Characterization of reactive oxygen species-involved oxidative damage in Hapalosiphon species crude extract-treated wheat and onion roots

Crude extract-induced oxidative damage using the cyanobacterium, Hapalosiphon sp., was investigated in wheat seedlings ( Triticum aestivum L. cv. Norin 61) and onion seedlings ( Allium cepa L. cv. Raputa II). The analysis of root cell viability or cell death using Evans blue uptake showed that the root-tip cells of wheat and onion lost viability after 24 h and 48 h treatment with 3 g dry weight (DW) L⁻¹ of the crude extract, respectively. Lipid peroxidation was induced in the roots of both species and the shoots of onion, whereas no increase in lipid peroxide formation was observed in the wheat shoots. In onion, the degree of random DNA fragmentation increased with the increasing concentration of the extract and laddering of the DNA was observed with 6 g DW L⁻¹ of the extract, but no apparent DNA ladder formation occurred in the wheat. Pretreatment for 1 h with the NADPH oxidase inhibitors, diphenyleneidonium or imidazole, reduced the crude extract-induced root cell death in both species. From the results, we suggest that the Hapalosiphon sp. crude extract might enhance reactive oxygen species (ROS) production, which causes membrane lipid damage and fragmentation of the DNA of plant cells, resulting in cell death and growth inhibition. The crude extract-mediated phytotoxic damage might be caused by ROS, triggered by NADPH oxidase.     

Inhibitory effects of aqueous extracts of black mustard on germination and growth of alfalfa

The present study investigated the allelopathic effects of various black mustard ( Brassica nigra L.) plant parts on alfalfa germination and seedling growth. Aqueous extracts of black mustard ( Brassica nigra L.) leaf, stem, flower and root plant parts were made to determine their effects on germination and dry weights of hypocotyl, and radicle length of 5-day-old alfalfa seedlings over a range of extract concentrations. All aqueous extracts significantly inhibited alfalfa seed germination and seedling growth when compared with the distilled water control. Increasing the aqueous extract concentrations of separated Brassica nigra plant parts significantly inhibited alfalfa germination, seedling length and weight. Based on significant radicle length reactions to aqueous extracts, the toxicity may be classified in the following order of decreasing inhibition: flower, leaf, mixture of all plant parts, root, and stem. Increasing the concentration of aqueous flower extracts significantly inhibited the water uptake by germinated alfalfa seeds. The greatest inhibition in water uptake occurred at a 12 g kg⁻¹ extract concentration for seeds soaked for 8 h.     

Crude extract of the cyanobacterium, Hapalosiphon sp., causes a cessation of root elongation and cell division in several plant species

The phytotoxic activities of the crude extract from the cyanobacterium, Hapalosiphon sp., on the initial growth and root cell division of several plant species were investigated. Although the germination percentages of the plants were not affected by the extract, their root and shoot growth were remarkably suppressed, depending on the extract concentrations. Roots were more sensitive to the extract than shoots and the sensitivity, determined by the I₅₀ concentrations between the susceptible and tolerant species, differed by 5 and 11 times in dicotyledonous and monocotyledonous plants, respectively. The mitotic index for wheat ( Triticum aestivum L. cv. Norin 61) root cells was not affected, but the mitotic index for onion ( Allium cepa L. cv. Raputa II) was reduced with the increasing concentration of the extract. In the onion root meristem, the number of cells in prophase, metaphase, anaphase, and telophase were decreased significantly with 6 g DW L⁻¹ (g dry weight of algae L⁻¹). This suggests that the extract might inhibit the processes regulating the entry of cells into mitosis.     

Litter-mediated allelopathic effects of kudzu (Pueraria montana) on Bidens pilosa and Lolium perenne and its persistence in soil

This study investigated the allelopathic effects of kudzu litter on the seed germination and early growth of Bidens pilosa and Lolium perenne . The bioassays, with various concentrations (10, 20, 30, 40, and 50 g L⁻¹) of aqueous kudzu litter leachate, significantly affected the germination percentages and radicle growth of both species. These parameters decreased progressively when the seedlings were exposed to increasing concentrations of leachate. The root and shoot length, dry weight, and chlorophyll concentration of the B. pilosa and L. perenne seedlings also were significantly affected when they were grown in leachate-amended soil. However, the chlorophyll fluorescence values of the seedlings subjected to different concentrations of aqueous kudzu litter leachate indicated that the mere presence of leachate in the soil was not stressful for the plants. The seedlings of both species were under stressful conditions only when they were grown in the soil that was treated with higher concentrations of leachate (30, 40, or 50 g L⁻¹). A 6 week decomposition study of the kudzu litter in soil was conducted to observe the retention of phenolic content in the soil. The results showed that, although the concentration of dissolved organic carbon decreased with increasing decomposition, the phenolic concentration was not significantly affected when the observation period ended. This suggests that the phytotoxic properties of kudzu litter remain stable in soil systems for a considerable amount of time after incorporation into the soil.     

