Infected Leaf Discs Research Articles

Potential of Lecanicillium species for dual microbial control of aphids and the cucumber powdery mildew fungus, Sphaerotheca fuliginea

Abstract Detached leaf disc bioassays were conducted against cucumber powdery mildew and three species of aphid with three entomopathogenic species of Lecanicillium; Lecanicillium longisporum (Vertalec®), Lecanicillium attenuatum (CS625), and an unidentified isolate (DAOM198499). The three Lecanicillium species had high virulence against the aphids Myzus persicae, Macrosiphum euphorbiae and Aulacorthum solani with the exception of DAOM 198499, which demonstrated reduced virulence to A. solani with an LT50 of 6.4 days. Otherwise, LT50 ranged between two and four days. Suspensions of conidia and blastospores of the Lecanicillium species were also applied onto 15 mm leaf discs dissected from cucumber plants previously inoculated with Sphaerotheca fuliginea. Powdery mildew did not develop when the Lecanicillium applications were made one and eight days after S. fuliginea inoculations. When Lecanicillium was applied to highly infected leaf discs 11 and 15 days after S. fuliginea inoculation, the application suppressed subsequent production of S. fuliginea spores as compared to the controls. These results suggest the potential of a dual role for Lecanicillium spp. as microbial control agents against aphids and powdery mildew.

Relationship between peroxidase activity and the amount of fully N-methylated compounds in bean plants infected by Pseudomonas savastanoi pv. phaseolicola

Changes in the level of endogenous formaldehyde (HCHO), some N-methylated compounds (choline and trigonelline) and peroxidase activity were examined in the leaves of bean genotypes (Phaseolus vulgaris L.) with different disease-sensitivity during ontogenesis in the stressfree condition and after natural infection by Pseudomonas savastanoi pv. phaseolicola (until the appearance of lesions). HCHO, as its dimedone adduct, and fully N-methylated compounds were determined by overpressured layer chromatography (OPLC) in different developmental stages and in the infected leaves/leaf discs. Peroxidase activity was measured by a spectrophotometric method. HCHO level decreased with ageing of the primary leaf and accordingly in the leaves at different developmental stages, then increased again in both cases due to the demethylation and methylation processes. Concentration of choline and trigonelline as potential HCHO generators decreased considerably while peroxidase activity increased with ageing of the plants. Comparing the symptomless and the Pseudomonas infected leaf discs (with watersoaked lesions) we found a decrease in the level of HCHO, choline and trigonelline and there was detectable increase in the peroxidase activity in the infected leaf tissues. Our findings are in accordance with previously published results that peroxidases play an important role in oxidative demethylation processes. Our hypothesis is that the high level of HCHO in the old leaves can originate from methylated components as the result of peroxidase activity and this high level may lead to the old leaf being resistant to pathogen. This conclusion is supported by the fact that the leaves of susceptible bean genotypes became resistant to Pseudomonas while growing older.

High temperature effects on the response of photosynthesis to light in sweet orange plants infected with Xylella fastidiosa

The objective of this study was to evaluate the high temperature effects on the response of photosynthesis to light in sweet orange plants infected with Xylella fastidiosa. This vascular bacterium is the causal agent of the citrus variegated chlorosis that causes severe economical losses to the Brazilian citrus industry. The responses of the photosynthetic oxygen evolution and the parameters related to chlorophyll a fluorescence to the increase in light intensity were evaluated at 35ºC and 45ºC in both healthy and infected leaf discs. The increase in temperature affected the photosynthetic apparatus of both healthy and infected plants, although infected plants showed higher photochemical sensitivity at the higher temperature (e.g. in the potential quantum efficiency of photosystem II, maximum and basal fluorescence yield, and in the relation between variable and basal fluorescence yield). This higher sensitivity of infected plants was not reflected in the overall photosynthetic reaction, since photosynthetic oxygen evolution values did not vary at 45ºC. Healthy and infected plants showed differences in photosynthetic oxygen evolution but displayed similar effective quantum efficiency of photosystem II as well as apparent electron transport rates at 35ºC. These results suggest that the limitations in photosynthesis observed on the infected plants might arise through impaired biochemical reactions.

