Leaf Wilt Research Articles

Cold stress tolerance mechanisms in plants. A review

The human population is increasing at an alarming rate, whereas at the same time agricultural productivity is decreasing due to the effect of various environmental problems. In particular, cold stress is a serious threat to the sustainability of crop yields. Indeed, cold stress can lead to major crop losses. Various phenotypic symptoms in response to cold stress include poor germination, stunted seedlings, yellowing of leaves (chlorosis), reduced leaf expansion and wilting, and may lead to death of tissue (necrosis). Cold stress also severely hampers the reproductive development of plants. The major negative effect of cold stress is that it induces severe membrane damage. This damage is largely due to the acute dehydration associated with freezing during cold stress. Cold stress is perceived by the receptor at the cell membrane. Then a signal is transduced to switch on the cold-responsive genes and transcription factors for mediating stress tolerance. Understanding the mechanism of cold stress tolerance and genes involved in the cold stress signaling network is important for crop improvement. Here, I review cold stress tolerance mechanisms in plants. The major points discussed are the following: (1) physiological effects of cold stress, (2) sensing of cold temperatures and signal transduction, and (3) the role of various cold-responsive genes and transcription factors in the mechanism of cold stress tolerance.

Identification and characterization of the Enterobacter complex causing mulberry (Morus alba) wilt disease in China

Mulberry wilt disease (MWD) was recently identified in Hangzhou, Zhejiang province, China. Typical symptoms of the disease are browning of vascular tissues, leaf wilt, defoliation, and tree decline. Unlike the symptoms of bacterial wilt disease caused by Ralstonia solanacearum, symptoms of MWD generally started from the bottom of the plants and moved upward. In inoculation experiments, four selected MWD strains caused mulberry shoot leaf wilt, discoloration, and defoliation. They also induced whole plant leaf wilt, defoliation and dark brown discoloration of vascular tissue. Based on Biolog metabolic profiles, fatty acid methyl ester analysis (FAME) and sequence analysis of the partial 16S rDNA and rpoB genes four MWD strains were identified as members of the genus Enterobacter. The 16S rDNA and rpoB gene sequences revealed a close relationship among two isolates, R2-2 and R6-2, and the E. asburiae type strain JCM6051. The isolates showed >98% similarity to E. asburiae JCM6051 in their rpoB gene. These results indicated that isolates R2-2 and R6-2 belonged to E. asburiae. No similarity in 16S rDNA sequences above 97% was found between either of the remaining isolates, R11-2 or R18-2, and any recognized Enterobacter species, suggesting that the two isolates may represent novel Enterobacter species. rpoB gene similarity values between the isolates and Enterobacter spp. type strains were <98%, providing further evidence that the two isolates may represent a novel species within the Enterobacter. The causal agent for MWD was previously reported to be E. cloacae, however, this study found that other Enterobacter spp. (E. asburiae and Enterobacter sp.) also cause MWD.

Phytophthora taxon niederhauserii, a New Root and Crown Rot Pathogen of Banksia spp. in Italy.

