Studies on the seed-borne phases of dark leaf spot Alternaria brassicicola and grey leaf spot Alternaria brassicaeof brassicas

Tests in Britain on samples of basic and commercial Brassica oleracea seed between 1976 and 1978 showed that many lots were infected with Alternaria brassicicola. A. brassicae was uncommon in basic seed in these years and in commercial seed harvested in 1976 and 1977 but was frequent in seed harvested in 1978. Most affected seeds were contaminated by surface-borne spores and mycelium of A. brassicicola but many were internally infected by the fungus situated within the seed-coat and in some seeds in the embryo tissues. Superficial contamination by the fungus declined rapidly after 2 yr in cabbage seeds stored at 10 °C, 50% r.h. but internal infection persisted for up to 12 yr. In some samples, internal infection was commonly associated with small shrivelled seeds. Surface contaminated and internally infected seeds transmitted the disease but seedling infection was more closely correlated with the latter.     

Pea seed infection by Mycosphaerella pinodes and Ascochyta pisi and its control by seed soaks in thiram and captan suspensions

Apparently normal pea seeds from pods bearing lesions of Mycosphaerella pinodes were often internally infected with the fungus. When infected seeds were sown in sterile grit almost all the seedlings showed disease lesions, at or below soil level, 4–6 weeks after sowing. Seed infected with Ascochyta pisi gave only 40% infection of seedlings: these showed lesions on the stem and first two leaves within 4 weeks of sowing. Infection of seeds by both pathogens could be eradicated by soaking the seeds for 24 hr. in 0.2% suspensions of thiram or captan at 30d?C. In laboratory or greenhouse tests these treatments did not check germination, but in the field the captan treatment reduced emergence. The treated seeds became fully imbibed but could be dried and stored: the thiram treatment was used for semi-commercial treatment of quantities of seed up to 3 cwt.     

The Role of White Mold-Infected White Bean (Phaseolus vulgaris L.) Seeds in the Dissemination of Sclerotinia sclerotiorum (Lib.) de Bary

Sclerotinia sclerotiorum survived in infected seeds of white beans as dormant mycelium in testa and cotyledons. The rate of survival averaged 85 to 89% and did not change appreciably over a 3-year period. When the infected bean seeds were sown in soil or sand, 88 to 100% failed to germinate. The seeds that failed to germinate, depending on the severity of seed infection, were rotted by S. sclerotiorum. In place of each seed, 3 to 6 sclerotia were formed. A low percentage of these sclerotia germinated carpogenically with or without preconditioning, (2.5 and 11.5% respectively). Myceliogenic germination of sclerotia with and without preconditioning was 35.5% and 70.5% on water agar and 81.0% and 93.0% on glucose agar, respectively. Both, preconditioning and nonpreconditioned sclerotia which were scattered on soil surface could germinate myceliogenically and infect bean leaves by contact. It is therefore, concluded that dormant mycelia in the infected seeds play an important role notonly in dissemination of the fungus but also in epidemiology of the disease.     

The effects of soil conditions, soil sterilisation and seed treatmen on the emergence of cabbage in controlled temperature cabinets and the field

Previous cropping history of the soil and the changes that occurred in soil moisture content and temperature were the major factors determining the emergence of cabbage seedlings from pathogen-free seeds. Seedling losses caused by soil-borne pathogens could be high but these losses were prevented by seed treatment with metalaxyl or soil sterilisation with methyl bromide prior to sowing. The beneficial effects of metalaxyl and methyl bromide treatments were confirmed in field experiments involving repeated sowings of cabbage seeds. In experiments with sterilised soil and cabbage seed infected with Alternaria brassicicola emergence at – 0·2 bars soil moisture potential was reduced by 7·8–23.5% at 5, 10, 20, and 25°C. In the field, losses due to A. brassicicola were greatest when the soil became capped after sowing. The results show that recently developed fungicides may facilitate emergence under conditions extremely favourable for seed- and soil-borne pathogens. For maximum benefit, selection of seed treatments should be based on knowledge of the range of pathogens likely to reduce emergence.     

