Shoot Dieback Research Articles

The use of Verticillium dahliae and Diplodia scrobiculata to induce resistance in Pinus halepensis against Diplodia pinea infection

The pathogenic fungi Verticillium alboatrum and Diplodia scrobiculata were assayed for biological control of Diplodia pinea on Aleppo pine (Pinus halepensis) in Catalonia (north-eastern Spain). Young shoots were pre-treated with inoculations of either V. dahliae or D. scrobiculata, by placing colonized agar plugs on wounds made by removing a single needle fascicle. An inoculation with D. pinea was performed 15 days later. Two months after the shoot inoculations, the canker length on the stems was measured and the percentage of shoot dieback calculated. Verticillium dahliae and D. scrobiculata were found to significantly reduce the canker length of D. pinea (P 0.05) were observed in the mean canker length between the two treatments. Trees pre-inoculated with V. dahliae resulted in 31.12% shoot dieback, while those pre-inoculated with D. scrobiculata resulted in 32.18% shoot dieback, compared with positive controls (42.85%).

Seasonal variation in photosynthesis in six woody species with different leaf phenology in a valley savanna in southwestern China

During 2003–2005, we examined the effect of seasonal drought on water status, gas exchange, δ13C, chlorophyll fluorescence and spectral reflectance in six woody species in a valley savanna near the Yuanjiang River (the upper Red River) in southwestern China. Three different phenological types of these woody species were compared, i.e., an evergreen species, Cyclobalanopsis helferiana, two winter-deciduous (WD) species, Buchanania latifolia and Symplocos racemosa, and three drought-deciduous (DD) species, Terminthia paniculata, Wendlandia tinctoria and Woodfordia fruticosa. We aimed to test the following three hypotheses: (1) the evergreen and WD species employ a drought avoidance strategy, whereas DD species employ a drought tolerance strategy; (2) the evergreen and WD species have a more economical water use strategy than the DD species and (3) the evergreen and WD species have a stronger photoprotection capacity through thermal dissipation than the DD species. At the end of a prolonged drought, the predawn leaf water potential (Ψpd) in C. helferiana and S. racemosa dropped to ca. −0.8 MPa, whereas the Ψpd in B. latifolia remained close to zero and DD species were leafless. In the rainy seasons, maximal photosynthetic rates of the evergreen (18.4 μmol m−2 s−1) and W. fruticosa (18.0 μmol m−2 s−1) were higher than those of the other four species (12.2−13.8 μmol m−2 s−1). The evergreen and WD species responded to drought by closing stomata and thus maintained a constant relative water content (RWC), which is a typical drought avoidance strategy; however, it is at the expense of carbon gain. DD species maintained a high photosynthetic capacity with a decrease in both stomatal conductance and RWC until the driest period, and then shifted from the drought tolerance strategy to the avoidance mechanism by shoot dieback. There was no significant difference in the means of δ13C across the phenological groups. The evergreen and WD species had stronger heat dissipation than the DD species in dry seasons. All species increased leaf spectral reflectance, probably because of degradation of chlorophyll as indicated by the leaf reflectance index, which should reduce light harvesting. All species showed a strong increase in the ratio of red to green spectral reflectance of leaves during dry seasons, indicating the accumulation of anthocyanin, which may contribute to screening sunlight and scavenging reactive oxygen species. Different responses to drought of savanna woody species with different leaf phenologies may facilitate the partitioning of resource use and hence their co-existence.

Control of "Diplodia pinea" and "D. scrobiculata" in "Pinus halepensis" by 5-Chloro-Salicylic Acid

