Microsite heterogeneity in litterfall risk to seedlings

Abstract Litterfall is an important cause of damage and mortality to seedlings in many forest ecosystems. This study is the first to investigate the contribution of variable risk of litterfall damage to microsite heterogeneity. Two hundred artificial seedlings were ‘planted’ in the ground at 2‐m intervals along transects in each of two New Zealand forests, and all plant species with foliage directly above each artificial seedling were recorded. Additional artificial seedlings were planted on the ground underneath treeferns and on their trunks (epiphytically). The artificial seedlings were monitored monthly for damage over 2 years. Three overtopping species that cause litterfall damage, along with one species that impedes litterfall, produced different microsites with a hierarchy of litterfall damage risk to seedlings (2–30% per year). This risk differed significantly among microsites (P < 0.0005). Seedlings differ in resilience to litterfall and, therefore, microsites with different litterfall risks provided the potential for regeneration niche differentiation. More seedlings were damaged beneath Cyathea dealbata (Cyatheaceae), which drops whole fronds, than on its trunks. The reverse was found for Dicksonia fibrosa (Dicksoniaceae), which retains dead fronds as a ‘skirt’. We suggest that shedding whole fronds, or producing a skirt of fronds, are alternative ‘strategies’ that can reduce competition from terrestrial and epiphytic seedlings, respectively.     

Fungal occurrence,disease incidence and severity,and yield of maize symptomatic for seedling disease in Mississippi

A study was conducted in Mississippi from 1995 to 1997 comparing soil rhizosphere fungi isolated from Pioneer 3167 hybrid maize (Zea mays L.) planted on Brooksville silty clay and Memphis silt loam soils. Maize seedlings were collected over four sampling dates from conventional and no-tillage plots. Eleven fungal genera consisting of nineteen species were isolated from these plants; Trichoderma spp. were most frequently isolated, followed by Fusarium spp. The highest disease incidence occurred in tilled plots of the latest planting date on Brooksville silty clay when samples were collected 17 days after planting. Root disease was most severe in 1996 from seedlings planted on the last planting date in tilled plots sampled 17 days after planting. Yields were significantly (P ≤ 0.05) higher on Brooksville silty clay soil than on Memphis silt loam in both 1995 and 1996. Yields were highest from no-tillage plots and from maize planted on the earliest date. There was a significant correlation between incidence of root infection and disease severity. There was no correlation between the incidence of root infection and yield or between disease severity and yield at either location. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tree fern trunks facilitate seedling regeneration in a productive lowland temperate rain forest

Seedling regeneration on forest floors is often impaired by competition with established plants. In some lowland temperate rain forests, tree fern trunks provide safe sites on which tree species establish, and grow large enough to take root in the ground and persist. Here we explore the competitive and facilitative effects of two tree fern species, Cyathea smithii and Dicksonia squarrosa, on the epiphytic regeneration of tree species in nutrient-rich alluvial forests in New Zealand. The difficulties that seedlings have in establishing on vertical tree fern trunks were indicated by the following observations. First, seedling abundance was greatest on the oldest sections of tree fern trunks, near the base, suggesting that trunks gradually recruited more and more seedlings over time, but many sections of trunk were devoid of seedlings, indicating the difficulty of establishment on a vertical surface. Second, most seedlings were from small-seeded species, presumably because smaller seeds can easily lodge on tree fern trunks. Deer browsing damage was observed on 73% of epiphytic seedlings growing within 2 m of the ground, whereas few seedlings above that height were browsed. This suggests that tree ferns provide refugia from introduced deer, and may slow the decline in population size of deer-preferred species. We reasoned that tree ferns would compete with epiphytic seedlings for light, because below the tree fern canopy photosynthetically active radiation (PAR) was about 1% of above-canopy PAR. Frond removal almost tripled %PAR on the forest floor, leading to a significant increase in the height growth rate (HGR) of seedlings planted on the forest floor, but having no effects on the HGRs of epiphytic seedlings. Our study shows evidence of direct facilitative interactions by tree ferns during seedling establishment in plant communities associated with nutrient-rich soils.     

