ALBERT

Notes: Polymyxa betae is the fungal vector of beet necrotic yellow vein virus (BNYVV), which is the causal agent of the sugar beet disease rhizomania. The within-season dynamics of the fungus are a crucial factor in the occurrence and severity of rhizomania. Late infection of the host by viruliferous fungi enables host resistance to the virus to develop and hence limits crop damage. A previously published mechanistic model for the dynamics of Polymyxa betae is extended in this paper to incorporate the effect of temperature on the germination of resting spores, and on the latent periods between infection and the production of secondary zoospores and new resting spores. It is shown that, for UK temperature conditions, the effect of sowing date on infection is greater than that of year-to-year variations in temperature associated with a single representative sowing date. The variation in inoculum build-up predicted when temperature data from a range of soil types were used in the model agreed with field observations, where higher levels of infection are observed on sandy soils than on black fen peat soils. The difference was most distinct when daily maximum soil temperature values were used to drive the model rather than rolling 24-hour average values.

Notes: Bait plants, comprising micropropagated and commercial seed tubers, were used to estimate the effects of rotation on the density and spatial pattern of inoculum of Rhizoctonia solani in large field plots of potatoes. Soilborne inoculum of R. solani produced economically significant levels of stem canker and its incidence and severity varied with rotation, with most disease in 2-year and less in 4- and 6-year rotations. The rates of loss of inoculum during intercrop periods differed amongst rotations with a rapid fall to low levels occurring after 1 year in a 6-year rotation and after 2 years in a 4-year rotation. Replenishment of inoculum to soil was rapid following the growth of a susceptible crop, with comparatively high levels of infection and disease, even in long rotations. Disease occurred in patches and the size of patches and the density of R. solani within patches differed with cropping frequency. The degree of spatial autocorrelation also differed amongst rotations but there was no evidence for any significant differences in the rate of change of spatial autocorrelation during intercrop periods in the three rotations.

Notes: The effects of different densities of tuber-borne inoculum, selected agronomic treatments (date of planting, irrigation and size of seed tubers) and their interactions on the temporal progress of stem canker (Rhizoctonia solani) on potato plants (Solanum tuberosum) were investigated in a multifactorial experiment. Data comprising stem number and the incidence and severity of stem canker from planting until tuber initiation in two consecutive growing seasons were analysed using linear contrasts, quadratic contrasts and the area under the disease progress or host growth curve. Differences in the incidence and severity of stem canker were dominated by the effect of different densities of tuber-borne inoculum. The majority of disease progress curves were nonmonotonic for the incidence and severity of stem canker with a rapid rise in disease up to stem emergence and a decline thereafter. Most treatments affected the area under the curve and to a lesser extent the average rate of increase in disease. Of the agronomic treatments, later dates of planting and pre-emergence irrigation reduced the levels of stem canker whereas size of seed tubers did not affect the progress of disease. Little additional information was revealed by scoring for the severity rather than the incidence of stem canker.

Notes: Relationships between diseases caused by Rhizoctonia solani on different parts of potato plants (Solanum tuberosum) at different stages of crop growth were investigated under selected agronomic conditions. The effects of different densities of tuber-borne inoculum, date of planting, irrigation, size of seed tubers and their interactions on the incidence of stem and stolon canker during crop growth, the incidence and severity of black scurf and the yield of progeny tubers at harvest were quantified in a multifactorial experiment. Differences in the incidence of stem canker, stolon canker and black scurf were dominated by the effect of density of inoculum on seed tubers at planting. Highly positive correlations between the disease variables indicated a close relationship between the incidence of disease at each stage of crop growth although the degree of association between variables measured at an early growth stage and those measured at progressively later stages of crop growth weakened as the time interval increased. Total yield of progeny tubers was not affected by the density of tuber-borne inoculum although there was a shift in the size distribution, with a decrease in the yield of main-sized tubers and an increase in the yield of baker- and oversized tubers at the higher density of inoculum. Of the remaining factors, the effect of season tended to be more pronounced than any of the agronomic treatments although the use of irrigation and later dates of planting did influence the incidence of infection to a limited extent.

Notes: The effects of soil temperature on infection of sugar-beet roots by the soil-borne fungus Polymyxa betae were investigated in controlled environments. Pre-germinated seeds were sown in pots of naturally infested soil and seedlings sampled at frequent intervals over a period of several weeks. Within the range 10-30°C, the optimum soil temperature for infection was c. 25°C; the time between sowing and the first detectable infection was shortest and the subsequent rate of infection most rapid at this temperature. No infection was observed over 80 days at 10°C.Both root and shoot dry weight were reduced on plants growing in infested soil at 15, 20 and 25 C compared with those growing in uninfested soil. In general, root growth was more severely affected than shoot growth and the effects were most pronounced at 20°C. These results were confirmed in a subsequent experiment in which P. betae-infected root material was used as the inoculum. In addition to its role as the vector of beet necrotic yellow vein virus (the cause of Rhizomania disease), the significance of P. betae as a plant pathogen in its own right is discussed.