Reduced rates of tank mixtures for red sprangletop (Leptochloa chinensis [L.] Nees) and greater club-rush (Scirpus grossus [L.] f.) control in rice

A study on the efficacy of tank-mix combinations on the growth of Leptochloa chinensis (L.) Nees and Scirpus grossus (L.) f. at the 3–4-leaf stage was conducted under glasshouse conditions. Sethoxydim plus cyhalofop-butyl at 8 + 25, 16 + 25, 32 + 25, 32 + 12.5, and 32 + 6.25 g ai ha⁻¹ provided good control of L. chinensis , ranging from 89–95%. However, a comparison of the costs of herbicide mixtures revealed that sethoxydim at 32 g ai ha⁻¹ plus cyhalofop-butyl at 6.25 g ai ha⁻¹ were the most cost-effective among the combinations. For Scirpus grossus , bensulfuron plus 2,4-D dimethylamine at 2.5 + 18, 5.0 + 18, 1.25 + 36, 2.5 + 36, and 5.0 + 36 g ai ha⁻¹ gave good control that ranged from 88–93%. Tank-mixing bensulfuron with 2,4-D at 1.25 + 36 g ai ha⁻¹ was, however, the most cost-effective mixture compared to the other mixtures. None of the tank mixtures tested in this study caused injury to the rice seedlings. However, further field studies need to be undertaken to verify these findings.     

Control of Penicillium expansum by plant volatile compounds

Nine plant volatiles were tested for their activity in vitro and in vivo against Penicillium expansum , the cause of blue mould of pear. In vitro spore germination and mycelial growth assay showed a consistent fungicidal activity by trans -2-hexenal, carvacrol, trans -cinnamaldehyde and citral, while hexanal (-)- carvone, p -anisaldehyde, eugenol and 2-nonanone exhibited a progressively lower inhibition. trans -2-Hexenal was the best inhibitor of conidial germination [MIC (minimum inhibitory concentration) = 24·6  µ L L⁻¹; ED₅₀ = 10·2  µ L L⁻¹], while carvacrol was the best inhibitor of mycelial growth (MIC = 24·6  µ L L⁻¹; ED₅₀ = 9  µ L L⁻¹). The four most active compounds in in vitro studies were tested in vivo as fumigants against blue mould on pear cv. Conference. Best control was achieved by trans -2-hexenal vapour treatments (12·5  µ L L⁻¹) when applied over a 24-h period, beginning 24 h after inoculation. In contrast, carvacrol (12·5–200  µ L L⁻¹), and trans -cinnamaldehyde (50–400  µ L L⁻¹) were ineffective and citral (200  µ L L⁻¹) showed only slight effect.     

Plant sensitivity to atrazine and chlorsulfuron residues in a soil-free system

The sensitivity of 22 major crops, pastures and weeds from the north-east grain region of Australia to atrazine and chlorsulfuron residues was determined in a glasshouse using a soil-free bioassay system. A logistic equation was fitted to the seedling fresh weights as a function of the logarithm of herbicide concentration by non-linear regression and used to calculate the doses for 10%, 30% and 50% inhibition of seedling growth (ID₁₀, ID₃₀ and ID₅₀). The ID₅₀ for atrazine ranged from 0.03 to 0.04 mg a.i. L^–1 for Salvia reflexa Hornem. and barley to 1.47 mg a.i. L^–1 for sorghum. The ID₅₀ for chlorsulfuron ranged from 0.19 to 0.21 μg a.i. L^–1 for lucerne and snail medic to 102 μg a.i. L^–1 for wheat. Based on ID₅₀ values measured, the predicted responses of each species to a range of concentrations of atrazine and chlorsulfuron were classified into four categories ranging from no damage to severe damage. These sensitivity data will assist in planning cropping sequences in soils previously treated with atrazine or chlorsulfuron.     