Diplodia pinea (syn. Sphaeropsis sapinea) and D. scrobiculata are destructive pathogens of conifer species in many parts of the world. The sensitivity of these fungi to externally applied 5-chloro-salicylic acid on Pinus halepensis was studied. Trees treated with 2 mM 5-chloro-salicylic acid were more resistant to the fungi than untreated trees. After 15 days of treatment shoot dieback affected 30% of trees inoculated with D. pinea, compared to 60% of untreated trees. D. scrobiculata caused shoot dieback in 30% of untreated trees but only in 20% of trees pretreated with 5-chloro-salicylic acid. The controls never developed tip blight. The direct effect of 5-chloro-salicylic acid on the mycelial growth of D. pinea and D. scrobiculata was tested in vitro using PDA amended with 5-chloro-salicylic acid at five concentrations (0.2, 1, 2, 2.5, and 3.0 mM). The radial growth of colonies was measured after 48 and 72 h of incubation at 24°C. After 48 h, 5-chloro-salicylic acid significantly inhibited mycelial growth of D. pinea at 3mM, although there was no longer any significant difference in growth rates after 72 h of incubation. D. scrobiculata was slightly more sensitive to 5-chloro-salicylic acid than D. pinea. After 48 h, significant differences were observed in the mean colony diameter of D. scrobiculata when directly exposed to 5-chloro-salicylic acid at dilutions from 0 to 3 mM. After 72 h, however, mycelial growth was reduced significantly only at the highest concentrations (2; 2.5 and 3 mM) (P-value <0.05).

First Report of Canker and Dieback Disease of Grevillea robusta in East Africa Caused by Botryosphaeria spp.

An outbreak of a canker and dieback disease was observed for the first time on Grevillea robusta in Uganda during October of 2001. Disease symptoms included dieback of shoots and branches, lesions and canker formation on the stems, clear or yellow-to-red exudates on stems and branches, formation of clusters of small and deformed leaves, and sapling and tree mortality. Isolations were made from the inner bark and wood of stems and branches in the Masaka and Rakai districts of Uganda in March and August to October of 2003. Samples from the border between healthy and necrotic tissues were plated on potato dextrose agar, vegetable juice agar, and water agar as well as on selective Phytophthora medium. Fungal isolates from several of the symptomatic trees were identified as belonging to the genus Botryosphaeria on the basis of morphological characteristics. Botryosphaeria-like fungi were not isolated from healthy trees. The internal transcribed spacer regions of the ribosomal DNA, as well as parts of the beta-tubulin and elongation factor 1-alpha genes, were sequenced from eight of these isolates. Five isolates matched Botryosphaeria parva in GenBank and one isolate matched B. rhodina. Two isolates matched an unidentified Botryosphaeria sp. previously isolated from Eucalyptus grandis in Uganda (GenBank Accession Nos. AY228102 and AY226852). Pathogenicity tests were performed by inoculating 10 18-month-old G. robusta seedlings with each isolate. A 10-mm3 malt agar plug with mycelium was inserted into a cut in the bark and the wound was sealed using grafting tape. One month after inoculation, the number of seedlings displaying death from the point of inoculation to the top (TM) and whole seedling mortality (SM) were recorded. Six of the isolates produced pathogenic responses in the seedlings: Botryosphaeria sp. isolate 1 (TM 10, SM 2), Botryosphaeria sp. isolate 2 (TM 10, SM 1), B. parva isolate 1 (TM 6, SM 1), B. parva isolate 2 (TM 3, SM 1), B. parva isolate 3 (TM 3, SM 0), and B. parva isolate 4 (TM1, SM 0). Inoculation with B. parva isolate 5 and B. rhodina, as well as sterile agar plugs, resulted in no TM or SM. The canker and dieback disease seems to be widespread in the East African Region since typical symptoms have been observed on G. robusta in Kenya (February 2004) and Ethiopia (February 2006). Botryosphaeria spp. are known to have multiple hosts and have been isolated from several other species in this region, including Eucalyptus spp. (1,2). From these findings, it appears that the same species of Botryosphaeria, which are previously known to be pathogenic on Eucalyptus, also cause serious disease on G. robusta.

First Report of Spirea Witches'-Broom Disease in China.