In the last 10 years, various species of Banksia (family Proteaceae) endemic to Australia have been introduced into Italy where cultivation as flower plants is expanding. In the spring of 2003, a decline associated with root and basal stem rot of 2- to 3-year-old plants of Banksia speciosa R. Br., B. baxteri R. Br., and B. prionotes Lindl. grown in the ground was observed in a commercial nursery in Liguria (northern Italy). Aboveground symptoms included leaf chlorosis and wilt. Plants collapsed within 1 to 2 weeks after the appearance of leaf symptoms. A Phytophthora species was consistently isolated from roots and basal stem on BNPRAH selective medium (3). On V8 juice agar (V8A), axenic cultures obtained by single hyphal transfers formed stellate to radiate colonies with aerial mycelium; on potato dextrose agar (PDA). the colonies showed stoloniform mycelium. Minimum and maximum growth temperatures on PDA and V8A were between 5 and 10°C and 38 and 40°C, respectively, with the optimum at 30°C on PDA (mean radial growth rate of 10 isolates ranged between 9.3 and 10.2 mm per day) and 25 to 30°C on V8A (14 mm per day). In saline solution and soil extract, all isolates produced catenulate hyphal swellings and ellipsoid, nonpapillate, persistent sporangia. Sporangia in saline solution varied from 47 to 70 × 30 to 44 μm (mean l/b ratio of 1.5). All isolates were A1 mating type and produced oogonia with amphyginous antheridia when paired with A2 mating type of P. drechsleri Tucker on V8A plus β-sytosterol (3). The electrophoretic patterns of total mycelial proteins and two isozymes (esterase and malate dehydrogenase) (2) of all isolates from Banksia plants were identical, but distinct from the patterns of isolates of other Phytophthora species, including P. drechsleri, P. megasperma sensu stricto, and P. sojae. Internal transcribed spacer (ITS) regions of rDNA were amplified with primers ITS4/ITS6 and sequences of two isolates, IMI 393960 from B. speciosa and 466/03 from B. baxteri (GenBank Nos. FJ648808 and FJ648809), were 100% identical to sequences of isolates identified as Phytophthora taxon niederhauserii Z. G. Abad and J. A. Abad (GenBank Nos. AY550916, AM942765, and EU244850). Pathogenicity tests were performed on 1-year-old potted plants of B. speciosa with isolates IMI 393960 and 466/03. Twenty plants per each isolate were transplanted into 12-cm-diameter pots containing infested soil prepared by mixing steam-sterilized sandy loam soil with 1% of inoculum produced on autoclaved wheat kernels. Twenty control plants were grown in autoclaved soil mix. Plants were kept in the greenhouse with natural light at 25 ± 2°C and watered to field capacity weekly. All Banksia plants transplanted in infested soil showed symptoms of wilt, leaf chlorosis, and basal stem rot within 2 to 3 weeks. Noninoculated plants remained healthy. P. taxon niederhauserii was reisolated solely from inoculated plants. P. taxon niederhauserii has been reported recently from Banksia spp. in Australia (1), but to our knowledge this is the first report from Italy. P. taxon niederhauserii may represent a threat to the cultivation of many ornamentals including Cystus spp., English ivy, and laurel (4).

Callistemon citrinus and Cistus salvifolius, Two New Hosts of Phytophthora taxon niederhauserii in Italy.

Bottlebrush (Callistemon citrinus (Curtis.) Skeels., Myrtaceae) and rock rose (Cistus salvifolius L., Cistaceae) are evergreen shrubs native to Australia and the Mediterranean Region, respectively. In the spring of 2003, approximately 2% of a nursery stock of 12-month-old potted plants of C. citrinus and 8% of a nursery stock of 12-month-old potted plants of Cistus salvifolius grown in the same nursery in Sicily, showed symptoms of leaf chlorosis, defoliation, and wilt associated with root and collar rot. A Phytophthora species was consistently isolated from roots and basal stems on BNPRAH selective medium (2). One isolate from rock rose (IMI 391708) and one from bottlebrush (IMI 391712) were characterized. On potato dextrose agar (PDA), the colonies showed stoloniform mycelium and irregular margins; on V8 juice agar (V8A), colonies were stellate to radiate. Minimum and maximum temperatures on PDA were 10 and 35°C, respectively, with the optimum at 30°C. Mean radial growth rate of isolates on this substrate was 9.9 and 11.3 mm/day, respectively. In saline solution (1), both isolates produced catenulate hyphal swellings and ellipsoid, nonpapillate, persistent sporangia with internal proliferations and dimensions of 52 to 70 × 30 to 42 μm and 51 to 85 × 39 to 45 μm. Mean l/b ratio of sporangia for both isolates was 1.8 ± 1. On V8A plus β-sytosterol, both isolates produced amphyginous antheridia and spherical oogonia in dual cultures with an A2 tester of P. drechsleri Tucker. Conversely, they did not produce gametangia with an A1 tester of P. cryptogea Pethybr., indicating they were A1 mating type. The internal transcribed spacer (ITS)-rDNA sequences of rock rose and bottlebrush isolates showed 100% similarity with those of two reference isolates of P. taxon niederhauserii from GenBank (Accession Nos. FJ648808 and FJ648809). On the basis of the analysis of the DNA, the species isolated from bottlebrush and rock rose were identified as Phytophthora taxon niederhauserii. Pathogenicity tests were carried out on 6-month-old potted plants of C. salvifolius and C. citrinus (10 plants of each plant species for each isolate) transplanted into pots (12 cm in diameter) containing a mixture of 1:1 steam-sterilized, sandy loam soil (vol/vol) with 4% inoculum produced on autoclaved kernel seeds. Plants were maintained at 25 to 28°C and watered to soil saturation once a week. After 2 to 3 weeks, all inoculated plants developed symptoms identical to those observed on plants with natural infections. Ten control plants transplanted into pots containing noninfested soil remained healthy. P. taxon niederhauserii was reisolated solely from inoculated plants. To our knowledge, this is the first report of P. taxon niederhauserii on C. citrinus and C. salvifolius in Italy. This Phytophthora taxon has been reported recently on rock rose in Spain (3).

