growth. The results of our surveys demonstrated the role of contaminated seeds and soil as primary inoculum sources, and secondary spread caused by agrotechnical activities. The patterns of secondary disease spread were demonstrated in a series of inoculation experiments involving contaminated knives, shears or hands on wet and dry plants, conducted under research‐greenhouse conditions. In parallel experiments using CGMMV‐specific antibody and secondary antibody conjugated to Alexa fluor 488, the viral coat protein was visualized in several plant tissues: phloem, xylem, trichomes and grasping tendrils. In addition, commercial‐greenhouse experiments were aimed at reducing the number of inoculum sources by identifying and removing infected plants from the plots (early monitoring) prior to agrotechnical activities and/or by adding intermediate medium (IM), such as virus‐free compost, to the planting pits at the planting stage. It is suggested that the use of IM combined with early monitoring, awareness of worker mobility (from contaminated structures to young planting areas) and proper sanitation (e.g. disinfection of agrotechnical tools) may reduce the yield losses caused by CGMMV below the economic threshold.     

The role of seed-tuber and stem inoculum in the development of gangrene in potatoes

To determine the relative importance of seed tubers and stems as sources of inoculum for potato gangrene in progeny tubers, different levels of inoculum of Phoma exigua var. foveata were established in field experiments by planting rotting or contaminated seed tubers and by inoculating stems shortly before haulm desiccation. The pathogen was only occasionally detected by isolation from inside green stems in June and July on plants growing from contaminated but unrotted seed. The incidence of pycnidia on desiccated stems increased with increasing concentration of inoculum contaminating seed tubers and with increasing time interval between desiccation and harvest. Stem infection was probably derived from inoculum on seed tubers spreading via the soil to the stem bases. Stem inoculation of plants growing from uninoculated or uncontaminated seed greatly increased both the gangrene potential of progeny tubers (defined as % wounds with gangrene after uniformly damaging tubers and storing them at 5°C for 12 wk), and tuber contamination (defined as % wounds with gangrene after spreading tuber-borne soil onto test tuber slices and storing them at 5°C for 8 wk). However, when stems of plants growing from rotting or contaminated seed were cut at ground level and removed before desiccation, gangrene potential of progeny tubers was only slightly less than that of tubers from untreated plots. There was no evidence that soil inoculum or aerial spread played a significant role in disease development. Gangrene potential and contamination of progeny tubers were related to Contamination levels on seed tubers. Some transmission also occurred from rotted seed tubers to progeny. Inoculum levels around progeny tubers increased rapidly after desiccation even in plots where stems had previously been cut at soil level and removed to eliminate pycnidial development above ground as a source of inoculum. Both gangrene potential and contamination of progeny tubers were greater on cv. Ulster Sceptre than on cv. Pentland Crown. The results showed that the inoculum on seed tubers, whether from rots or surface contamination, contributed more to the contamination of progeny tubers at harvest than did the inoculum from pycnidia on stems following desiccation of the haulm.     

Differential isolatoion of Pythium species from soil by means of selective media,temperature, and pH.

Pythium aphanidermatum, with an optimum temperature for growth at 35C, grew welland was readily isolated from soil on pimaricin-vancomycin medium (MPVM) when incubated for 24h at 38-40C. The pH of the medium affected recovery; maximum numbers developed above pH 6.0. Other Pythium spp. were recovered on MPVM at 20-25C, but were excluded by incubation at 38-40C. These Pythium spp. included P. ultimun, P. paroecandrum, P. irregular, P. mamillatum, and an unidentified Pythium sp. These species grew well and were readily siolated from soil on gallic acid medium (GAM) when incubated for 24-8h at 20 C.P. aphanidermatum and P. myriotylum grew from mycelium on GAM, but their oospores did not germinate nor could they be isolated from soilon this medium. P. myriotylum grew well on MPVM, but was only rarely isolated, evenfrom soils with known high potential for disease caused by P. myriotylum. Propagules of Pythium were enumerated by a plate-dilution frequency method or by a smearplateethod is valuable for studies on the ecology, survival, and inoculum potential in soils with mixed populations of P. aphanidermatum and other Pythium spp.     