Bumald spirea (Spiarea bumalda Burv.) is an important ornamental tree widely grown in northern China. In August of 2006, spirea plants exhibiting symptoms of witches'-broom, stunting, yellowing, and shoot dieback were found at an incidence of 5 to 15% in Qingzhou City, Shandong Province, China. Total DNA was extracted separately from 0.1 g of phloem tissue from leaf midribs and stems of six symptomatic and six asymptomatic plants with a modified cetyltriethylammonium bromide (CTAB) method (3). Resulting DNA samples were analyzed for phytoplasma DNA by a nested PCR assay using phytoplasma universal 16S rDNA gene primer pairs R16mF2/R16mR1 and R16F2n/R16R2 (2). These primers amplified 1.5- and 1.2-kb products, respectively, from DNA of all symptomatic plants only. Restriction fragment length polymorphism (RFLP) analysis of the 1.2-kb 16S rDNA product using enzymes AluI, MseI, and HhaI indicated that all symptomatic plants contained a group 16SrI (aster yellows group) subgroup B (16SrI-B) phytoplasma strain (4). A 16S rDNA sequence derived from this strain (GenBank Accession No. EF176608) was most similar (99.8 and 99.6%) to those of severe aster yellows (GenBank Accession No. M86340) and Maryland aster yellows (GenBank Accession No. AF322644) phytoplasmas, respectively, thereby confirming strain identity based on RFLP analysis. A phytoplasma (Spiarea stunt phytoplasma, GenBank Accession No. AF190228), which belongs to X-disease group (16SrIII), was reported to infect spirea and probably be lethal to S. tomentosa in New York (1,4). The phytoplasma reported here shared low identity (90.8%) with Spiarea stunt phytoplasma, but also caused dieback of spirea shoots. The epidemiology and economic impact of this disease need further intensive investigation. To our knowledge, this is the first report of spirea witches'-broom disease and of its association with a subgroup 16SrI-B phytoplasma in China.

Root characteristics and growth potential of container and bare-root seedlings of red oak (Quercus rubra L.) in Ontario, Canada

Root characteristics and field performance of container and bare-root seedlings of red oak (Quercus rubra L.) were compared during the first growing season after planting. Sixty seedlings of each stock type were planted on a clearfell and weed-free site near Restoule, Ontario. Twenty-four additional seedlings from each stock type were compared at the start of the study in terms of shoot and root parameters. Measurement of root and shoot parameters were repeated at three dates during the first growing season in the field. The root systems of container stock had a larger number of first order lateral long roots and were significantly more fibrous than bare-root stock. These differences were sustained throughout the first growing season. In terms of field performance, container seedlings had 100% survival and achieved significant increases in both biomass and shoot extension. Bare-root seedlings suffered 25% mortality, significant shoot dieback and more variable growth. The mean relative growth rate (RGR) of container seedlings increased throughout the study period to a maximum of 30 mg/g/day, whereas the mean RGR of bare-root stock remained close to or below zero. Overall, the container seedlings proved less prone to transplanting shock than the bare-root seedlings, most likely due to favourable root architecture and the pattern of root development. Further work may be warranted in container design, growing regimes and root architecture to fully realise the potential of container systems for the production of high quality red oak seedlings across a range of site conditions.

Growth responses of an African savanna tree, Bauhinia thonningii Schumacher, to defoliation, fire and climate

This study investigated the growth responses to defoliation, fire-exposure and climate factors of a widespread Africana savanna tree, Bauhinia thonningii Schumacher, at a site in central Zambia. Experimental trees (n = 47) were either exposed to fire (n = 12) in the first half of the dry season or protected from fire (n = 35). Some of the fire-protected trees (n = 12) were subjected to artificial defoliation in two consecutive years. Phenological responses (bud break, leaf flush and leaf production) to fire-exposure and defoliation were monitored on permanently marked sample shoots over a 2-year period. Radial tree growth (diameter at 1.3 m above ground) was measured annually over a 7-year period from 1998 to 2005. During the first two years, defoliation and fire-exposure advanced the onset of bud break and leaf flush but fire-exposed trees produced significantly less leaves than did trees protected from fire, probably because scorching caused more severe shoot die-back than is normal. Leaf production was also significantly affected by experimental treatments and their interaction with year. Although treatments had significant short-term effects on radial growth, previous-year growth significantly influenced current-year growth, thereby confirming the existence of autocorrelation in the time-series growth data of B. thonningii. The interaction between previous-year growth and climate factors explained a significant proportion (25–40%, P < 0.001) of the variance in annual tree growth. Variogram models predicted that a 2-year manual defoliation treatment would shorten the longer-term growth cycle while continuous fire-exposure extended the cycle by one year. The results are useful for the management of savanna trees.

