Effect of Bacillus subtilis on Growth and Protection of Onion against White Rot

The effect of isolates of Bacillus subtilis applied as seed treatments on plant growth and white rot on three onion cultivars was studied for two seasons on muck soil of Fraser valley of British Columbia. The isolates of B. subtilis provided significant season-long protection against onion white rot on all three cultivars. The most effective isolate (BACT 2) protected the onion cultivar Autumn spice against onion white rot in both years. plant height, number of leaves per plant, and bulb weight were not affected by treating seeds with the Bacillus isolates. Bulb neck diameter of the cultivar Autumn Spice was significantly reduced by two isolates in both seasons.     

Biocontrol of onion white rot by application of Trichoderma species formulated on wheat bran powder

In vitro, Trichoderma album, Trichoderma harzianum, Trichoderma koningii, Trichoderma viride and Trichoderma virens showed antagonistic effect against the most pathogenic isolate (Sc2) of Sclerotium cepivorum, the cause of onion white rot disease. Five Trichoderma preparations of each Trichoderma sp. were prepared on wheat bran powder to be used for controlling white rot disease of onion. Greenhouse and field experiments followed the same trend where T. harzianum and T. koningii were the most effective in reducing the incidence and severity of white rot disease compared with the control. Trichoderma species preparations caused promotion to vegetative parameters of onion plants in pots and increase bulb productivity in filed. In this regard, T. harzianum and T. koningii were the most effective. A positive correlation was found between the biocontrol activity of Trichoderma species preparations and enhancement of peroxidase, polyphenoloxidase and chitinase enzymes in onion plants to resist infection with S. cepivorum.     

Effect of some antibiotics on Sclerotium cepivorum Berk., the cause of white rot of onion

The effect of some antibiotics onSclerotium cepivorum, the cause of white rot of onion was studied in agar culture and soil. The growth ofS. cepivorum was inhibited in Czapek Dox yeast agar containing 50µg of gliotoxin, viridin, actidione and 100µg of patulin per ml of the medium. Lower concentrations of the antibiotics retarded the growth of the fungus. In soil, patulin had no effect in the control ofS. cepivorum infection of onion seedlings. Concentration of actidione of 5µg/g of soil completely controlled white rot infection but severely stunted the growth of onion seedlings; 40µg/g of actidione killed the seedlings. Despite the importance of actidione as a fungistatic agent its use on onion is limited by its phytotoxicity.     

Application of actinomycetes as biocontrol agents in the management of onion bacterial rot diseases

This study was conducted to achieve biological control for the post-harvest onion bacterial rot diseases with the aid of Egyptian isolates of actinomycetes. In this respect, 45 actinomycetes strains were isolated from Egyptian soils and screened for their antagonistic effect against onion bacterial rot pathogens; Erwinia carotovora subsp. carotovora and Burkholderia cepacia. The most two active strains were identified based on their cultural, morphological and molecular properties as Streptomyces lavendulae HHFA1 and Streptomyces coelicolor HHFA2, the latter was most potent and so was used in vivo (pots and field) for controlling onion bacterial rot. S. coelicolor HHFA2 application resulted in enhancement in the photosynthetic pigments and some foliar growth parameters of onion plants confirming its growth promoting effect. The results of the post-harvest estimation of the disease incidence (DI) of the onion bacterial rot throughout storage revealed that, the application of S. coelicolor HHFA2 reduced the DI pronouncedly comparing with the untreated control and confirm its successful role in the biological control of onion bacterial rot diseases.     

Production of Pectic Enzymes by Fusarium oxysporum f. sp. cepae and its Involvement in Onion Bulb Rot

Fusarium oxysporum f. sp. cepae produced an exo-polygalacturonase (exo-PG) and endopectin-trans-eliminase (endo-PTE) in a mineral medium supplemented with a restricted supply of either D-galacturonic acid or onion cell walls. These enzymes were also extracted from infected onion tissue, but only endo-PTE caused tissue maceration and cell death. The patterns of host tissue colonization and pectic enzyme production were followed during bulb rot development. Stem plates were invaded within two weeks of inoculation. The pathogen then remained confined to the stem plates for several weeks or months, before spreading to the outer fleshy scales to initiate a basal rot. In most cases the inner leaf sheaths containing the lateral bud remained healthy. Exo-PG activity m stem plate tissue was greatest at two weeks after inoculation, then it declined. Endo-PTE was not detected in newly invaded stem plate tissue, but was recovered from infected stem plates before decay and from the bases of bulb scales and leaf sheaths at the onset of bulb rot. There was no pectic enzyme activity in uninvaded onion tissue. Spread of the fungus and pectic enzyme production in two Caledon Globe genotypes susceptible or tolerant to F. oxysporum f. sp. cepae were similar, but the onset of bulb rot in tolerant genotypes was considerably delayed.     

