Root Dieback Research Articles

Biological and molecular characterization of Lasiodiplodia theobromae causing dieback disease of strawberry in Thailand

AbstractStrawberry (Fragaria × ananassa) is commercially grown in Chiang Mai Province of Northern Thailand. Strawberry dieback, whose symptoms include dieback of runners and roots followed by wilt and necrotic discoloration in crowns, was first observed in diseased strawberry cultivated in the greenhouse. Fungi were isolated from infected parts of diseased strawberry and a pathogenicity assay was conducted for confirmation of fungal pathogenesis. Morphological features of fungal isolates were observed using stereo and compound microscopes. For molecular characterization, internal transcribed spacer (ITS) and large subunit 28S rRNA (LSU) regions were amplified using polymerase chain reaction (PCR) and sequenced. Based on morphological and molecular identification, the fungi were identified as Lasiodiplodia theobromae. Koch's postulates were fulfilled by reisolating the pathogen. To the best of our knowledge, this is the first report of L. theobromae causing dieback of strawberry in Thailand.

Differences in Nutrient Uptake Can Influence the Performance of Citrus Rootstocks under Huanglongbing Conditions

Huanglongbing {HLB [Candidatus Liberibacter asiaticus (C Las)]} has been one of the biggest challenges in citrus (Citrus sp.) production in Florida and wherever it is present. HLB-affected trees show significant shoot and root dieback, fruit drop, and reduction in yield. Currently, there is no cure for HLB, and there is no commercial HLB-resistant germplasm. Nonetheless, intensive nutrient management has been promising for citrus growers. The nutrient requirement of HLB-affected trees seems to be greater than that of healthy citrus trees. By understanding the nutrient uptake potential of rootstocks, fertilizer programs can be customized accordingly to enhance the performance of a rootstock in existing groves. Moreover, a reduction in the application of nutrients is possible by planting rootstocks with a high nutrient absorption capacity. Use of rootstocks with good nutrient uptake efficiency can take some burden off the growers who are intensively managing HLB-affected citrus groves. Therefore, the objective of this study was to evaluate and understand the nutrient uptake potential of the citrus rootstocks. To achieve this objective, a 100% hydroponic greenhouse study was conducted with six rootstocks with a range of tolerance to HLB. Several physiological and molecular tools were applied to evaluate the rootstocks for their nutrient uptake potential. A+Volk × O-19 (HLB-tolerant) rootstock had greater nutrient uptake efficiency, whereas US-896 (HLB-susceptible) had lesser nutrient uptake efficiency. Swingle, one of the most popular pre-HLB rootstocks, had poor zinc uptake and the least expression of ZINC TRANSPORTER, suggesting that zinc applications should be emphasized in Swingle plantings. US-896 rootstock expressed the least level of nutrient transporter genes, such as IRON TRANSPORTER. UFR-4 (a good performer under HLB conditions) had a large root biomass, but the uptake efficiency for nutrients was poor, suggesting that the nutrient uptake potential is a complex process that is not solely dependent on root biomass. This study is unique because it is one of the first citrus studies to report nutrient uptake efficiency and the potential of rootstocks. The information presented can be used to improve performance or select better-performing rootstocks under HLB conditions.

First Report of Crown and Root Rot Caused by Phytopythium helicoides on Photinia × fraseriDress in China.