Neck rot (Botrytis allii) of bulb onions

Experiments on neck rot of onions, caused by Botrytis allii showed that, although the disease only became evident in store, a major source of the pathogen was samples of infected seeds. In 1972 and 1973, 39·5 and 71·4% respectively of commercial onion seed samples tested at Wellesbourne were infected. The pathogen was internal in seed and persisted for 3 ½ yr in infected seeds kept in a seed store at 10°C and 50% r.h. Seedlings raised from diseased seeds became infected by mycelial invasion of the cotyledon leaf tips from seed-coats many of which remained attached to the cotyledons when seedlings emerged from the soil. The fungus attacked the living tissues of these leaves symptomlessly, producing conidiophores only after the leaf tissue senesced and became necrotic. Because the fungus was symptomless, the rate of spread of the pathogen in onion crops was assessed by incubating successive samples of plants from the field in humid conditions when infected tissues developed conidiophores of the fungus. This method showed that the disease was progressive in onion crops spreading more rapidly in wet humid conditions (e.g. 1972) than in dry ones (e.g. 1973). The principal means of spread were probably fungal spores; conidiophores bearing spores being produced abundantly on plants in the field under high humidity. The fungus invaded the leaves of plants successively, first infecting each leaf at the tip and then growing downwards in the tissues and eventually invading the neck of the onion bulb via the leaves which emerged directly from the top of the neck. By harvest, the fungus was situated deep within the neck tissues of infected maturing onion bulbs.     

Transmission of Alternaria macrospora in Cotton Seeds

Alternaria macrospora was isolated from seeds only after the natural opening of the bolls and exposure of seeds to an environment in which the fungus was present. The fungus lacks the ability to penetrate the boll wall and reach the seed site. Attempts to isolate the pathogen from seeds of immature bolls at different developmental stages failed. Internal infection by slow injection resulted in seed infection and partial shedding of the injected plant parts which was high in buds and decreased with the ripening to mature bolls. Severity of seed infection was not dependent on either inoculum level, boll physiological age or even if the boll itself was not diseased. Infection of flowers under field conditions caused flower shedding. Naturally infected seeds or inoculated seeds with inoculum levels of 100 spores/ml and above resulted in diseased cotyledons, the incidence of which was, for inoculated seeds, positively correlated with inoculum level. A small difference was observed between cultivars in susceptibility to artificial inoculation at the cotyledon stage. A. macrospora survived on commercial cotton seeds and on post-season plants left growing at the field edges. Survival in plant debris under field conditions was minimal and may only have a minor effect on field reinfestation.     

Seed ecology of dust seeds in situ: a new study technique and its application in terrestrial orchids

A method is described by which seeds of terrestrial orchids are sown and retrieved in the field under almost natural conditions. For the first time it is possible to conduct a quantitative study of orchid germination in situ and observe seasonal growth and mortality of seedlings. The technique has also enabled us to investigate the relation between the site where the seeds are sown, the availability of an appropriate fungus to infect the seeds, and seedling establishment in the soil. Five local species were studied. Corallorhiza odontorhiza, Goodyera pubescens, and Galearis spectabilis all began to germinate in May–June, after 23-30 weeks in the soil. These species differed in their dependency on infection at germination time, but none of the seedlings developed beyond the point of rupturing the testa except when infected. Seeds of Liparis lilifolia and Tipularia discolor did not germinate within the first 12 months of the experiment. The implications and potential uses of this field sowing technique for further studies and for other kinds of minute seeds are discussed.     

Phenology and fungus-nematode relations of corky root disease of Douglas-fir

Symptoms of corky root disease of Douglas-fir seedlings include: (I) swollen tap- and lateral roots; (2) paucity of lateral roots; and (3) stunted shoot growth. In diseased roots, the primary cortex persisted and enlarged, but secondary vascular tissues, periderm, pericycle and endodermis were reduced. Cylindrocarpon destructans was the fungus most frequently isolated from cortical cells and symptoms were induced by massive inoculum of some isolates but not by dispersed inoculum nor by extracts from cultures. Whereas symptoms first appeared 2 months after seeds sown in May germinated, C. destructans was rarely isolated until 4 months after germination. Relatively more colonies of C. destructans were isolated from corky roots in the winter than in the summer, when Fusarium oxysporum or Mycelium radicis atrovirens were more abundant. Populations of the plant-parasitic nematode Xiphinema bakeri on roots and in adhering soil were highest 2 months after germination and during the summer, correlating positively with the per cent of tap-root affected and negatively with numbers of lateral roots.     