Incidence and pathogenicity ofFomes lignosus (Klotzch) bres. onTriplochiton scleroxylon K. Schum

The effect ofFomes lignosus (Klotzsch) Bress onTriplochiton scleroxylon K. Schum was determined by artificially inoculating 5 to 6-week old seedlings, as well as by conducting a field survey on 1 to 4-year old trees. Inoculated seedlings did not show any foliar symptoms of infection during 4 months of observation. Though heights of inoculated seedlings were not affected by the pathogen, diameters of such seedlings were significantly smaller (atP=0.05) than those of control seedlings. Naturally infectedT. scleroxylon trees in the field were very often found close together, usually around infected tree stumps. Leaves of diseased trees remained healthy-looking, thus making detection of the disease difficult in trees lacking signs (viz. rhizomorphs and fruitbodies) of the pathogen.     

Woody plant regeneration in three South Carolina Taxodium/Nyssa stands following Hurricane Hugo

Long-term monitoring began in 1990 to follow community changes resulting from hurricane disturbance. In addition, one-year-old Taxodium distichum seedlings were planted to determine if planting was feasible in saltwater-flooded areas. The canopy of the least impacted swamp recovered rapidly, but there were few seedlings growing in the understory. Planted seedlings survived well, but they grew very little. Both lack of seedlings and poor growth of planted seedlings were probably due to intense shading and flooding. Two impacted areas contained a greater number of seedlings, most of which were found growing on raised microsites like Taxodium knees. The majority of the seedlings in all areas were shrub species. Planted seedlings grew cell (30 cm/yr) where open canopy conditions allowed sunlight to reach the forest floor and no new saltwater has been introduced since the hurricane.     

The significance of antibiotic production by Pseudomonas aureofaciens PA 147-2 for biological control of Phytophthora megasperma root rot of asparagus

Pseudomonas aureofaciens PA147-2 produces an antibiotic (Af⁺) which inhibits the growth of fungal phytopathogens on phosphate buffered potato dextrose agar (PBPDA). To determine the role of the antibiotic in disease suppression in vivo, PA147-2 and an antibiotic-deficient Tn5 mutant (Af^-) PA109, were tested for their ability to suppress root rot of Asparagus officinalis seedlings caused by Phytophthora megasperma var sojae, in the glasshouse. Seedlings coinoculated with the pathogen and the wildtype strain PA147-2, showed a significantly reduced level of infection and disease severity compared to seedlings inoculated with the pathogen alone. However, 100% of seedlings treated with Af^- mutant PA109 were diseased. Furthermore, a strain derived from mutant PA109, restored to Af⁺ through allele replacement of Tn5 by homologous recombination, gave similar levels of disease suppression as the wildtype. This suggests the antibiotic produced by PA147-2 is important for the control of P. megasperma in vitro as well as in planta. Treatment of seedlings with PA147-2 and derived strains including the Tn5 mutant (Af^-) strain improved plant weight by 40–100% in the presence and absence of the pathogen suggesting PA147-2 also has a direct growth stimulatory effect independent of antibiotic production.     

Transmission of Bacterium Corynebacterium michiganense pv. michiganense by Seeds

The results of experiments on transmission of Corynebacterium michiganense pv. michiganense showed that the spread of the bacterium by artificially infested seeds varies, but it was confirmed that from artificially infested seeds few diseased plants can be grown. The results depend on the environmental conditions during experimentation and the method of assaying the seedlings. High relative humidity favours the spread of the pathogen. The use of selective media helped the detection ofthe pathogen in seemingly healthy plants. It was proved that host can have population of C. michiganense pv. michiganense in a resident phase. It is assumed that when conditions are favorable the pathogen multiplies and cause disease.     