Responses of otebo bean (Phaseolus vulgaris L.) to postemergence herbicides

Ontario otebo bean growers have few herbicide options available for weed management. Six field trials were conducted in Ontario, Canada, over a 2 year period (2003 and 2004) to evaluate the tolerance of otebo bean to the postemergence (POST) application of bentazon at 1080 and 2160 g ai ha⁻¹, fomesafen at 240 and 480 g ai ha⁻¹, sethoxydim at 500 and 1000 g ai ha⁻¹, quizalofop-p-ethyl at 72 and 144 g ai ha⁻¹, imazamox plus fomesafen at 25 + 200 and 50 + 400 g ai ha⁻¹, and imazamox plus bentazon at 25 + 600 and 50 + 1200 g ai ha⁻¹. All treatments, including the untreated control, were maintained weed-free during the growing season. The POST application of bentazon, imazamox plus fomesafen, and imazamox plus bentazon caused as much as 9% visual injury and reduced the plant height ≤ 12%, reduced the shoot dry weight ≤ 32%, and delayed maturity but had no adverse effect on the yield of otebo bean. Fomesafen, sethoxydim, and quizalofop-p-ethyl applied POST caused as much as 8% visual injury but this was transient and had no adverse effect on the plant height, shoot dry weight, seed moisture content, and yield of otebo bean, except for quizalofop-p-ethyl, which reduced the shoot dry weight as much as 18%. Based on these results, bentazon, fomesafen, sethoxydim, quizalofop-p-ethyl, imazamox plus fomesafen, and imazamox plus bentazon applied POST have an adequate margin of crop safety for weed management in otebo bean production in Ontario. However, care must be taken to avoid spray overlaps to prevent injury from bentazon, imazamox plus fomesafen, and imazamox plus bentazon.     

Mulched cover crops as an alternative to conventional weed management systems in vineyards

Conventional methods of weed management in vineyards rely primarily on herbicides and tillage. The desire to adopt alternatives to these methods is driven by environmental and economic reasons. Weed suppression and grape yield under mulched cover crop systems at two rainfed northern California vineyards were similar to, and at times exceeded, those under conventional tillage or herbicide management. Cover crop productivity was positively correlated with weed suppression and mulch decomposition rates and seemed to be determined primarily by location and then by cover crop type. The mulch from mowed cover crops averaged 603(± 94) gm⁻² at the two sites. Weed suppression was linked to light interception by the mulch cover for most weed species. Subterranean clover planted directly in the vine row significantly reduced weed cover where it established. The increased dominance of the perennial Convolvulus arvensis and reduction of certain annual species was indicative of species compositional changes in all treatments. Profits under the cover cropping systems exceeded those under conventional tillage and herbicide systems by €  794 ha⁻¹ averaged over the duration of the experiment at both locations.     

Susceptibilities of some aquatic ferns to paddy herbicide bensulfuron methyl

Paddy herbicides have the potential to cause adverse effects on non-target plants. Susceptibilities of some aquatic ferns ( Azolla japonica Franch. et Savat., Isoetes japonica A. Braun, Marsilea quadrifolia L. and Salvinia natans All.) and duckweeds ( Lemna minor L. and Spirodela polyrhiza Schleid.) to paddy herbicide bensulfuron methyl (BSM) were evaluated with a 20 day exposure experiment using 200 cm² pots. The BSM concentrations in the surface water of monitoring pots with no plants dissipated exponentially with half lives of 3.5 and 3.9 days at application rates of 15 and 150 g ha⁻¹, respectively. The BSM concentrations in the surface water 1 day after application in the culture pots were comparable among plant species, and were lower than those in the monitoring pots. Bensulfuron methyl reduced the plant growth in all species. I . japonica showed the lowest intrinsic relative growth rate (RGR) and the lowest susceptibility with an effective dose resulting in 50% growth inhibition (ED₅₀) of 21 g ha⁻¹. Except for I . japonica , the RGR of the duckweeds was similar to the ferns, and ED₅₀ for the duckweeds was higher than the ferns. ED₅₀ for Sa . natans , A . japonica and M . quadrifolia were 1.1, 1.8 and 1.2 g ha⁻¹, respectively, which were smaller than 1/20 of the recommended field dose (51–75 g ha⁻¹) and ranged from 1/2 to 1/6 of ED₅₀ for L . minor and Sp . polyrhiza (6.5 and 3.2 g ha⁻¹, respectively). These results suggest that BSM application in paddy fields and its runoff in some localities is expected to have adverse effects on the growth of Sa . natans , A . japonica and M . quadrifolia .     

