The effect of irrigation on powdery scab and other tuber diseases of potatoes

In field experiments seed tubers affected with powdery scab cankers were planted and the effect on disease incidence of timing of irrigation and some seed-tuber fungicides was investigated over 3 yr. For 2 yr, irrigation to maintain soil wetter than—20 centibars (—20 kPa) during the first half of the growing season increased disease compared to unirrigated plots. Disease incidence was not affected by irrigation at 2 wk intervals or when applied during the second half of the season. Little disease developed in 1983 even in irrigated plots, probably because of high soil temperatures. None of the fungicides tested gave consistent disease control. Common scab and silver scurf were both decreased by irrigation but in two years, black dot was increased. The relative importance of black dot could increase in irrigated crops where fungicides are used to control silver scurf.     

A revised infrageneric classification of Nothofagus (Fagaceae)

HILL, R. S. & READ, J., 1991. A revised infrageneric classification of Nothofagus (Fagaceae). An examination of cupule morphology, leaf architecture and cuticular morphology of Nothofagus species demonstrates that the existing infrageneric classifications are inaccurate. Four subgenera are proposed, based on these characteristics as well as other morphological information. This analysis indicates that the traditional pollen groupings of Nothofagus closely reflect the infrageneric taxonomy. It is hypothesized that deciduousness is primitive in Nothofagus , and the evergreen habit has arisen more than once, and therefore the deciduous or evergreen habit is invalid as a primary taxonomic character.     

A comparison of methods of seed tuber inoculation for assessing the susceptibility of potato cultivars to blackleg (Erwinia carotovora subsp. atroseptica) in the field

Experiments were done with the aim of developing a reliable method for assessing the susceptibility of potato cultivars to blackleg caused by Erwinia carotovora subsp. atroseptica, in the field. Over four years seed tubers were either stab-inoculated at the heel-end prior to planting, vacuum infiltrated with bacteria just after harvest or vacuum infiltrated prior to sprouting and after ‘cutting’ or ‘pricking’. Stab inoculation produced fewer diseased plants but generally a greater range of symptoms than vacuum infiltration. Vacuum infiltrated ‘cut’ seed gave most disease whereas infiltration at harvest and infiltration of ‘pricked’ seed gave similar disease incidence. The cultivar Maris Piper was more resistant than Desiree in 1981 and 1982, and Pentland Javelin than Ulster Sceptre in 1983 and 1984, largely irrespective of the inoculation treatment. Irrigation improved yields but did not affect disease symptoms. In further field experiments over three years, each with a minimum of 20 cultivars, seed tubers were sliced just before planting at a standard distance from the apex and inoculated by applying a pad impregnated with inoculum. Differences were shown between cultivars and it is suggested that the method could be adapted for testing for blackleg susceptibility under controlled environment conditions.     

Carbohydrate Transport in Trypanosoma gambiense*

SYNOPSIS. The uptake of ¹⁴C-labeled carbohydrates by Trypanosoma gambiense was studied. Glucose, mannose, glycerol, 2-deoxyglucose and fructose were rapidly absorbed by the parasite, and all had saturation kinetics. The glucose analog 3-O-methylglucose was not taken up by T. gambiense. Competitive inhibition experiments indicate that there are 2 transport loci for the tested substrates. It is suggested that there is a “glucose site” thru which glucose, mannose and glycerol, but not fructose, are transported and a “fructose site” thru which only fructose is transported. The specificity of the glucose-transporting mechanism appears to differ from those of other animals.     

Big flies, small repeats: the "Thr-Gly" region of the period gene in Diptera

The region of the clock gene period (per) that encodes a repetitive tract of threonine-glycine (Thr-Gly) pairs has been compared between Dipteran species both within and outside the Drosophilidae. All the non- Drosophilidae sequences in this region are short and present a remarkably stable picture compared to the Drosophilidae, in which the region is much larger and extremely variable, both in size and composition. The accelerated evolution in the repetitive region of the Drosophilidae appears to be mainly due to an expansion of two ancestral repeats, one encoding a Thr-Gly dipeptide and the other a pentapeptide rich in serine, glycine, and asparagine or threonine. In some drosophilids the expansion involves a duplication of the pentapeptide sequence, but in Drosophila pseudoobscura both the dipeptide and the pentapeptide repeats are present in larger numbers. In the nondrosophilids, however, the pentapeptide sequence is represented by one copy and the dipeptide by two copies. These observations fulfill some of the predictions of recent theoretical models that have simulated the evolution of repetitive sequences.      

Filamentous algal communities related to pH in streams in The Trossachs, Scotland

• 1 A survey of acidified streams in the Loch Ard area of The Trossachs, Scotland, was carried out between 1986 and 1988, to determine the influence of physical and chemical factors on the distribution of benthic algae. Samples were taken on thirty-five occasions from fifteen sites on ten streams. Forty-nine operational taxa of filamentous algae were distinguished. Relative abundance of taxa in samples was scored on a seven-point scale. Estimates of standing crop were made by pigment extraction and ash-free dry weight (AFDW) determination from biomass developed on artificial substrata.
• 2 No evidence was found for seasonality in standing crop, nor for an increase in standing crop with a decrease in pH. Seasonality in relative abundance was evident for few taxa. Species richness and diversity were highly correlated with pH and correlated chemical variables.
• 3 Canonical correspondence analysis (CCA) carried out using the CANOCO program ordinates species and sites and relates them directly to environmental variables. The most important variable was found to be pH (or an acidification-related variable that is highly correlated with pH), with percentage forest cover and total oxidized nitrogen being of secondary importance. The effect of subdividing the data set, using data based on presence-absence as well as estimated relative abundance, was tested. A comparison of the effects on the distribution of site and species values on the pH vector showed that the results are robust, confirming the value of this semi-quantitative sampling method for use with a difficult algal group. It was possible to infer pH from algal community structure using CANOCO.
• 4 The pH ranking of species derived from CCA revealed that larger cell diameter taxa do not predominate at low pH, implying that reduction of invertebrate grazing was not an important determinant of algal community composition.