Canola seed germination and seedling emergence from pre-hydrated and re-dried seeds subjected to salt and water stresses at low temperatures

Low soil temperatures and low water potentials reduce and delay the seed germination of canola (Brassica rapa L., B. napus L.) in western Canada. Germination is also very sensitive to the salinity effects of nitrogen fertiliser placed with the seed, especially when the seed bed is relatively dry. The effects of pre-hydration and re-drying treatment on canola (Brassica rapa L. cv. Tobin) seed germination and seedling emergence at 10°C subjected to either a water or salt stress were determined. Low water potentials, induced by polyethylene glycol (PEG 8000), low soil moisture, or high concentrations of salts, reduced both germination and seedling emergence, and increased the time to 50% germination and emergence of seeds at 10°C. At equal osmotic potentials, Na₂SO₄ was less inhibitory on low temperature germination than either NaCl or PEG, suggesting that the sulphate ion partially alleviated the inhibitory effects of low water potential. Solutions of NaCI produced more abnormal seedlings compared to Na₂SO₄, suggesting that NaCl was more toxic than Na₂SO₄ during seedling development. Pre-hydration and re-drying partially overcame the inhibitory effects of both low water potential and salts on seed germination and seedling emergence at 10°C. The seed treatment increased the germination rate in Petri dishes and seedling emergence from a sandy loam soil. Water potentials or soil water contents required to inhibit 50% germination or emergence at 10°C were lower for treated seeds compared to control seeds. Salt concentrations inhibiting 50% emergence were higher for treated seeds than control seeds. Neither treated nor control seeds produced seedlings which emerged if the soil water content was lower than 9% or when the soil was continuously irrigated with salt solutions of 100 mmol kg^-1 of NaCl or 50 mmol kg^-1 of Na₂SO₄. These results suggest that the pre-hydration and re-drying treatment did not lower the base water potentials at which seedling emergence could occur. Abnormal seedlings were observed in both treated and control seeds, particularly if the soil was watered with NaCl solutions; however, the seed treatment reduced the number of abnormal seedlings.     

Control of dark leaf spot (Alternaria brassicicola) of Brassica oleracea seed production crops with foliar sprays of iprodione

Three sprays of iprodione (Rovral 50% w. p.) at 0.5-1 kg a. i./ha applied to Brassica oleracea seed crops in experimental plots and field trials at 3-wk intervals from the young green pod stage until cutting controlled pod infection caused by A. brassicicola in seasons 1976–1979. As a result few seeds were infected, seed yields were increased and their germination was improved. Sprays of Bordeaux mixture were as effective as iprodione in years when disease levels were low but were ineffective when infection pressure was severe.     

Survival and perpetuation of Alternaria brassicae causing Alternaria blight of oilseed crucifers

Modes of survival and perpetuation of Alternaria brassicae attacking rapeseed and mustard were examined. The pathogen remained viable in diseased plant debris and seeds of infected plants which served as primary sources of inoculum. The pathogen was internally seed-borne and infected seeds resulted in damping-off of seedlings. Air-borne conidia produced on aerial parts of the infected plants were responsible for secondary spread of the disease.     

Studies on the etiology of black rot, Stemphylium radicinum (Meier, Drechsl. & Eddy) Neerg., and leaf blight, Alternaria dauci (Kühn) Groves & Skolko, on carrot crops; and on fungicide control of their seed-borne infection phases

Of nineteen commercial samples of carrot seed in use at Wellesbourne in 1963, eight were found to be infected with both Stemphylium radicinum and Alternaria dauci and five with S. radicinum alone. Both fungi caused damping-off of seedlings and when carrots were grown at high densities A. dauci caused severe foliar infection. Neither fungus gave rise to marked infection of ware crops at normal spacings but, when seed infected with S. radicinum was sown in the autumn, the resulting seed crop was heavily infected by the succeeding autumn. Spring-sown carrots grown in soil that had borne infected crops or contained debris infected with either fungus showed little or no infection in the autumn or after storage for 3 1/2 months; autumn-sown or spring-planted (steckling) carrots put into ground infected with S. radicinum showed severe infection in the following autumn. This suggested that ware-crop carrots were resistant to infection from the soil whereas the seed crop was not. Both fungi were eradicated from seed by a 24 hr. soak at 30d? C. in a 0.2% thiram suspension, but were not completely eliminated by seed dust treatments with a range of fungicides.     