Photinia × fraseriDress is a hybrid species of Rosaceae and Photinia genus which is widely cultivated in China.During 2020 and 2021, approximately 80% of plants growing in Xuanwu district of Nanjing, China, exhibited disease symptoms including blight, necrosis, and dieback of crowns and roots. Symptomatic root tissues collected from 2-year-old plants were rinsed with water, cut into 2-mm tissues which were surface-sterilized in 70% ethanol for 60 s, and plated onto 10% V8 PARP agar and incubated in the dark at 26°C for 3 days. Hyphae emerged from 70% of the samples. Two representative isolates (PF-he2, PF-he3) were obtained and deposited. Ten agar plugs (2×2 mm2) of each isolate were transferred into 10 mL of 10% V8 juice to produce mycelial mats. To stimulate sporangial production, 3-5 drops of soil extract solution (soil collected from healthy fields, immersed in sterile water, and filtered) were added to each plate. Sporangia were terminal, ovoid to globose or papillate. The zoospores were 7.1-9.3 µm in diameter. Each oogonium contained a single, smooth, spherical and aplerotic oospore with a diameter of 24.5-32.6 μm. These morphological properties resemble those of Phytopythium helicoides (CBS286.31 from S. F. Ashby). For molecular identification, the large subunit (LSU) rDNA, cytochrome c oxidase I (COXI) gene, and COXII gene were amplified using the primer pairs NL1/NL4 , FM55/FM52R , and FM66/FM58 . The LSU, COXI, and COXII sequences of isolate PF-he2 were 100% (763/763 nt), 98.07% (1066/1087 nt), 99.64% (561/563 nt) identical to isolate CBS 286.31 (AY598665.2), GDGJ6 (KT750956.1), and TC3 (MN952224.1), respectively. Based on the morphological and molecular analysis, the two isolates shared 100% homology were identified as Phytopythium helicoides. The pathogenicity of two isolates were tested on potted 2-yr-old (40-cm tall) P. × fraseri. Six plants were dug up to expose root balls which were wounded before inoculations with a sterile needle, and then inoculated with zoospore suspension (106 zoospore/mL). Controls were treated with ddH2O. Three seedlings/isolate were used for each treatment including controls. All plants were repotted using the original sterilized potting mix and pots. After inoculation, the plants were covered with plastic bags, and sterilized H2O was sprayed into the bags twice per day to maintain humidity and kept in a greenhouse at the day/night temperatures at 25/16 °C. All the inoculated plants showed lesions similar to those observed in the field after 23 days , whereas controls were asymptomatic. The isolates were reisolated from the lesions and sequenced as P. helicoides which has found causing root rot on Nelumbo nucifera, Rhododendron pulchrum, Zea mays in China, and also on Fragaria × ananassa in America, Peach Rootstock in California. Globally, this is the first report of P. helicoides causing crown blight and root rot of P. × fraseri. Management programs are under development to contain the spread of P. helicoides and treat diseased plants.

Early Physiological Plant Response and Systemic Effects of Huanglongbing Infection in Split-Root Plants.

Huanglongbing (HLB), caused by 'Candidatus Liberibacter asiaticus' (CLas), is a devastating disease of citrus. After initial infection, CLas quickly colonizes the root system before canopy symptoms develop. There is limited understanding of CLas movement from roots to canopy and local and systemic effects on root dynamics. Using split-root rhizoboxes and late summer below-the-split bud inoculation, effects of local infection on systemic disease development were studied. Upward bacterial movement from roots is linked to seasonal flushes and CLas population in roots. CLas stayed isolated to one side of the roots for at least 8 months, until the spring flush. HLB caused differential root responses depending on tree age at infection. Systemic effects, independent of CLas movement, occur very early after infection. Stimulation of root growth occurred on noninfected roots prior to CLas detection in 1.5-year-old trees but decreased in 2.5-year-old trees. Independent of tree age, root growth was stimulated during spring root flushes after CLas population stabilized. Root dieback began simultaneously with detection of CLas in roots (6 weeks postinoculation). Infection and tree age altered root lifespan. In total, 1.5-year-old CLas-infected roots from summer and fall flushes had 3 and 6 weeks reduced lifespan. In contrast, 2.5-year-old CLas-infected plants lifespan was unaffected. Season affected root lifespan with late summer root flush lifespan was three times shorter than fall or spring root flushes. Split-root inoculation allowed study of local and systemic effects of CLas infection in roots, information crucial to prolonging the productivity of HLB-affected trees.

Effects of Mob-Grazing on Soil and Range Quality Vary with Plant Species and Season in a Semiarid Grassland

Mob-grazing involves maintaining high densities of livestock for short periods so that most plants are either eaten or trampled, followed by long rest periods. This practice has been proposed as a mechanism to increase soil carbon (C) storage and range quality. However, mob-grazing has not universally achieved these objectives, possibly because many factors influence the effects of grazing on soil C dynamics and vegetation. This study examines factors that may mediate grazing impacts on soil C by comparing plant cover and the seasonal dynamics of roots, soil variables, and mycorrhizal fungal hyphae in experimental plots treated with traditional grazing, annual mob-grazing or no grazing for 18 yr. Root and soil variables were measured directly underneath a C4 grass, a C3 grass, and a forb up to 5 × during a 13-mo period. Mob-grazing did not influence total soil C, but it significantly increased soil organic matter (SOM), fine particulate organic matter, and nitrogen-15 (δ15N). Furthermore, mob-grazing increased soil compaction, decreased soil aggregate stability, decreased soil moisture, and tended to increase the abundance of two invasive plant species: Salsola tragus and Bromus tectorum. Soil compaction, soil aggregate stability, root biomass, particulate organic matter, and percent soil C and N varied significantly across seasons and among plant species. The density of mycorrhizal fungal hyphae varied with season but not with grazing treatment. A significant grazing by date interaction in root biomass and soil carbon-13 (δ13C) suggests that root dieback and inputs of pulverized plant material with a higher δ13C signature could be the source of higher SOM in mob-grazed plots. Compared with ungrazed plots, traditionally grazed plots had higher SOM without the adverse impacts on vegetation and soil properties observed in the mob-grazed plots. No single management strategy is universally beneficial. Range managers should carefully weigh the pros and cons of mob-grazing because, although it can increase SOM in surface soils, it may also negatively impact soil structure and composition of vegetation.