Genetic and virulence variation in Fusarium oxysporum f. sp. cepae causing root and basal rot of common onion in Iran

Root and basal rot of common onion (Allium cepae L.) caused by Fusarium oxysporum f. sp. cepae is one of the most important diseases causing tremendous losses in onion‐growing areas worldwide. In this study, random amplified polymorphic DNA (RAPD), intersimple sequence repeats (ISSR) and virulence studies were conducted to analyse 26 F. oxysporum f. sp. cepae isolates obtained from the main onion‐growing regions of Iran, including Fars, Azerbaijan and Isfahan states. Cluster analysis using UPGMA method for both RAPD and ISSR markers revealed no clear grouping of the isolates obtained from different geographical regions, and the isolates were observed to derive probably from the same clonal lineage. Pathogenicity test indicated that all F. oxysporum f. sp. cepae isolates were pathogenic on onion; however, virulence variability was observed among the isolates. The grouping based on virulence variability was not correlated with the results of RAPD and ISSR analyses.     

Identification of Fusarium species causing basal rot of onion in East Azarbaijan province,Iran and evaluation of their virulence on onion bulbs and seedlings

One of the economically important diseases of onion is the basal rot caused by various Fusarium species. Identification of the pathogenic species prevalent in a region is indispensable for designing management strategies, especially to develop resistant cultivars. Eighty Fusarium isolates are obtained from red onion bulbs on infected fields of East Azarbaijan province. Inoculating the onion bulbs with 38 selective isolates indicated that 17 isolates were pathogenic on onion. According to the morphological and molecular characteristics, these isolates were identified as F. oxysporum, F. solani, F. proliferatum and F. redolens. This is the first report of F. redolens on onion in Iran. On the other hand, the virulence of each pathogenic isolate was evaluated on onion bulbs and seedlings. F. oxysporum which causes severe rot and damping-off was considered as a highly virulent species in both conditions. While, F. proliferatum was considered as the most destructive on onion bulbs. Rot ability of F. solani was not considerable, and only the 4S isolate caused pre- and post-emergence damping-off more than 50%. Finally, F. redolens with less pathogenicity on onion bulbs was identified as the most virulent isolate on onion seedlings, which was explanatory of its importance on farm.     

Induced tolerance of Sclerotium cepivorum to antibiotics in the presence of onion exudates

The onion white rot pathogen Sclerotium cepivorum was cultured on agar media containing 2% malt extract and one of the antifungal antibiotics, endomycin, griseofulvin, venturicidin and cycloheximide at concentrations that reduced but did not prevent growth of mycelium. When onion seeds or agar discs impregnated with diffusates from onion bulbs were placed on the antibiotic media, radial growth of the fungus was greatly increased, and there was a profuse development of aerial mycelium. Gaseous diffusates from onion tissue and from impregnated agar discs were also effective. On the antibiotic media, tomato, cabbage and radish seeds did not stimulate the growth of S. cepivorum and the onion exudates did not stimulate the growth of four other fungi. This and other evidence is considered to show that the stimulation of growth of S. cepivorum was not caused by any direct effect on the antibiotics but by a tolerance of the fungus to them, which was specifically induced by an exudate from its host plant. The phenomenon may be related to the reported reversal by onion extracts of the inhibitory effects of soil mycostasis on germination of sclerotia of the fungus.     

Induction of defence-related enzymes in onion by combined application of fungal and bacterial biocontrol agents against Fusarium oxysporum f. sp. cepae

Basal rot disease of onion is a major problem in different onion growing regions of Tamil Nadu, India. Fungal and bacterial cultures were isolated and tested their efficiency against Fusarium oxysporum f. sp. cepae under in vitro conditions. Effective bacterial and fungal antagonists were tested alone and in combinations for the control of F. oxysporum f. sp. cepae in glasshouse experiments. Defence-related enzymes such as peroxidase, polyphenol oxidase and phenylalanine ammonia-lyase were induced and accumulated in onion treated with fungal and bacterial antagonists. Defence-related enzymes were significantly higher in onion pretreated with consortial formulation of Pf12 + Pf27 + TH3 at 5 days after the challenge inoculation with F. oxysporum f. sp. cepae and gave resistance to onion against basal rot disease.     

