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E. Lucienne Mansvelt and F.J. Calitz

`Packham's Triumph' (Pyrus communis L.) pear blossoms were inoculated with Pseudomonas syringae pv. syringae van Hall upon emasculation and then hand-pollinated at different times to assess the effects of bacterial colonization on fruit set. The pathogen significantly reduced fruit set by 24%. No lesions developed on pear flowers at any stage of the trials.

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Elzbieta Krzesinska and Anita Nina Miller

An excised twig assay was developed to evaluate cherry genotypes for their tolerance to Pseudomonas syringae pv. syringae. One-year-old wood was collected at monthly intervals from Oct. until Jan. of `Napoleon', `Corum', and a number of cherry rootstock. The rootstock included F/12-1, Giessen (GI), and M×M selections. Twigs were inoculated with one avirulent and three virulent strains. Evaluation of incision browning, callus, and gummosis production were made after incubation for 3 weeks. Based on gummosis is and browning ratings, all the rootstocks tested were found to be more tolerant than `Napoleon' and `Corum' to the 3 strains of Pseudomonas syringae used. No gummosis or browning was observed on twigs inoculated with water or the avirulent strain.

Plant signals extracted from cherry leaves have been shown to control expression of virulence genes in P. syringae. Crude aqueous extracts from `Napoleon' twigs induced the syrB::1acZ fusion in P. syringae strain B3AR132 Other rootstocks are currently being evaluated for their ability to induce virulence in P. syringae pv. syringae,

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Elzbieta Krzesinska and Anita Nina Miller

An excised twig assay was developed to evaluate cherry geno-types for their tolerance to Pseudomonas syringae pv. syringae. One-year-old wood was collected at monthly intervals from October until January of `Royal Ann', `Corum', and a number of cherry rootstock. The rootstock included; F/12–1 and Giessen (GI) and M × M selections. A 2-cm incision (“^”-shaped flap) was made on each twig. A 20-μl droplet of inoculum or water was placed onto each incision. The inoculum was prepared with one avirulent (K4) and three virulent strains (W4N54, AP2, B15) concentrations (105, 106, or 107 cfu). Inoculated twigs were placed in test tubes and incubated at 15C in high relative humidity for 3 weeks. After incubation, twigs were evaluated for gummosis production (0–3, 0 = no gummosis), incision browning (1–4, 1 = yellow pith), and callus production (0–1, 0 = no callus). The concentration of bacterial suspension had no effect on symptom development. No gummosis or browning was observed on twigs inoculated with water or the avirulent strain. Based on the gummosis and browning ratings, rootstock M × M 2, M × M 39, M × M 60, GI 148-1, GI 154-2, and GI 154-4 were found to be resistant to these three strains of P. syringae in this assay. Root-stock F 12-1, GI 169–15, GI 172–9, and GI 173-9 were found to be tolerant.

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Elzbieta Z. Krzesinska, Anita N. Azarenko, and Dennis C. Gross

The syrB gene required by Pseudomonas syringae pv. syringae van Hall to produce the phytotoxin syringomycin is activated by plant signal molecules. Extracts from twigs of 12 cherry (Prunus) genotypes were tested for their ability to induce syrB::lacZ fusion in P. syringae pv. syringae strain B3AR132 to determine whether signal activity is correlated with susceptibility to bacterial canker. One-year-old twigs of `Napoleon', `Corum', and 12 cherry rootstocks (F12/1, `Colt', M×M2, M×M39, M×M60, Gi 148-1, Gi 148-9, Gi 154-2, Gi 154-5, Gi 169-15, Gi 172-9, and Gi 173-9) were tested at concentrations of 0.2, 1.0, and 2.0 mg twig dry weight/ml solution for their ability to induce syrB::lacZ fusion, as measured by β -galactosidase activity. Extracts from all cherry genotypes induced syrB::lacZ fusion, but to varying degrees. The highest β -galactosidase activity was observed in `Napoleon' and `Corum'-the most susceptible genotypes; activities were two to four times higher than that of F12/1, a disease-resistant genotype. Activities higher than that of F12/1 were induced at the lowest extract concentration by rootstocks M×M60, Gi 148-1, Gi 148-9, and Gi 154-5, whereas rootstocks M×M2, M×M39, Gi 154-2, Gi 172-9, Gi 173-9, and `Colt' were not significantly different from F12/1. At the two highest extract concentrations tested, only `Napoleon' and `Corum' consistently had higher induct&m activity than F12/1. At high extract concentrations, interfering substances seemed to suppress or antagonize the induction of syrB::lacZ fusion. These results suggest that susceptible genotypes contain higher signal activities than resistant genotypes.