Splitting the Difference: Heterogeneous Soil Moisture Availability Affects Aboveground and Belowground Reserve and Mass Allocation in Trembling Aspen.

When exploring the impact of resource availability on perennial plants, artificial treatments often apply conditions homogeneously across space and time, even though this rarely reflects conditions in natural systems. To investigate the effects of spatially heterogeneous soil moisture on morphological and physiological responses, trembling aspen (Populus tremuloides) saplings were used in a split-pot experiment. Following the division of the root systems, saplings were established for a full year and then subjected to either heterogeneous (portion of the root system exposed to non-lethal drought) or homogeneous (whole root system exposed to non-lethal drought or well-watered) treatments. Above- and belowground growth and non-structural carbohydrate (NSC) reserves (soluble sugars and starch) were measured to determine how allocation of reserves and mass between and within organs changed in response to variation in soil moisture availability. In contrast to saplings in the homogeneous drought treatment, which experienced reduced shoot growth, leaf abscission and fine root loss, saplings exposed to the heterogeneous conditions maintained similar aboveground growth and increased root system allocation compared to well-watered saplings. Interestingly under heterogeneous soil moisture conditions, the portion of the root system that was resource limited had no root dieback and increased carbon reserve concentrations, while the portion of the root system that was not resource limited added new roots (30% increase). Overall, saplings subjected to the heterogeneous soil moisture regime over-compensated belowground, both in mass and NSC reserves. These results indicate that the differential allocation of mass or reserves between above- and belowground organs, but also within the root system can occur. While the mechanisms and processes involved in these patterns are not clear, these responses could be interpreted as adaptations and acclimations to preserve the integrity of the entire sapling and suggests that different portions of plant organs might respond autonomously to local conditions. This study provides further appreciation of the complexity of the mechanisms by which plants manage heterogeneous conditions and offers evidence that spatial and temporal variability of resource availability, particularly belowground, needs to be accounted for when extrapolating and modeling stress responses at larger temporal and spatial scales.

Pythiogeton manoomin, a new species causing root and basal stalk rot of wild rice in the United States

ABSTRACTA new species of Pythiogeton isolated from wild rice exhibiting rot and dieback of roots and stalks in California is described. Pythiogeton manoomin sp. nov. is characterized by coenocytic hyphae, club-like appressorium, and terminal or intercalary sporangia, which are often a short distance from the end of supporting hyphae. The protoplasm is discharged through a discharge tube into an elongate transient vesicle, which soon disappears, leaving the naked protoplasm to differentiate into reniform zoospores. Pythiogeton manoomin also produces thick-walled pigmented chlamydospores, not found in other Pythiogeton species. In greenhouse trials, Pg. manoomin did not infect economically important crops such as rice, bean, chard, corn, carrot, lettuce, oat, radish, sweet pepper, tomato, or wheat. Phylogenetic analysis based on ITS data supports the conclusion that this organism is a new species that is most closely related to Pg. ramosum. In this paper, we describe morphological characteristics, temperature–growth relationships, pathogenicity, and phylogenetic relationships that support the description of this taxon as a new species, Pythiogeton manoomin sp. nov.urn:lsid:zoobank.org:pub:4C63AAA4-4D4A-4679-A344-79B75121A5C6

Role of Fine Tree Roots in the Nitrogen Cycle of Boreal Forests

Despite the insignificant contribution to the total forest phytomass, fine roots form 30% of the production of forest ecosystems, playing a crucial role in biogeochemical cycles. However, estimates of the biomass, annual production, dieback, and age of fine roots remain understudied, especially in Russian forests, due to technical difficulties. There are only a few estimates of nitrogen consumption for the annual production of fine roots. It is believed that they comprise up to 75 kg N ha–1 in mature forests. This is up to 60% of the total consumption of nitrogen for annual yield and is comparable with mineralized soil nitrogen. However, these values are approximate because the methods of measuring fine roots production are debatable.

