Lethal bole rot disease of coconut in East Africa

A lethal bole, rot disease of coconut, caused by Marasmiellus cocophilus Pegler, sp. nov., reaches epidemic proportions in several areas along the coasts of Kenya and Tanzania. The first symptoms noticed on palms 8 or more years old are a frond wilt and a crown rot, but these follow a primary bole rot. Highest mortality is among seedlings and young palms up to 8 years old. Where the disease occurs sporophores are sometimes common on exposed roots, dead seedlings and the soil surface where diseased palms have been dug out. The fungus appears to be a persistent colonizer of coconut debris in the soil. Mycelial cultures from infected bole tissues and from sporophores were highly pathogenic to seedlings, and slowly invaded older tissue. M. cocophilus reaches the inner bole tissues only through the roots, and wounding of roots of their aerenchymatous protuberances is important in infection. Seedlings may become infected through roots damaged during transplanting from nurseries to the field; movement of seedlings from affected areas is one certain method of dispersal. Suggested control measures are: (1) selection of seedlings in nurseries and subsequent transplanting should be as early as possible; (2) seedlings should be transplanted very carefully, and damaged roots pruned and disinfected; (3) seedlings should never be taken from affected areas; (4) periodic soil sterilization of nurseries; and (5) no cultivation should be done between palms, especially where disease is present.     

Leafhopper transmission of Rhynchosia little leaf, a disease associated with mycoplasma-like organisms in Jamaica

Little leaf disease of Rhynchosia minima (RLL) in Jamaica is reported for the first time. The presence of phloem-restricted MLO in diseased but not healthy plants, the remission of symptoms induced in RLL-affected plants with soil drenches of tetracycline, but not penicillin, and the transmission of disease-associated MLO to R. minima test plants, suggests that RLL has an MLO aetiology. RLL is vectored by the cicadellid leafhopper Ollarianus balli, for which R. minima represents the specific field host. Healthy colonies of O. balli produced from eggs oviposited on the RLL-immune weed Asystasia gangetica suggest that RLL is not transovarially transmitted. O. balli acquired the RLL agent after access to infected plants for 5 days (shorter feeds were not tried), and there was a maximum latent period in the leafhopper of 21 days. Of the O. balli collected from heavily-infected field stands of RLL, 35% transmitted the disease, while, of those reared on RLL in captivity for 14–16 days, 56% transmitted. Male and female O. balli transmitted equally efficiently, while nymphs were less frequent vectors. O. balli also infected Cajanus cajan, an important small scale subsistence crop in Jamaica, and Catharanthus roseus. It did not, however, transmit coconut lethal yellowing (CLY) disease to test palms after natural or deliberate acquisition-feeding on RLL, acquisition-feeding on CLY-affected palms, or, after injection with CLY-affected phloem exudate. There was thus no evidence that RLL is related to CLY or that O. balli can act as a vector of CLY.     

Relative rates of nucleotide substitution at the rbcl locus of monocotyledonous plants

Summary We subjected 35 rbcL nucleotide sequences from monocotyledonous taxa to maximum likelihood relative rate tests and estimated relative differences in rates of nucleotide substitution between groups of sequences without relying on knowledge of divergence times between taxa. Rate tests revealed that there is a hierarchy of substitution rate at the rbcL locus within the monocots. Among the taxa analyzed the grasses have the most rapid substitution rate; they are followed in rate by the Orchidales, the Liliales, the Bromeliales, and the Arecales. The overall substitution rate for the rbcL locus of grasses is over 5 times the substitution rate in the rbcL of the palms. The substitution rate at the third codon positions in the rbcL of the grasses is over 8 times the third position rate in the palms. The pattern of rate variation is consistent with the generation-time-effect hypothesis. Heterogenous rates of substitution have important implications for phylogenetic reconstruction.Offprint requests to: M.T. Clegg     