Notes: Disease-progress curves of take-all, caused by Gaeumannomyces graminis var. tritici, were analysed for consecutive crops of winter wheat. Comparisons were made over 9 years amongst a sequence of consecutive wheats and first and second successive wheats grown after non-susceptible break crops (spring beans). The principal objectives were to identify differences in the shapes of the disease-progress curves that could be attributed to the rotational treatments. Shapes of disease-progress curves were summarized by average rate of disease increase and components for curvature together with the mean amount of disease. Analyses were done for curves based upon percentages of diseased plants and of diseased roots.Mean levels of disease in second and continuous wheats rose from the start of the experiment (1979) to reach a maximum in the middle years (1982-84) and thereafter declined. This pattern was absent in first wheats in which disease progress within years was rectilinear and comparatively slow. In second and continuous wheats, years of high disease were characterized by more marked curvature than years of low disease. For proportions of diseased plants, the rate of disease increase in second and continuous wheats slowed as the season progressed. Differences in shapes of disease-progress curves for second and continuous wheats were apparent when proportions of diseased roots rather than plants were considered. The average linear rate of increase in the proportion of diseased roots in continuous wheats was intermediate in magnitude between those of first and second wheats. Epidemics in second wheats were initially slow and accelerated as the season progressed, whilst in continuous wheats, an early faster rate of increase in disease subsequently slowed. Some epidemiological consequences of these effects are discussed in relation to the phenomenon of take-all decline, which is associated with the suppression of the disease in cereal monoculture. The effects of cropping history on the relative importance of infections arising from soil-borne inoculum and infections arising from the spread of disease from neighbouring infected roots are inferred and epidemiological hypotheses for take-all decline are advanced. Methods involving the use of weighted linear functions were used to overcome statistical problems of repeated observations within plots as well as variable sampling frequencies and intervals within and amongst years These methods, which have general applicability for epidemiological work, are described.

Notes: Mouse monoclonal antibodies (mAbs) and rabbit (polyclonal) antiserum were used to develop DIAGNOSTIC-ELISA, double-antibody-sandwich-ELISA (DAS-ELISA), DIP-STICK and immuno-fluorescence colony staining immunoassays for the specific detection of Rhizoctonia solani in soil. mAbs were raised against an anastomosis group 4 isolate of R. solani. Mice were immunized using either phosphate-buffered saline (PBS) suspensions of lyophilized mycelium plus Quil A adjuvant, or with a solubilized acetone precipitate prepared from cell-free surface washings from solid slant cultures. Polyclonal antisera were raised in rabbits using PBS suspensions of lyophilized mycelium and Quil A adjuvant. Hybridoma supernatants and rabbit antisera were screened by ELISA. Four of the cell lines raised produced mAbs that were species-specific. They recognized antigens from R. solani by ELISA and immunofluorescence, but not other related or unrelated species of soil-borne fungi. The remaining cell line produced mAbs that cross-reacted slightly, by ELISA, with antigens from R. cerealis. These mAbs did not recognize R. cerealis by immunofluorescence, or other related or unrelated soil-borne fungi, by ELISA.

Notes: The properties of a number of non-linear models are considered in relation to the description and interpretation of curves for disease progress of soil-borne pathogens. The curves include the logistic function, with and without allowance for growth of the host population, as well as ‘custom-built’ models with and without algebraic solution. The custom-built models are used to separate the epidemic into primary and secondary infection and allowance is made for growth of the host population and for decay of initial inoculum. The models are compared with a simple polynomial model. All models are fitted to two disease progress curves derived from an extensive data set for infection and disease of winter wheat by the take-all fungus, Gaeumannomyces graminis var. tritici. The results are discussed in relation to the role of primary and secondary infection, decay of inoculum and growth of the host population on epidemic development

Notes: The effect of sowing date on the infection of sugar-beet seedlings by Polymyxa betae was examined in a small-plot experiment on a naturally infested site. Seed was sown on seven occasions at weekly intervals from late March to early May. From each sowing, plant samples were taken at approximately weekly intervals over a period of 7 weeks. The extent of root infection by P. betae and the dry weight of plants was determined at each sampling date, and the progress of infection and rate of plant growth were examined against time and thermal time. Infection occurred sooner after sowing and the subsequent rate of fungal development was more rapid in late-sown than in early-sown plants. Early sowing allowed germination and growth of sugar beet at temperatures too low for fungal infection. The growth of late-sown plants appeared to be reduced by P. betae infection. The implications of these findings for the development of rhizomania disease are discussed.