Sporulation of Plasmopara viticola: Differentiation and Light Regulation

Abstract: The development of grape downy mildew (Plasmopara viticola) was followed histologically during the entire latent period until the appearance of mature sporangia. Production of sporangiophores and sporangia was assessed using low‐temperature scanning electron (LTSEM) and fluorescent light microscopy. Time‐course studies using attached leaves of Vitis vinifera cv. Müller‐Thurgau revealed that the production of sporangiophores and sporangia is a highly coordinated process and is completed within 7 h. As this differentiation is assumed to occur only in darkness, the influence of light was investigated. For this purpose, different light regimes were applied to infected leaf discs of V. viniferacv. Müller‐Thurgau. White light irradiation prevented formation of sporangia, although the growth of the mycelium was not affected. Many sporangiophores were observed that were abnormally shaped, i.e., short hyphae in clusters or thin, extremely elongated hyphae. For the formation of mature sporangia, a prolonged dark period was necessary. Light experiments suggest photosensitivity at the end of the latent period. A terminal white light irradiation caused an inhibitory effect, whereas a final phase of darkness promoted sporangium development. Different light qualities were tested, revealing an inhibition of sporangium development by blue light whereas neither red nor far‐red light were effective.

Brief Note : Stimulation of jasmonic acid production in Zea Mays L. infected by the maize rough dwarf virus - R韔 Cuarto. Reversion of symptoms by salicylic acid

In the present paper we study the possible biological relevance of endogenous jasmonic acid (JA) and exogenous salicylic acid (SA) in a plant-microbial system maize-virus. The virus disease "Mal de Río Cuarto" is caused by the maize rough dwarf virus-Río Cuarto. The characteristic symptoms are the appearance of galls or "enations" in leaves, shortening of the stem internodes, poor radical system and general stunting. Changes in JA and protein pattern in maize control and infected plants of a virus-tolerant cultivar were investigated. Healthy and infected-leaf discs were collected for JA measurement at different post-infection times (20, 40, 60 and 68 days). JA was also measured in roots on day 60 after infection. For SDS-PAGE protein analysis, leaf discs were also harvested on day 60 after infection. Infected leaves showed higher levels of JA than healthy leaves, and the rise in endogenous JA coincided with the enation formation. The soluble protein amount did not show differences between infected and healthy leaves; moreover, no difference in the expression of soluble protein was revealed by SDS-PAGE. Our results show that the octadecanoid pathway was stimulated in leaves and roots of the tolerant maize cultivar when infected by this virus. This finding, together with fewer plants with the disease symptoms, suggest that higher foliar and roots JA content may be related to disease tolerance. SA exogenous treatment caused the reversion of the dwarfism symptom.

Plant-mediated interactions between insects and a fungal plant pathogen and the role of plant chemical responses to infection

Diverse organisms simultaneously exploit plants in nature, but most studies do not examine multiple types of exploiters like phytophagous insects and fungal, bacterial, and viral plant pathogens. This study examined patterns of induction of antipathogenic peroxidase enzymes and phenolics after infection by the cucurbit scab fungus, Cladosporium cucumerinum, and then determined if induction mediated ecological effects on Colletotrichum orbiculare, another fungal pathogen, and two insect herbivores, spotted cucumber beetles, and melon aphids. Peroxidase induction occurred in inoculated, `local,' symptom-bearing leaves 3 days after inoculation, and in `systemic,' symptom-free leaves on the same plants 1 day later. Phenolics were elevated in systemic but not in local leaves 3 days after inoculation. Detached systemic leaves from plants inoculated with C. cucumerinum developed significantly fewer and smaller lesions after challenge with C. orbiculare. Spotted cucumber beetles did not show consistently significant preferences for infected versus control leaf disks in comparisons using local or systemic leaves, but trends differed significantly between leaf positions. In no-choice tests, beetles removed more leaf area from local but not from systemic infected leaves compared to control leaves, and melon aphid reproduction was enhanced on local infected leaves. In the field, cucumber beetle and melon aphid densities did not differ between infected and control plants. Antipathogenic plant chemical responses did not predict reduced herbivory by insects. Other changes in metabolism may explain the positive direction and spatially dependent nature of plant-mediated interactions between pathogens and insects in this system.