Responses to drought and flooding in tropical forage grasses

Seasonal drought and flooding severely limit pasture growth in tropical savannas. The objective of this study is to analyze and compare yield, biomass allocation, leaf growth rate and nutrient concentration of four important perennial C₄ forage grasses to short term flooding and moderate drought under controlled conditions. The grasses studied were the tufted Andropogon gayanus (CIAT 621) and Hyparrhenia rufa and the stoloniferous Echinochloa polystachya and Brachiaria mutica. All grasses were able to adjust their growth and development in response to flooding and drought: leaf growth and total biomass decreased under both treatments but the specific responses to these treatments differed markedly. Considering only total yield and leaf area, A. gayanus and H. rufa were relatively more tolerant to and less affected by drought whereas B. mutica and E. polystachya were more flood tolerant. In A. gayanus and H. rufa, both treatments reduced the proportion of assimilates devoted to roots and culms while increasing that of leaves decreasing the root/shoot ratio. In contrast, in B. mutica and E. polystachya only the proportion devoted to culms or stolons increased under flooding but the root/shoot ratio remained relatively stable under both treatments. All grasses produced adventitious rootlets except A. gayanus which was the most affected by flooding. Waterlogging decreased leaf nutrient concentration in all grasses which contributed to growth reduction. All species were relatively tolerant to both stresses. The results confirm the empirical observation that stoloniferous species B. mutica and E. polystachya are more tolerant to flooding thanks to adaptations typical of wetland plants such as hollow stolons which enhance oxygen diffusion to the roots and the development of adventitious rootlets that promotes water and nutrient absorption.     

Dynamics of energy and nutrient concentration and construction cost in a native and two alien C4 grasses from two neotropical savannas

In Venezuela, the alien grasses Melinis minutiflora Beauv. and Hyparrhenia rufa (Nees.) Stapf tend to displace the native savanna plant community dominated by Trachypogon plumosus (Humb. and Bonpl.) Nees. This occurs in either relatively wetter and fertile highland savannas or in drier and less fertile lowland savannas. Although the native and aliens are perennial C₄ grasses, higher net assimilation leaf biomass per plant and germination rate of the latter are some causes for their higher growth rates and for their competitive success. The objective of this study is to compare seasonal tissue energy, N, P and K concentrations and the calculated construction costs (CC) between the native grass and either one of the alien grasses from lowland and highland savannas. We predict that, in order to out-compete native plants, alien grasses should be more efficient in resource use as evidenced by lower tissue energy and nutrient concentrations and CC.Tissue energy and nutrient concentration were measured throughout the year and compared between M. minutiflora and the co-occurring local population of T. plumosus in a highland savanna and between H. rufa and its neighbor local population of T. plumosus in a lowland savanna. CC was calculated from energy, N and ash concentrations considering ammonium as the sole N source. Differences between co-occurring species, T. plumosus populations, seasons, and organs were analyzed with ANOVA.Highland and lowland grasses differed in concentration and allocation of energy and nutrients whereas the differences between alien and native grasses were specific for each pair considered. Highland grasses had higher energy, N, P and CC than lowland grasses. These variables were always lowest in the culms. In the more stressed lowland site, tissue energy and nutrient concentrations decreased significantly during the dry season except in the roots of both grasses which had the highest energy and nutrients concentrations during the drought. This seasonal response was more marked in the local lowland population of T. plumosus in which maximum CC alternated seasonally between leaves and roots. Energy and nutrient concentrations and CC were the lowest in H. rufa. In the lowland savannas, the higher efficiency of resource use in the invader grass contributes to its higher competitive success through increased growth rate. In the highlands, overall tissue energy concentration and CC, but not N nor P concentration, were lower in the fast growing M. minutiflora but seasonal differences were lacking. The higher leaf CC in T. plumosus can be attributed to the higher proportion of sclerenchyma tissue which is more expensive to construct. Considering CC, both fast growing alien grasses are more efficient in resource use than the co-occurring native grass. However, the role of CC explaining the competitive success of the former, through higher growth rates, is more evident in the more stressful environment of the lowland savanna.     

H+-ATPase as a biochemical marker for early detection of root (wilt) disease in coconut palms (Cocos nucifera L).

H+-ATPase activity in leaves and roots of coconut palms growing in 'root wilt disease-prevalent areas' was compared with that of coconut palms growing in 'disease-free areas'. The activity was found to be significantly less in the leaves and roots of palms in the disease-prevalent zone as compared to that in disease-free zone. Histochemical examination of the leaves showed results that corroborated the biochemical findings. The possible application of H+-ATPase activity as a marker for the early detection of wilt disease in coconut palms is suggested.     