Anthropogenic nitrogen deposition alters growth responses of European beech (Fagus sylvativa L.) to climate change.

Global change affects the functioning of forest ecosystems and the services they provide, but little is known about the interactive effects of co-occurring global change drivers on important functions such as tree growth and vitality. In the present study we quantified the interactive (i.e. synergistic or antagonistic) effects of atmospheric nitrogen (N) deposition and climatic variables (temperature, precipitation) on tree growth (in terms of tree-ring width, TRW), taking forest ecosystems with European beech (Fagus sylvatica L.) as an example. We hypothesised that (i) N deposition and climatic variables can evoke non-additive responses of the radial increment of beech trees, and (ii) N loads have the potential to strengthen the trees' sensitivity to climate change. In young stands, we found a synergistic positive effect of N deposition and annual mean temperature on TRW, possibly linked to the alleviation of an N shortage in young stands. In mature stands, however, high N deposition significantly increased the trees' sensitivity to increasing annual mean temperatures (antagonistic effect on TRW), possibly due to increased fine root dieback, decreasing mycorrhizal colonization or shifts in biomass allocation patterns (aboveground vs. belowground). Accordingly, N deposition and climatic variables caused both synergistic and antagonistic effects on the radial increment of beech trees, depending on tree age and stand characteristics. Hence, the nature of interactions could mediate the long-term effects of global change drivers (including N deposition) on forest carbon sequestration. In conclusion, our findings illustrate that interaction processes between climatic variables and N deposition are complex and have the potential to impair growth and performance of European beech. This in turn emphasises the importance of multiple-factor studies to foster an integrated understanding and models aiming at improved projections of tree growth responses to co-occurring drivers of global change.

Water relations in plants subjected to heavy metal stresses

Concentrations of heavy metals in soil seldom reach a level sufficient to cause osmotic disturbances in plants. It is likely that water entry to the roots is indirectly governed by other factors which are themselves affected by metals. Decreased elongation of the primary root, impaired secondary growth, increased root dieback, or reduced root hair caused by toxic ions all exert a deleterious effect on the root-absorbing area and water uptake. Moreover, metals are able to decelerate short-distance water transfer both in symplast and apoplast, which in turn reduce the movement of water into the vascular system and affect water supply to the shoot. Long-distance transport is limited also due to decreased hydraulic conductivity in the root, stem and leaf midrib caused by a reduction in the size of vessels and tracheids, and partial blockage of xylem elements by cellular debris or gums. Heavy metals influence water delivery to the shoot due to inhibition of transpiration as they decrease the size of the leaves and the thickness of the lamina, reduce intercellular spaces, affect the density of stomata and decrease their aperture. Stomata closure is induced by direct interaction of toxic metals with guard cells and/or as a consequence of the early effects of metal toxicity on roots and stems. In metal-stressed plants, root-derived ABA or ABA-induced signals might play a role in stomatal movement. Disturbances in water relations trigger differential regulation of aquaporin gene expression, which may contribute to further reductions in water loss.

Flood-Ring Formation and Root Development in Response to Experimental Flooding of Young Quercus robur Trees.

Spring flooding in riparian forests can cause significant reductions in earlywood-vessel size in submerged stem parts of ring-porous tree species, leading to the presence of ‘flood rings’ that can be used as a proxy to reconstruct past flooding events, potentially over millennia. The mechanism of flood-ring formation and the relation with timing and duration of flooding are still to be elucidated. In this study, we experimentally flooded 4-year-old Quercus robur trees at three spring phenophases (late bud dormancy, budswell, and internode expansion) and over different flooding durations (2, 4, and 6 weeks) to a stem height of 50 cm. The effect of flooding on root and vessel development was assessed immediately after the flooding treatment and at the end of the growing season. Ring width and earlywood-vessel size and density were measured at 25- and 75-cm stem height and collapsed vessels were recorded. Stem flooding inhibited earlywood-vessel development in flooded stem parts. In addition, flooding upon budswell and internode expansion led to collapsed earlywood vessels below the water level. At the end of the growing season, mean earlywood-vessel size in the flooded stem parts (upon budswell and internode expansion) was always reduced by approximately 50% compared to non-flooded stem parts and 55% compared to control trees. This reduction was already present 2 weeks after flooding and occurred independent of flooding duration. Stem and root flooding were associated with significant root dieback after 4 and 6 weeks and mean radial growth was always reduced with increasing flooding duration. By comparing stem and root flooding, we conclude that flood rings only occur after stem flooding. As earlywood-vessel development was hampered during flooding, a considerable number of narrow earlywood vessels present later in the season, must have been formed after the actual flooding events. Our study indicates that root dieback, together with strongly reduced hydraulic conductivity due to anomalously narrow earlywood vessels in flooded stem parts, contribute to reduced radial growth after flooding events. Our findings support the value of flood rings to reconstruct spring flooding events that occurred prior to instrumental flood records.

