Resistance to Sclerotium cepivorum in Allium and other genera

Stimulatory and non-stimulatory species of Allium and four other members of the Liliaceae were susceptible to infection by germinating sclerotia of Sclerotium cepivorum in unsterile soil. Plants belonging to other families were not infected in soil even when germinating sclerotia were in contact with their roots. Under aseptic conditions S. cepivorum infected plant species not closely related to Allium , particularly when large amounts of inoculum were used. Infection was slight in the presence of germinating sclerotia. It is suggested that within the genus Allium and a few related genera resistance results from the lack of ability to stimulate germination of sclerotia. Non- Allium plants also lack this ability but in addition possess tissue resistance to S. cepivorum.     

Pest and disease management by intercropping: Suppression of thrips and rust in leek

Abstract

Leek (Allium porrum L.) is an Important field vegetable in most of western and central Europe. Infestations of onion thrips (Thrips tabaci Lindeman) cannot be controlled with the range of insecticides now available in The Netherlands. Leek rust (Puccinia allii Rudolph) is now also difficult to control. Experiments in which no insecticides or fungicides were applied, were carried out to assess the effects on thrips populations and infection by leek rust when leek crops were undersown with subterranean clover (Trifolium subterraneum L.). To evaluate the economic aspects of this approach, both the quality and quantity of the leeks produced in the two systems were compared. Undersowing leeks with clover drastically reduced thrips infestations which was reflected in improved quality of leeks at harvest. Leek rust incidence was also reduced slightly by undersowing with clover and the quality of the leeks at harvest was also better. Although the quality of the leeks was improved when the crop was undersown with clover, the quantity of crop produced was reduced considerably as a result of plant competition.     

Studies on Botrytis spp. occurring on onions (Allium cepa) and leeks (Allium porrum)

Botrytis byssoidea (mycelial neck rot) was more prevalent than B. allii (sclerotial neck rot) on the leaves of field onions and the bulbs of stored onions grown in some of the areas where onions or onions and leeks had previously been grown sequentially.
B. byssoidea and B. porri were also isolated from leeks. Spores of B. allii, B. byssoidea (from onions and leeks), B. porri , and B. squamosa caused infection of seedlings of salad (green) and bulb onions.
Inoculation with B. squamosa spores caused severe infection of seedling leaves, but inoculation with mycelial discs caused little damage to onion bulb tissue. By comparison, mycelial discs of the remaining species were highly pathogenic to bulbs.
The practical implications of disease transfer of certain of these species between onions and leeks are discussed.     

Screening for resistance of Allium species to Sclerotium cepivorum with special reference to non-stimulatory resistance

Approximately 150 seed lots of Allium cepa and more than 30 different Allium species were tested in the greenhouse under strictly controlled conditions against Sclerotium cepivorum , the causal fungus of Allium white rot disease. The quantity and quality of the inoculum was determined using a simple and rapid test to achieve a high degree of standardization. Only the A. cepa cultivar "Sweet Sandwich' and A. porrum, A. rotundum, A. sphaerocephalon and two further unidentified Allium species showed significantly lower disease incidence than the cultivar 'Golden Bear'. The stimulatory effect of the root exudates on sclerotial germination was compared with that of diallyl sulphide. With the exception of A. species'PI 249549', extracts of all species that showed low levels of disease incidence in the greenhouse test stimulated sclerotial germination only weakly.     

New mite-borne virus isolates from rakkyo,shallot and wild leek species

Flexuous viruses were transmitted from rakkyo (Allium chinense) and wild leek species (especiallyA. commutatum) to plants of crow garlic (A. vineale), by transfer of dry bulb mites. By electron microscope decoration tests using three antisera and by inoculations onto test plants, it was concluded that from each of the two natural host species at least two viruses were isolated. The viruses from wild leeks are both pathogenic onAllium spp. and may be of economic importance. Decoration tests on a virus mixture from shallot obtained earlier, revealed another new mite-borne virus in this species. The mite-borne viruses ofAllium spp. appear to be very common; they are largely diverse and their identification remains difficult.     

Studies of dormancy in sclerotia of Sclerotium cepivorum

Sclerotia of Sclerotium cepivorum formed on onion bulbs in soil were shown to undergo a period of constitutive dormancy lasting 1–3 months during which they failed to respond maximally to the germination stimulant diallyl disulphide. Even after the breakdown of constitutive dormancy sclerotia were subject to exogenous dormancy from which they were only released by the presence of species of Allium or of germination stimulants known to be produced by them.     