Effects of defoliation and symbiosis on polyamine levels in pine and birch

 We report the effect of ectomycorrhizal fungi (Suillus variegatus, Paxillus involutus) and defoliation on polyamine concentrations in pine (Pinus silvestris) and birch (Betula pendula) foliage and roots. Symbiotic root tips showed consistently higher concentrations of putrescine than non-symbiotic roots. Partial defoliation had no effect on the polyamine levels in mycorrhizal pine or birch roots. The foliage of mycorrhizal pine seedlings had lower putrescine concentrations and higher spermidine than foliage of non-mycorrhizal plants, and defoliation reversed this pattern. The response to partial defoliation differed in birch foliage: mycorrhizal status had no effect and all new growth after defoliation had higher spermidine levels than in non-defoliated birch. The potential role of polyamines in mycorrhizal symbiosis is discussed. Accepted: 26 February 1997     

Effects of leaf water evaporative 2H‐enrichment and biosynthetic fractionation on leaf wax n‐alkane δ2H values in C3 and C4 grasses

Leaf wax n‐alkane δ²H values carry important information about environmental and ecophysiological processes in plants. However, the physiological and biochemical drivers that shape leaf wax n‐alkane δ²H values are not completely understood. It is particularly unclear why n‐alkanes in grasses are typically ²H‐depleted compared with plants from other taxonomic groups such as dicotyledonous plants and why C3 grasses are ²H‐depleted compared with C4 grasses. To resolve these uncertainties, we quantified the effects of leaf water evaporative ²H‐enrichment and biosynthetic hydrogen isotope fractionation on n‐alkane δ²H values for a range of C3 and C4 grasses grown in climate‐controlled chambers. We found that only a fraction of leaf water evaporative ²H‐enrichment is imprinted on the leaf wax n‐alkane δ²H values in grasses. This is interesting, as previous studies have shown in dicotyledonous plants a nearly complete transfer of this ²H‐enrichment to the n‐alkane δ²H values. We thus infer that the typically observed ²H‐depletion of n‐alkanes in grasses (as opposed to dicots) is because only a fraction of the leaf water evaporative ²H‐enrichment is imprinted on the δ²H values. Our experiments also show that differences in n‐alkane δ²H values between C3 and C4 grasses are largely the result of systematic differences in biosynthetic fractionation between these two plant groups, which was on average ?198‰ and?159‰ for C3 and C4 grasses, respectively.     

Interactions between Heterodera rostochiensis Woll. and Verticillium dahliae Kelb. on potatoes and the effect of CCC on both

H. rostochiensis (British pathotype A) increased the severity of the disease caused by Verticillium dahliae in potato plants grown in pots when there were more than ten eggs/g of soil and both pathogens parasitized the root system together. The growth-suppressing chemical CCC, which severely stunted plants, prevented the disease and decreased H. rostochiensis reproduction. An estimate of foliage symptoms measured the effects of the disease more precisely than did tuber yields.     

Allelopathic effect of Aconitum pendulum (Ranunculaceae) on seed germination and seedlings of five native grass species in the Tibetan Plateau

Aconitum pendulum is a dominant weed in bare‐land meadows on the Tibetan plateau and infected areas are very difficult to restore because of the allelopathic effects of this weed. We wanted to select native grasses to restore bare‐land meadows dominated by this toxic weed and tested the allelopathic effect of A. pendulum on seed germination and growth of roots and shoots of five native forage grasses (Elymus nutans, E. sibiricus, Poa crymophila, Festuca sinensis, Bromus inermis). Leaf exudates and high concentrations of root and stem exudates of A. pendulum inhibited seed germination as well as growth of roots and shoots. The allelopathic effect of A. pendulum leaf exudates was stronger than that of roots and stems. Poa crymophila was most resistant to inhibition by A. pendulum, whereas F. sinensis was the most susceptible grass. We conclude that P. crymophila is more suitable than the other four native grasses for sowing in pastures when restoring bare‐land meadows dominated by A. pendulum in the Tibetan plateau.     