Shoot development and dieback in progenies of Nothofagus obliqua

Shoot growth and dieback were compared among progenies of nursery-grown seedlings of Nothofagus obliqua belonging to seven progenies of the same provenance (Quila-Quina, Argentina). First-year shoots consisted of one growth unit (GU) and second-year shoots of one or two GUs. The probability of development of two GU was similar for all progenies. Progenies were different in terms of shoot size, terminal bud abscission, the extent of shoot dieback after shoot extension and the node of origin of the relay shoot on the first shoot. Plants with a second-year shoot consisting of two GUs had a thicker stem and more nodes than those with single-GU shoots. The selection of N. obliqua seed trees based on architectural traits suitable for forestry development at specific sites must contemplate variability among progenies and their probabilities of successful development under different conditions.

Forest health condition in New South Wales, Australia, 1996–2005. II. Fungal damage recorded in eucalypt plantations during forest health surveys and their management

Forests New South Wales (NSW) manages over 26 000 ha of young eucalypt (Eucalyptus and Corymbia) plantations, established since 1994, in NSW, Australia. Forest health surveillance of these plantations was initiated in late 1995: results for damage from fungal diseases, including foliar, stem and root fungi, are summarised for surveys from 1996 to 2005. The main pathogen in Corymbia spp. plantations was Quambalaria shoot blight, caused by Quambalaria pitereka, which caused shoot death, affecting growth and form. Kirramyces (=Phaeophleospora) epicoccoides caused significant defoliation (>75% severity) of Eucalyptus grandis and E. grandis×Eucalyptus camaldulensis clones in several locations from 2000 onwards, often resulting in infection by stem fungi leading to top-death and tree mortality. Kirramyces eucalypti caused significant defoliation (>95% severity) of Eucalyptus nitens over several years in two plantations, which also resulted in infection by stem fungi and top-death and tree mortality of many trees. Other leaf fungi (e.g. Mycosphaerella cryptica, Mycosphaerella marksii, Aulographina eucalypti and Pilidiella eucalyptorum) were common on several host species, but rarely reached a level of serious concern. Holocryphia eucalypti was associated with elongated perennial cankers, top-death and tree mortality of Eucalyptus dunnii in plantations severely defoliated by insects and fungi or that had experienced severe windstorms. Botryosphaeria spp. were associated with twig cankers and shoot dieback of Eucalyptus pilularis on stressed sites, but also with root-bole damage and scattered mortality of several hosts during drought. Caliciopsis sp. pleomorpha was associated with localised branch and stem cankers, mostly of stressed trees, on a range of hosts from 2003 onwards, and severe cankers leading to tree mortality of E. nitens. Phytophthora cinnamomi was associated with mortality of several hosts in years following episodes of high rainfall. Mortality from Armillaria luteobubalina was rare. Management options for these diseases are discussed, and include: (1) risk-site mapping to assist in targeting management options; (2) increasing tree resistance via tree breeding; and (3) increasing tree tolerance to damage, and recovery after damage, via silvicultural regimes such as fertiliser application and improved site-species matching.

Responses of three Acacia species to simulated herbivory in a semi-arid southern African savanna

Published Article.,This study investigated responses of juvenile Acacia karroo, Acacia nilotica and Acacia rehmanniana to simulated herbivory in a semi-arid southern African savanna. For each species, we clipped twigs in the dry season and measured the removed twig biomass. Seven months after clipping, we counted the number of resprouts on clipped stems and measured their lengths, diameters and biomass. We also measured dieback length on clipped stems and dieback biomass (DB). Removed twigs of A. rehmanniana had the greatest biomass, followed by A. karroo and least was A. nilotica. However, A. nilotica had the greatest resprout biomass, followed by A. rehmanniana and least was A. karroo. Consideration of DB as lost biomass resulted in A. karroo fully compensating lost biomass, A. nilotica overcompensating and A. rehmanniana under-compensating. Acacia rehmanniana had the shortest resprouts while A. nilotica had the longest. The number and the diameter of resprouts did not vary among the species. We conclude by predicting that in semi-arid African savannas, undercompensation has the greatest probability of occurrence among woody species prone to severe shoot dieback following herbivory.