Bacterial antagonists of Sunflower (Helianthus annuus L.) fungal pathogens

Bacteria isolated on nutrient agar and King's medium B from sunflower leaves, crown and roots inhibited in vitro growth of the leaf spot and wilt pathogens Alternaria helianthi, and Sclerotium rolfsii, respectively, and also the root rot pathogensRhizoctonia solani and Macrophomina phaseolina. Antagonistic bacteria from leaves were mainly actinomycetes and pigmented Gram-positive bacteria, while those from roots and crowns were identified asPseudomonas fluorescens-putida, P. maltophilia, P. cepacia, Flavobacterium odoratum andBacillus sp. In soil bioassays, when used as seed inoculum in the presence ofS. rolfsii, P. cepacia strain N24 increased significantly the percentage of seedling emergence. Bacterial strains which exhibited broad spectrum in vitro antagonistic activity were tested for colonisation of sunflower roots, when used as a seed inoculum. Good colonisers (10⁴ to 10⁶ bacteria/g root) were consistent in their ability to reduce disease and fungal wilt. A seedling having a primary root length < 5 cm with fewer lateral roots, necrosed cotyledons or crown and a wilted shoot indicated its diseased status. On an average, only 30% of seedlings were diseased when treated with the antagonistic strains, in the presence of the pathogen, while 60% of the seedlings were diseased in the presence of the pathogen alone. In microplots treated with strain N24, only 1 to 3% of the seedlings were wilted, while 14% of the seedlings were wilted in the presence of the pathogen alone. The results obtained show that bacterial antagonists of sclerotial fungi can be used as seed inocula to improve plant growth through disease suppression     

Epidemiological and molecular study on ‘Candidatus Phytoplasma prunorum’ in Austria and Hungary

The epidemiology of ‘Candidatus Phytoplasma prunorum’ was studied in Austria and Hungary from 2014 to 2018. Testing of root samples showed average infections rates of 61 and 40% of the Austrian Prunus spinosa and Prunus domestica spp. insititia samples, respectively. In Hungary, on average 21% of the P. spinosa and 13% of the feral Prunus cerasifera samples were infected. The pathogen was found in 18 out of 19 apricot orchards and PCR positive Cacopsylla pruni were observed at 11 out of 17 sampling locations in both countries. In cage experiments with C. pruni remigrants successful pathogen transmission to Prunus armeniaca, P. domestica and P. spinosa seedlings in budding and foliated developmental stages was recorded, an inoculation access period of 4 hr was sufficient for transmission. A field experiment with ungrafted apricot seedlings planted in 2012 and 2014 indicated a prominent role of the insect vectors for disease spread. In 2017, 40 and 28% of the trees planted in 2012 and 2014, respectively, were infected. Molecular characterisation based on the genes aceF and imp allowed the discrimination between 10 phytoplasma types in apricots. Around 70% of the phytoplasma types in apricots were also common in P. spinosa, in P. domestica spp. insititia and in remigrant C. pruni pointing to a possible pathogen exchange by insects between wild and cultivated Prunus spp. For disease control, vector management over the entire flight period of the remigrants seems necessary; when selecting active compounds, the short inoculation access period of not more than 4 hr should be considered.     

Spatial pattern of seedlings 1 year after fire in a Mediterranean pine forest

Summary The spatial distribution of seedlings of the dominant perennial plant species (Pinus halepensis, Cistus salviifolius, Rhus coriaria) and may annual species was studied after a wild fire in an eastern Mediterranean pine forest. The spatial distribution of all seedlings is affected by the location of the old burned pine trees. Seedling density of Pinus and Cistus is higher at a distance from the burned pine canopy and lower near the burned pine trunk. It is also higher beneath small burned pine trees than under big ones. Rhus seedling density is higher under big burned pine trees and also near the burned trunks. Seedlings of Pinus, Cistus and Rhus growing under the burned canopy of big pine trees tend to be taller than seedlings under small ones or outside the burned canopy. Most annual species germinate and establish themselves outside the burned canopies, and only a few annual species are found beneath them. It is suggested that variation in the heat of the fire, in the amount of ash between burned pine trees of different sizes, and in the distance from the burned canopy are responsible for the observed pattern of seedling distribution. The possible ecological significance of the spatial pattern of seedlings distribution and their differential growth rate are discussed.     