Relationships between Mycobacterium isolates from patients with pulmonary mycobacterial infection and potting soils

High numbers of mycobacteria, including known pathogenic species such as Mycobacterium avium, Mycobacterium intracellulare, and Mycobacterium chelonae, were recovered from aerosols produced by pouring commercial potting soil products and potting soil samples provided by patients with pulmonary mycobacterial infections. The dominant mycobacteria in the soil samples corresponded to the dominant species implicated clinically. Profiles of large restriction fragments obtained by pulsed-field gel electrophoresis demonstrated a closely related pair of M. avium isolates recovered from a patient and from that patient's own potting soil. Thus, potting soils are potential sources of infection by environmental mycobacteria. Use of dust-excluding masks should be considered during potting or other activities that generate aerosol with soil.     

Transmission Efficiency of Cucumber green mottle mosaic virus via Seeds,Soil, Pruning and Irrigation Water

Cucumber green mottle mosaic virus (CGMMV; genus Tobamovirus) infects frequently the grafted watermelon and is widely distributed in China. Investigating the transmission modes and their efficiency is urgently needed to understand the factors contributing to the epidemiology of this viral disease. In the present study, we found that the occurrence of CGMMV in a bottle gourd seed production base reached 100%, while the contamination rate and transmission rate were 100 and 0.92%, respectively. The bottle gourd plants showed obvious mottle symptom on leaves starting 36 days after seed sowing. The long latent period of CGMMV in seedlings implies a potential risk to use contaminated seeds in the production of grafted watermelon. This virus could overwinter in soil with debris of infected plants, and the infection rate of CGMMV from contaminated soils was 10.30%. CGMMV could be transmitted from infected watermelon plants to healthy ones by pruning at least to the ninth plant during the whole growing season. The transmission distance was 1.87 m by drip irrigation and 2.31 m by flow irrigation. This study suggested that contaminated seeds, contaminated soil, pruning and irrigation could transmit CGMMV at different efficiency, and all contribute to the epidemiology of CGMMV.     

An exploratory study of peat and sawdust as enhancers in the (bio)degradation of n-dodecane

Current practice for dealing with oil spills involves the use of adsorbent materials to contain the pollution prior to bioremediation of the contaminated soil and adsorbent. This work presents a study of the effects of bioavailable carbon sources in the adsorbents peat and sawdust as organic nutrients for microorganisms specialized in degrading n-dodecane in soil and sawdust contaminated with hydrocarbon mixtures. An experimental bioremediation system was developed using n-dodecane, biomass adapted to n-dodecane, inorganic nutrients and the two adsorbents (sterilized). Bioreactors containing peat enhanced cell growth the most and also evolved more CO(2). An advantage of peat is that its soluble carbon sources can sustain higher cell densities compared to sawdust, and this may prove decisive when cultivating endogenous microorganisms for the aerobic bioremediation of soils contaminated with hydrocarbons. However, at the end of the 68-day experiment slightly higher n-dodecane removal was identified in the system containing sawdust-n-dodecane (99.6%) than in that with peat-n-dodecane (98.5%), evidencing the higher hydrocarbon retention capacity of peat. Based on this study, the use of sawdust instead of peat is recommended when an adapted inoculum is available for aerobic bioremediation of organic contaminants, whereas the use of peat is advisable to boost cell densities in order to improve the probability of sustaining a viable biomass in unfavorable conditions.     

