Sources of resistance to downy mildew (<Emphasis Type="Italic">Peronospora parasitica</Emphasis> (Pers. (ex Fr.) Fr.) in Sicilian germplasm of cauliflower and broccoli

Downy mildew (Peronospora parasitica (Pers. ex Fr.) Fr.) is a serious disease of brassicas in several countries. Seedlings are very susceptible to this pathogen and crops require frequent fungicide treatments to reach a good marketable yield. The use of resistant cultivars can be the most economical, reliable and environmental friendly method for managing this disease. In this work 32 Sicilian landraces and 16 commercial cultivars of cauliflower and broccoli (B. oleracea) were screened for downy mildew resistance at the cotyledon stage using one P. parasitica strain from Portugal and one from Sicily (Italy). Seven-day old seedlings were inoculated by deposing a droplet of a spore suspension on the cotyledons, incubated under controlled environment and scored 7 days later using a seven-class scale of interaction phenotype (IP), which took into consideration host response and pathogen sporulation. There were no differences in virulence between the two P. parasitica isolates. Accessions ranged from very susceptible to highly resistant to downy mildew showing a variable number of resistant individuals per accession. Forty accessions were very susceptible to downy mildew and are of no interest as sources of resistance, since most of the seedlings were scored in the most susceptible IP classes. Seven accessions had intermediate resistance and included individuals that expressed some degree of resistance. Accession Cv 90 (Cavolfiore Torino) and Br 63 (Sparaceddu) showed the majority of seedlings in the resistant IP classes and may constitute valuable sources of resistance to downy mildew to be used in breeding programs.     

Distribution of violet root rot (Helicobasidium purpureum Pat.) of sugar beet and preliminary experiments on factors affecting the disease

Examination of sugar beet grown in eastern England over the period 1936-43 has shown that up to 5% of factory tarehouse samples contain roots infected with violet root rot. The results of the survey suggest that there are considerable areas where light attacks of the disease occur, as well as fields in which severe attacks develop when susceptible crops are grown. The disease occurred on roots grown in many types of soil but most frequently on those from light alkaline soils. It was found to reduce the sugar content and increase the dirt tare of sugar beet.
The causal fungus, Helicobasidium purpureum, has a wide host range and observations suggest that it persists in the field on weeds, as severe attacks on cultivated crops often occur in weed-infested fields. It also survives as sclerotia, which develop freely on the roots of susceptible crops. While the undisturbed soil conditions of leys and uncultivated land favour increase in infectivity, good cultural practices, trap cropping and application of nitrogenous fertilizers act as a check. These control measures will reduce infectivity even when applied to soil in which consecutive, susceptible crops are grown. Wet soil conditions did not favour the development of infection, and the frequent occurrence of the disease in wet places may be due to such soil conditions favouring survival of sclerotia.
Experiments with compost made from factory waste showed that although the fungus can survive the process, its virulence was so reduced that there is no likelihood that use of such compost would lead to severe violet root rot infection.     

Reducing airborne ectomycorrhizal fungi and growing non-mycorrhizal loblolly pine (Pinus taeda L.) seedlings in a greenhouse

Atmospheric spores of ectomycorrhizal (ECM) fungi are a potential source of contamination when mycorrhizal studies are performed in the greenhouse, and techniques for minimizing such contamination have rarely been tested. We grew loblolly pine (Pinus taeda L.) from seed in a greenhouse and inside a high-efficiency particulate air-filtered chamber (HFC) constructed within the same greenhouse. Seedlings were germinated in seven different sand- or soil-based and artificially based growth media. Seedlings grown in the HFC had fewer mycorrhizal short roots than those grown in the open greenhouse atmosphere. Furthermore, the proportion of seedlings from the HFC that were completely non-mycorrhizal was higher than that of seedlings from the greenhouse atmosphere. Seedlings grown in sterilized, artificially based growth media (>50% peat moss, vermiculite, and/or perlite by volume) had fewer mycorrhizal short roots than those grown in sand- or soil-based media. The HFC described here can minimize undesirable ECM colonization of host seedlings in greenhouse bioassays. In addition, the number of non-mycorrhizal seedlings can be maximized when the HFC is used in combination with artificially based growth media.     