Study of flagella-mediated interactions between plants and Pseudomonas syringae

The phytopathogenic bacterium P. syringae can be classified into more than 50 pathovars on the basis of their host plant species and the disease symptoms they cause. They enter apoplastic spaces of host plants via natural openings such as wounds and stomata in leaves, stems, and fruits, then multiply and cause leaf spots, necrosis, wilt and stem canker. Each P. syringae pathovar has its own host species. For example, P. syringae pv. tabaci (Pta) 6605 is the causal agent of wild fire disease in tobacco, but does not cause disease in nonhost plants such as tomato and Arabidopsis thaliana. In Gram-negative phytopathogenic bacteria, a hrp (hypersensitive reaction and pathogenicity) gene cluster is required to cause the hypersensitive reaction (HR) on nonhost plants and pathogenicity on host plants. The hrp gene cluster encodes components of the Hrp type III secretion apparatus, regulatory proteins for hrp gene expression and the harpin protein, a proteinaceous elicitor effective on nonhost plants. Harpin is encoded by hrpZ in the hrp gene cluster. We have investigated the hrpZ gene in several pathovars of P. syringae and revealed that hrpZ in strain Pta6605 has a deleterious internal deletion and frame shift, and thus does not produce active elicitor. Harpin is thought to be a major elicitor of HR in nonhost plants, and we predicted that Pta6605 has a different elicitor because Pta6605 does induce typical HR on nonhost tomato plants. By screening for active elicitor molecules in the culture supernatant of Pta6605, we identified the flagellin protein, a major component of the flagellar filament, as a strong elicitor of hypersensitive cell death in tomato.

Water Deficit Stress Responses of Three Native Australian Ornamental Herbaceous Wildflower Species for Water-wise Landscapes

Perennial wildflower species are important but not well-understood elements in water-wise landscaping that anchors urban water conservation programs in arid climates. Comparative growth and physiological responses to soil substrate drying of three herbaceous Australian ornamental species from habitats of variable moisture regimes were investigated in the context of isohydric and anisohydric behavior. Clonal Orthosiphon aristatus, Dianella revoluta ‘Breeze’, and Ptilotus nobilis plants were container-grown individually and competitively together in two separate studies. In both studies, plants were water-stressed through cyclical dry downs. We measured stomatal conductance (g S), soil water content, and water potential during each study and osmotic adjustment estimated from pressure-volume data and plant biomass at the end of each study. O. aristatus, a rainforest species, fit a general anisohydric model of high water use and more negative water potential during soil drying until stomatal closure and leaf wilting. D. revolata and P. nobilis, indigenous to Australia's dry interior, fit a general isohydric, drought-tolerant model of stomatal closure from water deficits that moderates leaf water potential but through different mechanisms. P. nobilis and D. revolata moderate water use and maintain acceptable aesthetic performance under water stress, suitable for mixed low-water landscape plantings. O. aristatus would not be suitable for low-water urban landscapes, either isolated or in mixed plantings, because of high soil water depletion and wilting.

First Report of Goss's Bacterial Wilt and Leaf Blight on Corn Caused by Clavibacter michiganensis subsp. nebraskensis in Indiana.