Availability and function of arbuscular mycorrhizal and ectomycorrhizal fungi during revegetation of dewatered reservoirs left after dam removal

Revegetation following dam removal projects may depend on recovery of arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungal communities, which perform valuable ecosystem functions. This study assessed the availability and function of AM and EM fungi for plants colonizing dewatered reservoirs following a dam removal project on the Elwha River, Olympic Peninsula, Washington, United States. Availability was assessed via AM fungal spore density in soils and EM root tip colonization of Salix sitchensis (Sitka willow) in an observational field study. The effect of mycorrhizal fungi from 4 sources (reservoir soils, commercial inoculum, and 2 mature plant community soils) on growth and nutrient status of S. sitchensis was quantified in a greenhouse study. AM fungal spores and EM root tips were present in all field samples. In the greenhouse, plants receiving reservoir soil inoculum had only incipient mantle formation, while plants receiving inoculum from mature plant communities had fully formed EM root tips. EM formation corresponded with alleviation of phosphorus stress in plants (lower shoot nitrogen:phosphorus). Thus, revegetating plants have access to AM and EM fungi following dam removal, and EM formation may be especially important for plant P uptake in reservoir soils. However, availability of mycorrhizal fungi declines with distance from established plant communities. Furthermore, EM fungal communities in recently dewatered reservoirs may not be as effective at forming beneficial mycorrhizae as those from mature plant communities. Whole soil inoculum from mature plant communities may be important for the success of revegetating plants and recovery of mycorrhizal fungal communities.     

Relationships between Mycobacterium Isolates from Patients with Pulmonary Mycobacterial Infection and Potting Soils

High numbers of mycobacteria, including known pathogenic species such as Mycobacterium avium, Mycobacterium intracellulare, and Mycobacterium chelonae, were recovered from aerosols produced by pouring commercial potting soil products and potting soil samples provided by patients with pulmonary mycobacterial infections. The dominant mycobacteria in the soil samples corresponded to the dominant species implicated clinically. Profiles of large restriction fragments obtained by pulsed-field gel electrophoresis demonstrated a closely related pair of M. avium isolates recovered from a patient and from that patient's own potting soil. Thus, potting soils are potential sources of infection by environmental mycobacteria. Use of dust-excluding masks should be considered during potting or other activities that generate aerosol with soil.     

Bioremediation of atrazine-contaminated soil by repeated applications of atrazine-degrading bacteria

Bioaugmentation has previously been unreliable for the in situ clean-up of contaminated soils because of problems with poor survival and the rapid decline in activity of the bacterial inoculum. In an attempt to solve these problems, a 500-l batch fermenter was investigated for its ability to deliver inoculum repeatedly to contaminated soils via irrigation lines. In a field experiment, mesocosms were filled with 350 kg soil containing 100 mg kg⁻¹ atrazine, and inoculated one, four or eight times with an atrazine-degrading bacterial consortium that was produced in the fermenter. After 12 weeks, no significant degradation of atrazine had occurred in soil that was inoculated only once; whereas, mesocosms inoculated four and eight times mineralized 38% and 72% of the atrazine respectively. Similar results were obtained in a laboratory experiment using soil contaminated with 100 mg kg⁻¹ [¹⁴C]atrazine. After 35 days, soil that was inoculated once with 10⁸ cfu ml⁻¹ of the consortium or with the atrazine-degrading bacterium, Pseudomonas sp. strain ADP, mineralized 17% and 35% of the atrazine respectively. In comparison, microcosms inoculated every 3 days with the consortium or with Pseudomonas sp. (ADP) mineralized 64% or 90% of the atrazine over this same period. Results of these experiments suggest that repeated inoculation from an automated fermenter may provide a strategy for bioaugmentation of contaminated soil with xenobiotic-degrading bacteria. Received: 20 November 1998 / Received revision: 8 February 1999 / Accepted: 12 February 1999     

Local and non-local soil microbiota impede germination of the endangered Acacia whibleyana

Inoculating soils with microbiota that benefit the germination and growth of endangered plant species could improve their revegetation success and conservation status. While ecosystem degradation can disrupt beneficial plant–soil-microbial interactions, the prospect of reintroducing native plant-associated soil microbiota during revegetation could help to restore these important ecological links and assist the recovery of key species. We address the role of soil microbiota on germination and seedling fitness traits of the endangered Acacia whibleyana (Fabaceae) through a 17-week greenhouse experiment. Soil treatments included local soil, potting medium, three inoculation ratios (3:1, 1:1, 1:3 local soil: potting medium), sterilized local soil and sterilized potting medium. Soil sterilization reduced the time to first seed germination, indicating a role of soil microbiota on germination. The 1:1 whole soil inoculation saw reduced germination rates compared with either pure local or potting-medium treatments, and the slower germination times observed in live soils confirmed the strong influence of soil microbiota on the timing of germination. We report evidence that poor inoculation strategies can adversely impact germination of this endangered Acacia. Furthermore, our findings suggest that careful assessment of microbiota associated with A. whibleyana could help to improve germination and recruitment during its revegetation and conservation management.     

