Botrytis cinerea Infection of Grape Flowers: Light and Electron Microscopical Studies of Infection Sites

ABSTRACT Specific floral organs including the calyptra, stigma, and receptacle area of glasshouse-grown grapevines (Vitis vinifera cv. Cabernet Sauvignon) were inoculated with aqueous suspensions of Botrytis cinerea conidia, and the initial steps involved in colonization and infection of the host tissues were studied for several days postinoculation using light microscopy as well as scanning and transmission electron microscopy. Conidia germinated on all floral organs examined and became attached to the host surface within 48 h after inoculation. However, in all cases the vast majority of conidia accumulated in a channel-like gap between the ovary and the calyx that extended in a narrowing fashion into the flower interior where the ovary joined the receptacle. Very few conidial germ tubes were detected in the style following inoculation of the stigma, and no evidence for their growth toward the ovaries could be found. In contrast, hyphae were more abundant in the receptacle area, regardless of the site of inoculation. Tips of the calyx became necrotic and mycelium formed in the gap between ovary and calyx within 72 h following inoculation, providing a major point of colonization and infection. B. cinerea colonized dehiscent calyptras within 72 h of inoculation, providing a potential source of inoculum from calyptras that remained stuck in the cluster. The results suggest that the grape flower's receptacle area is the predominant site of infection for B. cinerea, although a minor portion of infections may also occur through the stigma and style.     

Molecular and symptom analyses of phytoplasma strains from lettuce reveal a diverse population

ABSTRACT Epidemics of aster yellows in lettuce in Ohio are caused by at least seven distinct phytoplasma strains in the aster yellows (AY) group. Five of the strains are newly reported: AY-BW, AY-WB, AY-BD3, AY-SS, and AY-SG. All seven strains were characterized based on symptoms in aster and lettuce, and by polymerase chain reaction (PCR). Strain AY-BD2 (formerly 'Bolt') causes yellowing and leaf distortion in lettuce and bolting in aster, whereas strain AY-S (formerly 'Severe') causes stunting, leaf clustering, and phyllody. Strain AY-WB causes yellowing and wilting in lettuce and witches'-broom in aster. Strain AY-SG induces horizontal growth in lettuce and aster plants. Strain AY-BW causes chlorosis of emerging leaves and abnormally upright growth of leaf petioles. AY-SS causes symptoms similar to those caused by AY-S but has a different PCR-restriction fragment length polymorphism (RFLP) banding pattern. Strains AY-BD2 and AY-BD-3 cause mild leaf and stem distortion in lettuce but are differentiated by PCR-RFLP. All phytoplasma strains collected from lettuce in Ohio belong to the 16SrI group. AY-WB belongs to the 16SrI-A subgroup and the other six belong to the 16SrI-B subgroup. Five of the seven strains were distinguished from each other by primer typing. The results of phylogenetic analyses of sequences of the 16S rRNA genes were basically consistent with the classification based on PCR-RFLP, in which AY-WB clustered with phytoplasmas of the 16rIA subgroup and the other Ohio lettuce strains clustered with phytoplasmas in the 16SrI-B subgroup.     

Effectiveness of the resistance inducer prohexadione‐Ca against fireblight in shoots of apple trees inoculated with Erwinia amylovora1

Due to the lack of effective, non‐phytotoxic and publicly acceptable materials for controlling fireblight in pome fruit trees, novel strategies against Erwinia amylovora are being sought. Resistance‐inducing compounds, such as prohexadione‐Ca, represent promising alternatives. Prohexadione‐Ca is the active substance of the bioregulator Regalis, currently being introduced in several European countries and overseas. Prohexadione‐Ca reduces shoot elongation due to inhibition of gibberellin biosynthesis. Furthermore, it leads to significant changes in the spectrum of flavonoids and their phenolic precursors in pome fruits, which causes reduced susceptibility to fireblight and other pathogens. In 2002 and 2003, container‐grown apple trees of the cultivars ‘Idared’ and ‘Freedom’ were treated with different dosages of prohexadione‐Ca two weeks before inoculation with E. amylovora. The effect of prohexadione‐Ca against shoot blight was determined by measuring the lengths of necrotic lesions and symptoms on vascular bundles caused by the pathogen. Treatments with prohexadione‐Ca turned out to be much superior to the ones with streptomycin, kasugamycin and a bacterial antagonist, which were used for comparison. Acibenzolar‐S‐methyl (Bion), another resistance‐inducing compound, was included in some of the experiments and gave intermediate results. The simultaneous control of excessive shoot growth and shoot infections by fireblight is seen as a major advantage of using prohexadione‐Ca in pome fruit trees.     

Effects of foliar- and root-applied silicon on the enhancement of induced resistance to powdery mildew in Cucumis sativus

Two cucumber ( Cucumis sativus ) cultivars differing in their resistance to powdery mildew, Ningfeng No. 3 (susceptible) and Jinchun No. 4 (resistant), were used to study the effects of foliar- and root-applied silicon on resistance to infection by Podosphaera xanthii (syn. Sphaerotheca fuliginea ) and the production of pathogenesis-related proteins (PRs). The results indicated that inoculation with P. xanthii significantly suppressed subsequent infection by powdery mildew compared with noninoculation, regardless of Si application. Root-applied Si significantly suppressed powdery mildew, the disease index being lower in Si-supplied than in Si-deprived plants, regardless of inoculation treatment. The resistant cultivar had a more constant lower disease index than the susceptible cultivar, irrespective of inoculation or Si treatment. Moreover, with root-applied Si, activities of PRs (for example peroxidase, polyphenoloxidase and chitinase) were significantly enhanced in inoculated lower leaves or noninoculated upper leaves in inoculated plants of both cultivars. Root-applied Si significantly decreased the activity of phenylalanine ammonia-lyase in inoculated leaves, but increased it in noninoculated upper leaves. However, Si treatment failed to change significantly the activity of PRs in plants without fungal attack. Compared to the control (no Si), foliar-applied Si had no effects either on the suppression of subsequent infection by P. xanthii or on the activity of PRs, irrespective of inoculation. Based on the findings in this study and previous reports, it was concluded that foliar-applied Si can effectively control infections by P. xanthii only via the physical barrier of Si deposited on leaf surfaces, and/or osmotic effect of the silicate applied, but cannot enhance systemic acquired resistance induced by inoculation, while continuously root-applied Si can enhance defence resistance in response to infection by P. xanthii in cucumber.     