Resident bacteria, nitric oxide emission and particle size modulate the effect of Brassica napus seed meal on disease incited by Rhizoctonia solani and Pythium spp

Amendment of orchard soil with low-glucosinolate Brassica napus (rape) seed meal (RSM) suppresses infection of apple roots by Rhizoctonia solani but increases incidence of Pythium spp. infection. Following incorporation of Brassica sp. seed meals, soils were monitored for changes in populations of selected saprophytic and plant pathogenic microorganisms. When conducted in pasteurized soil, which possessed high numbers of Bacillus spp. and lower than detectable numbers of Streptomyces spp., RSM amendment did not provide control of R. solani. Populations of streptomycetes in RSM-amended soil increased to stable levels >20-fold higher than in non-amended soil. Disease suppressiveness was restored to pasteurized RSM-amended soil by adding any of several Streptomyces strains. Maximal rates of nitrification in orchard soil, determined by nitric oxide emission, were observed within two weeks following RSM amendment and inhibition of nitrification via application of nitrapyrin abolished the capacity of RSM to suppress R. solani infection of apple roots when seedlings were planted one day after soil amendment. Apple seedling mortality and Pythium spp. root infection were highest for seedlings planted immediately following incorporation of B. napus cv. Athena RSM, particularly when meal was added in a flake rather than powder form. Lower infection frequencies were observed for seedlings planted four weeks after RSM incorporation, even for soil in which densities of culturable Pythium spp. had not declined. Our results demonstrate that suppression of Rhizoctonia root rot in response to RSM amendment requires the activity of the resident soil microbiota and that initial disease control is associated with the generation of nitric oxide through the process of nitrification.     

Habitat and Population Effects on the Germination and Early Survival of the Invasive Weed, Lythrum salicaria L. (purple loosestrife)

In this study, we examined how environmental factors and differences among populations of Lythrum salicaria L. (Lythraceae; purple loosestrife) influenced the establishment of this species into both its preferred wetland habitat and secondary upland habitat. Seeds and seedlings from five parents from each of the three populations were planted in each wetland and upland of two sites in both disturbed and undisturbed plots. More seeds germinated in wetland than in upland sites (31% as compared to 11%) and, although a disturbance resulted in greater germination (23% as compared to 17% in undisturbed plots), there was no interaction with habitats. The results of a simultaneous greenhouse germination study in which seeds were planted under saturated and drained conditions suggest that the higher germination observed in wetland sites was due to the higher moisture present at such sites (79% vs. 45% in saturated and drained conditions, respectively). Disturbance enhanced both germination and growth of L. salicaria and this was especially so for seedlings. Dry weights of seedlings from disturbed plots were five times greater than those from undisturbed plots ( =9.2 vs. 1.8g in disturbed and undisturbed conditions, respectively). In addition to environmental factors, population differences are likely to play an important role in the spread of L. salicaria as we found differences between the populations in both life stages examined. One of the populations germinated equally well in both moisture treatments. In our seedling establishment experiment, one of the populations produced both the smallest plants and the most and earliest flowers, suggesting superior reproduction.     

Survival and perpetuation of Alternaria brassicae causing Alternaria blight of oilseed crucifers

Modes of survival and perpetuation of Alternaria brassicae attacking rapeseed and mustard were examined. The pathogen remained viable in diseased plant debris and seeds of infected plants which served as primary sources of inoculum. The pathogen was internally seed-borne and infected seeds resulted in damping-off of seedlings. Air-borne conidia produced on aerial parts of the infected plants were responsible for secondary spread of the disease.     

Photosynthetic characteristics of dipterocarp species planted on degraded sandy soils in southern Thailand

To elucidate whether dipterocarp species, dominant late-successional species of tropical forests in Southeast Asia, actually have a disadvantage when planted on open site in terms of their photosynthetic characteristics, we investigated photosynthesis in dipterocarp seedlings planted in the open on degraded sandy soils in southern Thailand. These species were compared with seedlings of Acacia mangium Willd., a fast-growing tropical leguminous tree, which is often planted on degraded open site in Southeast Asia. The dipterocarp seedlings had an irradiance-saturated net photosynthetic rate (P _N), stomatal conductance (g _s), carboxylation efficiency, and photosynthetic capacity comparable to or superior to those of A. mangium. In particular, seedlings of Dipterocarpus obtusifolius Teijsm. ex Miq. showed an irradian-ce-saturated P _N of 21 μmol m⁻² s⁻¹, a value higher than any previously reported for a dipterocarp species, accompanied by high g _s (0.7 mol m⁻² s⁻¹) and high photosynthetic capacity. Thus dipterocarp species do not necessarily have a disadvantage in terms of their photosynthetic characteristics on open sites with degraded sandy soils.     