In August of 2008, leaves of hybrid corn (Zea mays L.) and popcorn from Pulaski and Jasper counties in northwest Indiana were submitted to the Purdue Plant and Pest Diagnostic Lab with symptoms characteristic of a bacterial disease. Symptomatic leaves had large, tan-to-gray necrotic lesions with dark freckling present within the lesions. Shiny bacterial exudate was present on the surface of many of the lesions. Microscopic observation revealed no fungal structures within the lesions, and bacterial streaming was observed from the cut edge of symptomatic tissue under ×100 magnification with phase contrast. A commercially available ELISA test (Agdia Inc., Elkhart, IN) determined that samples were negative for Pantoea stewartii, the causal agent of Stewart's bacterial leaf blight and wilt. A bacterial suspension was prepared from symptomatic tissue and streaked onto King's B medium and subcultured on semiselective CNS medium (1,2). Axenic, peach-colored colonies present on the CNS medium tested gram positive with a KOH test. Analysis of fatty acid methyl esters (MIDI Inc, Newark, DE) indicated that the strain was very similar (0.611) to Clavibacter michiganensis. Amplification of the 500-bp 16S rRNA region of the bacterial gene and subsequent BLAST alignments of the resulting sequence indicated a 99% match for C. michiganensis subsp. nebraskensis (GenBank Accession Nos. AM410697 and U09763; D16S2 gene bacterial library, version 2.10; MIDI Inc,). Koch's postulates were used to confirm pathogenicity of the isolated bacteria on corn inbred B73. Eighteen plants were mechanically inoculated at growth stage V1 to V2 with a bacterial suspension of approximately 1 × 108 CFU/ml prepared from cultures grown on CNS for 10 days at 28°C (2). Inoculum was rubbed onto leaves dusted with Carborundum and 0.1 ml of the bacterial suspension was injected into stems with a hypodermic needle. Nine control plants were inoculated with sterile water. Plants were kept at greenhouse conditions (24°C) with supplemental 400W high-pressure sodium light. Within 5 to 8 days, leaves and stems of all 18 inoculated plants developed water-soaked, necrotic lesions. No symptoms were observed in control plants. Bacteria were reisolated from symptomatic plants on CNS medium as described above, and gram-positive colonies were obtained. Reisolated strains were identical to C. michiganensis subsp. nebraskensis by D16S2 DNA sequence analysis, confirming the causal agent of the disease. Disease incidence in affected fields ranged from 20 to 60% and significant yield loss was reported. This confirmation is of regulatory importance because of potential export restrictions of Indiana-grown seed corn and popcorn to select countries. To our knowledge, this is the first report of Goss's bacterial wilt and leaf blight on corn in Indiana.

Leaf water absorption and desorption functions for three turfgrasses

Summary Plant leaf can absorb water when the leaf is in contact with water. This happens when the rainfall is intercepted by plant leaves, where the intercepted part of rain remains on the leaf surface. When the intercepted water is either absorbed or subsequently evaporated into the atmosphere, the plant leaves can dissipate water through the desorption process until the plant is dry or rewatered. In this paper, two symptomatic models in the form of exponential functions for leaf water absorption and leaf water desorption were derived and validated by experimental data using leaves of three turfgrasses (Tall fescue, Perennial ryegrass and Kentucky bluegrass). Both the models and measured data showed that the rate of leaf water absorption was high at the low initial leaf water content and then gradually leveled off toward the saturated leaf water content. The rate of leaf water desorption was high at the high initial leaf water content then decreased drastically over time toward zero. The different plant leaves showed different exponents and other parameters of the functions which indicate the difference of plant species. Both the absorption and desorption rates were relatively higher for the Kentucky bluegrass and lower for the Tall fescue and Perennial ryegrass. The concept of specific leaf area (SLA) was used to understand the saturated leaf water content (Cs) of the three turfgrasses. Linear relationships were found between Cs and SLA. The leaf water absorption and desorption functions are useful for deriving physiological parameters of the plant such as permanent wilting leaf water content, naturally irreducible leaf water content, exponential leaf water absorption coefficient, and exponential leaf desorption coefficient, as well as for evaluating the effects of rainfall interception on plant growth and water use efficiency.

Creeping Bentgrass Putting Green Turf Responses to Two Summer Irrigation Practices: Rooting and Soil Temperature

Light and frequent (LF) and deep and infrequent (DI) irrigation are two common practices for golf course managers. Few studies have compared the effects of these two opposing irrigation practices on summer root performance in creeping bentgrass (Agrostis stolonifera L.). The objectives of this field study were to quantify summer root development and longevity in response to LF vs. DI irrigation in ‘Providence’ creeping bentgrass grown on a sand‐based rootzone. The LF plots were irrigated daily to moisten the upper 4 to 6 cm of soil, whereas DI plots were irrigated at leaf wilt to wet soil to a depth ≥24 cm. Root measurements were obtained using the minirhizotron imaging technique and included total root count, total root length (TRL), total root surface area (TRSA), and average root diameter. When averaged over the entire 0‐ to 24‐cm rootzone depth, DI‐irrigated creeping bentgrass produced a greater number of roots, longer root lengths, and a larger root surface area than LF‐irrigated turf. Average root diameters were smaller in DI‐irrigated creeping bentgrass in the summer of 2007. Compared with data collected in 2006, the 2‐yr‐old turf had 55 and 32% fewer roots in LF‐ and DI‐irrigated bentgrass by September 2007, respectively. There were similar reductions in TRL and TRSA between years in both irrigation regimes. Deep and infrequent irrigation stimulated root growth throughout the 0‐ to 24‐cm rootzone in May and June and promoted root longevity in summer.