First Report of a Pythiogeton sp. Causing Root and Basal Stalk Rot of Wild Rice in California.

In the summer of 2012, an outbreak of a newly discovered root and basal stalk rot of wild rice (Zizania palustris L.) cv. Franklin was observed in a 16-ha field in Big Valley, Lassen County, California (GPS coordinates 41°08'41.93″ N 121°10'07.49″ W). Infected plants exhibiting rot and dieback of roots and stalks were in various stages of decline, including death. Symptomatic stem and root tissues from affected plants were surface disinfected in 1% NaOCl for 90 s and placed on PARP agar plates, which were then incubated at 25°C in the dark for 1 week. Hyphal tips were used to start and maintain the organism in pure cultures. Isolates were transferred into petri plates with water and sterilized blades of turfgrass for the production of hyphae and reproductive structures. Isolates had coenocytic hyphae and produced zoospores 20 to 30 μm in diameter outside of sporangia (75 to 160 × 46 to 110 μm) from a naked mass of protoplasm, unlike from a vesicle, which is characteristic of Pythium spp. (2). Based on these morphological features, the isolate was tentatively identified as a Pythiogeton sp. Total genomic DNA was extracted from mycelia using the DNeasy Plant Mini Kit (Qiagen Inc., Valencia, CA). The internal transcribed spacers (ITS) 1 and 2 flanking the 5.8S rRNA regions were amplified by PCR and sequenced using universal ITS5 and ITS4 primers. A BLAST search of the 855-bp sequences revealed 98% similarity with a sequence of P. ramosum isolate Pg-164 (GenBank Accession No. JQ610190.1). The 21 nucleotide differences suggest that the isolate from wild rice may be an unreported species. The sequences were submitted to GenBank (KF719169). To fulfill Koch's postulates and confirm pathogenicity, 100 wild rice seeds were surface disinfected in 1% NaOCl for 90 s and placed in a 500 ml sterile pot with 250 g of autoclaved sand. Three 5 mm-diameter disks from the margin of a 7-day-old culture growing on PARP were placed in each of five pots. As a control, three 5 mm-diameter disks from a non-inoculated PARP plate were placed in five different pots, and five pots with autoclaved sand were not inoculated. All pots were kept in a randomized complete block design at 25°C for 14 days under a 14-h photoperiod. The pathogenicity test was repeated three times. After 14 days, the inoculated plants in all tests developed root and basal stalk rot, consistent with the symptoms observed on diseased wild rice in the field. The Pythiogeton sp. was consistently re-isolated on PARP from symptomatic plants but not from control plants. The non-inoculated wild rice plants remained asymptomatic. DNA sequences of the ITS region of the re-isolated Pythiogeton sp. revealed 100% identity with the isolate from the field. There have been reports of P. zeae on corn in Korea and P. zizaniae on water bamboo in Taiwan (1,2,3). This is the first report of a Pythiogeton sp. on wild rice.