Etiology and Real-Time Polymerase Chain Reaction-Based Detection ofGremmeniella- andPhomopsis-Associated Disease in Norway Spruce Seedlings

ABSTRACT In spring 2002, an unusual disease outburst was recorded on Norway spruce seedlings in southeast Norway. Extensive damage was recorded on 1- and 2-year-old Norway spruce seedlings that either had wintered in nursery cold storage or had been planted out in autumn 2001. The damage was characterized by leader shoot dieback and stem necroses on the upper or lower part of the shoot from 2001. Gremmeniella abietina and a Phomopsis sp. frequently were isolated from the diseased seedlings. Internal transcribed spacer (ITS) ribosomal (r)DNA sequence analysis and random amplified microsatellites profiling indicated that the G. abietina strains associated with diseased nursery seedlings belonged to the large-tree type (LTT) ecotype of the European race of G. abietina var. abietina, and inoculation tests confirmed their pathogenicity on Norway spruce. Based on ITS rDNA sequence analysis, the Phomopsis strains associated with diseased seedlings did not represent any characterized Phomopsis spp. associated with conifers. The Phomopsis sp. was not pathogenic in inoculation tests, indicating that it may be a secondary colonizer. ITS-based real-time polymerase chain reaction assays were developed in order to detect and quantify G. abietina and Phomopsis in the nursery stock. We describe here the G. abietina-associated shoot dieback symptoms on Norway spruce seedlings and conclude that the unusual disease outburst likely was related to the G. abietina var. abietina epidemic caused by the LTT on large Scots pines in 2001.

Differences in morphological and physiological responses to water-logging between two sympatric oak species (Quercus petraea[Matt.] Liebl.,Quercus roburL.)

Pedunculate (Quercus robur L.) and sessile (Q. petraea (Matt.) Liebl.) oaks are known to display different ecological requirements, par- ticularly relative to root hypoxia induced by water-logging. Q. robur is more tolerant to hypoxia than Q. petraea. We designed an experiment aiming at identifying morphological and physiological responses to root hypoxia that might differ between the two species. Potted seedlings were submitted during seven weeks to a water-logging treatment with O2 concentrations below 3 mg L −1 in the vicinity of roots. The treatment induced growth ces- sation in both species. Q. petraea displayed a lower tolerance to hypoxia as demonstrated by the higher number of seedlings suffering shoot dieback and leaf chlorosis as compared to Q. robur .T his difference should be related to the high number of adventitious roots and hypertrophied lenticels that were formed in Q. robur, compared to Q. petraea. In the fine roots of the two species, the activity of pyruvate decarboxylase (PDC), the key enzyme of the fermentative pathway, was stimulated after 24 h of water-logging. Transcripts of PDC increased after 48 h of water-logging in Q. robur and not in Q. petraea. Interestingly, transcripts of haemoglobin (Hb) (possibly involved in the putative nitric oxide cycle) followed the same pattern of response than those of PDC. Enzymes of the sucrose degradation pathway displayed decreased activities after 3 weeks of water-logging, probably due to decreased carbohydrate availability. Alcohol dehydrogenase (ADH), sucrose synthase (Susy), and pyruvate kinase (PK) activities were higher in Q. robur after 3 weeks of water-logging. This study provided a set of markers characterizing the differences of tolerance to hypoxia between the two species for further studies on intra and inter-specific diversity.