Screening legume green manures as nitrogen sources to succeeding non-legume crops

A buried bag incubation technique was proposed to monitor N release from soil and decomposing green manure. The technique would facilitate not only the screening of legumes as sources of N but also measurement of the N supplying capacity of soils. Several tropical legumes were incorporated into field plots followed either by maize (Zea mays L.) or by bare fallow. Soil samples from the plow layer containing the incorporated green manure were placed in low density polyethylene bags and buried within the plow layer under the maize crop for in situ incubation. Periodic withdrawal of the bags was accompanied by fallow soil profile sampling. Above ground N accumulation by maize was equally well correlated to N release measured by either method although the bag technique required much less labor. Supplemental experiments suggested that N accumulation in the bags was reduced due to inadequate O₂ diffusion but only when O₂ demand was high and soil water potential was high. The results show that in situ bag incubation alone or together with fallow soil sampling can be used to estimate the N supplying potential of soil and leguminous residues.     

Arsenic uptake and accumulation in rice (Oryza sativa L.) irrigated with contaminated water

Long-term use of arsenic contaminated groundwater to irrigate crops, especially paddy rice (Oryza sativaL.) has resulted in elevated soil arsenic levels in Bangladesh. There is, therefore, concern regarding accumulation of arsenic in rice grown on these soils. A greenhouse pot experiment was conducted to evaluate the impact of arsenic-contaminated irrigation water on the growth and uptake of arsenic into rice grain, husk, straw and root. There were altogether 10 treatments which were a combination of five arsenate irrigation water concentrations (0–8 mg As l^–1) and two soil phosphate amendments. Use of arsenate containing irrigation water reduced plant height, decreased rice yield and affected development of root growth. Arsenic concentrations in all plant parts increased with increasing arsenate concentration in irrigation water. However, arsenic concentration in rice grain did not exceed the maximum permissible limit of 1.0 mg As kg^–1. Arsenic accumulation in rice straw at very high levels indicates that feeding cattle with such contaminated straw could be a direct threat for their health and also, indirectly, to human health via presumably contaminated bovine meat and milk. Phosphate application neither showed any significant difference in plant growth and development, nor in As concentrations in plant parts.     

The Influence of the Inoculum Source of Rhizoctonia solani on Development of Black Scurf on Potato

Rhizoctonia solani, the causal agent of stem canker and black scurf on potato, survives as sclerotia on tubers, in soil and in plant residues. The objective of the present study was to evaluate the importance of inoculum source on disease development. Disease‐free minitubers and seed tubers contaminated with low levels of R. solani were planted in fumigated or artificially inoculated growth mixture in greenhouse experiments. Black scurf incidence and severity were significantly higher when the inoculum was present in both seed tubers and soil, compared with either of them separately. The severity of disease symptoms on the subterranean parts of the plant also were significantly higher in plots where both seed tubers and soil were contaminated, compared with plots where the inoculum source was either the seed tubers or the soil. Thus, both major sources of inoculum, seed tubers and soil, are important in disease development. However, when both sources are present, black scurf incidence and severity are increased, leading to economical damage to tuber yield and quality. Additional results from field trials support these findings. Disease incidence and severity on daughter tubers were correlated with levels of contamination in seed tubers and soil. When seed tubers and soil were heavily infested, the levels of black scurf incidence and severity on daughter tubers were very high; when seed tuber and soil infestation were very low, black scurf incidence and severity on progeny were also lower. Disease levels were reduced by in‐furrow fungicide treatment, but were less effective when the initial levels of the fungus on the seed tubers and in the soil were high.     