Contrasting impacts of defoliation on root colonization by arbuscular mycorrhizal and dark septate endophytic fungi of Medicago sativa

Individual plants typically interact with multiple mutualists and enemies simultaneously. Plant roots encounter both arbuscular mycorrhizal (AM) and dark septate endophytic (DSE) fungi, while the leaves are exposed to herbivores. AMF are usually beneficial symbionts, while the functional role of DSE is largely unknown. Leaf herbivory may have a negative effect on root symbiotic fungi due to decreased carbon availability. However, evidence for this is ambiguous and no inoculation-based experiment on joint effects of herbivory on AM and DSE has been done to date. We investigated how artificial defoliation impacts root colonization by AM (Glomus intraradices) and DSE (Phialocephala fortinii) fungi and growth of Medicago sativa host in a factorial laboratory experiment. Defoliation affected fungi differentially, causing a decrease in arbuscular colonization and a slight increase in DSE-type colonization. However, the presence of one fungal species had no effect on colonization by the other or on plant growth. Defoliation reduced plant biomass, with this effect independent of the fungal treatments. Inoculation by either fungal species reduced root/shoot ratios, with this effect independent of the defoliation treatments. These results suggest AM colonization is limited by host carbon availability, while DSE may benefit from root dieback or exudation associated with defoliation. Reductions in root allocation associated with fungal inoculation combined with a lack of effect of fungi on plant biomass suggest DSE and AMF may be functional equivalent to the plant within this study. Combined, our results indicate different controls of colonization, but no apparent functional consequences between AM and DSE association in plant roots in this experimental setup.

Extremely low fine root biomass in Larix sibirica forests at the southern drought limit of the boreal forest

Mongolia's Larix sibirica forests at the southern fringe of the Eurosiberian boreal forest belt are exposed not only to very low winter temperatures, but also to frequent summer droughts. It is not completely known how Siberian larch adapts to these stressors. We examined whether (i) these forests differ in their fine root bio- and necromass from more humid boreal forests further in the North and (ii) inter-annual fluctuations in fine root biomass are related to tree vitality. In two exceptionally dry summers, we found only 4–5g DM m−2 of fine root biomass (in 0–20cm depth), which is far less than typical conifer fine root biomass figures from boreal forests (c. 200–400gm−2) and the lowest forest fine root biomass reported worldwide; in a moist summer, fine root biomass was 20 fold higher. In contrast to fine root biomass, both necromass and non-tree root mass were high in all three years. From the large increase of fine root biomass in the moist summer and the generally high root necromass, we conclude that drought-induced fine root dieback was the likely cause of the very small amount of live root mass in the dry summers. Larch fine roots seem to be more drought-sensitive than shoots, since marked needle loss did not occur under the extreme conditions.

Phytophthora aquimorbida sp. nov. and Phytophthora taxon ‘aquatilis’ recovered from irrigation reservoirs and a stream in Virginia, USA

Two distinct subgroups (L2 and A−2) were recovered from irrigation reservoirs and a stream in Virginia, USA. After molecular, morphological and physiological examinations, the L2 subgroup was named Phytophthora aquimorbida and the A−2 designated as Phytophthora taxon ‘aquatilis’. Both taxa are homothallic. P. aquimorbida is characterized by its noncaducous and nonpapillate sporangia, catenulate and radiating hyphal swellings and thick-walled plerotic oospores formed in globose oogonia mostly in the absence of an antheridium. P. taxon ‘aquatilis’ produces plerotic oospores in globose oogonia mostly with a paragynous antheridium. It has semi-papillate, caducous sporangia with variable pedicels, but it does not have hyphal swelling. Analyses of ITS, CO1, β-tubulin and NADH1 sequences revealed that P. aquimorbida is closely related to P. hydropathica, P. irrigata and P. parsiana, and P. taxon ‘aquatilis’ is related to P. multivesiculata. The optimum temperature for culture growth is 30 and 20 C for P. aquimorbida and P. taxon ‘aquatilis’ respectively. Both taxa were pathogenic to rhododendron plants and caused root discoloration, pale leaves, wilting, tip necrosis and dieback. Their plant biosecurity risk also is discussed.

Heat stress and N fertilization affect soil microbial and enzyme activities in the creeping bentgrass (Agrostis Stolonifera L.) rhizosphere