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F.A. Hammerschlag

A detached-leaf bioassay was used to evaluate peach [Prunus persica (L.) Batsch] somaclone 122-1 (derived from callus produced on an immature embryo of peach cultivar Redhaven) for resistance to several virulent strains of Xanthomonas campestris pv. pruni [E.F. Sm.) Dows], causal agent of bacterial leaf spot, and to a virulent isolate of Pseudomonas syringae van Hall pv. syringae, causal agent of bacterial canker. The detached-leaf bioassay was also used to evaluate progeny of 122-1 for resistance to X. campestris pv. pruni virulent strain XP1. Somaclone 122-1 was significantly more resistant to most strains of X. campestris pv. pruni than was `Redhaven', and all of its progeny exhibited high levels of resistance to X. campestris pv. pruni strain XP1. Somaclone 122-1 exhibited significantly higher levels of resistance to Pseudomonas syringae pv. syringae than did `Redhaven' and this resistance was retained over time in the greenhouse and following a 2-year cycle of tissue culture propagation.

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Elzbieta Z. Krzesinska and Anita Nina Miller Azarenko

Au excised twig assay was developed to evaluate cherry rootstocks (hybrids of Prunus avium L.; P. canescens Bois.; P. cerasus L.; P. fruticosa Pall.; P. mahaleb L.; P. pseudocerasus Lindl.) for their tolerance to Pseudomonas syingae pv. syringae van Hall. Twigs of `Napoleon', `Corum', and F12/1 in 1988 and 10 more rootstocks in 1989 were inoculated with water, one avirulent (K4), or one of three virulent strains (W4N54, AP1, and B-15) of bacteria at 105, 106, and 107 colony forming units (cfu)/ml in 1988 and with 107 cfu/ml in 1989. Evaluation of browning and gummosis at the inoculation site after incubation for 4 weeks at 15C and high relative humidity revealed no gummosis or browning on twigs inoculated with water or the avirulent strain. The amount of browning and gummosis induced by concentrations of 106 and 107 cfu/ml of the virulent strains was, in general, not different within genotypes. `Napoleon' and `Corum' had significantly higher browning and gummosis ratings in 1989 than F12/1 and the 10 rootstock selections, most of which did not differ from F12/1. Rootstocks Gisela (Gi.) 172-9 and Gi. 169-15 had higher incision browning than F12/1 in some instances.

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R.R. Tripepi and M.W. George

Seeds of `Berken' mung bean [Vigna radiata (L.) R. Wilcz.] were surface-sterilized with NaOCl and then either aerated 24 hours before sowing (routine procedure), planted immediately after the NaOCl treatment, or treated with hot cupric acetate and antibiotics before planting. Nine- or 10-day-old seedlings were used in rooting bioassays. Up to 10% of the seedlings and 17% of the cuttings had collapsed upper stems or wilted leaves. None of the seed treatments completely eliminated the pathogen, but the combination of hot cupric acetate plus antibiotics reduced the quantity of diseased cuttings to 3.3%. A white and two yellow-pigmented (Y1 and Y2) bacteria were isolated from diseased cuttings and used in subsequent pathogenicity tests. The Y2 strain was nonpathogenic. Stems of plants inoculated with the white strain turned brown and collapsed 2 days after inoculation, whereas leaves of plants inoculated with the Y1 strain wilted after 7 days. Electron microscopy, fatty acid analysis, and standard biochemical and physiological tests were used to identify the white strain as Pseudomonas syringae pv. syringae van Hall and the Y1 strain as Curtobacterium flaccumfaciens ssp. flaccumfaciens (Hedges) Collins and Jones. These results emphasize that seeds of mung bean should be checked for seedborne pathogens to avoid experimental artifacts.

