Effects of impacting rain drops on the growth and development of powdery mildew fungi

The effects of rain on different stages of powdery mildew development were investigated quantitatively for Erysiphe graminis on barley, Erysiphe pisi on pea and Sphaerotheca pannosa on rose. Water drops 4 mm in diameter released from a height of 2·5 m onto target leaves reduced the numbers of conidia on the impacted surfaces and inhibited subsequent mildew development, the effects declining as the interval between inoculation and impaction increased. Simulated rainfall applied after inoculation reduced the numbers of conidia on leaves and inhibited mildew development, the effect declining as the interval between inoculation and treatment lengthened. Simulated rainfall applied to infected plants reduced their capacity to inoculate other plants, the effect declining as the interval between treatment and use as inoculum lengthened. Controlled exposure of infected peas and roses to natural rain reduced the numbers of conidia on leaves and inhibited subsequent mildew development, especially on upper leaf surfaces.     

Biological Control of Botrytis cinerea in Cyclamen with Ulocladium atrum and Gliocladium roseum Under Commercial Growing Conditions

ABSTRACT The effect of treatments with conidial suspensions of Ulocladium atrum and Gliocladium roseum on leaf rot of cyclamen caused by Botrytis cinerea was investigated under commercial greenhouse conditions. Spraying U. atrum (1 x 10(6) conidia per ml) or G. roseum (2 x 10(6) conidia per ml and 1 x 10(7) conidia per ml) at intervals of 2 to 3 weeks during the production period and spraying U. atrum (1 x 10(6) conidia per ml) at intervals of 4 to 6 weeks resulted in a significant reduction of natural infections of petioles by B. cinerea. U. atrum or G. roseum (1 x 10(7)conidia per ml) was as effective as the standard fungicide program. B. cinerea colonized senesced leaves within the plant canopy and infected adjacent petioles and leaves later. The antagonists colonized senesced leaves and reduced B. cinerea development on these leaves. Thus, the inoculum potential on petioles adjacent to necrotic leaf tissues was reduced. The fate of U. atrum conidia on surfaces of green cyclamen leaves during a 70-day period after application was studied. The number of conidia per square centimeter of leaf surface remained relatively constant during the entire experiment. Sixty percent of the conidia sampled during the experiments retained the ability to germinate. When green leaves were removed from the plants to induce senescence and subsequently were incubated in a moist chamber, U. atrum colonized the dead leaves. Senesced leaves also were colonized by other naturally occurring fungi including B. cinerea. On leaves treated with U. atrum from all sampling dates, sporulation of B. cinerea was significantly less as compared with the untreated control. Our results indicate that early applications of U. atrum before canopy closure may be sufficient to achieve commercially satisfactory control of Botrytis leaf rot in cyclamen.     

Magnaporthe oryzae conidia on basal wheat leaves as a potential source of wheat blast inoculum

Wheat blast caused by Magnaporthe oryzae Triticum causes significant losses on wheat during outbreak years in several South American countries. Despite reports of wheat blast leaf lesions on some wheat cultivars, the importance of inoculum originating from leaves in severely affected commercial fields is disputed. It is generally considered that leaf lesions and/or sporulation on leaves do not usually appear before the occurrence of spike blast in wheat. The purpose of this study was to (i) determine the occurrence of wheat blast on basal leaves, (ii) estimate the number of conidia produced on these leaves, and (iii) determine the impact of current fungicide application practices on inoculum produced from sporulating lesions on basal wheat leaves. Inoculations at the three‐leaf stage showed that certain cultivar and isolate combinations caused more disease on old wheat leaves than young expanding leaves. Under optimum conditions, M. oryzae had the potential to produce tens to hundreds of thousands of conidia on small amounts of wheat basal leaves. A mean of 1 669 000 conidia were produced on 1 g dry basal leaves of a highly susceptible cultivar under optimum conditions for sporulation. Conidia production on leaves coincided with spike emergence under both greenhouse and field conditions. When field studies were conducted under natural epidemic conditions, foliar fungicide applications reduced the amount of M. oryzae conidia on basal leaves by 62–77% compared to non‐sprayed controls. An earlier application of foliar fungicides might reduce inoculum if conidia from basal leaves contribute to wheat spike blast development.     