Germination of <Emphasis Type="Italic">Plasmodiophora brassicae</Emphasis> resting spores stimulated by a non-host plant

Plant-induced germination of Plasmodiophora brassicae resting spores was studied in a laboratory experiment. Spore reaction was analysed in nutrient solution with exudates from growing roots of different plant species – one host plant (Brassica rapa var. pekinensis) and four non-host plants (Lolium perenne, Allium porrum, Secale cereale and Trifolium pratense) – and in controls with distilled water and nutrient solution. It was found that root exudates from L. perenne stimulated spore germination more than exudates from the other plants, including those from the host plant. The effect could not be explained by differences in the nutritional composition of the solutions due to differential uptake of the plant species, or by differences in root activity, measured as exudation of soluble sugars. This is the first time such a separation of factors has been done in analysing the influence of plants on P. brassicae germination. Although stimulation of P. brassicae resting spore germination is not restricted to the presence of host plants, it seems to vary depending on the plant species.     

Flavor and odour characteristics of species of Allium in relation to their capacity to stimulate germination of sclerotia of Sclerotium cepivorum

Of many species or cultivars of Allium tested only six ornamental species showed little or no capacity to stimulate germination of sclerotia of Sclerotium cepivorum. All six species had S-methyl-L-cysteine sulphoxide as their principal flavour and odour precursor and their overall flavour and odour levels were low. All other species and cultivars were highly stimulatory, contained considerahle amounts of S-1 or S-2-propenyl-L-cysteine sulphoxide as flavour and odour precursors and, with a few exceptions, they possessed high overall flavour and odour levels. These included several species which are thought to be related to cultivated edible forms. With the possible exception of A. caeruleum and A. cyaneum no evidence of resistance to infection by S. cepivorum was detected.     

Pathology and taxonomy of Cladosporium leaf blotch of onion (Allium cepa) and leek (A. porrum)

Cross-inoculations to leek and onion plants using isolates of Cladosporium from leaf blotches on leeks or onions showed that no cross-infection occurred between isolates and their respective hosts. The species causing leaf blotch on leeks was morphologically distinct from that causing a similar disease on onions and the differences, augmented by the results obtained from the cross-pathogenicity tests, indicate that two taxa should be recognized at the rank of species. Following an examination of type or authentic material it was concluded that the name C. allii (Ell. & G. W. Martin) P. M. Kirk & Crompton comb. nov. should apply to the species causing leaf blotch on leeks and C. allii–cepae (Ranojevi) M. B. Ellis to that on onions.     

Sources, survival and transmission of Cladosporium allii and C. allii-cepae, leaf blotch pathogens of leek and onion

Neither Cladosporium allii nor C . allii-cepae was detected in seed samples of 46 commercial cultivars of leek and nine of onion.
In glasshouse tests where flower inflorescences of onions and leeks were dusted with conidia of C. alliicepae and C. allii , respectively, few onion seeds and no leek seeds became contaminated. In field crops of leeks some spathes and inflorescences became diseased and gave 0 002% C. allii contaminated seeds. No infected onion seeds were obtained from naturally-infected plants in the field.
Conidia of both fungi survived for less than 6 weeks in sterile and non-sterile soil (during which time the temperature varied from -8 to +11 C); pseudothecial bodies survived longer.
Both pathogens survived for 3 months in debris of onion or leek on the surface of soil in pots under similar conditions. In debris buried at a depth of 7 cm in sterile or unsterile soil they survived for 2 months. Although C. allii survived only 1 month in debris on the soil surfaces following a leek crop, conidia of the fungus were trapped from the air above the area for at least 6 months and healthy leeks transplanted into this area developed the disease. No leaf blotch developed in leeks grown on land which had not previously borne that crop.     

长叶红砂和红砂种子萌发对光照、温度和水分的响应

长叶红砂(Reaumuria trigyna)和红砂(Reaumuria soongorica)均为荒漠地区超旱生小灌木,长叶红砂为红砂属珍稀种,而红砂为广布种。通过种子萌发对光、温和土壤水分需求的研究表明,长叶红砂对光照不敏感,而黑暗有利于红砂的萌发。种子萌发的适宜温度均为恒温20-25℃或15/25℃变温。25℃后,随着温度升高萌发率下降。土壤含水量在3%以上时,有利于两物种的萌发,萌发适宜的含水量都在12%。在各种条件下,珍稀种长叶红砂的萌发均高于广布种红砂,说明种子质量和萌发并没有造成长叶红砂的珍稀濒危。但长叶红砂高萌发率却不利于应对荒漠地区极端和不可预测的环境变化,增加了生存的风险性,可能是其濒危原因之一。     