Creeping Bentgrass Putting Green Turf Responses to Two Irrigation Practices: Quality, Chlorophyll, Canopy Temperature, and Thatch–Mat

Frequent vs. infrequent irrigation are two common irrigation practices. This field study was conducted on ‘Providence’ creeping bentgrass (Agrostis stolonifera L.) grown on a sand‐based rootzone and maintained as a putting green. The objectives of this study were to measure several creeping bentgrass performance and physiological factors as influenced by light and frequent (LF) vs. deep and infrequent (DI) irrigation. The LF plots were irrigated daily to moisten the upper 4 to 6 cm of soil, while DI plots were irrigated at leaf wilt to wet soil to a depth of ≥24 cm. The LF‐irrigated creeping bentgrass exhibited very good color and quality throughout most of 2006 and 2007, whereas DI‐irrigated bentgrass exhibited acceptable quality in 2007 but not in 2006. The DI‐irrigated bentgrass had lower chlorophyll levels in leaf and sheath tissue in 2006, but developed better color and quality and had higher chlorophyll levels in late summer of 2007 vs. LF‐irrigated bentgrass. Creeping bentgrass subjected to DI irrigation developed a less thick thatch–mat layer that contained less organic matter than that found in LF‐irrigated plots. Twice the amount of water was applied to the LF‐ vs. the DI‐irrigated plots.

Response of Container-Grown Girdled Grapevines to Short-Term Water-Deficit Stress

The response of container-grown, girdled vines to water-deficit stress was examined by investigating the leaf net CO₂ assimilation rate (<i>A</i>), stomatal conductance (<i>g</i>_<i>s</i>), transpiration rate (<i>E</i>), predawn leaf water potential (Ψ_PD), and the fluctuations of stem diameter in vines. At the beginning of withholding irrigation, <i>A</i>, <i>g</i>_<i>s</i>, and <i>E</i> were lower in girdled vines than in control vines. <i>A</i> and <i>g</i>_<i>s</i> decreased 4 and 5 days after the start of withholding irrigation in control and girdled vines, respectively. Ψ_PD reached −1.13 MPa in the control vines and −0.67 MPa in girdled vines at 6 days after withholding irrigation. The higher soil water potential in water-deficient girdled vines than in control vines showed lower water consumption in girdled vines. Daytime stem contraction was smaller in girdled vines than in control vines while drying. Hence, results indicated that decreased water use as a result of girdling reduced the depletion of soil water in the containers, which caused high Ψ_PD and subsequent low stem contraction in girdled vines. Combined treatment of girdling and short-term strong water-deficit stress did not cause leaf wilt or decrease shoot growth in the subsequent growing season. These results reveal the response of girdled vines to water-deficit stress, especially severe and short-term water-deficit stress, in controlled conditions under which vines were grown in containers in a vinyl greenhouse.

Biological management of gray mold in pot roses by co-inoculation of the biocontrol agent Ulocladium atrum and the mycorrhizal fungus Glomus mosseae

In pursuit of a biological disease and stress management strategy for ornamentals, a three-factorial experiment was performed to examine the effects of plant darkness stress, Ulocladium atrum and Glomus mosseae on gray mold caused by Botrytis cinerea and plant vigour in potted roses. While darkness stress significantly enhanced gray mold, U. atrum negated the stress effect, reducing disease incidence to comparable levels in both stressed and non-stressed plants. G. mosseae interacted positively with U. atrum under non-stress conditions and negatively in darkness. G. mosseae significantly reduced non-specific leaf wilt incidence. Darkness significantly reduced the number of leaves formed above the pruning stub; U. atrum efficiently counteracted this effect, but had no effect under non-stress conditions. G. mosseae and U. atrum additively increased the total number of leaves per plant, and increased plant top dry weight. However, G. mosseae had no effect on dry weight under stress conditions. G. mosseae root colonization increased significantly when U. atrum was applied, but only under normal light conditions. In conclusion, a biological approach combining a foliar biocontrol agent and an arbuscular mycorrhizal fungus for gray mold protection seems promising.