Engineering Genetic Resistance against Insects in Japanese Persimmon Using the cryIA(c) Gene of Bacillus thuringiensis

Japanese persimmon (Diospyros kaki L. `Jiro') was transformed using a disarmed strain of Agrobacterium tumefaciens, EHA101, carrying the binary plasmid vector, pDU92.710. The T-DNA region of pDU92.710 contained the kanamycin resistance gene (nptII), the β-glucuronidase gene (uidA), and a synthetic reconstruct of cryIA(c) encoding the insecticidal crystal protein fragment of Bacillus thuringiensis subsp. kurstaki HD-73. Leaf discs made from leaves of shoot cultures were cocultivated with Agrobacterium and cultured on a callus-induction medium containing kanamycin and cefotaxime. Among 720 infected leaf discs, 17 putative transformed callus lines showing kanamycin resistance were obtained after 8 weeks of culture. When these were cultured on a regeneration medium containing kanamycin, 15 formed adventitious buds. Of the 15 shoot lines, 11 grew well on a shoot-proliferation medium containing kanamycin, while 4 lines did not grow well. Of the 11 shoot lines, 10 showed GUS activities by fluorometric assay and were subjected to polymerase chain reaction (PCR) and Southern analyses. Except for two lines, all results were consistent with a stable integration of T-DNA into the persimmon genome. The production of CryIA(c) protein in transformed shoot lines was confirmed with Western analysis using anti-CryIA(c) serum. Insect bioassays were conducted with 10 lines showing GUS activity. Many of these lines showed high significant mortality of the test insects, Plodia interpunctella Hüber and Monema flavescens Walker, when compared to nontransformed controls.

Dynamic changes in the activities of glucose-6-phosphate dehydrogenase, ribulose bisphosphate carboxylase and ribonuclease in tobacco leaves, leaf discs and mesophyll protoplasts in relation to TMV multiplication

Three metabolic pathways of virus-RNA biosynthesis (reductive and/or oxidative pentose phosphate cycles, rRNA degradation pathway) were investigated using three tobacco systems: leaves, leaf discs and mesophyll protoplasts infected with tobacco mosaic virus (TMV).TMV multiplication differed in each system. The maximum concentration of TMV (7.2 mg TMV g−1fresh wt) was reached at 18 days post inoculation (p.i.) in leaves, but in leaf discs the TMV multiplication curve culminated at 9 days p.i. and virus content was lower (2.04 mg TMV g−1fresh wt). In protoplasts, most virus was synthesized between 8 and 30 h p.i. and TMV concentration reached 45.6 μg TMV per 106living protoplasts at 72 h p.i.Virus infection significantly increased ribonuclease (RNase) activity in all three systems. Maximum enhancement to 140% was observed at 16 days p.i. in leaves, to 241.8% at 8 days p.i. in discs and to 146.1% at 30 h p.i. in protoplasts in comparison with mock-inoculated healthy controls (100%). Significant linear correlation between enhanced RNase activity and TMV content was found in leaves and discs (r=0.872+++and r=0.979+++, respectively) but not in protoplasts.Activity of glucose-6-phosphate dehydrogenase increased to 128% 12 days p.i. in infected leaves. The enhancement of enzyme activity linearily correlated with the TMV accumulation (r=0.781+++) over the whole monitored period. In leaf discs the activity of this enzyme was initially enhanced to as much as 148.7% at 6 days p.i., and then a slow decrease was observed. In protoplasts this enzyme was not detectable.Activity of RUBISCO did not differ significantly during the first 10 days between healthy and infected plants, but then a rapid fall to 60–65% of healthy control values was found in infected leaves. In infected leaf discs the enzyme activity gradually decreased after inoculation. At the time of the culmination of the TMV multiplication curve the activity was 16% of control mock-inoculated disc tissues. Similarly, in protoplasts this enzyme was not detectable.

Evidence for a primary role of active oxygen species in induction of host cell death during infection of bean leaves withBotrytis cinerea

Bean genotypes (Phaseolus vulgaris) selected for high or low sensitivity to oxidants showed similar stress responses to ozone, paraquat and to infection withBotrytis cinerea. The potentially oxidative nature of the fungal attack was investigated on bean leaf discs floated in diluted malt broth medium and inoculated with conidia ofB. cinereaisolates differing in aggressiveness. Hydrogen peroxide and OH* radicals were measuredin situduring infection. Additionally, peroxidases, cellulases, xylanases and polygalacturonases (PG) diffusing into the incubation medium during infection were analysed. An assay based on the oxidative degradation of 2-deoxyribose enabled the specific detection ofin situgenerated OH* radicals. The aggressiveness of six isolates ofBotrytiswas closely correlated with the amount of H2O2and OH* radicals detected in the leaf tissue during infection. Aggressive isolates induced peak signals of OH* radical formation within 24 h and strongly elevated levels of H2O2within 48 h post-inoculation (hpi), in a phase of most rapidly progressing infection. No such increases in active oxygen species (AOS) were detected in non-aggressive interactions. All fungal isolates tested suppressed plant peroxidase activity as compared to non-inoculated leaf discs. However, aggressive isolates were significantly more suppressive than non-aggressive ones, suggesting a role for peroxidases in plant resistance as scavengers of harmful AOS. Hydrogen peroxide production measured in infected leaf discs was stronger than its accumulation in the incubation medium, pointing to sources of H2O2inside the plant. The scavengers of AOS, catalase and D-mannitol significantly reduced severity of infection. Secretion of cellulases and xylanases occurred later than the start of infection, suggesting a role of these cell-wall degrading enzymes in the digestion of dead rather than the killing of living host tissue. PGs were produced by all fungal isolates much earlier (12 hpi) but the production did not reflect the isolate aggressiveness. This is the first study to directly detect toxic AOSin situduring infection of plant tissue byB. cinerea. It is concluded that AOS, besides their probable role in plant resistance, might be instrumental for necrotrophs likeB. cinereato kill the host tissue in initial stages of infection.