Biological control of slugs in winter wheat using the rhabditid nematode Phasmarhabditis hermaphrodita

A field experiment on winter wheat in autumn 1991 investigated the effect of the rhabditid nematode, Phasmarhabditis hermaphrodita, applied to soil at five dose rates (10⁸ - 10¹⁰ infective larvae ha^-1) immediately after seed sowing, on slug populations and damage to seeds and seedlings. The nematode was compared with methiocarb pellets broadcast at recommended field rate immediately after drilling and no molluscicide treatment. Slug damage to wheat seeds and seedlings was assessed 6 and 13 wk after drilling. Seedling survival increased and slug grazing damage to seedlings declined linearly with increasing log nematode dose. These two measures of slug damage were combined to give an index of undamaged plant equivalents, which also increased linearly with increasing log nematode dose. ANOVA showed that, after 6 wk, there were significantly more undamaged plant equivalents on plots treated with the two highest nematode doses (3 × 10⁹ and 1 × 10¹⁰ ha^-1) than on untreated plots, but the number of undamaged plant equivalents on methiocarb-treated plots was not significantly greater than that on untreated plots. Slug populations were assessed by refuge trapping and soil sampling. Deroceras reticulatum was the commonest of several species of slugs recorded. During the first 4 wk after sowing, significantly more slugs were found under refuge traps on plots treated with certain doses of P. hermaphrodita than under traps on untreated plots and more showed signs of nematode infection than expected from the prevalence of infection in slugs from soil samples, suggesting that the presence of P. hermaphrodita altered slug behaviour. Application of P. hermaphrodita had no significant impact on numbers or biomass of slugs in soil during a 27 wk period after treatment, except after 5 wk when slug numbers were inversely related to log nematode dose. However, by this time, numbers in soil samples from untreated plots had declined to levels similar to those in plots treated with the highest dose of nematodes. During the first 5 wk after treatment, c. 20% of slugs in soil samples from untreated plots showed symptoms of nematode infection. It is suggested that this represented the background level of infection in the experimental field rather than spread of infection from treated plots. The apparent lack of impact of P. hermaphrodita on slug numbers and biomass in soil suggests that its efficacy in protecting wheat from slug damage was through inhibition of feeding by infected slugs.     

Ethylene formation by germinating,Drechslera graminea — Infected barley (Hordeum sativum) grains: A simple test for fungicides

Grains of barley (Hordeum sativum L.); infected with the parasitic, systemic fungus Drechslera graminea, produce more ethylen than uninfected controls. Treatment of infected grains with mercury-free fungicides yields a differentiated suppression of the ethylene evolution 7 d after the beginning of germination. Suppression of visible symptoms (chlorotic stripes on leaves) appearing six to eight weeks after germination of infected, untreated seeds correlates with the decrease in ethylene formation after treatment with fungicides. The gaschromatographic ethylene determination thus allows for an early and reliable (significance higher than 99.9%) differentiation of fungicidal activities against the barley stripe disease.Abbreviations ACC I-aminocyclopropane-1-carbonic acid - CD critical difference     

Verticillium dahliae (Kleb.) Infecting Pumpkin Seed

This study investigated the natural occurrence of Verticillium dahliae (Kleb.) infection in pumpkin (Cucurbita pepo L.) seed. The mean incidence of infection was found to be 21.0%. Isolates recovered from seeds were pathogenic to pumpkin (cultivar ‘Jamaican squash’). Surface sterilization by immersion in 0.6% sodium hypochlorite for 20 min eradicated V. dahliae from infected pumpkin seeds without affecting germinability. Plating of seed components revealed that the fungus was present in the seed coat but not in the embryo or cotyledons. In a growing‐on test, 25% of 6‐week‐old plants grown from untreated seeds were infected. Germination and production of normal seedlings were unaffected by V. dahliae infection of seeds. Verticillium dahliae in pumpkin seed was found to be external and transmissible to plants. The findings of this study are important in devising disease control strategies.     

Seed treatment for the control of halo-blight of beans (Pseudomonas phaseolicola)

Streptomycin and kasugamycin were applied as soaks, slurries or dusts to dwarf bean seeds either naturally (internally) infected or externally contaminated with Pseudomonas phaseolicola. Slurries of streptomycin (2–5 g a. i./kg seed) or kasugamycin (0–25 g a.i./kg seed) were the most effective treatments against both types of infection and were generally non-phytotoxic at these rates. Combined analysis of eight field experiments made over a 3 yr period showed that on average both compounds applied in slurries reduced primary infection from infected seeds by 98 %. The method thus shows considerable promise as a commercial control treatment.     

Grass seed infection following inundation with Pyrenophora semeniperda

Seed infection by the seed-borne pathogen Pyrenophora semeniperda was demonstrated following inundative applications of the fungus to several annual grass weeds and wheat. The optimum time for applying inoculum was found to be around mid-anthesis in wheat. Applications of inoculum during and after anthesis in field experiments resulted in seed infection that manifested as stromatal development which affected germination or reduced seedling vigour. In an initial field experiment, >70% of Bromus diandrus seeds exhibited stromata of P. semeniperda. In a field comparison of inoculum types, conidial suspensions resulted in the greatest level of seed infection of B. diandrus compared with several types of inoculum containing mycelium fragments. Inundation of Avena fatua, Lolium rigidum, Hordeum leporinum, Vulpia bromoides and B. diandrus under field conditions with conidia of P. semeniperda also resulted in either the failure of infected seeds to germinate or a reduction of seedling vigour. It is concluded that the use of P. semeniperda as a seed-borne bioherbicide may be a biologically reasonable tactic, and the many logistical and technological constraints impeding its development are discussed.     