First-year growth response of cold-stored, nursery-grown aspen planting stock

This research examined the first year growth characteristics of cold stored and transplanted nursery-produced aspen (Populus tremuloides) seedlings (container and bareroot (BR)) and compared it to the growth of seedlings that had not been transplanted (established from germinants in the field) and therefore had an unrestricted root system (UR). Prior to planting, nursery-produced seedlings were placed in cold storage (−3°C) and root growth potential (RGP) and total non-structural carbohydrate (TNC) root reserves were tested at 0, 10, 75 and after 150 (container) and 190 days (BR) of storage. Both container and BR stock had much lower root to shoot ratios (RSRs) and root carbohydrate reserves compared to UR seedlings after 170 days. During storage, root reserves in container stock declined faster than in the BR and UR seedlings. RGP in all nursery stock was the highest after 75 days of storage, while longer storage resulted in shoot dieback and reduced root growth. After the first growing season, UR seedlings were one tenth the size of the nursery stock; however, in the second growing season they had no stem dieback and grew twice the height and stem diameter. The higher RSRs and root reserves in the UR seedlings was likely caused by early bud set in its first year of growth. This suggests that inducing bud set earlier in the growing regime might allow seedlings to increase root mass and carbohydrate reserves.

Spatial relationships between nitrogen status and pitch canker disease in slash pine planted adjacent to a poultry operation

Pitch canker disease ( Fusarium circinatum Nirenberg & O'Donnell) causes serious shoot dieback, reduced growth and mortality in pines found in the southern and western USA, and has been linked to nutrient imbalances. Poultry houses with forced-air ventilation systems produce nitrogen (N) emissions. This study analyzed spatial correlations between pitch canker disease and foliar, forest floor, soil, and throughfall N in a slash pine ( Pinus elliottii var. elliottii Engelm.) plantation adjacent to a poultry operation in north Florida, USA. Tissue and throughfall N concentrations were highest near the poultry houses and remained elevated for 400 m. Disease incidence ranged from 57–71% near the poultry houses and was spatially correlated with N levels. Similarly, stem mortality ranged from 41–53% in the most heavily impacted area, and declined to 0–9% at distances greater than 400 m. These results suggest that nutritional processes exacerbate changes in disease susceptibility and expression in slash pine.

Terminal bud failure of black cottonwood (Populus trichocarpa) exposed to salt-laden winter storms

At coastal sites, trees are exposed to marine aerosols that may cause foliar necrosis and shoot dieback, which can result in deformed crowns and contorted stems. A six-year study of leaf primordia in terminal buds of black cottonwood trees (Populus trichocarpa Torr. & Gray) on Heimaey Island off the south coast of Iceland was undertaken to elucidate the physiological events associated with salt-deposition-related bud failure. Leaf and bud lengths, dry mass, water content and chloride concentrations were monitored and related to four phenological stages: (1) bud set; (2) dormancy induction; (3) dormancy release; and (4) bud break. The trees set buds in July and shed their leaves by late September. Leaf primordia generally stopped growing by September 10 +/- 22 days and attained midwinter water content in late September. Leaf growth commenced in the terminal buds by March 2 +/- 16 days, but mean dates of bud swelling and bud break were April 29 +/- 19 and May 10 +/- 12 days. In summer and until November, chloride concentrations in leaf primordia were low, but increasing. Chloride concentrations remained stable from December to February, even though the dormant trees were exposed to large amounts of marine aerosols. In February and March, three events occurred more or less simultaneously: (1) leaf extension growth commenced; (2) chloride concentration surged in the leaf primordia; and (3) the leaf primordia began to hydrate. Following dormancy release, growth and hydration of leaf primordia were negatively related to chloride concentration in the leaf primordia, with inhibition of leaf growth, tissue hydration and chloride acquisition occurring at a chloride concentration threshold estimated at 7.3 mg Cl- g(-1) tissue water. Necrosis of leaf primordia was observed above 14 mg Cl- g(-1) tissue water. Growth and hydration of leaves at bud break in mid-May was explained by a three-parameter logistic model of chloride concentration in leaf primordia at the end of March. By mid-May, 90% of all buds remained non-necrotic, but only 56% the terminal buds had broken. Salt alone explained the observed growth suppression of leaf primordia in the buds and the resultant failure of terminal buds to break by mid-May.

First Report of Dieback and Leaf Lesions on Rhododendron sp. Caused by Phytophthora hedraiandra in the United States.