High summer temperatures often cause damage to bentgrass on putting greens in transition zone regions. One of the most damaging effects is a depression of rooting. Although heat stress effects on plant functions are considered as a main reason for the damage, heat stress also may be related to organic matter (OM) accumulation and poor gas exchange into the rhizosphere. The OM accumulation and the often-observed root dieback suggest that soil microbial processes play a role in summer bentgrass decline. In this study, the impact of high temperature on soil microbial properties and enzyme activities was examined using creeping bentgrass (Agrostis stolonifera) growing in a phytotron controlled environment chamber. The high temperature exposures (34/30°C versus 22/18°C for controls) lasted for four weeks and the bentgrass cultures received mineral N at two rates. Our working hypothesis was that not only did high temperatures stimulate overall soil microbial and enzyme activity but also selectively modified microbial catabolic functions. To test this hypothesis, we compared temperature sensitivities and Q10 values of microbial substrate utilization patterns using a Biolog plate approach and soil enzyme activities. The results indicated that soil enzyme activities had similar responses to assay temperatures and their Q10 values averaged ∼2 with changes of laboratory assay temperatures from 12 to 22°C and from 22 to 34°C. Such positive responses of microbial activity to high temperatures were supported by parallel increases in rates of microbial substrate utilization. Total substrate availability in Biolog plates also increased with laboratory assay temperatures. This enhancement could not be explained by the overall stimulation of high temperature on microbial activity, but instead by selective modification of microbial community functions. Nitrogen fertilization significantly changed soil biological activities. Phenol oxidase activity was reduced by the high rate of N fertilization, whereas β-glucosidase and β-glucosaminidase activities were increased. Interactions on soil enzyme activities between growth chamber temperatures and N fertilization rates also occurred. Soil peroxidase activity was ∼three-fold greater for bentgrass subjected to heat stress and the low rate of N fertilization. Our results indicated that summer heat stress and the associated increases in root and OM degradation in bentgrass systems are related with overall temperature stimulations on soil microbial and enzyme activities as well as with modifications in functional components of the microbial community.

Protein accumulation in leaves and roots associated with improved drought tolerance in creeping bentgrass expressing an ipt gene for cytokinin synthesis

Cytokinins (CKs) may be involved in the regulation of plant adaptation to drought stress. The objectives of the study were to identify proteomic changes in leaves and roots in relation to improved drought tolerance in transgenic creeping bentgrass (Agrostis stolonifera) containing a senescence-activated promoter (SAG12) and the isopentyl transferase (ipt) transgene that increases endogenous CK content. Leaves of SAG12-ipt bentgrass exhibited less severe senescence under water stress, as demonstrated by maintaining lower electrolyte leakage and lipid peroxidation, and higher photochemical efficiency (Fv/Fm), compared with the null transformant (NT) plants. SAG12-ipt plants had higher root/shoot ratios and lower lipid peroxidation in leaves under water stress than the NT plants. The suppression of drought-induced leaf senescence and root dieback in the transgenic plants was associated with the maintenance of greater antioxidant enzyme activities (superoxide dismutase, peroxidase, and catalase). The SAG12-ipt and NT plants exhibited differential protein expression patterns under well-watered and drought conditions in both leaves and roots. Under equivalent leaf water deficit (47% relative water content), SAG12-ipt plants maintained higher abundance of proteins involved in (i) energy production within both photosynthesis and respiration [ribulose 1,5-bisphosphate carboxylase (RuBisCO) and glyceraldehyde phosphate dehydrogenase (GAPDH)]; (ii) amino acid synthesis (methionine and glutamine); (iii) protein synthesis and destination [chloroplastic elongation factor (EF-Tu) and protein disulphide isomerases (PDIs)]; and (iv) antioxidant defence system (catalase and peroxidase) than the NT plants. These results suggest that increased endogenous CKs under drought stress may directly or indirectly regulate protein abundance and enzymatic activities involved in the above-mentioned metabolic processes, thereby enhancing plant drought tolerance.

Fungal diseases in forest nurseries in Finland

Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and silver birch (Betula pendula) are the major tree species grown in Finnish forest nurseries where 99% of the seedlings are grown in containers first in plastic-covered greenhouses and later outdoors. The main diseases on conifer seedlings are Scleroderris canker (Gremmeniella abietina), Sirococcus blight and cankers (Sirococcus conigenum), snow blights (Herpotrichia juniperi and Phacidium infestans) and needle casts (Lophodermium seditiosum and Meria laricis). Also grey mould (Botrytis cinerea) and birch rust (Melampsoridium betulinum) are among the diseases to be controlled with fungicides. During last years Scleroderris canker has been a problem on Norway spruce, which has been since 2000 the most common species produced in Finnish nurseries. Root die-back (uninucleate Rhizoctonia sp.) on container-grown spruce and pine was a problem in the 1990s. Today the disease has become less common in modern nurseries due to improvements in hygiene and cultivation practice. Since 1991 stem lesions and top dying caused by Phytophthora cactorum has been a problem on birch. The ongoing climate change has already had effect on rusts and powdery mildews as well as other fungi infecting leaves. All diseases, which gain high precipitation and warm and long autumns. For same reasons winter stored seedlings need sprayings against grey mold. Fungal infections are also possible during short-day (SD) treatment, that is necessary for summer and autumn plantings and a beneficial step prior freezing temperatures outside or in freezer storage. Growers are encouraged to use cultural and integrated pest management techniques such as better nursery hygiene, including removing plant debris in nursery growing areas and hot water washing of containers plus removal of diseased, spore-producing seedlings and trees around the nursery.