Acclimation to changing light conditions of long-term shade-grown beech (Fagus sylvatica L.) seedlings of different geographic origins

 Effects of changing light conditions on the ecophysiological condition behind survival were examined on beech from two different populations. Plants were grown in a greenhouse under simulated understorey and canopy gap light conditions. Upon exposure to high light maximum photosynthesis of shade-acclimated leaves increased followed by a reduction over several days to between high- and low-light control rates. In the reciprocal transfer, the decrease in maximum photosynthesis was rapid during the first 2–3 days and then levelled off to values comparable to low-light controls. Seedlings from Sicily (Madonie) showed generally higher maximum photosynthetic rates than those from Abetone. Leaf conductance varied in the same direction as photosynthesis in high- to low-light seedlings but to a lesser degree. Leaves grown under low light and exposed to high light experienced photoinhibition. The Abetone population was more susceptible to photoinhibitory damage than the seedlings from Sicily. Exposure to high light of shade-acclimated seedlings resulted in intermediate chlorophyll concentrations between levels of the high-light and low-light seedlings. Carotenoid concentration was unaffected by treatments. Seedlings grew more in high light, but had a lower leaf area ratio. Light-limited seedlings showed a shift in carbon allocation to foliage. Leaves formed in the new light regime maintained the same anatomy that had been developed before transfer. Seedlings from Sicily had thicker leaves than those of seedlings from Abetone. Seedlings from Abetone were found to be more susceptible to changing light conditions than seedlings from Sicily. We conclude that small forest gaps may represent a favorable environment for photosynthesis and growth of beech regeneration as a result of the limited ability of seedlings to acclimate to sudden increases in high irradiance and because of the moderate levels of light stress in small gaps. Received: 11 April 1997 / Accepted: 11 December 1997     

Characterisation of powdery mildew resistance in a segregating diploid rose population

Powdery mildew (Podosphaera pannoso) is one of the most serious fungal diseases on both greenhouse and field grown roses. Improvement of disease resistance is a major selection aim for garden rose breeders. For rose cultivars, being mostly tetraptoid, it is complicated to develop molecular markers for resistance. Hence, a segregating diploid population was established from a cross between 'Yesterday', a commercial available rose variety susceptible to powdery mildew, and R. wichurana, a rose species with resistance to certain isolates of powdery mildew. A progeny of 94 seedlings was planted in the field. The segregation of powdery mildew resistance was studied in this population by means of a bioassay with two different monoconidial isolates of powdery mildew. Based on the response to these inoculations different groups were selected: a first group of genotypes was susceptible to both isolates, other groups were susceptible to one of both isolates and a last group was resistant to both tested isolates. The disease resistance inherits for both isolates in a quantitative way. A genetic map based on AFLP and SSR markers was established and will be used for QTL analysis of powdery mildew resistance.     

Plant Growth‐promoting Fungus Penicillium oxalicum Enhances Plant Growth and Induces Resistance in Pearl Millet Against Downy Mildew Disease

Plant Growth‐promoting Fungus (PGPF) Penicillium oxalicum was isolated from rhizosphere soil of pearl millet and was tested for its ability to promote growth and induce systemic resistance in pearl millet against downy mildew disease. The fungal isolate P. oxalicum UOM PGPF 16 was identified as P. oxalicum using ITS sequencing and morphological analysis and sequence was deposited at NCBI with accession number KF150220. Pearl millet susceptible seeds were treated with three different inducers (CS, CF and LCF) of PGPF P. oxalicum and all the inducers significantly reduced the downy mildew disease and enhanced plant growth. Among the inducers tested, CS treatment recorded highest seed germination of 91% and 1427 seedling vigour followed by LCF and CF treatments. The vegetative growth parameter and NPK uptake studies under greenhouse conditions revealed that the CS treatment of P. oxalicum remarkably enhanced the parameters tested when compared to control plants. A significant disease protection of 62% and 58% against downy mildew disease was observed in plants pretreated with CS of P. oxalicum under greenhouse and field conditions, respectively. The spatio‐temporal studies revealed that inducers P. oxalicum required a minimum of 3 days for developing maximum disease resistance which was maintained thereafter. The maximum Peroxidase (POX) activity (62.7 U) was observed at 24 h in seedlings treated with CS of PGPF P. oxalicum and the activity gradually reduced at later time points after pathogen inoculation. Chitinase (CHT) activity was significantly higher in inducer treated seedlings when compared to control seedlings inoculated with pathogen after 48 h and remained constant at all time points.     