Environmental Factors Affecting Production, Release, and Field Populations of Conidia of Alternaria alternata, the Cause of Brown Spot of Citrus

ABSTRACT Alternaria brown spot, caused by Alternaria alternata pv. citri, affects many tangerines and their hybrids, causing loss of immature leaves and fruit and reducing the marketability of the remaining fruit. Conidial production of A. alternata was greatest on mature leaves moistened and maintained at near 100% relative humidity (RH) for 24 h, whereas leaves that had been soaked or maintained at moderate RH produced few conidia. Conidial release from filter paper cultures and infected leaves was studied in a computer-controlled environmental chamber. Release of large numbers of conidia was triggered from both substrates by sudden drops in RH or by simulated rainfall events. Vibration induced release of low numbers of conidia, but red/infrared irradiation had no effect. In field studies from 1994 to 1996, air sampling with a 7-day recording volumetric spore trap indicated that conidia were present throughout the year with periodic large peaks. The number of conidia captured was not closely related to rainfall amounts or average wind speed, but was weakly related to the duration of leaf wetness. Likewise, disease severity on trap plants placed in the field weekly during 1995 to 1996 was not closely related to conidial numbers or rainfall amounts, but was weakly related to leaf wetness duration. Sufficient inoculum appears to be available to allow infection to occur throughout the year whenever susceptible host tissue and moisture are available.     

Effectiveness of systemic resistance in bean against foliar and soilborne pathogens as induced by biological and chemical means

Unifoliate leaves of 9-day-old green bean, Phaseolus vulgaris cv. Redlands Pioneer, were inoculated with 10⁴ conidia/ml Colletotrichum lindemuthianum , causing local lesions, or sprayed with 20 μg 2, 6-dichloro-isonicotinic acid/ml formulated by Ciba-Geigy Ltd as CGA 41396. At various times afterwards (7–16 days), first, second or third trifoliate leaves of these plants were challenge-inoculated with 10⁵ conidia/ml C. lindemuthianum or with the rust pathogen, Uromyces appendiculatus. The numbers of anthracnose lesions or rust uredinia resulting from challenge-inoculation were reduced to similar extents by both pre-treatments compared with control plants. Halo blight, caused by Pseudomonas syringae pv. phaseolicola , was reduced in first trifoliates following treatment of unifoliate leaves 6 days earlier with CGA 41396. Induced resistance to root-infecting pathogens was not observed when stems of either 14- or 16-day-old plants were inoculated with mycelial plugs of Fusarium solani f. sp. phaseoli , or when 11- and 15-day-old plants were inoculated with Rhizoctonia sp., Treatment with CGA 41396 did not protect seedlings when they were transplanted into a mix containing the Fusarium sp. 1 day later.     

Growth and sporulation of <Emphasis Type="Italic">Stemphylium vesicarium</Emphasis>, the causal agent of brown spot of pear,on herb plants of orchard lawns

The inoculum sources of ascospores of Pleospora allii and of conidia of its anamorph Stemphylium vesicarium were investigated in relation to the brown spot disease epidemiology on pear. Dead and living leaves of three pear varieties (Abate Fétel, Conference and William), seven grasses (Poa pratensis, Festuca rubra, Festuca ovina, Lolium perenne, Digitaria sanguinalis and Setaria glauca) and Trifolium repens, which are used in pear orchard lawns, were inoculated with conidia of Stemphylium vesicarium virulent on pear and incubated under controlled-environment. Stemphylium vesicarium was always re-isolated from dead leaves of the considered plants, but not from symptomless green or yellowish living leaves. The fungus was occasionally re-isolated from leaf segments showing unspecific necrosis. Inoculation of pear leaves with isolates from grasses demonstrated that the fungus did not lose pathogenicity. Pseudothecia, ascospores and conidia were produced on all the dead inoculated leaves; differences between specimens were found for phenology of pseudothecia, their density and size, and for the number of conidia produced. Pseudothecia were produced faster in the lawn species than in pear leaves, and their density was higher, especially for S. glauca, L. perenne and P. pratensis. Ascospore maturation and ejection was more concentrated for the pseudothecia developed on pear leaves than for those on F. ovina and S. glauca. All the lawn species produced more conidia than pear leaves.     