Root rot of onion caused byPythium irregulare andPythium coloratum

Pythium irregulare andPythium coloratum were isolated consistently from roots of onion plants exhibiting root rot and associated foliar symptoms in fields located in Organe County, NY.P. coloratum predominated following extremely wet weather in 1984 and 1986, whileP. irregulare was prevalent following moderately wet weather in 1985. Both species produced root rot symptoms similar to field symptoms when 12 week old onion plants (cv Downing Yellow Globe) were inoculated and incubated in a growth chamber at 14°C for 10 days. This is the first report ofP. irregulare andP. coloratum as causes of root rot of onion.     

The effects of wood decay type on the growth of bryophyte gametophytes

Deadwood is an important habitat for bryophytes in boreal and subalpine forests. The type of decay in wood (white, brown, and soft rot) caused by fungal colonizers has been revealed to affect bryophyte communities. However, little is known about the effects of decay type on the growth of bryophytes. We tested the effect of wood decay type on gametophyte growth for two common bryophyte species, Scapania bolanderi Austin and Pleurozium schreberi (Brid.) Mitt., which dominate the logs in subalpine coniferous forest on Mt. Ontake, in central Honshu, Japan. We used pot culture experiments in an open-sky nursery field. After eight months of cultivation, the growth of S. bolanderi was larger on brown rot wood than white rot wood, but the growth of P. schreberi was not. Mixed cultures of the two species also showed greater growth on brown rot wood. However, growth of S. bolanderi was significantly smaller than P. schreberi in mixed culture. These results suggest that brown rot wood enhances growth of S. bolanderi, but growth may be reduced under competition from P. schreberi. The results are in agreement with the field observation that brown rot wood has a positive association with S. bolanderi coverage on deadwood.     

Direct determination of the chromosomal location of bunching onion and bulb onion markers using bunching onion–shallot monosomic additions and allotriploid-bunching onion single alien deletions

To determine the chromosomal location of bunching onion (Allium fistulosum L.) simple sequence repeats (SSRs) and bulb onion (A. cepa L.) expressed sequence tags (ESTs), we used a complete set of bunching onion–shallot monosomic addition lines and allotriploid bunching onion single alien deletion lines as testers. Of a total of 2,159 markers (1,198 bunching onion SSRs, 324 bulb onion EST–SSRs and 637 bulb onion EST-derived non-SSRs), chromosomal locations were identified for 406 markers in A. fistulosum and/or A. cepa. Most of the bunching onion SSRs with identified chromosomal locations showed polymorphism in bunching onion (89.5%) as well as bulb onion lines (66.1%). Using these markers, we constructed a bunching onion linkage map (1,261 cM), which consisted of 16 linkage groups with 228 markers, 106 of which were newly located. All linkage groups of this map were assigned to the eight basal Allium chromosomes. In this study, we assigned 513 markers to the eight chromosomes of A. fistulosum and A. cepa. Together with 254 markers previously located on a separate bunching onion map, we have identified chromosomal locations for 766 markers in total. These chromosome-specific markers will be useful for the intensive mapping of desirable genes or QTLs for agricultural traits, and to obtain DNA markers linked to these.     

Fusarium avenaceum and Fusarium crookwellens cause onion basal rot in Iran

Abstract

Basal rot is the main and economically soil-borne disease of onion that caused by various Fusarium species worldwide. To identify the prevailing Fusarium species, 140 Fusarium isolates were obtained from red onion bulbs farms in 10 regions of East and West Azarbaijan provinces in 2015. By inoculating 80 selected isolates, 40 of them were pathogenic on onion. These 40 isolates were identified as F. oxysporum with 43.62%, F. subglutinans with 44%, F. culmorum with 50.66%, F. avenaceum with 51%, F. solani with 42.41%, F. crookwellens with 55%, F. proliferatum with 47.16% and F. redolens with 55.5% virulence. Their frequency were 20%, 2.5%, 7.5%, 5%, 42.5%, 2.5%, 15% and 5%, respectively. Forty studied isolates demonstrating that, 14.2% were highly virulent, 26.1% virulent, 40.3% moderately virulent and 19.4% weakly virulent. This is the first report of F. avenaceum and F. crookwellens as the causal agents of red onion basal rot in Iran.     