Importance of soil-water relations in assessing the endpoint of bioremediated soils

This study was conducted to investigate water movement in hydrocarbon contaminated soils. Three soils were studied, a hydrocarbon contaminated soil, the same soil after 3 years of bioremediation, and a control soil from the same site. There was a critical soil water content around 18% (bioremediated soil) and 20% (contaminated soil), above which the sorptivity of the contaminated soil was near that of the control soil. For soils with water contents below this value, there was a strong divergence in sorptivity between contaminated and control or bioremediated soils. Results suggest that water availability in contaminated soils will be highly dependent on soil water properties as water potential approaches the permanent wilting point (-1.5 MPa matrix potential).Infiltration of water into air dry (2% m.c. w/w) hydrocarbon contaminated soils was up to three orders of magnitude slower than for the control soil. For air dried soils, the infiltration rate of the contaminated and bioremediated soils was constant with time. This was in contrast to the control soil where infiltration rate was a function of the reciprocal of the square root of time.     

Distribution of Legionella longbeachae serogroup 1 and other legionellae in potting soils in Australia

Legionella longbeachae serogroup 1 and other Legionella spp. were isolated from 73% of 45 potting soils made in Australia by 13 manufacturers but were not detected in 19 potting soils made in Greece, Switzerland, and the United Kingdom examined between March 1989 and May 1990. Several Legionella species were isolated from a small number of samples of uncomposted pine sawdusts, but it is not known whether sawdust was the source of some of the legionellae found in potting soils. Legionella spp. persisted for periods ranging from 3 to 10 months in a potting soil held at temperatures between -20 and 35 degrees C. Isolates of L. longbeachae serogroup 1 from soil did not grow at 43 degrees C, a temperature which was also lethal for this species in soil. Most Legionella spp. isolated from potting and natural soils belonged to one distinct group according to analysis of ubiquinones and were serologically related to several known species in this group. A small number of potting soils contained L. pneumophila and L. micdadei.     

Distribution of Legionella longbeachae serogroup 1 and other legionellae in potting soils in Australia.

Legionella longbeachae serogroup 1 and other Legionella spp. were isolated from 73% of 45 potting soils made in Australia by 13 manufacturers but were not detected in 19 potting soils made in Greece, Switzerland, and the United Kingdom examined between March 1989 and May 1990. Several Legionella species were isolated from a small number of samples of uncomposted pine sawdusts, but it is not known whether sawdust was the source of some of the legionellae found in potting soils. Legionella spp. persisted for periods ranging from 3 to 10 months in a potting soil held at temperatures between -20 and 35 degrees C. Isolates of L. longbeachae serogroup 1 from soil did not grow at 43 degrees C, a temperature which was also lethal for this species in soil. Most Legionella spp. isolated from potting and natural soils belonged to one distinct group according to analysis of ubiquinones and were serologically related to several known species in this group. A small number of potting soils contained L. pneumophila and L. micdadei.     

Is plant performance limited by abundance of arbuscular mycorrhizal fungi? A meta-analysis of studies published between 1988 and 2003

We conducted meta-analyses of 290 published field and glasshouse trials to determine the effects of various agricultural practices on mycorrhizal colonization in nonsterile soils, and the consequence of those effects on yield, biomass, and phosphorus (P) concentration. Mycorrhizal colonization was increased most by inoculation (29% increase), followed by shortened fallow (20%) and reduced soil disturbance (7%). The effect of crop rotation depended on whether the crop was mycorrhizal. Increased colonization resulted in a yield increase in the field of 23% across all management practices. Biomass at harvest and shoot P concentration in early season were increased by inoculation (57 and 33%, respectively) and shortened fallow (55 and 24%). Reduced disturbance increased shoot P concentration by 27%, but biomass was not significantly affected. Biomass was significantly reduced in 2% of all trials in which there was a significant increase in colonization. Irrespective of management practice, an increased mycorrhizal colonization was less likely to increase biomass if either soil P or indigenous inoculum potential was high.     