Dynamics of Neotyphodium endophyte infection in ageing seed pools: incidence of differential viability loss of endophyte,infected seed and non-infected seed

Symbiotic associations between grasses and vertically transmitted endophytic fungi are widespread in nature. Within grass populations, changes in the frequency of infected plants are driven by influence of the endophyte on the fitness of their hosts and by the efficiency of endophyte transmission from parent plants to their offspring. During the seed stage, the endophyte might influence the fitness of its host by affecting the rate of seed viability loss, whereas the efficiency of endophyte transmission is affected by losses of viability of the fungus within viable seeds. We assessed the viability losses of Lolium multiflorum seeds with high and low level of infection of the endophyte Neotyphodium occultans, as well as the loss of viability of the fungus itself, under accelerated seed ageing and under field conditions. Starting with high endophyte-infected accessions of L. multiflorum, we produced their low endophyte-infected counterparts by treating seeds with a fungicide, and subsequently multiplying seeds in adjacent plots allowing pollen exchange. In our accelerated ageing experiments, which included three accessions, high endophyte-infected seeds lost viability significantly faster than their low endophyte-infected counterpart, for only one accession. High endophyte-infected seeds of this particular accession absorbed more water than low endophyte-infected seeds. In contrast, the endophyte lost viability within live seeds of all three accessions, as the proportions of viable seeds producing infected seedlings decreased over time. In our field experiment, which included only one accession, high endophyte-infected seed lost viability significantly but only slightly faster than low endophyte-infected seed. In contrast, the loss of viability of the endophyte was substantial as the proportions of viable seeds producing infected seedlings decreased greatly over time. Moving the seeds from the air to the soil surface (simulating seed dispersion off the spikes) decreased substantially the rate of seed viability loss, but increased the rate of endophyte viability loss. Our experiments suggest that, in ageing seed pools, endophyte viability loss and differential seed mortality determine decreases in the proportions of endophyte-infected seeds in L. multiflorum. Endophyte viability loss within live seeds contributes substantially more to infection frequency changes than differential viability losses of infected and non-infected seeds.     

Leaf position-dependent effect of Alternaria brassicicola development on host cell death,photosynthesis and secondary metabolites in Brassica juncea

During the first 24 hours of infection, Alternaria brassicicola developmental parameters such as conidial germination, germ tubes and appressoria formation on each of the five mature Brassica juncea leaves, correlated with a leaf position showing stronger development of the pathogen on older leaves than on young ones. As a consequence of fungal development, the black spot disease was observed during 96 hours of infection on a macroscopic scale, as well as via confocal microscopy. Degradation of the chloroplast thylakoids and plastoglobule appearance during infection, followed by the decrease in chlorophyll a fluorescence parameters i.e. maximum quantum yield of PSII (F_v/F_m), non-photochemical quenching (NPQ) and chlorophyll a:b ratio, have been observed. Also, after an initial increase of carbohydrates (glucose, fructose and sucrose), content far below the respective control values was found. The content of secondary metabolites such as flavonoids and glucosinolates increased in a leaf position-dependent manner in infected leaves, with a lower level in older leaves than in younger ones. Although, the total phenolic compounds (TPCs) content did not differ significantly in infected leaves compared to control leaves, TPCs level in both control and infected leaves was leaf position-dependent. To the best of our knowledge, this is the first report on leaf position-dependent effect on the B. juncea biochemical response to A. brassicicola infection.     

The control of Septoria on celery seed

In studies on the control of Septoria on celery seed, ¼ and 1 lb (113 and 454 g) samples of infected seed treated by the thiram-soak method produced disease-free seedlings under glass and disease-free plants at maturity in the field. Hot-water treatment of infected seeds, however, failed to give complete disease control in either situation. Thiram soaking was not harmful to seed germination and produced field crops of equal quality to those obtained from hot-water-treated seeds. In N.A.A.S. extension trials a total of 18½ lb (8.3 kg) of celery seed was treated by the thiram-soak method and effective disease control was obtained in crops raised from this seed throughout the country.