The degree of oligophagy in Locusta migratoria (L.)

• 1 Locusta migratoria in the field feeds mainly on grasses, including cultivated species, but there are numerous reports of feeding on other monocotyledons. Feeding on dicotyledons is reported, but in many cases non-grasses are only accepted in the absence of grasses. Previous laboratory work also indicates that grasses comprise the main food, though other plants are eaten.
• 2 Field observations on nymphal bands in Australia show that the insects select grasses within the habitat; dicotyledons are generally rejected.
• 3 The feeding response to 209 species of plants from fifty-six families was tested. The grasses and some Cyperaceae and Juncaceae were generally fully eaten. Some other monocotyledons, such as palms, were also accepted though only small amounts were eaten; orchids and lilies were rejected. Very few dicotyledons were eaten. The dicotyledons which were eaten after 5h without food were rejected after only 1 h; the grasses were readily accepted after the shorter interval
• 4 The degree of oligophagy depends on the choice of food available and the time for which the insect has been deprived of food and water. Grass is normally eaten if available; in its absence, insects which have been without food for some hours will feed on a wide range of plants, but the amounts eaten are usually small.

     

Stilt Root Structure in the Neotropical Palms Iriartea deltoidea and Socratea exorrhiza1

Many arborescent palms develop a stilt root cone that provides increased mechanical support on steep hillsides, better root aeration under waterlogged conditions, resprouting after mechanical damage, and rapid stem elongation. However, for most species the role of stilt roots is not well understood. We determined how palm size and slope inclination affected the structure of stilt roots in the neotropical palms Iriartea deltoidea and Socratea exorrhiza. We expected palm height to be lower on steep slopes because the effectiveness of root support could decrease as slope inclination increases, and thus, the structure of the root cone would vary mostly with slope. Alternatively, if stilt root development is determined by palm size, their production should match stem height, with taller palms having larger cones. We measured the stilt root cone of 31 Iriartea and 36 Socratea palms in San Ramón Biological Reserve and Golfito Wildlife Refuge, Costa Rica. We divided the cone into five variables (horizontal projections of the cone base and stilt root height up and down the slope, and width of the cone base), from which we extracted the first two principal components and used them to measure the effects of slope and palm size on stilt root structure. We found stilt roots to be determined by palm size, not by slope conditions. Stilt roots matched palm size, with larger palms having taller and larger cones, maintaining stability under diverse slope conditions.     

Arbuscular Mycorrhizal Assemblages in Native Plant Roots Change in the Presence of Invasive Exotic Grasses

Plant invasions have the potential to significantly alter soil microbial communities, given their often considerable aboveground effects. We examined how plant invasions altered the arbuscular mycorrhizal fungi of native plant roots in a grassland site in California and one in Utah. In the California site, we used experimentally created plant communities composed of exotic (Avena barbata, Bromus hordeaceus) and native (Nassella pulchra, Lupinus bicolor) monocultures and mixtures. In the Utah semi-arid grassland, we took advantage of invasion by Bromus tectorum into long-term plots dominated by either of two native grasses, Hilaria jamesii or Stipa hymenoides. Arbuscular mycorrhizal fungi colonizing roots were characterized with PCR amplification of the ITS region, cloning, and sequencing. We saw a significant effect of the presence of exotic grasses on the diversity of mycorrhizal fungi colonizing native plant roots. In the three native grasses, richness of mycorrhizal fungi decreased; in the native forb at the California site, the number of fungal RFLP patterns increased in the presence of exotics. The exotic grasses also caused the composition of the mycorrhizal community in native roots to shift dramatically both in California, with turnover of Glomus spp., and Utah, with replacement of Glomus spp. by apparently non-mycorrhizal fungi. Invading plants may be able to influence the network of mycorrhizal fungi in soil that is available to natives through either earlier root activity or differential carbon provision compared to natives. Alteration of the soil microbial community by plant invasion can provide a mechanism for both successful invasion and the resulting effects of invaders on the ecosystem.     

