Mercury residues in breast muscle of wild ducks, 1970-71.

Samples of breast muscle from 327 ducks collected from October 1970 to March 1971 in the conterminous United States were analyzed for total mercury by flameless atomic absorption spectrometry. Mercury levels for the entire collection ranged from less than 0.01 to 3.91 ppm wet weight with a median of 0.10 ppm. Twenty-five ducks had levels equalling or exceeding the 0.5 ppm guideline for fish and shellfish established by the Food and Drug Administration, U.S. Department of Health, Education, and Welfare. Dabbling ducks, which are shallow-water feeders and mostly vegetarian in fall, winter, and spring, usually had lower levels than diving and sea ducks. Levels were generally higher in ducks collected in areas where environmental mercury levels were known to be greater than in ducks from nonsuspect areas. Despite the mobility of the ducks, levels seemed more closely linked to local environmental contamination than to various factors associated with large geographic areas.     

Prepenetration stages in infection of clematis by Phoma clematidina

A detailed study of conidial germination, germ-tube growth and the formation of infection structures in Phoma clematidina , the causal agent of clematis wilt, is described for two clematis varieties differing in disease resistance. On both the resistant and susceptible varieties, the fungus entered leaves and stems by direct penetration of the cuticle, often, but not always, following the formation of infection structures. More germ tubes per conidium were formed on the susceptible host, but these germ tubes were on average shorter than on the resistant host. Although germ tubes regularly entered the plant via trichomes, stomata were not found to be sites of entry. Following penetration of the cuticle of resistant plants, germ-tube growth was sometimes restricted to the subcuticular region, and halo formation occurred at the sites where penetration was attempted. Subcuticular growth and halo formation were not observed on susceptible plants. These observations may partly explain the resistance of small-flowered clematis varieties to P. clematidina .     

Non-structural plum pox potyvirus proteins detected by immunogold labelling

Plum pox potyvirus (PPV) induces in infected Nicotiana clevelandii cells characteristic crystalline inclusions known as nuclear inclusions (NI) when located in the nucleus and as dense material (Dm) when located in the cytoplasm. Crystalline inclusions contain protease (NIa) and RNA-dependent RNA polymerase (NIb) proteins. It is now well established for all potyviruses that cylindrical inclusions contain CI helicase ATPase protein (Martin et al., 1992). The intracellular location of other non-structural PPV proteins remains unknown. Using Escherichia coli expression vectors, specific antibodies were obtained against P1, P3, 6K2 and NIb PPV proteins for which antibodies were not yet available. As expected, NIb antiserum labelled crystalline inclusions. P1, P3 and 6K2 proteins were present in both types of crystalline inclusions found in the nucleus and in the cytoplasm of PPV-infected leaves of N. clevelandii, suggesting that nuclear inclusions and dense material were composed of the same proteins. This composition is discussed.     

The effects of ultraviolet-B (UV-B: 290–320 nm) radiation on blister blight disease of tea (Camellia sinensis)

Sunlight is known to reduce the incidence of blister blight disease of tea, caused by Exobasidium vexans . The effects of the ultraviolet-B (UV-B: 290–320 nm) component of solar radiation on the disease and the development of E. vexans were studied in the field in Sri Lanka using UV-screening filter materials held over a commercial crop. Exclusion of UV-B radiation by Polyester, which reduces fluxes in the UV-B region by approximately 75–85%, increased both the number of translucent spots (immature sites of infection) and number of sporulating blisters (mature sites of infection), but it had little or no effect on the sporulation of the pathogen. When basidiospores were artificially inoculated on leaves, and were exposed to full or filtered solar radiation, their survival and germination increased when UV-B wavelengths were removed; significant differences were found 62 h after inoculation. However, UV-B did not affect the extension of germ tubes. This study indicates that the UV-B component of solar radiation plays an important role in the natural regulation of blister blight disease in the field.     

A model of plant virus disease epidemics in asynchronously-planted cropping systems

A mathematical model was developed of the dynamics of a plant virus disease within a spatially-referenced lattice of fields of a host crop. The model can be applied to crops in continuous, contiguous cultivation such as tropical irrigated rice. Disease progress in each field of the host crop was assumed to be logistic and determined by incidence within the field itself as well as incidence in neighbouring fields, depending on the gradient of disease spread. The frequency distribution of planting dates (represented by the proportion of the total number of fields planted in successive months) was assumed to follow a normal distribution and the variance of planting date was used as a measure of cropping asynchrony. Analysis of the model revealed that disease incidence within the lattice (i.e. mean incidence over all fields) depended upon the infection efficiency, the slope of the dispersal gradient, and the variance in planting date. Disease endemicity depended mainly on planting date variance and disease persisted in the lattice if this variance exceeded a certain threshold. Above the threshold for persistence, the response of mean disease incidence to planting date variance was non-linear and the region of greatest sensitivity was closest to the threshold. Thus, disease systems that show moderate rather than high cropping asynchrony are more likely to be influenced by changes in the variance of planting date. Implications for the area-wide management of rice tungro virus disease are discussed.