Effect of soil type on infectivity and persistence of the entomopathogenic nematodes Steinernema scarabaei, Steinernema glaseri, Heterorhabditis zealandica, and Heterorhabditis bacteriophora

We tested the effect of soil type on the performance of the entomopathogenic pathogenic nematodes Steinernema scarabaei, Steinernema glaseri, Heterorhabditis zealandica, and Heterorhabditis bacteriophora. Soil types used were loamy sand, sandy loam, loam, silt loam, clay loam, acidic sand, and a highly organic potting mix. Infectivity was tested by exposing third-instar Anomala orientalis or Popillia japonica to nematodes in laboratory and greenhouse experiments and determining nematode establishment in the larvae and larval mortality. Infectivity of H. bacteriophora and H. zealandica was the highest in potting mix, did not differ among loamy sand and the loams, and was the lowest in acidic sand. Infectivity of S. glaseri was significantly lower in acidic sand than in loamy sand in a laboratory experiment but not in a greenhouse experiment, and did not differ among the other soils. Infectivity of S. scarabaei was lower in silt loam and clay loam than in loamy sand in a greenhouse experiment but not in a laboratory experiment, but was the lowest in acidic sand and potting mix. Persistence was determined in laboratory experiments by baiting nematode-inoculated soil with Galleria mellonella larvae. Persistence of both Heterorhabditis spp. and S. glaseri was the shortest in potting mix and showed no clear differences among the other substrates. Persistence of S. scarabaei was high in all substrates and its recovery declined significantly over time only in clay loam. In conclusion, generalizations on nematode performance in different soil types have to be done carefully as the effect of soil parameters including soil texture, pH, and organic matter may vary with nematode species.     

Distribution and decline of human pathogenic bacteria in soil after application in irrigation water and the potential for soil-splash-mediated dispersal onto fresh produce

Aims: To improve our understanding of the survival and splash‐mediated transfer of zoonotic agents and faecal indicator bacteria introduced into soils used for crop cultivation via contaminated irrigation waters. Methods and Results: Zoonotic agents and an Escherichia coli marker bacterium were inoculated into borehole water, which was applied to two different soil types in early‐, mid‐ and late summer. Decline of the zoonotic agents was influenced by soil type. Marker bacteria applied to columns of two soil types in irrigation water did not concentrate at the surface of the soils. Decline of zoonotic agents at the surface was influenced by soil type and environmental conditions. Typically, declines were rapid and bacteria were not detectable after 5 weeks. Selective agar strips were used to determine that the impact of water drops 24–87 μl could splash marker bacteria from soil surfaces horizontal distances of at least 25 cm and heights of 20 cm. Conclusions: Soil splash created by rain‐sized water droplets can transfer enteric bacteria from soil to ready‐to‐eat crops. Persistence of zoonotic agents was reduced at the hottest part of the growing season when irrigation is most likely. Significance and Impact of the Study: Soil splash can cause crop contamination. We report the penetration depths and seasonally influenced declines of bacteria applied in irrigation water into two soil types.     

Stabilization and solidification of hydrocarbon‐contaminated soils in concrete

This article describes an experimental program developed to investigate the potential for using hydrocarbon‐contaminated soils as a fine aggregate replacement in concrete. Five different contaminated soil types with a total petroleum hydrocarbon content of less than 1% were investigated. For each soil type, three concrete mixtures were obtained by replacing sand with contaminated soils (10, 20, and 40% replacement ratio). The resulting concrete was tested for setting times, compression strength, flexural strength, durability, and teachability of benzene to water.

The results indicate that the addition of hydrocarbon‐contaminated soil adversely affects the strength of concrete. The strength reduction at each soil replacement level depends on contamination concentration, contaminant type, and soil type. The durability of the tested concrete is comparable to normal concrete. For all five soils at a 40% replacement ratio, the leachability of benzene was nondetectable after 24 h and after 10 d. After testing the leachability of artificially contaminated soils (0.5 and 3% neat benzene contamination) for 24 h, it was found that the leaching of benzene increases with the percentage of contamination. However, the fraction of benzene that leached was about 95% lower than the values for loose soils.