Infection and disease development of Quambalaria spp. on Corymbia and Eucalyptus species

Quambalaria spp. are eucalypt leaf and shoot pathogens of growing global importance, yet virtually nothing is known regarding the manner in which they infect and colonize their hosts. A study of the infection process of Q. pitereka and Q. eucalypti on Corymbia and Eucalyptus species was thus undertaken using light, scanning and transmission electron microscopy after artificial inoculation. Conidial germination was triggered when relative humidity levels exceeded 90% and commenced within 2 h in the presence of free water. Light reduced germination but did not prevent germination from occurring. Conidial germination and hyphal growth occurred on the upper and lower leaf surfaces with penetration occurring via the stomata or wounds on the leaf surface or juvenile stems. There was no evidence of direct penetration of the host. Following penetration through the stomata, Q. pitereka and Q. eucalypti hyphae grew only intercellularly without the formation of haustoria or interaction apparatus, which is characteristic of the order Microstromatales. Instead, the presence of an interaction zone is demonstrated in this paper. Conidiophores arose through stomatal openings producing conidia 7 days after infection.     

Survey for viruses infecting onion, garlic and leek crops in Iran

Surveys to identify virus diseases affecting garlic ( Allium sativum ), onion ( Allium cepa ) and Persian leek ( Allium ampeloprasum var. persicum ) were conducted from 1999 to 2002. Surveys covered different regions of Iran (Tehran [different vegetable markets, farmer fields and cultivation areas], Noushahr, Chalous, Roudbar, Sari, Hamadan, Touyserkan, Ghazvin and Jiroft). A total of 2045 (1285 garlic, 525 onion and 230 leek) samples showing symptoms of virus infection were collected and tested by ELISA; and in some cases tests were also confirmed by immunoelectron microscopy (IEM) for the presence of Allium viruses. ELISA results showed that the following viruses were detected: Onion yellow dwarf virus (OYDV), Leek yellow stripe virus (LYSV) (genus Potyvirus , family Potyviridae ), Garlic common latent virus (GarCLV), Shallot latent virus (SLV) (genus Carlavirus ), Garlic virus D (GarV-D), Garlic virus B (GarV-B) and Garlic virus C type (GarV-C) (genus Allexivirus ). None of the samples reacted with antibodies to Shallot yellow stripe virus (SYSV) genus Potyvirus , family Potyviridae ), Shallot virus X (ShVX) and Garlic virus A (GarV-A, genus Allexivirus ). GarCLV, SLV, GarV-D, GarV-B and GarV-C are reported for the first time from Allium crops in Iran.     

Infection of cultivars of onion, leek, garlic and A llium fistulosum by Sclerotium cepivorum

In two field experiments on land severely infested with Sclerotium cepivorum largo differences in the incidence of white rot in four Allium species were recorded. There was inuch less disease ° Cultivars of leek than in those of onion, garlic or A. fistulosum. Differences between cultivars within species were either small or absent.     

Crude extract of the cyanobacterium, Hapalosiphon sp., causes a cessation of root elongation and cell division in several plant species

The phytotoxic activities of the crude extract from the cyanobacterium, Hapalosiphon sp., on the initial growth and root cell division of several plant species were investigated. Although the germination percentages of the plants were not affected by the extract, their root and shoot growth were remarkably suppressed, depending on the extract concentrations. Roots were more sensitive to the extract than shoots and the sensitivity, determined by the I₅₀ concentrations between the susceptible and tolerant species, differed by 5 and 11 times in dicotyledonous and monocotyledonous plants, respectively. The mitotic index for wheat ( Triticum aestivum L. cv. Norin 61) root cells was not affected, but the mitotic index for onion ( Allium cepa L. cv. Raputa II) was reduced with the increasing concentration of the extract. In the onion root meristem, the number of cells in prophase, metaphase, anaphase, and telophase were decreased significantly with 6 g DW L⁻¹ (g dry weight of algae L⁻¹). This suggests that the extract might inhibit the processes regulating the entry of cells into mitosis.     

