Modelling the microbial breakdown of pesticides in soil using a parameter estimation technique

Results of field measurements and laboratory experiments were used to simulate the behaviour of 1,3-dichloropropene (1,3-D) in a loamy sand soil. Microbial activity was described using pre-set conditions to compute transformation rates as dependent on compound concentrations and temperature. These kinetics could only be analysed using non-linear transformation rates. To link the development of microbial populations and the consumption of the compound over time, an iterative technique was used to estimate the necessary parameters.     

Variability among Spanish citrus tristeza virus isolates revealed by double-stranded RNA analysis

A survey for citrus tristeza virus (CTV) strains, based on double-stranded RNA (dsRNA) analysis, was carried out in five locations on the eastern citrus-growing area of Spain. CTV was recovered from 137 trees of different ages, citrus species and varieties, sampled in 53 orchards. The best months for dsRNA recovery were April, May, September, October, and November, and the highest dsRNA yield was obtained from sweet orange cultivars. Sixteen dsRNA profiles differing by the number and/or position of subgenomic bands were detected. One of these profiles was detected in more than half the trees analysed. Maximum diversity of dsRNA patterns was found in the location with the oldest citrus orchards and the highest CTV incidence (Alzira-Carcaixent). In many instances, several dsRNA profiles were detected in neighbouring trees of the same orchard, notably in Alzira-Carcaixent, where 70% of the plots sampled contained more than one profile. The possible causes for the diversity of CTV isolates found in this specific area are discussed.     

Characteristics and host range of tomato powdery mildew

The anamorphic state of a powdery mildew, which first appeared on tomato in the UK in 1987, currently listed as Erysiphe orontii (anamorph Oidium violae ), was examined microscopically on both tomato and alternative hosts. Host range studies were also carried out. In 1993–95, species and cultivars of plants from 26 of the 30 families listed as hosts of E. orontii were inoculated with tomato powdery mildew in the glasshouse and assessed for susceptibility on a 0–3 scale based on degree of sporulation and growth. Thirteen families contained species or varieties that were susceptible and a total of 80 varieties, including many economically important Cucurbitaceae and Solanaceae, supported sporulation to some extent (susceptibility score 1–3). Necrosis was not a constant feature of plants exhibiting resistance, as it could accompany sporulation on some susceptible varieties. Microscopic measurements showed that conidial size, conidiophore length, conidiophore foot-cell length and width, and the number of cells distal to the foot-cell were remarkably constant between samples of powdery mildew taken from tomato each year and from the various alternative hosts. Based on the production of conidia singly, the presence of nipple-shaped to moderately lobed appressoria, and occurrence of straight conidiophores, it is concluded that this fungus on tomato should more correctly be named Oidium lycopersicum until the sexual stage is discovered. The highly polyphagous nature of this powdery mildew would suggest that it represents a potential risk to UK agriculture and horticulture, but the failure to recover any powdery mildew from 120 samples of potato, including varieties Cara and Maris Piper, on which the tomato powdery mildew could sporulate in the glasshouse, suggests that environmental factors may play a key role in preventing establishment of this powdery mildew on alternative hosts on a commercial scale in the field.     

Lr46: a gene conferring slow-rusting resistance to leaf rust in wheat

ABSTRACT Wheat (Triticum aestivum) cultivar Pavon 76 carries slow-rusting resistance to leaf rust that has remained effective in Mexico since its release in 1976. 'Pavon 76' was crossed with two leaf rust-susceptible wheat cultivars, Jupateco 73S and Avocet S, and between 118 and 148 individual F(2) plant-derived F(3) and F(5) lines were evaluated for adult-plant leaf rust resistance at two field sites in Mexico during different seasons. Evaluation of F(1) plants and parents indicated that the slow-rusting resistance was partially dominant. Segregation in the F(3) and F(5) indicated that the resistance was based on two genes with additive effects. Monosomic analysis was carried out to determine the chromosomal locations of the resistance genes. For this purpose, two or three backcross-derived cytogenetic populations were developed by crossing 'Pavon 76' with a monosomic series of adult-plant leaf rust-susceptible cultivar Lal-bahadur. Evaluation of such BC(2)F(3) and BC(3)F(3) lines from 16 confirmed 'Lalbahadur' monosomics indicated that one slow-rusting gene was located in chromosome 1B of 'Pavon 76'. This gene, designated as Lr46, is the second named gene involved in slow-rusting resistance to leaf rust in wheat.     