Host-plant acceptance on mineral soil and humus by the pine weevil Hylobius abietis (L.)

1 The pine weevil Hylobius abietis (L.) (Coleoptera, Curculionidae) is an economically important pest of conifer forest regeneration in Europe and Asia. 2 Soil scarification, which usually exposes mineral soil, is widely used to protect seedlings from weevil attack. However, the mechanism behind this protective effect is not yet fully understood. 3 Field experiments were conducted to determine the pine weevil's responses to visual and odour stimuli from seedlings when moving on mineral soil and on undisturbed humus surface. 4 One experiment measured the number of pine weevils approaching seedlings, with and without added host odour, on mineral soil and undisturbed humus. Seedlings with added host odour attracted more weevils on both soil types. Unexpectedly, somewhat more weevils approached seedlings surrounded by mineral soil. 5 In a similar experiment, feeding attacks on seedlings planted directly in the soil were recorded. Only half as many seedlings were attacked on mineral soil as on undisturbed humus. 6 In the first experiment, the weevils were trapped 2.5 cm from the bases of the seedlings' stems, whereas they could reach the seedlings in the experiment where seedlings were planted directly in the soil. We conclude that the pine weevils' decision on whether or not to feed on a seedling is strongly influenced by the surrounding soil type and that this decision is taken in the close vicinity of the seedling. The presence of pure mineral soil around the seedling strongly reduces the likelihood that an approaching pine weevil will feed on it.     

The relative contributions of sexual reproduction and clonal propagation in Opuntia rastrera from two habitats in the Chihuahuan Desert

1 The clonal cactus Opuntia rastrera shows predominantly sexual reproduction in grasslands (GH) and clonal propagation in nopaleras (NH). We assessed the effects of light, herbivory, water availability and the habitat an offspring came from on the survival and growth of sexual or clonal offspring (i.e. seedlings and cladodes), through 3- and 4-year common garden and short-term greenhouse experiments.
2 Shading by nurse plants increased seedling survival in the field by an order of magnitude, and a small additional advantage due to predator protection by grasses was observed. Strong herbivory transforms a facultative nurse–protégé relationship for seedlings into an obligatory one.
3 In the greenhouse seedlings grew better under shade, but in the field the production of the first cladode was delayed in seedlings in the more shaded GH. Competition for soil resources may be more intense under a dense grass tussock than under a open shrub, thus affecting the nurse–protégé relationship. Seedling survival under nurse plants was similar in GH and NH, but higher plant cover suggests that a larger number of seedlings will establish in GH in the long term.
4 Cladode survival was higher in NH. Cladodes were more successful than seedlings at establishing in intercanopy areas, possibly due to physiological differences as well as their ability to survive partial predation. Cladode survival in intercanopy areas may explain the enhanced clonal propagation in the more open NH scrubland, together with their susceptibility to the flooding which affects GH.
5 The high seedling and cladode survival in the greenhouse experiments contrasted with that observed in the field, indicating that survival is determined by the interaction between herbivores, plants and abiotic conditions rather than the physiological aptitude of the plants.     