Characterization of aryl acylamidase activity from propanil-resistant barnyardgrass (Echinochloa crus-galli [L.] Beauv.)

The enzyme, aryl acylamidase, was characterized in propanil-susceptible and propanil-resistant barnyardgrass with respect to kinetic parameters, the effects of inhibitors, and the levels of activity in dark- and light-grown tissues. The enzyme reaction in the resistant tissue preparation proceeded linearly with time over a 5 h time course, while activity in the susceptible tissue preparation was 2- to 4-fold lower and the activity tended to decrease after 2 h. The apparent K_m values were 62.1 mmol L⁻¹ and 3.1 mmol L⁻¹ for the enzyme activity in the susceptible and resistant tissue preparations, respectively. Two herbicides (anilofos and piperophos), previously shown to synergize propanil injury against the resistant biotype, were found to be potent inhibitors of the in vitro aryl acylamidase activity.     

Enhanced effect of thiobencarb on bispyribac-sodium control of Echinochloa phyllopogon (Stapf ) Koss. in California rice (Oryza sativa L.)

Bispyribac-sodium {sodium 2,6-bis[(4,6-dimethoxy-2-pyrimidinyl)oxy] benzoate} has recently been introduced to California where it effectively controls Echinochloa spp. in rice ( Oryza sativa L.). However, biotypes of early watergrass ( Echinochloa oryzoides (Ard.) Fritsch) and late watergrass ( E. phyllopogon (Stapf ) Koss.) have evolved resistance to this herbicide. In 2001 and 2002, greenhouse and field experiments evaluated interactions between thiobencarb { S -[(4-chlorophenyl) methyl] diethylcarbamothioate} and bispyribac-sodium on resistant (R) and susceptible (S) late watergrass in California rice. Synergism was assessed using Colby's test and regression analysis. In the greenhouse, thiobencarb at 4480 and 5333 g ai ha⁻¹ synergistically reduced bispyribac-sodium GR₅₀ values on the R and S biotypes by 50–70% without increasing toxicity to rice. Synergism was also observed on S late watergrass in the field when 10 g ai ha⁻¹ bispyribac-sodium was mixed with 1120–2240 g ai ha⁻¹ thiobencarb. These effects could be related to interactions between thiocarbamates and enzymes in Phase I reactions of herbicide metabolism. This synergism results in better control at lower rates allowing a reduction in weed control costs, the herbicide load on the environment and a lower selection pressure towards resistant weed biotypes.     

Red rice (Oryza sativa L.) and barnyardgrass (Echinochloa spp.) biotype susceptibility to postemergence-applied imazamox

Pot experiments were conducted to evaluate the level of imazamox tolerance in five red rice ( Oryza sativa L.) and four barnyardgrass (three Echinochloa crus-galli (L.) Beauv. and one Echinochloa oryzoides (Ard.) Fritch) morphologically distinct biotypes collected from rice fields in northern Greece. The susceptibility of barnyardgrass biotypes to propanil was also studied. Red rice biotypes were not controlled by imazamox applied at 40 g ha⁻¹. In contrast, 80 g imazamox ha⁻¹ provided 56–84% red rice control (averaged across shoot number and fresh weight reduction). Not all barnyardgrass biotypes were susceptible to imazamox applied postemergence. However, propanil applied at 2.6 kg ha ⁻¹ controlled the E. crus-galli biotypes well, but propanil applied at rates of 2.6 and 5.2 kg ha ⁻¹ was not effective in reducing the shoot number and fresh weight of the E. oryzoides biotype. Propanil applied at 10.4 kg ha ⁻¹ reduced the shoot number and fresh weight of this biotype by 78 and 85%, respectively. In most cases, a linear equation ( y  = % of control, x  = g ha⁻¹) provided the best fit for regressions between red rice or barnyardgrass shoot number or fresh weight and imazamox rates. The results of this study suggest that postemergence application of imazamox is not effective against all red rice and barnyardgrass biotypes found in the rice fields of Greece and that significant variability regarding herbicide efficacy among biotypes might exist.     

Plant growth inhibitors: Patchoulane-type sesquiterpenes from Cyperus rotundus L.