Evidence for the involvement of ascochitine in phoma leafspot-wilt disease of Clematis

Ascochitine, a phytotoxic metabolite, was purified from cultures of Phoma clematidina and identified by 1 H and 13 C nuclear magnetic resonance spectroscopy and electron impact and chemical ionization mass spectroscopy. This toxin-induced electrolyte leakage from leaf discs of Clematis cultivars that were susceptible to fungal infection, while there was no significant electrolyte leakage from leaf discs of the cultivar most resistant to fungal infection. The level of ascochitine production in vitro by P. clematidina isolates was related to isolate virulence. The fungal isolates could be characterized into two groups: (1) high virulence, high ascochitine production, and (2) low virulence, low ascochitine production. Ascochitine was isolated from P. clematidina -infected leaf discs, indicating the toxin is produced in vivo . Leaf tissues exposed to ascochitine solutions showed black flecking in proportion to the concentration of ascochitine. Scanning electron and light microscopy of infected leaves indicated that the fungal hyphae were well behind the necrotic zone in leaf spots, while transmission electron and light microscopy suggested that mitochondria and chloroplasts were the major organelles affected by ascochitine, although extensive cellular damage was evident. These observations suggest that ascochitine may be involved in the pathogenesis of P. clematidina against Clematis , by killing plant cells prior to hyphae ramification through the necrotic tissue.

Inhibition of infectivity of potato virus X by flavonoids

Infectivity of potato virus X (PVX) in Chenopodium quinoa was strongly inhibited (80%) by low concentrations (< 1 μg ml− 1) of quercetin. Morin and 3-O-methylgalangin were also strong inhibitors. Methoxylation of quercetin reduced the inhibitory effect. Quercetin did not induce resistance, did not affect symptom development, had no direct viricidal effect and did not affect multiplication of PVX in infected leaf discs. Quercetin did not reduce infectivity when applied either before or after PVX inoculation. When PVX-RNA was used as inoculum, quercetin stimulated infectivity by 75%. It is proposed that quercetin interferes with an interaction between PVX coat protein and a host receptor essential for infection.

Factors affecting the rate of T-DNA transfer from Agrobacterium tumefaciens to Nicotiana glauca plant cells.

Different factors involved in the early steps of the T-DNA transfer process were studied by using a beta-glucuronidase gene (gusA) as a reporter in Nicotiana glauca leaf disc transformation experiments. The levels of transient expression of the gusA gene in leaf discs infected with several strains or vir mutants correlated well with their virulence phenotype, except for virC mutants. The rate of T-DNA transfer was shown to be stimulated in the case of non-oncogenic strains by the co-transfer of small amounts of oncogenic genes. It was found that the location of the T-DNA in the Agrobacterium genome affected the T-DNA transfer rate especially in virC mutants. The virC mutants transferred the gusA-containing T-DNA located on a binary vector more efficiently than the oncogenic T-DNA of the Ti plasmid. Although wild-type strains induced high levels of gusA expression early after infection, the gusA expression appeared to be lost late after infection in the infected leaf discs. In contrast, in leaf discs infected by virC mutants the level of gusA expression increased steadily in time. A model explaining these results is presented.