Analysing scots pine defence‐related transcripts and fungal DNA levels in seedlings single‐ or dual‐inoculated with endophytic and pathogenic Rhizoctonia species

SummaryFungal DNA and induction of host defence‐related transcripts were monitored by real‐time PCR in young Scots pine seedlings inoculated with pathogenic uninucleate (UNR) and endophytic binucleate (BNR) Rhizoctonia species. The UNR (teleomorph Ceratobasidium bicorne) causes root dieback in conifer seedlings following invasion of the vascular cylinder via root apex and destroying apical meristems whilst the BNR, representing anastomosis group AG‐I of genus Ceratobasidium, is primarily restricted to the cortex in basal root regions. In the experiment 1 the fungi were simultaneously inoculated on roots, while in experiment 2, BNR was pre‐inoculated 168 h before inoculation with UNR. Nucleic acids were extracted from infected roots at intervals up to 192 h post‐infection (hpi), and the genomic DNA levels of the host and fungi and the transcript levels of a house‐keeping gene (glyceraldehyde‐3‐phosphate dehydrogenase) and nine putative defence genes were quantified. In simultaneous inoculation UNR was more competitive than BNR whereas pre‐inoculation of BNR suppressed but did not completely prevent root colonization by UNR. Stilbene synthase (STS) transcription was significantly up‐regulated in single‐inoculations with both fungi and in dual inoculation in both experiments. Maximum STS transcript levels were observed in roots single‐inoculated with UNR; the peak level at 48 hpi in experiment 2 was significantly higher than in seedlings single‐inoculated with BNR or co‐inoculated with both fungi, the latter two treatments showing relatively similar STS transcript levels. Similarly, transcript levels of phenylalanine ammonia lyase at 48 hpi in experiment 2 were significantly higher in roots single‐inoculated with UNR compared with BNR or in UNR+BNR co‐inoculations. The other seven putative defence genes monitored did not show any clear‐cut up‐regulation following fungal inoculation. We conclude that BNR suppresses UNR in Scots pine roots via direct competition for infection sites, since the studied transcripts showed no evidence of BNR induced resistance against UNR.

Influence of Temperature on Pathogenicity of Pythium volutum Toward Creeping Bentgrass.

Symptoms of Pythium root dysfunction (PRD) in creeping bentgrass (Agrostis stolonifera) are most common in the summer during periods of heat and drought stress. However, recent observations in North Carolina indicate that Pythium volutum, a causal agent of PRD, is most active during the fall and spring. Soil temperature thresholds for this pathogen are needed so that preventive fungicide applications can be timed accurately. A mycelial growth assay was performed by incubating 11 P. volutum isolates at 10 temperatures ranging from 10 to 31°C. To determine the optimal temperature range for infection by P. volutum, five isolates of P. volutum were used to inoculate 5-week-old 'Penn A-1' creeping bentgrass plants. Inoculated plants were transferred to growth chambers at constant 12, 16, 20, 24, 28, or 32°C (12-h day/night cycles) for 4 weeks to permit root infection, then the temperature in all chambers was increased to 32/26°C day/night to induce foliar symptoms. P. volutum grew most rapidly in vitro when temperatures were between 18 and 26°C. Typical PRD foliar symptoms developed in the 12, 16, 20, and 24°C treatments 2 weeks after the temperature was elevated to 32/26°C day/night. Disease severity was greatest when plants were incubated at 16°C after inoculation. Reductions in root depth and/or root mass were observed prior to raising the temperature to 32/26°C in the 12, 16, and 20°C temperature treatments. Once exposed to 4 weeks of heat treatment, extensive root dieback occurred in the 12, 16, 20, and 24°C treatments. These results demonstrate that P. volutum is most active at temperatures prevalent during the fall and spring in North Carolina, supporting the hypothesis that the majority of root infection occurs during this time and that fungicides should be applied when soil temperatures are between 12 and 24°C to achieve preventative control of PRD symptoms in the summer.