Session VI - Biological Control: within taxon
Diversity and interactions among strains of Fusarium
oxysporum: application to biological control
INRA laboratoire de recherches
sur la flore pathogne dans le sol, 21034 DIJON Cedex, France.
The species Fusarium oxysporum is well represented among the communities of soil fungi, in every type of soils all over the world. Some strains are pathogenic on different plant species; they provoke either root-rot or tracheomycosis. These pathogenic strains are characterised as formae speciales in relation to the plant species they are able to infect. But soils also harbour very large populations of non-pathogenic strains of F. oxysporum, which play an important role in the soil microbial balance, especially in the soils suppressive to fusarium wilts. Indeed, it has been well established that non-pathogenic F. oxysporum competing with the pathogen are responsible for the suppressiveness of several soils to fusarium wilts.
The paper will address first the characterisation of diversity among strains of Fusarium oxysporum. Then the paper will focus on interactions among strains of F. oxysporum in soil and the rhizosphere. Competition at the root surface and indirect interaction through the plant will be described. Finally the application of F. oxysporum to biological control will be discussed in relation to screening and modes of action of efficient strains and mass production, formulation and delivery of effective inoculum.
The Use of Avirulent Mutants of Ralstonia solanacearum
to Control Bacterial Wilt Disease
CABI Bioscience, UK Centre
(Egham), Bakeham Lane, Egham, Surrey TW20 9TY, UK.
Bacterial wilt disease affects a large variety of hosts, is geographically widely distributed, of considerable economic importance and yet remains one of the most intractable bacterial plant diseases. Although a great deal of information currently exists on the causative organism, Ralstonia (Pseudomonas) solanacearum, disease control is hampered by our, as yet, incomplete knowledge of infraspecific diversity, modes of dissemination, infection and disease development. Certainly, at present no effective control measures exist for bacterial wilt disease in a large number of economically significant crops. Studies are currently underway to develop and evaluate biological control agents for this disease using avirulent mutants of the pathogen itself. In particular, efforts have been directed against one of the most homogeneous pathosystems; R. solanacearum Race 3 on potatoes. Indigenous populations of R. solanacearum affecting potato in Kenya have been extensively characterised using a multifaceted approach. From these studies a number of centres of diversity have been described within the indigenous population. Representative of each group has then been selected and avirulent mutants constructed by transposon-induced mutagenesis, directed towards the hrp gene cluster. These strains are currently being evaluated and results will be presented from greenhouse and controlled, field-testing which demonstrate the potential of this approach in the control of bacterial wilt disease.
Cross Protection in Cucurbits: Problems and Prospects
of Virology, The Volcani Center, ARO, Bet Dagan 50-250, Israel.
The zucchini yellow mosaic potyvirus (ZYMV) is considered the most damaging virus in cucurbits. This virus is a newly emerging aphid-borne virus that was first reported in Italy only in 1981. Then, in less than 10 years, it became a limiting factor for most cucurbits-growing regions in the world. Breeding for resistance, in squash, melons and watermelons did not yield commercial cultivars. The major control measure presently adopted to curb epidemics is based on frequent applications (once or twice a week) of mineral oils. Unfortunately, mineral oil treatments are not preventive in cases of very high infection pressure. In the late 1980s, a weak strain of ZYMV (ZYMV-WK) was isolated in France (Lecoq et al., 1991). This strain has been tested and found stable and protective in squash-growing regions of the world mainly for squash. Despite the successful experimental results, the commercial use of ZYMV-WK for cross protection remained limited. The limited use of cross protection is attributed to labor costs involved in large-scale inoculation of seedlings and to the concern of growers from introducing a virus in their crops. In Israel, successful application of cross protection in squash, watermelons and melons was attained. The total area that has been protected between 1996 and 1999 exceeded 2,500 hectares. This large-scale protection was achieved by developing an automated inoculation machine that ensures between 75% (in watermelons) to 90% (in squash and melons) mild virus infection. In addition, the farmers were offered professional advice throughout the growing season. The automatic inoculation was tailored for application on cucurbit seedlings in the nursery. Optimization of the cross protection conditions was needed. The factors tested included the buffer type and pH, the seedling stage to be inoculated and the duration of inoculation of the mild virus prior to exposure. This procedure ensured exposure of protected seedlings in the field, ready to resist severe infection. In last five years, cross protection was tested in regions that differ in climate, soil or growing conditions. In squash, the mild virus (in absence of severe infection) caused up to 15% reduction in yield. Surprisingly, in watermelons, there was a slight (10-15%) addition in yield in protected plots compared to non-protected (in absence of virus epidemics). As expected, in cases of severe epidemics, cross protection greatly added to the income of the growers.
Several problems were encountered in the process of developing cross protection: spread in cases where the rate of protection was incomplete (less than 50% of the seedlings), increase in severity of the mild virus when plants were co-infected with cucumber mosaic virus or when plants were grown in stress conditions. Other risks of heteroencapsidation were recorded in the past in the laboratory. The ways to reduce these risks are discussed.