The ecological role of dew in assisting seed germination of the annual desert plant species in a desert environment,northwestern China

It is important to understand the effects of dew events on non-mucilaginous seed germination of annual desert plant species during dry seasons, which is critical to maintaining long-term soil seed banks in a harsh desert environment. We hypothesize that dew deposition also assists in the non-mucilaginous seed germination of annual desert species. A common field dew treatment experiment was conducted in the Linze Inland River Basin Research Station to investigate the effects of dew deposition on the seed germination of four annual species, including Agriophyllum squarrosum, Corispermum mongolicum, Bassia dasyphylla and Halogeton arachnoideus. The results showed that the presence of dew significantly increased seed germination percentages and decreased the nonviable seed percentages of B. dasyphylla and H. arachnoideus, whereas there was no such trend for the seeds of C. mongolicum and A. squarrosum. The ecological effects of dew on the seed germination and viability of the annual desert plants were species specific. Although dew wetting is insufficient to cause seed germination, it may help in priming the seeds.     

Evaluation of morphology of infection structures in distinguishing between different Allium rust fungi

Fifteen isolates of rust fungi were collected in the United Kingdom and in the Netherlands from severalAllium species. The samples represented three putative rust species. The morphology of the telia, teliospores and urediniospores was investigated. From urediniospores infection structures were induced on leek, and their morphology was described. Telia and teliospores were not available on every sample. Morphology of the infection structures clearly differentiated between ‘leek’ type and ‘chive’ type isolates. The morphology of infection structures ofPuccinia mixta is very similar to that ofUromyces ambiguus, but clearly distinct fromPuccinia allii sensu Jennings from leek. Quantitative differences in urediniospore characters differentiated between the putative species, but there was overlap between the taxa. We conclude that morphology of infection structures of urediniosporelings is a useful trait for identification of rust isolates fromAllium. This is especially true where more than one rust species may occur on the same host species, as withAllium fistulosum.     

不同条件下韭菜汁对香蕉枯萎病菌4号小种活性抑制的研究

研究了不同温度、pH值、杀虫剂和杀菌剂的影响下,韭菜汁对香蕉枯萎病菌4号小种的抑制作用,以确定韭菜汁在不同条件影响下对病菌的活力。结果表明,在试验的各温度处理后,55℃以上的温度对韭菜汁的成分有破坏作用,菌落的生长受到显著影响。韭菜根汁对病菌孢子的抑制作用不受pH值的影响。酸性至弱碱性情况下,韭菜茎汁对病菌孢子萌发的抑制率为100%;在强碱性条件下,韭菜茎汁中物质的活性促进了孢子的萌发。供试杀虫剂和杀菌剂对韭菜汁的成分无明显破坏作用。     

韭菜等几种植物根系分泌物对棉花黄萎病菌的影响

通过水培法收集韭菜、大葱、野薄荷、藿香和薰衣草的根系分泌物,研究其对大丽轮枝菌菌丝生长、孢子萌发的影响,并通过室内盆栽试验研究了植物根系分泌物对大丽轮枝菌微菌核萌发的影响。结果表明,5种植物根系分泌物对大丽轮枝菌菌丝生长和孢子萌发均表现为抑制作用;随着浓度的升高,抑制作用增强,而在同一浓度下,随着处理时间的延长,根系分泌物对孢子萌发的抑制作用逐渐减弱。盆栽试验表明,植物根系分泌物对大丽轮枝菌微菌核的萌发均表现为抑制作用,且微菌核对韭菜根系分泌物最为敏感。5种植物中,韭菜根系分泌物的抑制作用最强,野薄荷次之,藿香最弱。     

Effects of temperature, relative humidity and duration of wetness period on germination and infection by conidia of the pear scab pathogen (Venturia nashicola)

Experiments were conducted to determine the effects of temperature, relative humidity (RH) and duration of wetness period on in vitro germination of conidia and infection of detached pear leaves by Venturia nashicola , the causal agent of pear scab. Conidia germinated only in near-saturation humidity (RH > 97%). The final percentage germination (24 h after inoculation) at 100% RH without free water was less than half that in free water. Conidia germinated over the range of temperatures tested (5–30°C); the optimum temperature for germination was ≈21°C. Changes in percentage germination of conidia over time were fitted by logistic models at each individual temperature. Polynomial models satisfactorily described the relationships between two (rate and time to 50% of maximum germination) of the three logistic model parameters and temperature. The minimum length of the wetness period for successful infection of detached pear leaves by conidia was observed at several temperatures. The shortest length of wetness period required for infection was 7 h at 22°C. Two polynomial models fitted well the relationship between the minimum wetness duration required for infection, and temperature.