Surveys for Phytophthora ramorum in Minnesota nurseries revealed the presence of P. hedraiandra de Cock & Man in't Veld and several other Phytophthora species but not P. ramorum. Symptomatic leaf and stem tissues from diseased Rhododendron and Quercus species were cultured on PARP, a selective growth medium for Phytophthora (3). The Phytophthora isolates obtained were later identified by sequencing the internal transcribed spacer (ITS) region of the rDNA and comparing the sequences with those in GenBank using BLAST searches (1). The ITS sequences of six cultures (GenBank Accession Nos. DQ139804-DQ139809), isolated during 2003 from various Rhododendron cultivars exhibiting leaf lesions and shoot dieback, showed 100% identity with the ITS sequence of P. hedraiandra (GenBank Accession No. AY707987) (2). This is a recently described pathogenic species from the Netherlands responsible for causing leaf spots on Viburnum spp. Since the ITS sequence of P. hedraiandra differs little from that of P. cactorum (2), we verified one isolate to be P. hedraiandra by sequencing the mitochondrial cytochrome c oxidase subunit I gene (cox1) (GenBank Accession No. DQ139810). Comparison of this sequence with the P. hedraiandra voucher specimen in GenBank (Accession No. AY769115) showed 99% identity, which was the closest match. Reproductive structures were measured on V8 juice agar. The average oogonium diameter for three isolates was 29 μm with a range of 26 to 32 μm, while the average antheridium length was 13 μm (11 to 15 μm). Sporangium length and width averages on crushed hemp seeds were 32 μm (28 to 36 μm) and 26 μm (21 to 30 μm), respectively, with the average length to width ratio of 1.25 (1.23 to 1.29). Pathogenicity tests on Rhododendron cv. Mikkeli were carried out using three of our P. hedraiandra isolates. Spore suspensions of 2 × 104 zoospores per ml were used to mist-spray shoots of five, 3-year-old plants for each isolate. Five controls were mist sprayed with water. After inoculation, plants were placed in plastic bags in a dark growth chamber (22°C) for 7 days and then moved to a greenhouse. Leaf blotches and shoot dieback were apparent 5 days after inoculation, and P. hedraiandra was reisolated from those symptomatic tissues and identified by an exact match of the ITS sequence. Necrotic areas lengthened from the shoot tips to the main stems of the plants while expanding into petioles and leaves. No symptoms were observed on control plants. To our knowledge, this is the first report of P. hedraiandra in the United States as well as the first report of Koch's postulates performed with P. hedraiandra on Rhododendron cv. Mikkeli. The significance of this disease to other woody plants in nurseries or the landscape is unknown, and further study is needed to determine the host range and extent of the disease that may occur from this introduction.

Response of Selected Nursery Crop Plants to Inoculation with Isolates of Phytophthora ramorum and Other Phytophthora Species

Many nursery crops are susceptible to root and foliage diseases caused by numerous species of Phytophthora. Phytophthora ramorum causes sudden oak death of trees and ramorum leaf blight and shoot dieback on numerous nursery plants, including rhododendron (Rhododendron spp.), viburnum (Viburnum spp.), pieris (Pieris spp.), and camellia (Camellia spp.) in Europe, the United States, and British Columbia, Canada. We sought to evaluate relative susceptibility of a selection of ornamental nursery crops by inoculating detached leaves with several species of Phytophthora known to infect rhododendrons, and to compare the relative virulence on those species to isolates of P. ramorum. The results indicated that many plants were susceptible under these experimental conditions, while others were not. On a given host, symptoms caused by all species of Phytophthora were identical except for differences in pathogen virulence. Plant species were identical except for differences in pathogen virulence. Plant species within genera or cultivars within species varied in susceptibility to isolates of P. ramorum and other species of Phytophthora. Phytophthora ramorum, P. citricola, P. citrophthora, and P. nicotianae were the most virulent pathogens on most of the host plants inoculated. Some plants were susceptible to several species of Phytophthora, while others were susceptible only to P. ramorum. Inoculation of detached leaves of `Nova Zembla' rhododendron, lilac (Syringa vulgaris), or doublefile viburnum (Viburnum plicatum var. tomentosum) under controlled conditions with different species of Phytophthora or isolates of P. ramorum (both mating types) indicated significant relative differences in species or isolate virulence.