A serological test for detecting Botrytis allii, the cause of neck rot of onion bulbs

Polyclonal antisera for the immunological detection of Botrytis allii, the cause of onion neck rot, were raised from cell wall and cytoplasmic fractions of the fungus, and from surface plate washings of two isolates of the pathogen. These antisera were used to develop an indirect ELISA for the detection of latent B. allii infection in stored onion bulbs. The antisera did not react with healthy onion tissue and showed only minimal recognition of fungi from other genera. Some cross-reactivity was observed with B. cinerea but not with other Botrytis spp. tested. In many instances following infection, fungal antigen could be detected earlier by ELISA than by direct culture. In tests on bulbs from a commercial store, the level of B. allii infection was found to be 3.3% by both ELISA and direct plating, but this result was achieved 7 days sooner by ELISA. In field tests, the indirect ELISA gave a good indication at harvest of the probable incidence during storage of neck rot in untreated and artificially inoculated plants, whilst culturing gave results consistently lower than the ELISA until bulbs had been in store for several months, when results for ELISA and culturing were the same. Spores applied to growing plants during the growing season led to greater levels of neck rot than did infested seed, indicating that field debris and waste from cold stores may be more important sources of inoculum in some years than seed contamination.     

Comparative studies on simple lipids ofSclerotium cepivorum,the causal agent of white rot of onion,and other fungi of the class Deuteromycetes

Summary The simple lipids ofSclerotium cepivorum, the causal agent of white rot of onion and nine other fungal species of the same class were investigated. The fatty acid composition of the simple lipids of these fungi were determined by GLC. The main fatty acids common to these fungal species were C₁₆ (saturated) and C₁₈ (unsaturated) acids. The sterol fraction was isolated by column chromatography and its components were detected by GLC and mass spectrometry. Ergosterol and γ-Ergostenol were found mostly in all fungal species under investigation. However, two fungal species namelyAlternaria alternata andScolecobasidium constrictum showed no Ergosterol.     

Studies on the biology of Botrytis species on overwintered salad onions

In the years 1973/4–1974/5, Botrytis cinerea was responsible for losses of 26% and 37% respectively in overwintered salad (green) onion crops grown at Wellesbourne. The fungus invaded the tips of cotyledonary and young true leaves and then grew downwards and inwards through the leaf axils to colonise the internal tissues and produce a collar rot which caused plants to collapse and die. More direct infection of the junction between an older and younger leaf produced a similar effect. Symptoms were rarely noticeable (except occasionally in the later stages of the disease) and affected plants disappeared unobtrusively. The incidence of collar rot increased as the rate of leaf production fell with the drop in temperatures in winter months but decreased as leaf growth resumed with increases in temperature in the spring. The fungus grew and sporulated well at low temperatures (5°C) and more inoculated seedlings developed collar rot when maintained at simulated winter temperatures (7°C) than when kept at simulated spring temperatures (15°C). B. squamosa occurred spasmodically producing white leaf lesions but caused no loss of plants. B. allii although present at a low incidence did not increase in the crops but in common with B. cinerea caused some damage to the bases of plants at harvest.     

Mango stem end rot pathogens — Infection levels between flowering and harvest

During flowering and fruit set of mango (Mangifera indica L.), colonisation by fungi (Alternaria alternata, Cladosporium cladosporioides, Dothiorella dominicana, Dothiorella mangiferae, Dothiorella sp., Epicoccum purpurascens and Pestalotiopsis sp.) increased as the flowers senesced and young fruit formed. In the third week after flowering, the incidence of Dothiorella dominicana and Dothiorella mangiferae associated with mango fruit-pedicel connection tissue declined coincidentally with early fruit-fall, suggesting that early infections by Dothiorella spp. may cause fruitlet abortion. Dothiorella spp. levels in fruit-pedicel connection tissue remained low for the subsequent 6 weeks, after which they increased. By 16 weeks after flowering, the incidence of Dothiorella spp., determined by isolation from fruit-pedicel connection tissue, was similar to the incidence of stem end rot caused by Dothiorella spp., which developed in fruit harvested at that time. By contrast, the frequency of detection of Dothiorella spp. in peduncle tissue peaked 11 weeks after flowering, when the levels of stem end rot developing in fruit were already similar to the levels recorded in fruit harvested at 16 weeks and later. The results suggest that in fruit and fruit-pedicel tissue, colonisation might arise from Dothiorella spp. occurring endophytically in the peduncle. The earliest indicator of stem end rot incidence at harvest was the infection level in peduncle tissue sampled 11 weeks after flowering. Early assays of peduncle tissue for Dothiorella spp. might prove useful for selecting crops with low stem end rot infection levels.     