Host finding in the pine weevil Hylobius abietis: effects of conifer volatiles and added limonene

The attraction of Hylobius abietis (L.) (Coleoptera: Curculionidae) to -pinene was recently shown to be inhibited by limonene. This study shows that added limonene also affects the response of H. abietis to odours from natural host material. The experiments included both pieces of Scots pine suspended in pitfall traps and planted pine seedlings that had either been manually wounded or left intact. By adding a limonene dispenser, the catch of pine weevils in pine-baited traps was reduced by about 80–90% in spring and autumn and by about 40–50% in summer. Limonene substantially reduced the rate at which manually wounded seedlings were attacked by weevils in spring and autumn. For intact seedlings, a limonene-induced reduction in attack rate was found only in autumn. In the absence of limonene, a small wound made on the stem of a seedling increased its probability of being attacked by about fivefold. This figure was estimated by comparing data from all experimental periods with a model assuming a constant ratio of attack rates for the two kinds of seedlings.     

The Occurrence and Economic Impact of <Emphasis Type="Italic">Plasmodiophora brassicae</Emphasis> and Clubroot Disease

The significance of Plasmodiophora brassicae Woronin and clubroot disease which it incites in members of the family Brassicaceae is reviewed as the focus for this special edition of the Journal of Plant Growth Regulation. This is a monographic treatment of recent research into the pathogen and disease; previous similar treatments are now well over half a century old. Vernacular nomenclature of the disease indicates that it had a well-established importance in agriculture and horticulture from at least the Middle Ages onward in Europe and probably earlier. Subsequently, the pathogen probably spread worldwide as a result of transfer on and in fodder taken by colonists as livestock feed. It is a moot point, however, whether there was much earlier spread by P. brassicae into China and subsequently Japan as Brassica rapa (Chinese cabbage and many variants) colonized those lands in archaeological time. Symptoms, worldwide distribution, and economic impact are briefly described here to provide a basis for understanding subsequent papers. Clubroot disease devastates both infected field and protected vegetable and agricultural Brassica crops. Particular importance is placed on recent reports of crop losses in tropical countries, albeit where the crops are grown in cooler altitudes, and in the Canadian prairie land canola crops. The latter is of enormous importance because this crop is the single most important and essential source of vegetable oils used in human foodstuffs and in industrial lubricants where mineral oils are inappropriate.     

Biological control of seedling blight of wheat caused by Fusarium graminearum with beneficial rhizosphere microorganisms

Fusarium graminearum is associated with the cereal damping-off complex which reduces germination, seedling stand and yield. Fifty-two bacterial strains and six Trichoderma spp. isolated from the wheat rhizosphere were evaluated for biocontrol of seedling blight of wheat caused by F. graminearum. Their potential as biocontrol agents was tested in vitro and in the greenhouse. Isolates varied in their ability to inhibit the mycelial growth of F. graminearum in agar plate bioassays by 0–79%. This parameter was not related with biocontrol efficacy of in vivo assays. In greenhouse trials, all isolates were initially evaluated for reducing disease on wheat cultivars Klein Centauro (moderately resistant to F. graminearum) and Pro INTA Oasis (susceptible) planted in sterilized soil artificially infested with the pathogen. Among the 25 bacteria and six fungal isolates that exhibited a pronounced suppressive effect, the most efficient 10 for both cultivars were further assayed on eight cultivars (Buck Candil, Buck Catriel, Buck Chambergo, Buck Poncho, Buck Topacio, Klein Cacique, Klein Centauro and Pro INTA Oasis) potted in cultivated–inoculated soil. Three weeks after sowing, plant stand, percentage of diseased emerging seedlings, plant height and dry weight were evaluated. Among the antagonists only Stenotrophomonas maltophilia was significantly better than the control for the average of the eight cultivars for plant stand, height and dry weight. Stenotrophomonas maltophilia also caused a non-significant decrease in the percentage of diseased plants. Three strains of Bacillus cereus and one isolate of Trichoderma harzianum gave also a good control in some cultivars. The ability of these isolates to affect the infection of wheat seedlings by F. graminearum may be of potential value in field trials.     