Two sesquiterpene ketones, cyperotundone and α-cyperone, were isolated from dried tubers of purple nutsedge ( Cyperus rotundus L.) as major constituents: ≈0.26% and 0.1% of dried tuber, respectively. These allelochemicals affect plant growth, but we consider that these terpenoids undergo modification when they are released into the rhizosphere from the donor plant. For the structure–activity relationship study, cyperotundone was oxidized with selenium dioxide in acetic acid to 4-patchoulene-2,3-dione and 4-patchoulene-2,3,6-trione. Subsequent hydrogenation of 4-patchoulene-2,3-dione and 4-patchoulene-2,3,6-trione gave hydroxylated derivatives, cyperotundon-2-ol and 3-hydroxy-4-patchoulene-2,6-dione, respectively. 4-Patchoulene-2,3-dione inhibited the hypocotyl growth of lettuce seedlings but promoted radicle elongation at 0.1−2 mg L⁻¹ concentration without chlorosis. Moreover, this radicle elongation recovered with the addition of galactose and was not antagonized by gibberellin A₃. The effect of 4-patchoulene-2,3,6-trione showed a similar chlorosis, caused by 3,6,9-sugetriol triketone, against lettuce seedlings. These ketones did not show the radicle elongation.     

A rapid reliable test for screening aryloxyphenoxy- propionic acid resistance within Alopecurus myosuroides and Lolium spp. populations

A reliable seedling bioassay was developed and tested for the rapid screening for resistance to aryloxyphenoxypropionic (APP) herbicides in Alopecurus myosuroides and Lolium spp. populations. It is based upon the difference in coleoptile length of resistant and susceptible A. myosuroides and Lolium seedlings, respectively, exposed to fenoxaprop-P acid and diclofop acid solution for 6 days in a plastic box. A 6 mg L⁻¹ fenoxaprop-P acid solution was selected as the best concentration for a reliable screening of resistant biotypes within A. myosuroides populations. At this concentration, coleoptile lengths of susceptible and resistant seedlings were shorter and longer than 10 mm respectively. Similarly, resistant seedlings within Lolium populations were easily detected at 10 mg L⁻¹ diclofop acid. At this concentration, coleoptile lengths of susceptible and resistant seedlings were shorter and longer than 20 mm respectively. For both populations, the coleoptile length distributions appear to discriminate between two kinds of APP-resistant biotypes (highly and slightly resistant).     

Response of winter wheat (Triticum aestivum L.) and weeds to tank mixtures of 2,4-D plus MCPA with clodinafop propargyl

Field and greenhouse experiments were conducted in 2004 and 2005 to study weed control and the response of winter wheat to tank mixtures of 2,4-D plus MCPA with clodinafop propargyl. The field experiments were conducted at Yazd and Oroumieh, Iran, with factorial combinations of 2,4-D plus MCPA at 0, 975, and 1300 g ai ha⁻¹ and with clodinafop propargyl at 0, 64, 80, 96, and 112 g ai ha⁻¹ in four replications. The greenhouse experiments further evaluated the effect of these tank mixtures on weed control, where each herbicide mixture was considered as one treatment and the experiment was established in a randomized complete block design with four replications. In the field experiments, the herbicides were applied at wheat tillering, while in the greenhouse experiments they were applied at the beginning of the tillering stage and at the four-leaf stage of the grass and broadleaf weeds, respectively. The results indicated antagonistic effects between 2,4-D plus MCPA and clodinafop propargyl. The best tank mixture with regard to weed control efficacy was 2,4-D plus MCPA at 975 g ai ha⁻¹ with clodinafop propargyl at 96 g ai ha⁻¹. The wheat grain yield was also increased by the tank mixture of clodinafop propargyl with 2,4-D plus MCPA. Generally, to inhibit clodinafop propargyl efficacy reduction due to tank-mixing with 2,4-D plus MCPA, it is recommended that the application dose of 64 g ai ha⁻¹ should be increased to 96 g ai ha⁻¹.     

New weed management strategy using subterranean clover reseeding under different tillage systems: Numerical experiments with the subterranean clover-tillage dynamics model

Subterranean clover ( Trifolium subterraneum L.) appears to be a suitable winter legume cover crop for Japan because the subterranean clover stands maintain themselves by reseeding in autumn and the large seeds facilitate establishment and enable early fall production. However, there is little information on the relationship between reseeding and tillage systems for subterranean clover under Japanese climatic conditions. A weed-tillage population dynamics model was developed to investigate the effects of the tillage method and timing on the reseeding of subterranean clover. The field experiments were conducted in a silage corn and subterranean clover rotation system. The life cycle of subterranean clover was modeled to describe the seed production competing with weed growth, seasonal changes in the buried seeds' viability, and the movement of seeds by tillage treatments. In the numerical simulation, the effect of the tillage method on the seedling population of reseeded subterranean clover was investigated and the simulation results showed good agreement with the experimental results. Rotary tilling immediately after subterranean clover seed maturation successfully produced a good subterranean clover stand the following spring. However, rotary tillage conducted 2 months after seed maturation killed the emerging subterranean clover seedlings and the field was dominated by winter weeds. These simulation results suggest that a suitable tillage system can maintain successful subterranean clover re-establishment from year to year.     

