The effect of solar heating of soil on natural and inoculated agrobacteria

Solarization trials were carried out over 3 years and in two countries to control crown gall disease on fruit trees and eliminate Agrobacterium . In 1992, agrobacteria in naturally infested soils of two Italian nurseries were monitored before and after solarization. Agrobacteria populations decreased by 99% and 92% after the treatment; however, crown gall incidence did not decrease. In 1993 and 1994 solarization was tested in Oregon in fields artificially infested with two marked strains of A. tumefaciens . In sandy loam soil, the target bacteria were eliminated in 4 weeks, while in silty clay soil the populations were markedly reduced after 2 months of treatment. Crown gall incidence on cherry rootstocks transplanted to the field at the end of 1993 was 3.7% in the sandy loam soil control plots, while no tumours were observed on plants from solarized plots. The use of solarization in combination with reduced doses of metham-sodium was also evaluated.     

Short-distance wind dispersal of the fungal pathogens causing Sigatoka diseases in banana and plantain

Airborne spores of the fungal pathogens causing Sigatoka diseases in banana and plantain were monitored using rotorod spore traps, sited at various heights within an infected plantation in Costa Rica from December 1993 to February 1994. Different capture patterns of ascospores and conidia were found and the relationship between wind behaviour and spore catches was investigated. This information has enabled an assessment to be made of the reliability of point measurements of airborne spores for monitoring spore movements on the plantation scale. The use of such information in forecasting the airborne movement of these spores and the likely role of the wind in the spread of this disease to uninfected areas is discussed.     

Role of acquisition and inoculation time in the expression of clonal variation for BYDV-PAV transmission in the aphid species Rhopalosiphum padi

Vector efficiency of 20 Rhopalosiphum padi clones, originating from Europe, North America and North Africa and exhibiting different types of life cycle, was evaluated by transmitting a French BYDV-PAV isolate to barley plants under five different acquisition/inoculation sequences (AAP/IAP). Differences between clones in transmission efficiency were found only when a short AAP was followed by a long IAP (6 h/120 h) and, to some extent, when a long AAP (48 h) was followed by a short IAP (6 h), but no differences were found when the conditions for virus transmission were optimal, i.e. long AAP followed by long IAP (48 h/120 h). There were no differences in transmission rates by clones of different geographical origins and with different life cycles. As a consequence, clonal variation is probably of little importance in the vector aspects of the epidemiology of PAV serotypes transmitted by R. padi , but the availability of a range of clones exhibiting transmission differences under limiting AAP or IAP conditions could be of interest for studies of virus–vector relationships.     

The effect of component number on Rhynchosporium secalis infection and yield in mixtures of winter barley cultivars

Mixtures of winter barley cultivars containing up to six components were grown over three years with and without fungicide treatment. Yield increases were recorded for mixtures compared with the mean of their monoculture components and there was a significant trend towards greater benefit from increased number of components. These benefits were partially attributable to a corresponding increase in control of Rhynchosporium secalis as component number increased. The potential for exploitation of mixtures in cereals for control of splash-dispersed pathogens is discussed.     

Transfer from Erwinia herbicola to Escherichia coli of a plasmid associated with biocontrol of fire blight

Strains of Erwinia herbicola effective in the biocontrol of fire blight of hawthorn were used to investigate the possibility that the antagonistic activity is coded by plasmid-born genes. Agarose gel electrophoresis of isolated plasmids from four antagonistic Erw. herbicola strains showed a band of a supercoiled 12 kb plasmid in each strain, with a second band greater than 16.2 kb consistently seen in two strains. Erw. herbicola strains showed resistance to penicillin-G, which could be conferred on penicillin-G sensitive Escherichia coli TG1 by transformation with a pure Erw. herbicola plasmid preparation. Transformed strains of Esc. coli appeared to contain the Erw. herbicola 12 kb plasmid, but not the > 16.2 kb plasmid. In an agar plate assay, Esc. coli transformants produced an inhibition zone against Erw. amylovora similar to those produced by the original Erw. herbicola strains. In two biocontrol assays, the transformed Esc. coli strains had a suppressive effect on disease development on infected pear fruit slices and hawthorn blossoms.     