Unique arbuscular mycorrhizal fungal communities uncovered in date palm plantations and surrounding desert habitats of Southern Arabia

The main objective of this study was to shed light on the previously unknown arbuscular mycorrhizal fungal (AMF) communities in Southern Arabia. We explored AMF communities in two date palm (Phoenix dactylifera) plantations and the natural vegetation of their surrounding arid habitats. The plantations were managed traditionally in an oasis and according to conventional guidelines at an experimental station. Based on spore morphotyping, the AMF communities under the date palms appeared to be quite diverse at both plantations and more similar to each other than to the communities under the ruderal plant, Polygala erioptera, growing at the experimental station on the dry strip between the palm trees, and to the communities uncovered under the native vegetation (Zygophyllum hamiense, Salvadora persica, Prosopis cineraria, inter-plant area) of adjacent undisturbed arid habitat. AMF spore abundance and species richness were higher under date palms than under the ruderal and native plants. Sampling in a remote sand dune area under Heliotropium kotschyi yielded only two AMF morphospecies and only after trap culturing. Overall, 25 AMF morphospecies were detected encompassing all study habitats. Eighteen belonged to the genus Glomus including four undescribed species. Glomus sinuosum, a species typically found in undisturbed habitats, was the most frequently occurring morphospecies under the date palms. Using molecular tools, it was also found as a phylogenetic taxon associated with date palm roots. These roots were associated with nine phylogenetic taxa, among them eight from Glomus group A, but the majority could not be assigned to known morphospecies or to environmental sequences in public databases. Some phylogenetic taxa seemed to be site specific. Despite the use of group-specific primers and efficient trapping systems with a bait plant consortium, surprisingly, two of the globally most frequently found species, Glomus intraradices and Glomus mosseae, were not detected neither as phylogenetic taxa in the date palm roots nor as spores under the date palms, the intermediate ruderal plant, or the surrounding natural vegetation. The results highlight the uniqueness of AMF communities inhabiting these diverse habitats exposed to the harsh climatic conditions of Southern Arabia.     

Life expectancy of oil palm (Elaeis guineensis) infected by Ganoderma boninense in coastal soils,Malaysia: a case study

Ganoderma boninense basal stem rot poses a serious threat to the oil palm industry. The effects of external disease symptoms and coastal soils (Briah – Typic Endoaquepts, Jawa – Typic Sulfaquepts, and Selangor – Typic Humaquepts) on the life expectancy of the infected palms, from disease detection to death, were studied. Six-monthly censuses on disease classes for each palm were recorded between 2004 and 2012. Survival curves of disease symptoms and soil types were compared using Kaplan–Meier and log-rank methods, respectively. Ganoderma-infected palms in acid-sulphate (AS) and potential AS soils recorded lower life expectancy. Survival duration of infected palms with foliar symptoms was 12-months shorter. External factors, such as soil type may influence the survival of infected palms and soil types may pre-dispose oil palm to higher risk of Ganoderma infection. More effective Ganoderma management for palms planted on Coastal soils (with and without AS layer) have been proposed.     

PALEOZOIC SEED FERNS: HETERANGIUM KENTUCKYENSIS SP. NOV., FROM THE UPPER CARBONIFEROUS OF NORTH AMERICA

Heterangium kentuckeyensis sp. nov. is described from Lower/Middle Pennsylvanian sediments of eastern Kentucky, based on permineralized stems, petioles, frond members, laminar foliage, and roots, including several organs in attachment. Stems 2.2 to 5.8 mm in diameter are known in several developmental stages. The considerable variability in stelar and cortical histology within this one species emphasizes the need to reassess variability within previously described taxa. Fronds of H. kentuckyensis are at least twice pinnate and bear primary pinnae alternately at approximately right angles. Laminar pinnules have dichotomous venation, are at least 2-lobed, and comparable to foliage of the Sphenopteris-type. Stomata possess 6–7 subsidiary cells with abaxial papillae. The characters used to distinguish the subgenera of Heterangium are evaluated and found to be unreliable. Moreover, suggested phylogenetic schemes both within Heterangium and between this taxon and other lyginopterid pteridosperms based on these features are inconsistent with stratigraphic data. Until reproductive features are known, the classification of Heterangium species is best based on characters of the vegetative sporophyte, including stelar organization (particularly protoxylem architecture), cortical histology, and frond morphology.     