First Report of Fusarium solani Causing Wilt of Olea europaea in Nepal

Leaf drop, wilt, and mortality were observed in September of 2007 on approximately 10% of 1- to 2-year-old olive (Olea europaea cv. Leccino) plants shipped from Europe and growing in a nursery in the District of Makwampur, Nepal. Roots of symptomatic and asymptomatic plants were disinfected in 1% NaOCl, cut into 1 cm long pieces, plated on 2% potato dextrose agar, and maintained at 20°C with 14 h of light per day. Colonies with white mycelium developed after 3 days. Microconidia and three-septated macroconidia averaged 11 × 3.9 μm and 38 × 5 μm, respectively. Chlamydospores were produced singly and in pairs. On the basis of culture characteristics, the fungus was identified as Fusarium solani (2). The ITS1-5.8S-ITS2 DNA sequences of 10 monoconidial cultures shared 99% identity with F. solani strains available on the NCBI databases (GenBank Accession Nos. 1115947 and 1115999). Pathogenicity tests were conducted with F. solani isolates NR1 and NR2 obtained from symptomatic plants. Twelve-month-old rooted cuttings of O. europaea cv. Leccino were transferred to pots containing a soilless mix and F. solani-infected oat grains (10:1 vol/vol). Fifteen plants of each F. solani isolate were inoculated. Noninfested sterilized oat grains were used for the control treatment. Symptoms on inoculated plants included leaf abscission followed by wilting and plant death approximately 10 days after inoculation and resembled those observed on the naturally infected plants. Noninoculated control plants remained healthy. The fungus was reisolated from roots of symptomatic tissues and was identical in appearance to the isolates used to inoculate the plants. No colonies of F. solani were isolated from noninoculated plants. F. solani has been reported as weakly pathogenic on olive in Spain (4) and highly aggressive on olive in Argentina (1) and India (3). To our knowledge, this is the first report of F. solani causing wilt and mortality of young olive plants in Nepal.

Abscisic acid, in combination with sucrose, is effective as a pulse treatment to suppress leaf damage and extend foliage vase-life in cut Eustoma flowers

SummaryWhile pulse treatment with sucrose extends the vase-life of cut Eustoma grandiflorum flowers, it may also cause leaf damage. Here, the effects of abscisic acid (ABA) treatment on leaf damage and the vase-life of Eustoma ‘Mira Coral’ cut flowers were investigated. Cut flowers with five open florets and four buds were treated with distilled water, 4% (w/v) sucrose including biocide, 10 µM ABA, or 4% (w/v) sucrose plus 10 µM ABA including biocide, at 23ºC and 66% relative humidity, in the dark, for 21 h. After pulse treatment, 50% of the cut flowers treated with sucrose alone had at least one damaged leaf, compared to none of the cut flowers treated with sucrose plus ABA. Sucrose plus ABA treatment resulted in a greater reduction in solution uptake than did treatment with sucrose alone. Fresh weight and the number of open florets in flowers decreased more slowly in flowers treated with sucrose plus ABA than in flowers treated with sucrose alone. Both the sucrose, and the sucrose plus ABA treatments extended flower vase-life compared to the control. Moreover, ABA and sucrose plus ABA treatments showed marked delays in leaf wilting. These findings indicate that a pulse treatment with sucrose plus ABA suppresses leaf damage and improves the quality of cut Eustoma flowers.

XANTHOMONAS SPECIES CAUSING BACTERIAL SPOT OF TOMATO IN THE RUSSIAN FEDERATION

Bacterial spot of tomato, caused by Xanthomonas euvesicatoria (Group A), X. vesicatoria (Group B), X. perforans (group C) and X. gardneri (Group D) (Jones et al., 2004), formerly known as X. campestris pv. vesicatoria, has become very important in the Russian Federation. Leaf spots and wilt symptoms were observed in 2006 in tomato fields located in southern European part of Russia. Field symptoms were first observed in early July of 2006 and had spread to over 30% of the plants in some fields by late August. Yellow-pigmented Xanthornonas-like bacteria were isolated from plants using yeast extract-CaCO 3 agar. Forty-three original strains were cloned and characterized based on morphologic and biochemical properties, by genetic analysis including rep-PCR, AP-PCR and gene sequencing. Reference strains included XV153 (group A), NCPPB 422 1 (Group B), XV 938 (Group C), XV GA2, XV444 (Group D), and 15 strains stored since 1947. Phenotypic and genetic properties of newly isolated and archived Russian strains were similar. Twent y-three strains that were not amylolytic or pectolytic and failed to utilize cis-aconitic acid were identified as X. gardneri and 18 strains that were strongly amylolytic and pectolytic were identified as X. vesicatoria; neither X. euvesicatoria nor X. perforans were found.