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Peter L. Sholberg, Paul Randall, and Cheryl R. Hampson

Acetic acid (AA) fumigation of rootstocks and dormant shoots was explored as a method of eliminating plant pathogens from propagation material. Dormant shoots were tested in early winter to determine the rate of AA vapor that they could tolerate before being damaged. Apricot (Prunus armeniaca), apple (Malus ×domestica), and peach (Prunus persica) shoots collected from a single site in Dec. 1999 tolerated 30, 12, or 6 mg·L–1 AA, respectively. Vineland 3 (V3) and Malling-Merton 106 (MM.106) rootstock liners fumigated with 1 mg·L–1 AA were adequately surface-sterilized although the effect on growth was not recorded. A similar experiment with Malling 9 (M9) rootstocks showed that 12 mg·L–1 AA would eliminate most surface microorganisims from roots although it delayed shoot growth when the trees were planted. The higher 15 mg·L–1 rate delayed tree growth and appeared to kill some trees. The 12 mg·L–1 rate prevented growth of Erwinia amylovora and Pseudomonas syringae pv. syringae bacteria on shoots even when an enrichment technique was used to detect them. Finally, when 96 `Jonagold' apple shoots known to be infected by Podosphaera leucotricha were fumigated with AA in 2001, none developed powdery mildew, although 99% of the control shoots did. These promising results suggest that further research should be done toward adapting AA fumigation for use by commercial nurseries.

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Freddi A. Hammerschlag, Ghazala Hashmi, Robin Huettel, Dennis Werner, and David Ritchie

One approach for obtaining useful genetic variation is to select for somaclonal variants generated by tissue culture techniques. Increased levels of resistance to bacterial leaf spot (Xanthomonas campestris pv. pruni) have been observed in toxin-selected and unselected peach regenerants in vitro, in the greenhouse and under field conditions. Peach regenerants have also demonstrated increased levels of bacterial canker (Pseudomonas syringae pv. syringae) and root-knot nematode (Meloidogyne incognita) resistance. Random amplified polymorphic DNA (RAPD) primers have been used to study genetic variation at the DNA level among the somaclonal variants. Sixty RAPD primers (10-mers) were screened and 10 proved useful as markers to detect polymorphisms, thus establishing a genetic basis for somaclonal variation. These studies demonstrate the feasibility of using tissue culture techniques to generate fruit trees with increased levels of disease resistance.

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Gregory L. Reighard, Eldon I. Zehr, and Freddi Hammerschlag

Peach tree short life (PTSL) is a serious peach tree disease syndrome on replant orchard sites in the Southeast. Pseudomonas syringae pv. syringae is a bacterial disease often associated with tree injury and death on these PTSL sites. Rootstocks that have better tolerance to ring nematodes such as Lovell have less PTSL death. Tissue-cultured peach embryos and/or explants have shown increased resistance to Pseudomonas syringae and Xanthomonas campestris pv. pruni, another bacterial peach pathogen, in laboratory and greenhouse screenings. Tissue-cultured `Redhaven' (RH), `Redskin' (RS), and `Sunhigh' (SH) peach cultivars on their own roots were planted with SH seedlings and RH and RS budded to Lovell rootstock on a severe PTSL site in South Carolina. Treatments beside cultivar/rootstock combination included preplant fumigation vs. nonfumigation. PTSL appeared in the third year and by year 4 significant tree death occurred. Tissue-cultured RH, RS, and SH trees had 54%, 55%, and 88% PTSL death, respectively, compared to RH (17%) and RS (29%) on Lovell or the SH seedlings (25%). Fumigation significantly decreased PTSL in both RS combinations but not RH. These data suggest that the tolerance of the cultivar root system to PTSL-inducing factors such as ring nematodes was more important in PTSL than scion resistance to bacteria.