Seedling growth of four fire-following Mediterranean plant species deprived of single mineral nutrients

1. Anthyllis vulneraria, Cistus creticus, Hippocrepis unisiliquosa and Pinus brutia are frequent post-fire colonizers whose seedlings are subjected to the mineral conditions imposed by ash in the early stages of growth. We test the hypothesis that the effective internal supply of mineral elements in the seeds of these species may complement the external availability of specific nutrients.
2. Newly germinated seedlings were grown in nutrient solutions, each deficient in one of the following elements: nitrogen, phosphorus or potassium. Control treatments consisted of full nutrients and distilled water. Seedling growth was monitored over 12weeks. The final dry masses of the seedlings were taken as a measure of the availability of each element from the seeds' own stored reserves.
3. For the two legume species nitrogen was the most limiting element followed by phosphorus. Potassium deficiency had no effect on the final legume dry biomass. For Cistus, the order was nitrogen followed by potassium, while phosphorus deprivation had no effect on growth during the 12weeks of the experiment. Deprivation of single minerals had no significant effect on seedling growth of Pinus over the same period. The two legumes and Cistus when denied nitrogen had relatively greater relative root ratios than control plants.
4. The relative seedling requirements of each species for early external supplies of the various nutrients for establishment in the field are discussed with reference to the adaptation of the four species to post-fire regeneration.     

Nitrogen dynamics and growth of seedlings of an N-fixing tree (Gliricidia sepium (Jacq.) Walp.) exposed to elevated atmospheric carbon dioxide

Summary Seeds of Gliricidia sepium (Jacq.) Walp., a tree native to seasonal tropical forests of Central America, were inoculated with N-fixing Rhizobium bacteria and grown in growth chambers for 71 days to investigate interactive effects of atmospheric CO₂ and plant N status on early seedling growth, nodulation, and N accretion. Seedlings were grown with CO₂ partial pressures of 350 and 650 bar (current ambient and a predicted partial pressure of the mid-21st century) and with plus N or minus N nutrient solutions to control soil N status. Of particular interest was seedling response to CO₂ when grown without available soil N, a condition in which seedlings initially experienced severe N deficiency because bacterial N-fixation was the sole source of N. Biomass of leaves, stems, and roots increased significantly with CO₂ enrichment (by 32%, 15% and 26%, respectively) provided seedlings were supplied with N fertilizer. Leaf biomass of N-deficient seedlings was increased 50% by CO₂ enrichment but there was little indication that photosynthate translocation from leaves to roots or that plant N (fixed by Rhizobium) was altered by elevated CO₂. In seedlings supplied with soil N, elevated CO₂ increased average nodule weight, total nodule weight per plant, and the amount of leaf nitrogen provided by N-fixation (as indicated by leaf ¹⁵N). While CO₂ enrichment reduced the N concentration of some plant tissues, whole plant N accretion increased. Results support the contention that increasing atmospheric CO₂ partial pressures will enhance productivity and N-fixing activity of N-fixing tree seedlings, but that the magnitude of early seedling response to CO₂ will depend greatly on plant and soil nutrient status.     

Sensibilité du Melon àSphaerotheca fuliginea en Fonction de L'Etage Foliaire

Effect of leaf position on the susceptibility of melon plants to artificial infection with powdery mildew, Sphaerotheca fuliginea The leaf position of melon plants seems to play a role on their susceptibility when they are artificially infected with powdery mildew Sphaerotheca fuliginea. The cotyledons are generally very susceptible, while the first leaf relatively resistant; the susceptibility again continues up to the 4th–5th leaf (but less susceptible than the cotyledons) and then after it decreases; these results can be obtained on plants in greenhouse or on detached leaves in Petri dishes. From this observation, we think that the screening of melon genotypes for resistance to powdery mildew can be evaluated neither on the cotyledons which are very susceptible nor on the first leaf which is resistant, but on the third leaf which is moderately susceptible. In fact, there is a good correlation between the reaction of the third leaf and the resistance or susceptibility of genotypes.     

THE FUNGICIDAL CONTROL OF LETTUCE DOWNY MILDEW, CAUSED BY BREMIA LACTUCAE

Protection experiments carried out in the greenhouse showed that preparations of thiram, zineb, ferbam and ziram were superior to cuprous oxide and copper oxy-chloride preparations, in controlling downy mildew of lettuce and in lack of phytotoxic effect.
Tests made in winter on lettuce seedlings grown in Dutch lights, showed that Bremia lactucae attack reduced the stand of seedlings, diminished their size and weight, and, when the crop was planted out into the open ground, reduced survival in the field and slightly delayed the date of reaching maturity. The best overall results in protecting foliage against downy mildew were given by thiram and zineb preparations.     