OCCURRENCE OF XANTHOMONAS CAMPESTRIS PV. RAPHANI ON TOMATO PLANTS IN THE RUSSIAN FEDERATION

Bacterial spot of tomato, caused by X. vesicatoria (Group B) and X. gardneri (Group 0) (Jones et al., 2004) has become a very important disease of tomato in the Russian Federation. It was reported previously that this plant can be affected by Xantlwmonas campestris pv. raphani as well (White, 1930). Leaf spots and wilt symptoms were observed in 2006 on tomato fields located in Krasnodar, Stavropol, and Alania-Osetia regions. Forty-tree original strains were cloned and characterized based on colony characteristics and biochemical properties (Jones et al., 2004). Reference strains included XV 153 (group A), NCPPB 4221 (Group B), XV 938 (Group C), XV GA2, XV444 (Group D), and several X. campestris (NCPPB 528T, PHW 231, and NCPPB 1946T)• The genes gyrB, operon Xcc0006-Xcc0007 (genome of Xcc ATCC 33913; da Silva et al., 2002), and gene cytP450 were amplified and sequenced in the strains. Analysis of the nucleotide sequences revealed a unique group (seven strains) isolated from diseased tomato plants collected in Alania-Osetia (North Caucasus) with symptoms of leaf spots and black rot-like wiltin. Those strains were grouped together with X. cainpestris pv. raphani NCPPB 1946 from radish. The strains were amylolytic, pectolytic, and most similar to X. vesicatoria in other biochemical properties. This is the first report of X. campestris pv. rap hani on tomato in Russia.

The ER luminal binding protein (BiP) mediates an increase in drought tolerance in soybean and delays drought-induced leaf senescence in soybean and tobacco

The ER-resident molecular chaperone BiP (binding protein) was overexpressed in soybean. When plants growing in soil were exposed to drought (by reducing or completely withholding watering) the wild-type lines showed a large decrease in leaf water potential and leaf wilting, but the leaves in the transgenic lines did not wilt and exhibited only a small decrease in water potential. During exposure to drought the stomata of the transgenic lines did not close as much as in the wild type, and the rates of photosynthesis and transpiration became less inhibited than in the wild type. These parameters of drought resistance in the BiP overexpressing lines were not associated with a higher level of the osmolytes proline, sucrose, and glucose. It was also not associated with the typical drought-induced increase in root dry weight. Rather, at the end of the drought period, the BiP overexpressing lines had a lower level of the osmolytes and root weight than the wild type. The mRNA abundance of several typical drought-induced genes [NAC2, a seed maturation protein (SMP), a glutathione-S-transferase (GST), antiquitin, and protein disulphide isomerase 3 (PDI-3)] increased in the drought-stressed wild-type plants. Compared with the wild type, the increase in mRNA abundance of these genes was less (in some genes much less) in the BiP overexpressing lines that were exposed to drought. The effect of drought on leaf senescence was investigated in soybean and tobacco. It had previously been reported that tobacco BiP overexpression or repression reduced or accentuated the effects of drought. BiP overexpressing tobacco and soybean showed delayed leaf senescence during drought. BiP antisense tobacco plants, conversely, showed advanced leaf senescence. It is concluded that BiP overexpression confers resistance to drought, through an as yet unknown mechanism that is related to ER functioning. The delay in leaf senescence by BiP overexpression might relate to the absence of the response to drought.

ASSOCIATION OF FUSARIUM SOLANI WITH ROLABC AND WILD TYPE TROYER CITRANGE

SUMMARY To determine if rolABC genes of Agrobacterium rhizogenes could modify the susceptibility of Troyer citrange to dry root rot, conidial suspensions of a Fusarium solani strain isolated from citrus roots (PVCT1A) were used in cutting, leaf midrib and root inoculation assays of wild type (wt) and rolABC Troyer citrange. In all assays, the pathogen was reisolated equally well from wt and transformant. Interveinal chlorosis, wilt and defoliation of inoculated cuttings and leaf midribs, observed both in rolABC and wt, were more severe in transgenic lines. Two months after root inoculation with the pathogen, root weight was significantly reduced in transgenic lines. No root rot symptoms were observed on rolABC or wt lines, but wilting was more severe on wt. Cell-free fungal culture filtrate induced leaf wilt and defoliation within 10 days from immersion in the filtrate, without any significant difference between rolABC and wt lines. Scanning electron microscopy observations of infected roots showed that fungal penetration and colonization was similar in both lines. Evidence is presented that F. solani colonizes Troyer citrange tissues even in absence of visible symptoms and without differences between wt and rolABC lines.

