Restriction fragment length polymorphisms (RFLPs) and the relationships of some host-adapted isolates of Verticillium dahliae

The relationships of two host-adapted pathotypes of Verticillium dahliae have been examined at the molecular level using restriction fragment length polymorphisms. Isolates obtained from and adapted to Mentha × piperita (peppermint), which were presumed to be haploid, formed a distinct subspecific group (referred to as M) related to the previously described non-host-adapted subspecific group A of V. dahliae. The limited molecular variation found among the four group M isolates was not related to geographic origin. Isolates from several cruciferous hosts (and one from Beta vulgaris (sugar beet)), which are thought to be natural, stable diploids, formed another distinct group (referred to as D) that was markedly different from all previously described subspecific groupings in both V. dahliae and V. alboatrum. This group of isolates might better be regarded as a separate species. Again, only limited variation was found within the D group. Polymorphisms revealed by two probes distinguished two isolates derived from Brassica rapa (Chinese cabbage) from the six other isolates (four from Brassica napus (oilseed rape) and one each from Raphanus raphanistrum (wild radish) and Beta vulgaris).     

Resistance to Peronospora viciae expressed as differential colony growth in two cultivars of Pisum sativum

Peronospora viciae sporulated frequently following localized inoculation of leaflets of Pisum sativum cv. Krupp Pelushka but rarely on cv. Early Onward (60 and < 1% of plants respectively). Similar results were obtained following inoculation of seedling shoot apices. The proportions of sporangia that germinated, formed appressoria, invaded via stomata or cuticle, and established colonies, were similar on leaflets of both cultivars. After 2 days, total hyphal lengths of most colonies were greater than 100 μm (94% in Krupp Pelushka, 64% in Early Onward), but in Krupp Pelushka there were five times as many haustoria per unit length of hypha. Subsequently, only occasional colonies in Early Onward grew to the same extent as in Krupp Pelushka, with 8% and 88% respectively attaining lengths > 800 μm after 4 days. Reduced growth of hyphae in Early Onward was associated with increased frequency of host cells stained by trypan blue.     

Studies on the biology of black mould (Aspergillus niger) on temperate and tropical onions. 1. A comparison of sources of the disease in temperate and tropical field crops

Aspergillus niger colonies were present on 60–90% of the leaves sampled from onion crops grown under hot, tropical conditions (Shambat, Sudan), but only on 21% of leaves in crops grown under temperate conditions (Wellesbourne, UK). The seed stocks used in the Sudan were naturally contaminated with A. niger , whereas the fungus was absent from seed used in the UK. Inoculation of seed with A. niger resulted in an increase in the incidence of the fungus on onion plants (mean 83% of plants with affected leaves) in the field and on bulbs in store in crops grown in the UK. In the UK, A. niger is infrequent in the soil and air, but it is a major component of the soil and air mycoflora in the Sudan. Most of the bulbs grown in Sudan were contaminated with A. niger at harvest because of the high incidence of the fungus in the field.     

The role of Poa annua in the epidemiology of barley yellow dwarf virus in autumn-sown cereals

Studies of cereal aphids and barley yellow dwarf virus (BYDV) from 1989 to 1992 revealed that Poa annua is an abundant weed of commercial winter barley crops during the summer months. P. annua was frequently infected with BYDV, and there were usually similarities with the BYDV infection of the surrounding barley crop. These P. annua weeds were often infested by cereal aphids both in July (pre-harvest) and in September (in cereal stubble fields). Poa -infested cereal stubbles may be major local sources of viruliferous aphids, increasing the risk of BYDV in nearby winter cereals. P. annua plays an important role in the epidemiology of S. avenae -transmitted BYDV.     

Influence of barley variety mixtures on severity of scald and net blotch and on yield *

Two susceptible barley cultivars and two moderately resistant advanced breeding lines were grown as pure stands and as the 11 possible equi-proportional mixtures over three field seasons, in the presence of scald and net blotch. Plots were either inoculated with infested straw, or non-inoculated and sprayed with fungicide. On average, the variety mixtures restricted leaf disease development by 12%. However, mixtures of either or both susceptible cultivar/s with the moderately resistant line 1861018 restricted disease severity by 20-32%. Generally, the mixtures did not increase yield relative to the mean of the pure stands. However, in the year of highest disease severity, yield was increased by 7% owing to mixing in the inoculated plots. In addition, one mixture consistently increased yield (mean of 4%) and one mixture had consistently lower yields (mean of 6%)) over years and inoculation treatments. Thus, careful evaluation is required in this environment and with these genotypes in order to identify mixtures with positive effects on disease control and yield.     

