Soilborne pathogens, especially Fusarium oxysporum, are responsible for damping‐off and root necrosis in Eucalyptus nurseries. New technologies are increasingly considering strategies for plant disease control other than chemical fungicides. Among these, natural fungal antagonists, which are colonizers of the root cortex, are potential biocontrol agents. An in vitro system was used: (1) to test the pathogenic effects of F. oxysporum strain Foeu1 which was recovered from a forest nursery soil; (2) to explore the potential of the nonpathogenic F. oxysporum strain Fo47, which is known for its efficiency in biological control, to suppress damping‐off of Eucalyptus seedlings; (3) to compare the patterns of root colonization and host response to invasion by the two Fusarium strains inoculated separately in a time‐course study. Root inoculation of E. viminalis with F. oxysporum strain Foeu1 caused damping‐off in young seedlings in vitro, whilst disease symptoms were not visible in plants inoculated with F. oxysporum strain Fo47 or when both strains (Foeu1 + Fo47) were inoculated simultaneously. Each strain showed similarities in patterns of root tissue colonization, and in the processes of root penetration and initial colonization. Differential effects on root tissue were observed with fungal development within the cortex: ingress of strain Foeu1 was accompanied by severe host‐cell alterations whilst no tissue damage occurred with development of strain Fo47.

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Pinus ponderosa seedlots from eight seed orchards in Western Patagonia, Argentina, were evaluated for the presence of Fusarium species, and isolates obtained were subjected to pathogenicity tests. The following species were isolated: Fusarium oxysporum, Fusarium equiseti, Fusarium verticillioides, Fusarium incarnatum, Fusarium acuminatum, Fusarium graminearum and Fusarium proliferatum. With the exception of F. proliferatum and F. graminearum all Fusarium species were recovered from P. ponderosa seedlings with symptoms of damping-off. Five Fusarium species were re-isolated from necrotic roots of seedlings showing chlorosis (F. oxysporum, F. equiseti, F. incarnatum, F. acuminatum and F. verticillioides). High damping-off losses occurred in Fusarium-contaminated seed of P. ponderosa and low disease incidence occurred on less contaminated seed. The role of seed-borne inoculum in disease spread within forest nurseries is discussed.

The quality of planted seeds has a critical influence on the ability of crops to become established and to realize their full potential of yield. Loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii Engelm.) are forest trees species, both native to the southern United States of America, mostly planted in Northeastern Argentina for site restoration and forest management due to their fast growth on diverse sites. Numerous diseases attack these two pine species in forest nurseries: damping-off and root rot. Many Fusarium species present on seeds were found to be associated with damping-off and root diseases in loblolly and slash pine seedlings. Seed of Pinus elliottii and P. taeda were collected from Northeastern Argentina and analyzed for seed-borne Fusarium. Seed treatments as running water for 24 and 48 h were tested for effectiveness in changing Fusarium populations on seeds. Seven different species of Fusarium were isolated from the seed lots assayed: F. solani, F. verticillioides (= F. moniliforme), F. oxysporum, F. proliferatum, F. incarnatum (= F. pallidoroseum = F. semitectum), F. equiseti and F. acuminatum. Fusarium populations detected on seeds changed after running water treatments for 24 and 48 h but this changes varied among seedlots. This is the first report on Fusarium species associated with P. elliottii and P. taeda seeds in Argentina.

Fusarium species are soil-borne fungal pathogens that produce a variety of disease symptoms when attacking crop plants. The mode of root colonization of Eucalyptus viminalis seedlings by a pathogenic F. oxyporum strain (Foeu1) at the ultrastructural level and changes in cell wall pectin during host pathogen interactions are described. Root systems of E. viminalis plants were inoculated with F. oxysporum in an in vitro model system. Hyphae of F. oxysporum adhered to the outer epidermal cell walls through fibrillar material, and after penetration they spread into the internal tissues. They developed intercellularly and intracellularly in the root cortex and invaded vascular tissues. Papillae were induced, and the host plasma membrane ruptured in colonized cells, causing rapid host tissue and cell damage. Changes in distribution and occurrence of nonesterified and methyl-esterified pectins were evaluated after root colonization by F. oxysporum using two monoclonal antibodies, JIM 5 and JIM 7, respectively. Nonesterified pectin in control roots was mainly localized in the epidermal cell walls and middle lamellae in parenchymal cortex, whereas methyl-esterified pectin accumulated more in primary cell walls of the cortex and phloem. Decreases in immunodetected nonesterified and methyl-esterified pectins were associated with extensive plant tissue degradation after root colonization by the pathogenic fungus.

Removal of topsoil, mainly for making bricks, is one of the main causes of soil loss around large urban centres of the Humid Pampa, Argentina. In about 7 per cent of La Plata County, the 20±40 cm thick A-horizon has been removed for that purpose. Most of the affected areas were originally prime farmland; however, with removal of the A-horizon they became unsuitable for agriculture, including grazing, since the exposed Bt-horizon is unsuited for plant growth due to its high clay content (45±65 per cent) and the low nutrient levels. Since trees survive better on poor soils than do agricultural crops, the possibility of afforestating desurfaced soils has been studied. Eucalyptus are one of the major species used in tree planting programmes aimed at reclaiming degraded soils since they are fast growing and can grow to commercial size in a wide range of soils and climatic conditions. The work reported here was done in a desurfaced Vertic Argiudoll and a similar non-desurfaced soil (control). Three Eucalyptus species were tested, i.e. E. camaldulensis, E. viminalis and E. dunnii. Their height and diameter (dbh) growth were 47Á9 to 75Á8 per cent less and timber volume 86Á5 to 98Á5 per cent less on the desurfaced soil. E. camaldulensis grew best in all the parameters in the desurfaced soil. Although tree growth was poor, afforestation may be an alternative use for desurfaced soils where agriculture and livestock breeding are not possible.