QTL analysis of powdery mildew resistance in cucumber (Cucumis sativus L.)

A population of F₇ recombinant inbred lines (RILs) was made from a cross between susceptible (‘Santou’) and resistant (PI197088-1) lines of cucumber in order to study powdery mildew resistance loci. Susceptibility to powdery mildew in the F₇ RIL individuals showed a continuous distribution from susceptible to resistant, suggesting that powdery mildew resistance is controlled by quantitative trait loci (QTLs). A QTL analysis identified two and three loci for powdery mildew resistance under 26 and 20°C conditions, respectively. One QTL was found in the same position under both temperature conditions. Therefore, it is more likely that one major QTL acts under both temperature conditions and that other QTLs are specific to the two temperature conditions. The above results suggest that the four QTLs are controlled in a different temperature manner, and that their combination played an important role in expressing a high level of resistance to powdery mildew in this cucumber population. Sequence-tagged site (STS) markers associated with each QTL were developed and would be useful for breeding a cucumber line with a high level of powdery mildew resistance. Y. Sakata and N. Kubo contributed equally to this work and are considered as first authors.     

Nodulation and Nitrogen Fixation in Cliffrose (Cowania mexicana var. stansburiana (Torr.) Jeps.)

Nodulation and acetylene reduction were demonstrated for cliffrose (Cowania mexicana var. stansburiana [Torr.] Jeps.) in a greenhouse trial. Nitrogen fixation was also verified with ¹⁵N. Seedlings were grown in a soil known to nodulate both bitterbrush species (Purshia tridentata (Pursh) D.C. and P. glandulosa [Curran]) to which a suspension of crushed bitterbrush nodules had been added. Bitterbrush seedlings grew faster and reduced more acetylene than cliffrose seedlings planted in the same pots, but the specific activity of the nodules and the nodule mass to root mass ratios were approximately the same.     

Growth and Mitochondrial Respiration of Mungbeans (Phaseolus aureus Roxb.) Germinated at Low Pressure

Mungbean (Phaseolus aureus Roxb.) seedlings were grown hypobarically to assess the effects of low pressure (21-24 kilopascals) on growth and mitochondrial respiration. Control seedlings grown at ambient pressure (101 kilopascals) were provided amounts of O₂ equivalent to those provided experimental seedlings at reduced pressure to factor out responses to O₂ concentration and to total pressure. Respiration was assayed using washed mitochondria, and was found to respond only to O₂ concentration. Regardless of total pressure, seedlings grown at 2 millimoles O₂ per liter had higher state 3 respiration rates and decreased percentages of alternative respiration compared to ambient (8.4 millimoles O₂ per liter) controls. In contrast, seedling growth responded to total pressure but not to O₂ concentration. Seedlings were significantly larger when grown under low pressure. While low O₂ (2 millimoles O₂ per liter) diminished growth at ambient pressure, growth at low pressure in the same oxygen concentration was enhanced. Respiratory development and growth of mungbean seedlings under low pressure is unimpaired whether oxygen or air is used as the chamber gas, and further, low pressure can improve growth under conditions of poor aeration.     

Comparison of Different Inoculation Techniques to Screen Resistance of Pea Lines to Downy Mildew

Pea lines with partial resistance to downy mildew were tested with different inoculation techniques. Lines with superior resistance to leaf and pod infection in the field also show superior resistance to artificial inoculation of germinating seeds or seedlings at different ages in greenhouse experiments. Increased resistance of older seedlings was found for all genotypes including the susceptible control. Juvenile leaves at the top of the seedlings plants at time of inoculation, developed the strongest symptoms. Selection for partial resistance can probably be carried out in the greenhouse to improve resistance of adult plants in field.     