Variability amongst strains of Fusarium poae assessed by vegetative compatibility and RAPD polymorphism

Vegetative compatibility tests and random amplification of polymorphic DNA (RAPD) were used to assess genetic relationships amongst 54 strains of Fusarium poae obtained from various geographical regions. Twenty-seven strains were assigned to eight multiple member vegetative compatibility groups (VCGs), while the other 27 isolates were found to form single-member VCGs. There was a partial correlation between VCG and geographical origin, but the relationship was not always clear. However, no correlation was observed between the VCG and the host plant of origin. RAPD patterns were closely associated with VCGs in all cases. Members of VCGs that were interconnected by bridging isolates formed common branches in the phenogram constructed on the basis of the RAPD patterns, while strains that belonged to single-member VCGs were scattered throughout the phenogram. These data demonstrate that the combination of traditional and molecular methodologies allows reliable intraspecific subdivisions in an asexual fungus, which is a secondary invader of a wide range of host plants, and so has never been subject to the intense selection pressure of a single host species and lacks pathogenic subgroups.     

Effects of neemgard on phytophagous and predacious mites and on Spiders

The effects of Neemgard, an acaricidal and fungicidal formulation obtained from neem (Azadirachta indica) seed kernels, on the phytophagous miteTetranychus cinnabarinus, the predacious mitePhytoseiulus persimilis, and the predatory spiderChiracanthium mildei, were investigated in laboratory experiments. Neemgard was highly toxic toT. cinnabarinus but had no toxic effect onC. mildei orP. persimilis. Another neem formulation—the insecticidal Neemix 45—caused conspicuous repellency, but no mortality, inT. cinnabarinus.     

Host status effect of cowpea and sunflower on the populations ofMeloidogyne javanica andRotylenchulus reniformis

A highly susceptible cowpea,Vigna sinensis cv. Baladi plants were tested as trap plants for eitherMeloidogyne javanica orRotylenchulus reniformis under greenhouse conditions. The plants were gathered by cutting them above the surface of the soil or by uprooting them, 1/2, 1, 3, 6, 12, 24, 36 and 48 days after nematode inoculation. Both of the mentioned nematodes began to mature and lay eggs after the 12th day from their inoculation. Hence, it is advised to pull up cowpea plants from 3–12 days after nematode inoculation. After planting sunflower,Helianthus annus cv. Miak replacing cowpea, the nematode populations were higher, in most cases, on sunflower plants replacing cutting cowpea than those on sunflower replacing uprooted cowpea. The highest percentages of nematode reduction were 98.55 and 99.57 forM. javanica and 95.09 and 92.90% forR. reniformis on sunflower plants replacing cutting and uprooted cowpea plants after 12 days from nematode inoculation, respectively.M. javanica andR. reniformis decreased the length and weight of sunflower plants as affected by planting time and method of cowpea harvest. This method of nematode control is cheaper, easy and pollution free.     

Biological and molecular characterization of Tomato spotted wilt Virus in Israel

Received April 24, 1997; received in final form June 29, 1997. Symptoms resembling tomato spotted wilt virus (TSWV) infections were documented among ornamental and vegetable crops in commercial greenhouses and open fields in Israel. Plants exhibiting these symptoms were collected from January 1992 to December 1996. Among cultivated plants analyzed for TSWV by enzyme-linked immunosorbent assay (ELISA), 19 species representing five families were found to be infected; natural infection was also recorded in six plant species of weeds. Virus identity was characterized by host range, serology and electron microscopy. Serological reaction with the isolates, found in Israel, using antisera from different sources as well as the sequence analysis of the nucleocapsid gene, demonstrated that the Israeli isolates of TSWV are a member of tospovirus serogroup I, type I (BR-01 strain). No virus transmission was found in seeds collected from virus-infected vegetable and ornamental crops. A non-radioactive molecular probe derived from the cloned nucleocapsid isolate enables specific detection of the virus in crude sap from infected plants. The detection of TSWV in Israel constitutes a severe potential threat to the ornamental and vegetable industry.