Ecology of black beetle Heteronychus arator. Influence of plant species on larval consumption, utilization and growth

The consumotion, utilization and growth of third-instar larvae of black beetle, Heteronychus arator (F.) feeding on roots of ryegrass (Lolium perenne), paspalum (Paspalum dilatatum) and white clover (Trifolium repens) were studied. Consumption was greater on the grasses than on white clover when assessed by gravimetric methods. Larvae had a higher growth rate on the grasses. Utilization of the roots of the three species was similar indicating that poor growth on white clover is the result of low consumption probably induced by the presence of feeding deterrents in the roots.This work is part of a study carried out by the senior author for the degree of D. Phil. at the University of Waikato.     

Seasonal respiration of foliage, fine roots, and woody tissues in relation to growth, tissue N, and photosynthesis

Autotrophic respiration may regulate how ecosystem productivity responds to changes in temperature, atmospheric [CO₂] and N deposition. Estimates of autotrophic respiration are difficult for forest ecosystems, because of the large amount of biomass, different metabolic rates among tissues, and seasonal variation in respiration rates. We examined spatial and seasonal patterns in autotrophic respiration in a Pinus strobus ecosystem, and hypothesized that seasonal patterns in respiration rates at a common temperature would vary with [N] for fully expanded foliage and fine roots, with photosynthesis for foliage, and with growth for woody tissues (stems, branches, and coarse roots). We also hypothesized that differences in [N] would largely explain differences in maintenance or dormant‐season respiration among tissues. For April–November, mean respiration at 15 °C varied from 1.5 to 2.8 μmol kg^?1 s^?1 for fully expanded foliage, 1.7–3.0 for growing foliage, 0.8–1.6 for fine roots, 0.6–1.1 (sapwood) for stems, 0.5–1.8 (sapwood) for branches, and 0.2–1.5 (sapwood) for coarse roots. Growing season variation in respiration for foliage produced the prior year was strongly related to [N] (r² = 0.94), but fine root respiration was not related to [N]. For current‐year needles, respiration did not covary with [N]. Night‐time foliar respiration did not vary in concert with previous‐day photosynthesis for either growing or fully expanded needles. Stem growth explained about one‐third of the seasonal variation in stem respiration (r² = 0.38), and also variation among trees (r² = 0.43). We did not determine the cause of seasonal variation in branch and coarse root respiration, but it is unlikely to be directly related to growth, as the pattern of respiration in coarse roots and branches was not synchronized with stem growth. Seasonal variations in temperature‐corrected respiration rates were not synchronized among tissues, except foliage and branches. Spatial variability in dormant‐season respiration rates was significantly related to tissue N content in foliage (r² = 0.67), stems (r² = 0.45), coarse roots (r² = 0.36), and all tissues combined (r² = 0.83), but not for fine roots and branches. Per unit N, rates for P. strobus varied from 0.22 to 3.4 μmol molN^?1 s^?1 at 15 °C, comparable to those found for other conifers. Accurate estimates of annual autotrophic respiration should reflect seasonal and spatial variation in respiration rates of individual tissues.     

Adhesion of fimbriated nitrogen-fixing enteric bacteria to roots of grasses and cereals

Summary The role of fimbriae in enterobacterial adhesion to roots of grasses and cereals is discussed. All nitrogen-fixing enteric bacteria isolated in Finland had fimbriae. AllEnterobacter isolates had mannose-binding type-1 fimbriae, whereas most of theKlebsiella isolates had both type-1 and type-3 fimbriae. The strains were isolated from a total of ten different grass species, and no specific association was found between grass species and bacterial fimbriation, biogroup or serogroup. Purified, radiolabeled fimbriae bound to roots ofPoa pratensis in vitro, and bacterial adhesion was inhibited by Fab fragments specific for fimbriae.Klebsiella strains carrying type-3 fimbriae adhered to roots of various grasses and cereals more efficiently than type-1- or nonfimbriated strains, and it was concluded that type-3 fimbriae are the major adhesions ofKlebsiella. Immunofluorescence studies revealed that the bacteria preferentially adhered to root hairs, and to a lesser extent, to the zone of elongation and the root cap mucilage. No strict host specificity in enterobacterial adhesion was observed.