The relative importance of conidia and ascospores as primary inoculum of Venturia inaequalis in a southeast England orchard

Apple scab, caused by Venturia inaequalis, can lead to large losses of marketable fruit if left uncontrolled. The disease appears in orchards during spring as lesions on leaves. These primary lesions are caused by spores released at bud burst from overwintering sources; these spores can be sexually produced ascospores from the leaf litter or asexual conidia from mycelium in wood scab or within buds. The relative importance of conidia and ascospores as primary inoculum were investigated in an orchard in southeast England, UK. Potted trees not previously exposed to apple scab were placed next to (c. 1 m) orchard trees to trap air‐dispersed ascospores. Number and position of scab lesions were assessed on the leaves of shoots from both the potted trees (infection by airborne ascospores) and neighbouring orchard trees (infection by both ascospores and splash‐dispersed, overwintered conidia). The distribution and population similarity of scab lesions were compared in the two tree types by molecular analysis and through modelling of scab incidence and count data. Molecular analysis was inconclusive. Statistical modelling of results suggested that conidia may have contributed approximately 20–50% of the primary inoculum in early spring within this orchard: incidence was estimated to be reduced by 20% on potted trees, and lesion number by 50%. These results indicate that, although conidia are still a minority contributor to primary inoculum, their contribution in this orchard is sufficient to require current management to be reviewed. This might also be true of other orchards with a similar climate.     

Overwintering of brown leaf spot fungus, Mycochaetophora gentianae, in infected gentian leaves as the primary inoculum source

Overwintering of the brown leaf spot fungus, Mycochaetophora gentianae, in infected gentian leaves was studied in Iwate, northern Japan. Sporophores were produced on overwintered, infected leaves when they were sampled from January to July, but not in August after incubation in high humidity at 15 °C. Symptoms developed on gentian plants grown in soil artificially infested with overwintered, infected leaves that were either left throughout the experiments or removed before planting. Few lesions developed when plants were grown in soil infested with conidia. These results indicate that M. gentianae can overwinter in infected leaves, which act as the primary inoculum source.     

Virus-induced changes in the response of faba bean to infection by Botrytis

Prior infection of faba bean with the viruses bean yellow mosaic and bean leaf roll increased host susceptibility to subsequent infection by Botrytis fabae and B. cinerea. Cell necrosis beneath inoculum droplets, rate and extent of lesion spread and sporulation of B. fabae were all increased on detached leaves from virus-infected compared with healthy plants. Changes were most marked in young leaves showing conspicuous symptoms of systemic virus infection and in plants virus-infected for at least 2-4 weeks. Localized lesions produced by B. cinerea or a low concentration of B. fabae conidia (10³ spores/ ml) showed increased cell necrosis but were not transformed into aggressive, spreading lesions on virus-infected leaves.     

Effects of leaf wetness duration, relative humidity, light and dark on infection and sporulation by Didymella rabiei on chickpea

No infection occurred at less than 95% relative humidity (r.h.) when chickpea plants were dried after inoculation with conidia of Didymella rabiei. Infection was significant when the dry leaves were exposed to 98% r.h. for 48 h. When inoculated plants were subjected to different leaf wetness periods, some infection occurred with 4 h wetness, and disease severity increased with wetness duration according to an exponential asymptote, with a maximum value after about 18 h. Germination of conidia and germ tube penetration increased linearly with increasing wetness periods when recorded 42 h after inoculation. With a 24-h wetness period, germination of conidia was first observed 12 h after inoculation and increased linearly with time up to 52 h (end of the experiment). Dry periods immediately after inoculation, followed by 24-h leaf wetness, reduced disease severity; as the dry period increased the severity decreased. Disease severity increased with increasing periods of darkness after inoculation. The number of pycnidia and the production of conidia on infected leaves increased only slightly with high r.h. (either in the light or in the dark), but large increases occurred over an 8-day period when the leaves were kept wet.     

Suppression of conidial germination and appressorial formation by silicate treatment in powdery mildew of strawberry

The mode of action of soluble silicon against strawberry powdery mildew (Sphaerotheca aphanis var. aphanis) was investigated in four experiments. First, silicon-treated leaves from plants grown with silicate (Si+) and control leaves were excised, inoculated with conidia, and subsequent germination and formation of appressoria in a petri dish was assessed after 24 h. The germination rate was 49.7% on Si+ leaves, and was 67.2% on control leaves (t-test, P < 0.01). Second, we soaked cellulose membranes in various solvents and then placed the membranes on 4% water agar, dusted the membranes with conidia, and examined after 12 h. No difference was apparent between any treatment and the control (distilled water). Third, strawberries growing hydroponically with additional silicon in the medium were inoculated with conidia, and leaves were observed with a scanning electron microscope 1–2 days after inoculation. Germ tubes and secondary hyphae were shorter and had fewer branches on Si+ leaves than on the control. Moreover, penetration appeared to be inhibited. Fourth, the cuticle was separated from leaves from plants grown as in the third experiment, placed on water agar, and dusted with conidia. Germination of conidia, observed with a light microscope, on Si+ leaves was suppressed markedly to 40%–60% of that of the control. These results suggested that soluble silicon induced physiological changes in the cuticle layer after absorption by the plant. In addition, soluble silicate reduced germination of conidia, formation of appressoria, and possibly the penetration of powdery mildew.     