Management of late leaf spot of groundnut (Arachis hypogaea) with chlorothalonil-tolerant isolates of Pseudomonas aeruginosa

Fifteen groundnut-associated bacterial isolates that inhibited by > 90% the in vitro conidial germination of Phaeoisariopsis personata , causal agent of late leaf spot disease of groundnut, were applied as a prophylactic spray (10⁸ cfu mL⁻¹) and tested for control of the disease in the glasshouse. Two groundnut seed-associated bacterial isolates, GSE 18 and GSE 19, identified as Pseudomonas aeruginosa , reduced the lesion frequency (LF) by up to 70%. A 90-day-old peat-based formulation of P. aeruginosa GSE 18 reduced LF measured 15 days postinoculation by up to 60%. Both P. aeruginosa GSE 18 and GSE 19 were tolerant to chlorothalonil (Kavach®) up to 2000  µ g mL⁻¹ in LB broth. In glasshouse trials, GSE 18 and GSE 19 tested in combination with reduced concentrations of chlorothalonil were highly efficient in management of the disease. The disease was completely controlled by chlorothalonil (> 250  µ g mL⁻¹), and in the presence of GSE 18 or GSE 19, 100  µ g mL⁻¹ chlorothalonil was equally effective. Application of rifamycin-resistant mutants of GSE 18 or GSE 19 together with chlorothalonil significantly increased the survival of these isolates in the groundnut phylloplane. In the field, a combination of GSE 18 and 500  µ g mL⁻¹ chlorothalonil reduced disease severity comparable to 2000  µ g mL⁻¹ chlorothalonil alone. Use of chlorothalonil-tolerant pseudomonads together with a quarter concentration of the recommended field dose of chlorothalonil doubled pod yield compared with the untreated unsprayed control.     

Control of weed barley species in winter wheat with sulfosulfuron at different rates and times of application

Field experiments were conducted at five locations in the major wheat production regions of Iran to evaluate the efficacy of sulfosulfuron in controlling weed barley species (including Hordeum spontaneum , Hordeum murinum , Hordeum distichon , and Hordeum vulgare ) in the 2004–2005 and 2005–2006 growing seasons. Sulfosulfuron was applied either postemergence (POST) or preplant-incorporated (PPI) at 0, 20.25, 30.75, 40.5, 51.0, 60.75 or 71.25 g ai ha⁻¹ to plots arranged in a randomized complete block design with four replications. Sulfosulfuron at the recommended rate (20.25 g ai ha⁻¹) failed to provide acceptable control of the weed barley species. However, the level of control increased with the application rate, particularly at rates >51.0 g ai ha⁻¹. Generally, PPI-applied sulfosulfuron resulted in markedly greater control levels than those of a POST application and complete control of H. murinum and H. vulgare was achieved with PPI-applied sulfosulfuron at all rates >20.25 and 30.75 g ai ha⁻¹, respectively. In most cases, the wheat yield increased with the application rate without any crop injury. The highest yield increase (186%) was obtained with a PPI application of 71.25 g ai ha⁻¹.     

A semiselective medium for the isolation of Acidovorax valerianellae from soil and plant debris

Bacterial black spot of corn-salad ( Valerianella locusta ) caused by Acidovorax valerianellae appeared in France in the beginning of the 1990s. The disease is now widespread in the main area of corn-salad production and is responsible for high depreciation and significant economic losses every year. A semiselective medium is described for the detection of the bacterium in various environments and to identify primary sources of inoculum. The medium TSAV (tryptic soy broth, 3 g L⁻¹, agar 15 g L⁻¹, 5-fluorouracyl 5 mg L⁻¹, novobiocin 5 mg L⁻¹, propiconazole 5 mg L⁻¹) increased chances of obtaining cultures of A. valerianellae from naturally infested soil and from root-debris. Using TSAV, the pathogen was detected in root-debris up to 39 days after the harvest of a diseased crop. As only a few days usually separates harvest from the next sowing date, soil is likely to be an important source of inoculum for the next crop.