Studies on the suitability of trap-tubs to control noxious rodents in forests

Various types of trap-tubs were tested both under controlled conditions in standardised trial series and in the field. New trap-tubs were poorly accepted by the animals, whereas weathered tubs were readily accepted within a very short period of time. The trapping mechanisms of tubs used in Lower Saxony and improved tubs in Hesse were both found to be suitable. The trapping method proved to be highly selective since long-tailed mice (Apodemus spp.) can easily escape by jumping onto the edge of the tub. Voles (Microtus agrestis, Clethrionomys glareolus) explored the inside of the tub and worked its exits with decreasing intensity during the time of captivity. They died of hypothermia within a few hours. Elimination of the trapped rodents by their natural enemies was not observed. The trials showed that trap-tubs are suitable for selective reduction of the density of field voles and red-backed voles in forests. The results also contribute to assessing the humaneness of the control method.     

Relationships Between Tillage and Spatial Patterns of Heterodera glycines

ABSTRACT The dynamics of Heterodera glycines spatial patterns were studied under different tillage systems in two naturally infested soybean fields in Iowa from 1994 to 1997. At each location, there were four different tillage treatments (conventional tillage, reduced tillage, ridge tillage, and no tillage). Soil samples were taken from 98 contiguous quadrats (5.2 m(2)) per plot in the fall of 1994, before any tillage was performed, and in the spring of the following 3 years shortly after planting. Cysts were extracted from soil samples by elutriation and counted, and eggs were extracted from cysts and enumerated. Spatial patterns of H. glycines populations were characterized by geostatistical analysis and variance-to-mean (VM) ratios. Semivariance values were calculated for cyst and egg densities and semivariograms were constructed. In general, there was greater spatial dependence among cyst populations than egg populations. In one field with a strongly aggregated initial H. glycines population, tillage practices resulted in changes in spatial patterns of H. glycines populations, characterized by spherical-model semivariogram parameters (sill, nugget effect, and range of spatial dependence). These parameters indicated increasing aggregation over time in no tillage and ridge tillage treatments, but decreasing aggregation in reduced and conventional tillage treatments. There was an increase of 350% in sill values (maximum semivariance) for cyst populations after 3 years of no tillage, but in the conventional tillage treatment, sill values remained unchanged or decreased over time as tillage was implemented. Semivariograms for cyst and egg population densities revealed strong anisotropy (directional spatial dependence) along soybean rows, coincident with the direction of tillage practices. VM ratios for cyst counts increased each year in the no tillage and ridge tillage treatments, but decreased for 2 years in reduced tillage and conventional tillage treatments. Final VM ratios for cyst and egg counts were highest in the no tillage treatment. In a second field, with low initial aggregation of H. glycines populations, there was little measurable change in semivariogram parameters after 3 years of no tillage, but in the conventional tillage treatment, populations became less aggregated, as the range, sill, and the proportion of the sill explained by spatial dependence decreased for cyst population densities. Our results indicated that in soybean fields with initially aggregated populations of H. glycines, no tillage and ridge tillage systems promoted aggregation of the nematode population, whereas conventional and reduced tillage systems resulted in a less aggregated spatial pattern.     

Interference between triazine-resistant Brassica napus and Panicum miliaceum

Atrazine carryover often limits growers to production of atrazine-tolerant crops the year following application, and allows the increase of triazine-tolerant weed species such as Panicum miliaceum L. (wild proso millet). Tiriazine-resistant Brassica napus L. cv. ‘Triton’ (oilseed rape) was tested to characterize the nature of interspecific interference with P. miliaceum. In a greenhouse study, atrazine at 2.2 kg ha^?1 depressed oilseed rape fruit (siliqua) number and fruit dry weight, and delayed flowering, but did not significantly affect height or weight of shoots, Oilseed rape fruit weight was reduced at 200 P. miliaceum plants m^?2. fruit number and shoot weight were inhibited at 400 weeds m^?2. and height was reduced and flowering delayed at 600 weeds m^?2. Number and weight of fruits were reduced by one-third after 8 weeks of interference as compared to oilseed rape grown with the weed for 4 weeks. Oilseed rape height was reduced by 29% and shoot weight by 55% by 600 weeds m^?2 and 2–2 kg ha^?1 atrazine, while fruit number and weight were reduced by 72%. Oilseed rape shoot weight was reduced by 74% by 600 weeds m^?2 for 12 weeks of interference, while fruit number and weight were reduced by 85% and 82%. respectively. In a field study, fluazifop reduced early season P. miliaceum cover by 72%, but did not increase oilseed rape cover. Mid-season P. miliaceum shoot weight was decreased by 97% by fluazifop and oilseed rape shoot weight was increased by 34%. P. miliaceum control increased oilseed rape biomass by 38% at 89 days, but biomass of oilseed rape sown at 11.2 kg ha^?1 with 2.2 kg atrazine ha^?1 was not decreased by P. miliaceum interference at 89 days.