Use of carbendazim and carbendazim-resistant yeasts to create different yeast densities on wheat leaves for field studies on biological control

Phyllosphere yeasts may play an important role in suppressing necrotrophic pathogens. In a randomized block design, yeast densities on flag leaves and second leaves of field-grown wheat were left unchanged (D), or were lowered by sprayings with carbendazim (A), or were raised by sprayings with a mixture ofSporobolomyces roseus, Cryptococcus laurentii var.flavescens and nutrients (B), or with carbendazim, carbendazim-resistantS. roseus andC. laurentii var.flavescens and nutrients (C). The application of carbendazim to plots A and C would avoid a biased interference of carbendazim due to other effects than reducing the natural yeast population. Prolonged differences in yeast density would be reflected in differences in severity of diseases caused by carbendazim-insensitive necrotrophic pathogens.Both treatments B and C enhanced the yeast density about 10-fold at the beginning of leaf colonization, 2 to 3 weeks after leaf emergence. Untreated leaves (D), however, were rapidly leaves. Initially, the yeasts were suppressed by carbendazim (A). However, progressively the yeast population consisted of carbendazim-resistant strains which made the carbendazim sprayings ineffective. Therefore, the substantial differences in yeast density were of limited duration. The effect of the treatments on naturally occurringMycosphaerella graminicola (anam.Septoria tritici), Puccinia recondita f. sp.tritici and the total necrotic leaf area (D-value) is discussed.Samenvatting Fyllosfeergisten kunnen misschien een belangrijke rol spelen in de onderdrukking van necrotrofe pathogenen. In een blokkenproef werd de dichtheid van gisten op vlagbladeren en tweede bladeren niet beïvvloed (D) of verlaagd door bespuitingen met carbendazim (A), of verhoogd door bespuitingen met een mengsel van gisten (Sporobolomyces roseus enCryptococcus laurentii var.flavescens en voedingsstoffen (B), of met een mengsel van carbendazim, carbendazim-resistenteS. roseus enC. laurentii var.flavescens en voedingsstoffen (C). De grootste verschillen in dichtheden van de gistpopulaties werden verwacht tussen de behandelingen A en B of C. Door alle veldjes A en C met carbendazim te behandelen wordt een eenzijdig effect van carbendazim, berustend op andere eigenschappen dan de onderdrukking van de natuurlijke gistpopulatie, voorkomen. Langdurige verschillen in de populatiedichtheden van de gisten zouden weerspiegeld kunnen worden in verschillen in aantasting door carbendazimongevoelige necrotrofe pathogenen.Aan het begin van de bladkolonisatie door gisten, 2 tot 3 weken na het verschijnen van het blad, leidde zowel behandeling B als C tot, een 10-voudige verhoging van de gistdichtheid. De onbehandelde bladeren (D) werden echter spoedig door de van nature voorkomende gisten gekoloniseerd en het verschil tussen de onbehandelde en met gist bespoten bladeren werd snel kleiner. Aanvankelijk werden de gisten door carbendazim (A) onderdrukt. Geleidelijk bestond de gistpopulatie echter uit carbendazim-resistente stammen, zodat de carbendazim bespuitingen nauwelijks meer effect hadden. Hierdoor werden grote verschillen in gistdichtheden niet langdurig gerealiseerd. Het effect van de behandelingen op de natuurlijke infectie doorMycosphaerella graminicola (anam.Septoria tritici), Puccina recondita f. sp.tritici en op het totale dode bladoppervlak (D-waarde) wordt besproken.     

Oxytetracycline residues in giant freshwater prawn (Macrobrachium rosenbergii)

The distribution of oxytetracycline (OTC) in male and female Macrobrachium rosenbergii was examined after the prawns had been given medicated feed containing OTC at levels of 2.5 and 5.0 g/kg of feed for 1 week and nonmedicated feed thereafter. OTC levels in the heads of both male and female prawns were consistently higher than in the muscles. Batches of both male and female prawns treated with higher dosages had significantly higher OTC residues in both head and muscle tissues than the batches treated with lower dosages. During treatment, peak concentrations in the head and muscle from each group were reached within 4-6 days. After drug treatment ceased, 13 days for the head and 10 days for the muscle were required to reduce OTC residues to safe levels in all batches of both sexes and dosages. In practice, to account for variations in water temperature, drug dosage, duration of therapy, and other environmental conditions, a withdrawal period of 21 days is recommended.