Carbohydrate Metabolism in Creeping Bentgrass as Influenced by Two Summer Irrigation Practices

This field study was conducted to investigate carbon metabolic responses to deep and infrequent (DI) versus light and frequent (LF) irrigation in ‘Providence’ creeping bentgrass (Agrostis stolonifera L.). LF irrigation was performed daily to wet soil to a depth of 4 to 6 cm, whereas DI irrigation was performed at leaf wilt to wet soil to a depth of ≥24 cm. The creeping bentgrass was seeded into a sand-based root zone in 2005 and was maintained as a putting green during the 2006 and 2007 study years. Canopy net photosynthesis (Pn) and whole plant respiration (Rw) were monitored, and water-soluble carbohydrates [WSC (i.e., glucose, fructose, and sucrose)], storage carbohydrates [SC (i.e., fructan and starch)], and total nonstructural carbohydrates [TNC (i.e., the sum of water soluble and storage sugars)] in leaf and root tissue were quantified. Creeping bentgrass subjected to DI irrigation had a lower Pn and a generally similar Rw compared with LF-irrigated bentgrass. DI irrigated bentgrass generally had greater levels of WSC and TNC in leaf tissue in 2006 and similar levels in 2007 when compared with LF-irrigated bentgrass. Leaf SC levels were higher in DI- than LF-irrigated bentgrass in both years. Creeping bentgrass roots subjected to DI irrigation generally had greater SC and TNC levels in both years than were found in LF-irrigated plants. Root WSC levels were higher (2006) or similar (2007) in DI- versus LF-irrigated bentgrass. Irrigating creeping bentgrass at wilt rather than daily to maintain moist soil generally resulted in higher carbohydrate levels in leaves and roots, which may enable creeping bentgrass to better tolerate and recover from drought and other stresses.

Bacterial Wilt of Mulberry (Morus alba) Caused by Enterobacter cloacae in China.

In August of 2006, a new bacterial disease was noted in Hangzhou mulberry orchards of Zhejiang Province, China where bacterial wilt of mulberry caused by Ralstonia solanacearum was previously reported (3). In the summer, the disease caused severe wilt, especially on 1- or 2-year-old mulberry plants, that resulted in premature plant death. Leaf wilt symptoms generally started on older leaves at the bottom of the plant and spread to the younger leaves. The leaves of infected plants became withered and dry, turned dark brown, and eventually the plants became defoliated. The root xylem of infected plants was moist and discolored with brown stripes. The phloem was asymptomatic, however, in severe infections, the phloem was decayed. The observation of wilting proceeding from the bottom of the plant to the top distinguishes this disease from bacterial wilt caused by R. solanacearum. Five bacterial strains isolated from infected mulberry plants showed characteristics similar to those of the standard reference strain of Enterobacter cloacae subsp. cloacae IBJ0611from China, but differed from R. solanacearum IBJ35, E. cancerogenus LMG2693T, and E. cloacae subsp. dissolvens LMG2683T from the University of Gent, Belgium in phenotypic tests, including the Biolog Identification System version 4.2 (Biolog Inc., Hayward CA), pathogenicity tests, transmission electron microscopy (TEM,KYKY-1000B, Japan) observation, and gas chromatographic analysis of fatty acid methyl esters (FAMEs) using the Microbial Identification System (MIDI Company, Newark, DE) with the aerobic bacterial library (TABA50). Isolates were gram negative, facultative anaerobic, rod shaped, 0.3 to 1.0 × 1.0 to 3.0 μm with peritrichous flagella. Colonies on nutrient agar were light yellow, smooth, circular, entire, and convex with no green fluorescent diffusible pigment on King's medium B (3). Weak hypersensitive reaction was observed on tobacco 3 days after inoculation. All five strains were identified as E. cloacae with Biolog similarity of 0.662 to 0.863 and FAMEs similarity of 0.632 to 0.701. Inoculation of 10 6-month-old intact mulberry plants of cv Husang with cell suspensions containing 109 CFU/ml by pinprick at the base of the stem reproduced symptoms observed in natural infections. No symptoms were noted on the two control plants inoculated by the same method but with sterilized distilled water. The bacterium was reisolated from the symptomatic mulberry plants. E. cloacae has been reported from the United States as the cause of internal yellowing of papaya fruits (1) and rhizome rot of edible ginger (2). To our knowledge, this is the first report of mulberry wilt caused by E. cloacae in China.