A new approach to modelling the dynamics of oospore germination in Plasmopara viticola

In Argentina, celery is grown in the outskirts of the principal cities in the provinces of Buenos Aires, Santa Fe, and Mendoza. Since the late 1990s, plants growing in La Plata (Buenos Aires) have shown rapid decline in commercial fields. Symptoms are delayed growth, yellowed leaves, discoloured vascular tissue and reduced root system. The objectives of this work were to determine the causal agent of the disease and to analyze the effects of inoculum levels on the disease. Fusarium oxysporum (Fo) was consistently isolated on potato-dextrose-agar (PDA). The "forma specialis" was identified by pathogenicity tests where each of seven selected strains of Fo on colonized rice that was dried and ground were mixed into soil. Initial Fusarium wilt symptoms were recorded 50 days after inoculation, and Fo was reisolated from affected roots, stems, crowns, and petioles suggesting that the pathogen is Fo f. sp. apii (Foa). In another assay different concentrations of inoculum (101 to 104 cfu per g of soil) were tested to determine the effects of Foa on the plants (cv. Golden Boy). Wilt severity, chlorosis, plant height, and dry weight were assessed weekly. Typical symptoms developed at all levels of inoculum. Disease severity increased when the inoculum density was increased. To our knowledge, this is the first report of Foa in Argentina. The possibility that the pathogen was introduced on imported seed during the last few years is discussed.