Vegetative growth and sporulation of Bipolaris sorokiniana on infected leaves of Poa pratensis exposed to postemergence herbicides

Poa pratensis was exposed to the postemergence herbicides 2,4-D, MCPP, and dicamba at concentrations of 10−6 M applied to the soil surface. The four youngest leaf blades were inoculated with Bipolaris sorokiniana and subsequently detached to determine the effect of the herbicides on vegetative hyphal growth and sporulation on the leaf-blade tissue. Hyphal growth from infected leaf disks cut from detached leaves was stimulated by 2,4-D and dicamba. MCPP did not affect hyphal growth. All herbicides increased sporulation by B. sorokiniana on the detached leaves. Increases in sporulation did not differ between 2,4-D and MCPP. Dicamba induced the heaviest sporulation on leaf blades. Key words: dicamba, MCPP, mecoprop, 2-methoxy-3,6-dichlorobenzoic acid, 2(2-methyl-4-chlorophenoxy)propionic acid, 2,4-dichlorophenoxyacetic acid.

Involvement of an O 2− generating system in the induction of necrotic lesions on tobacco leaves infected with tobacco mosaic virus

Leaf discs of the tobacco cultivar Samsun NN [a cultivar of Nicotiana tabacum containing the N gene for resistance to tobacco mosaic virus (TMV)], inoculated with TMV and grown at 30 °C for 36 h to allow infection, formed ring-shaped necrotic lesions 6–7 h after transfer to 20–26 °C. The infected leaf discs showed a marked O2− generating activity almost immediately after transfer to 20 °C, which decreased rapidly and then underwent a cyclic oscillation three times within a further 4 h incubation at 20 °C. The net activity detected for the initial 10 min after transfer to 20 °C was significantly correlated with the number of necrotic lesions which appeared later. Leaves of non-infected Samsun NN, and those of TMV-infected and non-infected Samsun (a cultivar without the N gene) showed little O2− generating activity after transfer from 30 to 20 °C. The accumulation of formazan at the sites of subsequent lesion development in TMV-infected Samsun leaf discs, infiltrated with nitroblue tetrazolium (NBT) at the beginning of the 20 °C incubation, suggested that O2− generation may be activated in sites of TMV-multiplication. A membrane-rich fraction isolated from TMV-infected Samsun NN leaf discs incubated at 30 °C showed an enhanced O2− generating activity dependent on NADPH and Ca2+ at 20–26 °C, but not at temperatures higher than 28 °C. Infiltration of TMV-infected Samsun NN leaves with NADP+, SOD or catalase caused a reduction in necrotic lesion formation following transfer from 30 to 20 °C. These results indicate that the activation of an NADPH-dependent O2− generating reaction in leaves of TMV-infected tobacco cultivars carrying the N gene may be involved in the induction of the necrotic lesions caused by virus infection.

Effects of hornonal balance during leaf-disc infection of Nicotiana plumbaginifolia with Agrobacterium tumefaciens nopaline strain C58

Abstract The technique of leaf-disc transformation has been used in order to study the influence of different hormonal balances on the process of transformation of Nicotiana plumbaginifolia tissues with Agrobacterium tumefaciens nopaline-strain C58. C58 Agrobacterium strain incites withish, compact undifferentiated calli on Nicotiana tabacum, whilst allows the recover of shoots in N. plumbaginifolia. Shooting can be modulated through the hormonal balance of the culture medium mostly during the tissue culture phase (phase 2) subsequent to the period of infection (phase 1). Complete absence of hormones in phase 1 and 2 does not result in any callus formation or shooting on the edges of the infected leaf-discs, thus indicating that in this case the transformed cells need hormones at least in initial phase of development. In both the phases, addition of 2,4-D, NAA and kinetin resulted necessary in the stimulation of callus and shooting, namely in phase 2, and kinetin played a determinant role. Direct shooting...

The Development of the Multinucleate Condition ofPeronospora TabacinaSporangia

The development of the multinucleate condition of Peronospora tabacina sporangia was studied at 15 and 23 C using the fluorescent stain mithramycin. Sporangiophores and sporangia from infected leaf disks were collected 7 h after induction of sporulation and thereafter at 30 min intervals for 5 h. Since the developmental age of sporangia varied among sporangiophores within samples, length of sporangia was used as a proxy class variable for time. Initially, one nucleus migrated from the sporangiophore into each small (3 μm). spherical sporangium. A predominant number of the developing sporangia in the 5–10 μm classes incubated at 15 C had either 2, 4 or 8 (2n) nuclei. Chi-square analyses of the number of sporangia in these nuclear classes supported the hypothesis that nuclei were derived from 1 (22) or 2 (23) synchronous divisions of the daughter nuclei from the initial nucleus and not from random divisions or nuclear migration from the sporaqgiophore. Nuclei in developing sporangia incubated at 23 C exhibited one synchronous division (22). Subsequent divisions occurred randomly. The average number of nuclei in mature sporangia of the 3 isolates examined was approximately 15.