Temperature-dependent development rates of <Emphasis Type="Italic">Bracon vulgaris,</Emphasis> a parasitoid of boll weevil

The duration of development of Bracon vulgaris Ashmead, parasitoid of the boll weevil Anthonomus grandis Boheman, was determined at nine constant temperatures between 18°C and 38°C. Nonlinear regression analysis was used to test the fit of temperature-dependent development rates to the Sharpe and DeMichele and Lactin et al. models. At the highest tested temperature (38°C) all the parasitoid eggs died before hatching and no evidence of development was observed. The high values of R ² for the models of Sharpe and DeMichele (0.8432 to 0.9834), and Lactin et al. (0.9071 to 0.9795) indicated that these models are suitable to estimate the development rate of B. vulgaris as a function of temperature. B. vulgaris showed tolerance to high temperature which is represented by the high value of H _H (change in enthalpy associated with high-temperature inactivation of the enzyme) for the prepupa stage of this insect obtained with the Sharpe and DeMichele model. According to that model, B. vulgaris exhibits thermal stress at 35.7°C, which indicates that maximum thermal stress estimated by this model was close to the real one.     

Over-winter predation of Abutilon theophrasti and Setaria faberi seeds in arable land

To improve understanding of over-winter weed seed predation in arable fields, we used data from winter exclosure trials to determine the amount of predation and the influence of crop habitats on predation of Abutilon theophrasti and Setaria faberi seed in 2-year (maize/soyabean) and 4-year (maize/soyabean/small grain+lucerne/lucerne) crop rotation systems between 2005 and 2008. Crop habitat influenced seed predation, and had similar impacts on the two weed species. Mean A. theophrasti predation ranged from 31% in the 2-year soyabean habitat to 99% in the 4-year lucerne habitat. Mean S. faberi predation ranged from 31% in the 2-year soyabean habitat to 97% in the 4-year lucerne habitat. Results suggest that a combination or interaction of cover and substrate may have affected crop habitat preference by seed predators. Future research should further examine the influence of physical habitat on seed predation to determine characteristics of cropping systems that encourage predation, particularly during over-winter periods, so as to routinely incorporate seed predators into long-term weed management strategies.     

Characterisation of a novel ilarvirus causing grapevine angular mosaic disease

Unusual symptoms were observed on ‘Baresana’ x ‘Baresana’ Vitis vinifera hybrid vines in the Grapevine Variety Collection of the Grapevine Institute, Athens. The affected vines showed sharp angular mosaic on leaves, along the veins and in vein angles, malformations, abortive flowers or very few berries with smaller, wrinkled and non-germinating seeds, as well as gradual decline, severe stunting and death of the vine. Serological tests on diseased vines for the presence of 13 known grapevine viruses gave negative results. An infectious agent was transmitted mechanically to several herbaceous indicator plants. Koch’s Postulates were fulfilled, and the agent, proven to be a virus, was named Grapevine angular mosaic virus (GAMV). Serological tests have been developed for the virus. The most conserved polymerase region showed significant similarity of GAMV with members of subgroup 1 of the Ilarvirus genus; however ML phylogenetic analysis could not support its clustering within this subgroup. GAMV differs serologically and in particle morphology from Grapevine line pattern virus (GLPV) a putative member of the Ilarvirus genus that infects grapevine. It is proposed that GAMV is a novel member of the Ilarvirus genus.