Stimulation of seedling root growth by coal-derived sodium humate

Coal-derived sodium humate was found to stimulate primary root growth of seedlings of cantaloupe (Cucumis melo L.) at 1000 mg L^–1, lettuce (Lactuca sativa L.) and onion (Allium cepa L.) at 500 and 1000 mg L^–1, as well as hypocotyl growth of cantaloupe at 1000 mg L^–1. Growth enhancement was not due to release of nutrient elements by the product, and, in the case of lettuce and onion, was not due to increased availability or uptake of mineral elements. Growth stimulation of cantaloupe, however, was dependent on the provision of nutrient solution. Growth stimulation of onion roots under axenic conditions indicated an effect of the humate per se rather than a response mediated via microbial breakdown products.     

Bulb and Root Rot in Lily (Lilium longiflorum) and Onion (Allium cepa) in Israel

In the past 10 years, there has been a substantial increase in reports, from growers and extension personnel, on bulb and root rots in lily (Lilium longiflorum) in Israel. Rot in these plants, when grown as cut flowers, caused serious economic damage expressed in reduction in yield and quality. In lily, the fungal pathogens involved in the rot were characterized as binucleate Rhizoctonia AG‐A, Rhizoctonia solani, Pythium oligandrum, Fusarium proliferatum (white and purple isolates) and F. oxysporum, using morphological and molecular criteria. These fungi were the prevalent pathogens in diseased plants collected from commercial greenhouses. Pathogenicity trials were conducted on lily bulbs and onion seedlings under controlled conditions in a greenhouse to complete Koch's postulates. Disease symptoms on lily were most severe in treatments inoculated with binucleate Rhizoctonia AG‐A, P. oligandrum and F. proliferatum. Plant height was lower in the above treatments compared with the control plants. The least aggressive fungus was R. solani. In artificial inoculations of onion, seedling survival was significantly affected by all fungi. The most pathogenic fungus was F. proliferatum w and the least were isolates of F. oxysporum (II and III). All fungi were successfully re‐isolated from the inoculated plants.     

Plant odours with potential for a push–pull strategy to control the onion thrips, Thrips tabaci

We compared the efficacy of four plant essential oils to repel onion thrips, Thrips tabaci (Lindeman) (Thysanoptera: Thripidae), in the presence of an attractive odour, ethyl iso‐nicotinate in a pasture field. Four horizontal white sticky plates were placed adjacent to (directions: N, S, E, W) a central horizontal white plate (C). After 24 h, in the treatment combination where the four plates were sprayed with essential oil surrounding a central sticky plate sprayed with ethyl iso‐nicotinate, fewer onion thrips were found on the plates treated with sweet marjoram [Origanum majorana L. (Labiatae)] or clove basil [Ocimum gratissimum L. (Labiatae)] (87 and 71% less, respectively) compared to the control treatment of four water‐sprayed plates surrounding a central plate with ethyl iso‐nicotinate. We also compared the distribution of onion thrips on the plates. Relative thrips numbers on each plate were compared with similar (N, S, E, W, and C) plates in the control treatment. There were relatively lower thrips numbers on the south (23% reduction) and west (26% reduction) O. majorana‐treated plates and higher numbers (37% increase) on the central attractant‐treated plate indicating a short‐distance push–pull effect. When four plates were sprayed with the thrips attractant surrounding a central sticky plate sprayed with an essential oil or water (control), only O. majorana reduced the number of thrips on the attractant‐sprayed plates (62% reduction). The distribution of thrips on the different plates within this treatment combination did not change substantially when compared to the distribution in the water‐control treatment. Other essential oils tested (wormwood [Artemisia arborescens L. (Compositae)]) and tea tree [Melaleuca alternifolia (Maiden. & Betche.) Cheel. (Myrtaceae)]) were not effective in any of the treatments. It appears that O. majorana is a promising thrips repellent which could be used for further testing in a push–pull system with the attractant ethyl iso‐nicotinate. The field setup used also proved to be a valuable tool for evaluating the potential of repellents to control onion thrips.