Comparative ecophysiology of two representativeQuercus species appearing in different stages of succession

Ecophysiological comparisons were made of the growth and photosynthetic characteristics between seedlings of deciduousQuercus serrata and evergreenQuercus myrsinaefolia. Q. myrsinaefolia seedlings naturally occurring in secondary coppice forests showed exponential-like growth in height with age, while sympatricQ. serrata seedlings were considerably smaller in height, their growth being limited by shortage of light. The photosynthetic characteristics measured under laboratory conditions showed no bases for the differences in growth between the two species on the forest floor: Light compensation points of the seedlings raised under 5% daylight were almost identical for the two species, being about 6.0 μE·m⁻²·s⁻¹. Growth analysis of seedlings planted in a coppice forest showed that bothQ. serrata andQ. myrsinaefolia could hardly grow during the summer under the shrub layer, when relative photon flux density (RPFD) was 0.9±0.5%. In the winter, when RPFD under the leafless canopy increased to 29.3±2.7%, the dry matter production of the evergreen seedlings ofQ. myrsinaefolia was much improved. Current-year seedlings of the species showed NAR of 0.102±0.021 g·dm⁻²·mo⁻¹ during the winter. Temperature dependency of photosynthesis and increment of leaf temperature by direct solar beam also indicated active photosynthesis ofQ. myrsinaefolia on the forest floor during the winter.     

The role of saltwort (Batis maritima L.) in regeneration of degraded mangrove forests

While saltwort (Batis maritima L.) is common in the fringe mangrove forests of southwest Florida, its role in regeneration of degraded mangrove communities is not known. Given the potential encroachment and subsequent degradation of mangrove communities by sea-level rise, it is important to quantify the effect of early-colonizing vegetation to early mangrove seedling survival. A greater number of mangrove seedlings were observed in existing B. maritima patches compared to surrounding mudflats. A planting experiment was designed to determine whether B. maritima was responsible for the observed pattern. Black mangrove (Avicennia germinans L.) seedlings, raised in a nursery, were planted in previously established B. maritima patches and on mudflats with and without nursery-raised B. maritima. There was significantly lower mortality of A. germinans seedlings when planted in existing B. maritima patches (69%), compared to seedlings planted on the mudflats (93%), demonstrating that existing B. maritima improved A. germinans seedling survival. Nursery-raised B. maritima had lower mortality on open mudflats (28%), suggesting that it can tolerate conditions, which make it an early colonizer of newly available habitats. The primary mechanism proposed for improving seedling success is a slight increase in elevation provided by the dense root network of established B. maritima. These findings have implications for scientists and managers anticipating the response of mangroves to sea-level rise.     

Modeling the impact of a biocontrol agent, Puccinia lagenophorae, on interactions between a crop, Daucus carota, and a weed, Senecio vulgaris

The impact of a biocontrol agent spreading from a point source on crop–weed interactions was modeled. The model encompassed: (i) severity of crop–weed competition as affected by a rust pathogen, (ii) velocity of spread of the rust pathogen, and (iii) density of infected plants introduced in the weed population as starting points (inoculum sources) for an epidemic. The model was parameterized for a study system encompassing the crop Daucus carota (carrot), the weed Senecio vulgaris (common groundsel), and its antagonist Puccinia lagenophorae. The parameters of (i) were estimated in a greenhouse study using a response-surface design. Estimates of the parameter of (ii) were obtained from the literature. The density of infected plants (iii) was varied to simulate crop loss as function of density. Simulations were run for various densities of the weed and various velocities of rust pathogen spread. The results of the simulations indicated a crop-loss ranging from 5 to 10% at levels of relatively weak D. carota–S. vulgaris competition. Density of inoculum sources and velocity of P. lagenophorae spread had only minor effects on crop loss. In contrast, density of inoculum sources and velocity of spread had major effects on crop loss at levels of intermediate (range of 10–35% loss) and severe competition (range of 30–70% loss). The results are discussed both with respect to biological control of S. vulgaris using P. lagenophorae as biocontrol agent and as a general model describing the impact of the spatial dynamics of a pathogen (natural enemy) on plant competition.