Spatial and temporal variation of reactive oxygen species and antioxidant enzymes in o-hydroxyethylorutin-treated tomato leaves inoculated with Botrytis cinerea

Pretreatment of detached tomato leaves with o -hydroxyethylorutin reduced the percentage leaf area affected, and delayed the appearance of necrosis, following inoculation with conidial suspensions in droplets of the grey mould fungus Botrytis cinerea . o -Hydroxyethylorutin delayed, but did not inhibit, in vitro germination of conidia, although overall percentage germination was reduced compared with water controls. Both the reactive oxygen species (ROS) – superoxide anions and hydrogen peroxide – increased twice as much in o -hydroxyethylorutin-treated leaf tissue 2 and 6 h postinoculation with B. cinerea conidia in tissues under inoculation drops, as well as in surrounding tissues, whereas in plants not pretreated with the compound ROS generation was noticed later, and only in tissues under inoculation drops. Compared with these compounds, changes in the levels of hydroxyl radicals, lipid peroxidation and the activity of the enzymes superoxide dismutase, guaiacol peroxidase, ascorbate peroxidase and catalase were largely unchanged. In stimulating ROS in inoculated tomato tissue, B. cinerea appeared to be affected directly pre- and postinfection, but indirect effects increasing host resistance cannot be excluded.     

Effects of Ascochyta blight caused by Mycosphaerella pinodes on the translocation of carbohydrates and nitrogenous compounds from the leaf and hull to the seed of dried-pea

The effects of Ascochyta blight (caused by Mycosphaerella pinodes ) on changes in dry weight and in water, carbohydrate, nitrogen and free amino acid contents were studied in the leaf, the hull and the seeds on the second fructifer node of pea ( Pisum sativum ). Pea plants were grown in a glasshouse and inoculated with various concentrations of conidia at the beginning of seed filling, with uninoculated plants as controls. Disease induced a premature water loss of hulls and leaves, accelerated seed desiccation and reduced seed weight. Biochemical analyses revealed a decline in the carbohydrate content and a lower nitrogen remobilization in diseased leaves and hulls. Thus, Ascochyta blight alters carbohydrate metabolism, protein remobilization and free amino acid translocation from these organs. Disease also reduced carbohydrate and nitrogen content in seeds and, in case of high disease severity, the carbohydrate/nitrogen ratio in the seeds was also affected (seed protein concentration increased and starch concentration decreased).     

Effects of inoculum density,leaf wetness duration and nitrate concentration on the occurrence of lettuce leaf spot

In Ehime Prefecture, Japan, lettuce leaf spot (Septoria lactucae) caused huge losses in marketable lettuce yields. To explore potential measures to control disease outbreaks, the effects of inoculum density, leaf wetness duration and nitrate concentration on the development of leaf spot on lettuce (Lactuca sativa) were evaluated. Conidia were collected from diseased plants in an infested field by single-spore isolation and were used to inoculate potted lettuce plants with different conidial concentrations. Lesions developed on inoculated lettuce plants at inoculum concentrations from 10⁰ to 10⁶ conidia/ml. The disease was more severe when the inoculum exceeded 10² conidia/ml, and severity increased with increasing concentrations. Assessment of the relationship between disease development and the duration of postinoculation leaf wetness revealed that symptoms appeared when the inoculated plants remained wet for 12 h or longer. The number of lesions and total nitrogen content in the lettuce leaves both increased when nitrate was applied.     

Fungal development and plant response in detached onion, onion bulb scales and leaves inoculated with Botrytis allii, B. cinerea, B. fabae and B. squamosa