Monohybrid and dihybrid segregations in the progenies of tobacco transformed for kanamycin resistance with a Ti-vector system

A chimeric DNA construction having nopaline synthase promoter, coding sequences of neomycin phosphotransferase gene conferring resistance to antibiotic kanamycin and OCS (octopine synthase) polyadenylation sequences bracketed by T-DNA ends was transferred to tobacco. Leaf discs were infected withA. tumefaciens containing disarmed, cointegrate plasmid pGV3850:: 1103 and allowed to form a callus in the presence of kanamycin. Shoots regenerated from infected leaf discs either through the callus or arising directly were further selected for their ability to root in kanamycin-containing media. Among the nine transgenic plants that were progeny tested, the transferred bacterial gene segregated as monohybrid ratio (3 KanR: 1 Kans) in seven. Segregation data of two plant progenies indicated the presence of two independent loci of KanR DNA insertion (15 KanR: 1 Kan s ). Back-cross segregation data were consistent with the monohybrid or independent assortment of duplicate factors. Thus in the two cases, a minimum independent integration of two copies of T-DNA each with a KanR marker is inferred.

Human leukocyte interferon and the antiviral factor (AVF) from virus-infected plants stimulate plant tissues to produce nucleotides with antiviral activity

Leaves of Nicotiana glutinosa and tobacco, as well as tobacco callus cultures, were treated with the plant antiviral factor AVF or with a purified subspecies of human leukocyte interferon. After incubation, a fraction containing short oligonucleotides was extracted directly from the plant tissue. In addition, a synthetase preparation was fractionated from treated tissues, which polymerized ATP in the presence of polyinosinic, polycytidylic acid (poly(I:C)). The various plant nucleotides were applied to tobacco mosaic virus (TMV)-infected leaf disks, the TMV content of which were determined by an enzyme-linked immunosorbent assay. The nucleotide fraction extracted directly from TMV-infected leaves exhibited a considerable antiviral activity, whereas similar fractions from AVF- or interferon-treated leaves did not, even though an antiviral state was induced in the tissue by these agents. However, when the synthetase fraction from TMV-infected, AVF-treated, or interferon-treated tissues was incubated with ATP and poly(I:C), the resultant, heat-stable, acid-soluble, polymerized ATP markedly inhibited TMV multiplication. It is concluded that the presence of double-stranded RNA is obligatory for the formation of the antiviral nucleotides. The analogy to interferon-induced resistance in animal tissues is discussed.

Inhibition of turnip yellow mosaic virus synthesis by pyrimidine analogues.

The pyrimidine analogues 2-thiouracil, 2-thiouridine, 6-azauracil and 6-azauridine all inhibited the synthesis of turnip yellow mosaic virus (TYMV) and increased the synthesis of empty virus protein shells in infected Chinese cabbage leaf discs. Uracil and uridine reversed these effects. 2-Thiouracil also reduced the UTP pool in TYMV infected leaf discs. The results are consistent with the suggestion that these analogues or their in vivo derivatives affect virus synthesis by inhibiting the biosynthesis of uridylic acid, possibly by inhibiting orotidylic acid decarboxylase.

Erysiphe cichoracearum on Arctium lappa: Cleistocarp dehiscence, ascospore germination, and infection of cucumber by ascospores and conidia

Cleistocarps of Erysiphe cichoracearum (D.C.) Merat on Arctium lappa L. which were subjected to weathering became progressively better sources of ascospores as judged by tests at intervals on wet leaf samples at 20°C. So, in varying degrees, did cleistocarps on air-dried leaves kept at 5, 15 or 25° and those on moist leaves kept on soil at 5°. Cleistocarps on moist leaves kept on soil at 15–20° discharged progressively fewer ascospores in successive tests owing to dehiscence during storage. Cleistocarps on air-dried leaves stored at fluctuating temperatures (5–15°) consistently discharged few ascospores in all tests. Ascospores germinated within the range 3–30° and optimally at c. 20°. At 20° they germinated well from 100 to 32% r.h. In these respects ascospores behaved like conidia, but they also germinated well in water, whereas the conidia do not. Both ascospores and conidia infected leaf disks and leaves of cucumber. On cucumber plants infection was confined mainly to the oldest leaves. Cleistocarps formed on these leaves were morphologically similar to, but larger than, those on A. lappa.