Deicing Salts Reduce Cold Hardiness and Increase Flower Bud Mortality of Highbush Blueberry

Injury has been observed since the early to mid-1990s to highbush blueberries (Vaccinium corymbosum L.) growing along roads in southern Michigan. Symptoms include shoot dieback, flower bud mortality, and reduced yields. To determine if this injury was the result of deicing salts applied to roads, salt (sodium chloride, NaCl) spray was applied to potted blueberry plants, and to the plant root zones. Bushes sprayed six times during the winter with NaCl solutions (0, 0.034, 0.068, 0.137, 0.274, 0.548 m) developed the same injury symptoms observed in roadside fields, and injury severity was proportional to the spray concentration. The root media of other potted plants was saturated with NaCl solutions (0, 0.017, 0.051, 0.154, and 0.462 m) in Mar. 2002. Pots were then rinsed with fresh well water when growth began in April to determine if soil salt caused similar damage. The highest soil salt levels killed most above ground growth, and damage diminished with decreasing salt levels. Twigs were also excised from branches sprayed twice with NaCl solutions or water and frozen incrementally to measure the temperature resulting in 50% flower bud mortality (LT50). Salt exposure reduced the LT50 of flower buds, by as much as 11.5 °C, relative to the control, even within 2 days of treatment. Additional studies with chloride salts (NaCl, KCl, CaCl2, MgCl2) and sodium salts (NaCl, Na-acetate, Na2SO4) indicated that most reduced the cold tolerance of blueberry flower buds to some degree.

Quambalaria leaf and shoot blight onEucalyptus nitensin South Africa

Quambalaria spp. cause leaf and shoot dieback diseases on young Eucalyptus trees in Australia, Thailand, South America and South Africa. The disease was first recorded in South Africa in the early 1990s but has been restricted to nurseries in the subtropical north-east coastal area of the country, without resulting in great effect. Recent disease surveys in the Mpumalanga Province of South Africa have revealed extensive shoot and leaf dieback, as well as stem cankers on 1-year-old E. nitens trees. Some symptoms of the disease resembled Quambalaria leaf and shoot blight. However, this was the first time it had occurred on the stems of larger trees, on E. nitens or in the cold temperate region of the country. The aim of this study was to identify the causal agent of the disease and to test different Eucalyptus spp. and clones of relevance to the South African forestry industry for their susceptibility to the pathogen. Comparisons of DNA sequence data for the ITS and 5.8S regions were used to identify the fungus. Results showed that the pathogen represented Q. eucalypti. Inoculation trials showed that all the material tested was susceptible to infection by Q. eucalypti. This study shows that Q. eucalypti has become elevated from a relatively minor nursery pathogen to one that can cause significant damage on a wide range of Eucalyptus spp. in both temperate and sub-tropical areas of South Africa.

Specific and sensitive detection of the conifer pathogen Gremmeniella abietina by nested PCR

BackgroundGremmeniella abietina (Lagerb.) Morelet is an ascomycete fungus that causes stem canker and shoot dieback in many conifer species. The fungus is widespread and causes severe damage to forest plantations in Europe, North America and Asia. To facilitate early diagnosis and improve measures to control the spread of the disease, rapid, specific and sensitive detection methods for G. abietina in conifer hosts are needed.ResultsWe designed two pairs of specific primers for G. abietina based on the 18S rDNA sequence variation pattern. These primers were validated against a wide range of fungi and 14 potential conifer hosts. Based on these specific primers, two nested PCR systems were developed. The first system employed universal fungal primers to enrich the fungal DNA targets in the first round, followed by a second round selective amplification of the pathogen. The other system employed G. abietina-specific primers in both PCR steps. Both approaches can detect the presence of G. abietina in composite samples with high sensitivity, as little as 7.5 fg G. abietina DNA in the host genomic background.ConclusionThe methods described here are rapid and can be applied directly to a wide range of conifer species, without the need for fungal isolation and cultivation. Therefore, it represents a promising alternative to disease inspection in forest nurseries, plantations and quarantine control facilities.