Adaptations of the Photosynthetic Mechanism of Sugar Maple (Acer saccharum) Seedlings Grown in Various Light Intensities

Sugar maple (Acer saccharum Marsh.) seedlings were grown in a nursery for three years in 13, 25, 45 and 100 per cent of full daylight. During the third year of growth, the rates of their apparent photosynthesis and respiration were measured periodically with an infra-red gas analyzer at various light intensities and normal CO₂ concentration. In addition, the rates of apparent photosynthesis of a single attached leaf of the same seedlings were measured at saturating light intensity, hut varying CO₂ concentrations. An increase in the light intensity in which seedlings were grown had no effect on their height or mean leaf area, hut resulted in thicker leaves, an increase in the total leaf area per seedling due to an increase in the number of leaves, an increase in the dry weight especially of roots and a decrease in the chlorophyll content of leaves. Throughout the growing season seedlings grown in full daylight, as compared with those grown in lower light intensities, had the lowest rates of apparent photosynthesis measured at standard conditions (21,600 lux light intensity and 300 ul/l of CO₂), when this was expressed per unit leaf area, hut the highest rates on a per seedling basis. Thus dry matter production attained at the end of the growing season correlated positively with the photosynthetic rate per seedling, but not per unit leaf area. The rates of apparent photosynthesis of seedlings grown at lower light intensities were more responsive to changes in light intensity or CO₂ concentration than those of seedlings grown in full daylight intensity.     

Plant regeneration in Kentucky bluegrass (Poa pratensis L.) via coleoptile tissue cultures

Summary Plant regeneration in Kentucky bluegrass (Poa pratensis L. cv. Touchdown) via culture of seedling tissues was investigated. When coleoptile, leaf, and stem sections of dark-germinated seedlings were cultured on Murashige and Skoog (MS) medium, different types of callus were produced, depending on the expiant source and growth regulator combinations. Only compact-friable callus (type 3) and moderately compact, friable callus (type 2) produced shoots upon subculture. The nonstructured watery callus (type 4) produced roots without shoots. Shoot differentiation from callus tissues was highest when the culture medium contained 0.2 mgL^–1 picloram + 0.01 mgL^–1 -naphthaleneacetic acid (NAA). Calli grown from coleoptiles had higher shoot regeneration frequency (32%) than that obtained from either stem sections (12%) or young leaf tissues (2%) of the same seedlings. Some organogenic callus lines produced exclusively green plants, while others produced albino shoots or a mixture of green and albino shoots. The green plants were multiplied in a medium containing 0.1 mgL^–1 BAP plus either 0.2 mgL^–1 picloram or 0.1 mgL^–1 indole-3-acetic acid (IAA). Over 90% of the cultures in the shoot proliferation medium produced roots in 4 weeks. The rooted plants were successfully established in soil medium and grown in the greenhouse.Abbreviations BAP 6-benzylaminopurine - 2,4-D 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid - MS Murashige and Skoog (1962) medium - NAA -naphthaleneacetic acid - picloram 4-amino-3,5,6-trichloropicolinic acid - TDZ thidiazuron     

Diurnal Modulation of Arginine Decarboxylase Activity of Chenopodium rubrum L. by Temperature and Light

Arginine decarboxylase (ADC), one of the key enzymes of polyaminemetabolism in plants, was investigated in Chenopodium rubrumL. seedlings under constant and alternating temperature andlighting conditions. With seedlings grown at constant temperature,ADC activity of the whole seedling increased rapidly betweenthe second and the third day after sowing. This effect was alwayshigher under continuous light than in continuous darkness. Fromthe third to the seventh day after sowing, there was a markeddecrease in ADC activity of the whole seedling almost down tothe level of the second day. Under "normal" lighting and temperatureconditions (32.5?C/10?C, light/dark) there was a marked increasein ADC activity when plants were transferred to 10?C and a rapiddecrease when they were transferred to 32.5?C. The same timecourse was observed when an "inverse" light-temperature program(32.5?C/10?C; dark/light) was applied. This means that the timecourse of ADC activity in the seedlings is slightly light-dependent,but strongly temperature-dependent. The data are discussed withrespect to the chronopathological effects of the "inverse" light-temperatureprogram. (Received March 5, 1985; Accepted October 2, 1985)