Fungal development and plant responses were examined in detached leaves and mid-bulb scales of Allum cepa. Following inoculation with suspensions of 105 conidia/ml distilled water Botrytis squamosa consistently produced spreading lesions in leaves and bulb scales. B. allii produced spreading lesions at most sites in bulbs but was very inconsistent in its infection of leaves; lesions were often confined to inoculation sites. Limited lesions were usually produced by B. cinerea but R. fabae failed to produce symptoms at most sites. Extensive colonization by B. allii and B. tauamosa required rapid penetration and totally necrotrophic fungal growth. During development of a spreading lesion, plant cell walls became very swollen around intramural hyphae and wall swelling appeared to precede epidermal cell death. Resistance to colonization was due to poor germination, failure to produce distinct infection hyphae (associated with accumulation of deposits of granular reaction material [RM] in underlying live cells) or restriction of infection ryphae amongst small groups of dead cells (limited lesion formation). Only B. fabae germinated poorly, and germ-tubes produced often failed to attempt penetration but grew over the leaf or bulb scale surface. Reducing numbers of conidia increased the frequency of sites associated with RM accumulation; granular deposits being particularly common at sites inoculated with low numbers of B. allii conidia. Electron microscopy revealed that RM granules were osmiophilic aggregates formed between the plasma membrane and epidermal cell wall. In the absence of RM, growth of avirulent species was restricted within the swollen walls of dead epidermal cells. Results ae compared with those from studies on tulip and broad bean leaves.     

Plant waste-based composts suppressive to diseases caused by pathogenic Fusarium oxysporum

The suppressive ability of three plant residue-based composts that could serve as components of soilless media for several vegetable crops was tested on four different formae speciales of Fusarium oxysporum: melonis, basilici, radicis-lycopersici and radicis-cucumerinum. The composts were prepared under controlled conditions from a mixture of separated cow manure (SCM) with orange peels (OP), wheat straw (WS), or dried tomato plants that had been removed from the greenhouse after the end of the season (TP). Disease development in melon, tomato and cucumber seedlings growing in the three composts was significantly less than that observed in peat. Plant inoculation was achieved by conidia produced in culture, conidia naturally produced on infected stems and soil inoculum produced by enriching the soil with infected tissues. Pathogen colonization of the roots and stems of infected melon plants grown in TP–SCM and OP–SCM composts was significantly lower than that of peat-grown plants. Sterilization by gamma irradiation reduced the suppressive capability of TP–SCM and OP–SCM composts, whereas it did not affect the disease development and final disease incidence in peat. Tested formae speciales exhibited differing decline rates of the conidia incorporated in the composts, compared with the rate in the peat control, which suggests that different mechanisms may be involved in the suppression of the different pathogens. The present study shows that composts based on plant-waste residues suppress diseases caused by different formae speciales of Fusarium oxysporum.     

Occurrence of New Powdery Mildew on Greenhouse Tomato Cultivars

During a year-round survey on the occurrence of powdery mildew on greenhouse-cultivated tomato plants, the disease was most severe in June and July. All tomato plants (45 commercial cultivars and 11 breeding lines) tested were infected with the pathogen but had different degrees of susceptibility. The pathogen was epiphytic and produced white, round pustules mainly on leaves of tomato plants. The pathogen produced conidia singly on conidiophores and forked appressoria on inoculated tomato leaves and seemed to be an Oidium sp. of Erysiphe polygoni type. Received 18 December 2000/ Accepted in revised form 22 July 2001     

Strawberry Plant Extracts Stimulate Secondary Conidiation by Colletotrichum acutatum on Symptomless Leaves

ABSTRACT Conidial suspensions of Colletotrichum acutatum were prepared in 1:27, 1:45, and 1:81 (wt/vol) dilutions of an extract of strawberry (cv. Tristar) flowers or leaves in water. Strawberry leaves and plastic coverslips were sprayed with the conidial suspensions, incubated at 25 degrees C and continuous wetness for 48 h, and the number of conidia and appressoria were counted. In another experiment, leaves and coverslips were sprayed with a conidial suspension in water, incubated for 72 h to establish C. acutatum populations, and placed in a growth chamber under dry conditions for up to 6 weeks. At each sampling time, leaves and coverslips were sprayed with flower extracts, leaf extracts, or water, incubated for 48 h at 25 degrees C and continuous wetness, and the number of conidia and appressoria were counted. Flower extracts significantly (P 相似文献   

Effects of virus infection of faba bean on subsequent infection by Uromyces viciae-fabae

Prior infection of faba bean with bean yellow mosaic virus (BYMV) and bean leaf roll virus decreased pustule density on leaves subsequently infected by Uromyces viciae-fabae. Changes were most marked on young leaves showing conspicuous symptoms of systemic virus infection and in plants virus-infected for at least 2-4 weeks. Pustule density progressively decreased when inoculations were made with uredospore generations successively produced on and inoculated to BYMV-infected leaves.
Uredospore germination and germ tube length were similar on virus-free and BYMV-infected leaves and on agar seeded with spores produced for one or several generations on BYMV-infected or virus-free plants. Formation of appressoria was slightly reduced on BYMV-infected leaves but changes in the post-penetration development of the rust probably accounted for much of the decreased pustule production encountered.