Fusarium basal rot (FBR), which is caused predominantly by Fusarium oxysporum and F. proliferatum, is the main limiting factor of onion crops. Resistant cultivars obtained in other countries do not behave as such in Argentina crop fields. The cultivars Antártica-INTA, Grano de Oro-Seminis, Valcatorce-INTA and TW-2007 (reported as tolerant) were tested with five Fusarium spp. isolates, using four inoculum concentrations. Disease incidence was recorded along 28 days and the area under disease progress curve was calculated. Diverse epidemiological models were fitted to experimental data. There were significant differences in the resistance level among cultivars, with TW-2007 being the most tolerant. Local Fusarium isolates were the most virulent ones. The concentration of 10,000 microconidia/ gram was the most lethal for all isolates. The absence of resistance to Fusarium in the four cultivars tested was confirmed. Inoculum concentration and isolate are critical factors in screening for resistance to FBR. Breeding based on the selection of genotypes against low virulence strains of Fusarium spp. and the presence of more aggressive strains in local fields may be one of the causes why varieties reported as resistant or tolerant behave as susceptible in our environment.

The use of a Neubauer chamber is a broadly employed method when cell suspensions need to be quantified. However, this technique may take a long time and needs trained personnel. Spectrophotometry has proved to be a rapid, simple and accurate method to estimate the concentration of spore suspensions of isolates of the genus Fusarium. In this work we present a linear formula to relate absorbance measurements at 530nm with the number of microconidia/ml in a suspension.

Fusarium basal rot (FBR) is one of the main onion diseases in Argentina. The Onion Breeding National Program, held at INTA Experimental Station in La Consulta, Argentina, has developed onion breeding lines with increased resistance to FBR through a selection scheme performed under field conditions. The aim of the present work was to evaluate the achieved response to selection in the FBR resistance trait, comparing two strategies: selection under field environment and artificial inoculations performed in controlled conditions. Seeds of the breeding line 17-1289 (P 0) were inoculated with Fusarium oxysporum f.sp. cepae (isolate FOC LJC10081), one seed per cap. After 15 days 162 surviving plants were treated with fungicide and transplanted. The resulting bulbs were planted and recombined avoiding contamination with foreign pollinators. The obtained seeds were labelled as PCC (Progeny generated under Controlled Conditions). Another portion of P 0 seeds was grown under field conditions in a FBR-conductive soil. The best bulbs were selected at harvest based on their FBR resistance score, root system quality and agronomic features. These bulbs later produced seeds by recombination in isolation and were labelled as PFC (Progeny generated under Field Conditions). The P 0 population and the PCC-PFC progenies were then compared through a standardized test for determination of FBR resistance, using two inoculum concentrations in the substrate. The incidence of FBR was significantly reduced in PCC at high inoculum concentrations, in comparison to the original population P 0. Disease progression was delayed and lower rates were achieved in PCC. The described lab selection technique may constitute an interesting and useful tool to select materials that carry genetic resistance and contribute to an integrative approach, complemented with selection under field conditions, in onion breeding programs for FBR resistance.

The cucurbit powdery mildew (CPM) caused by different fungal species is a major concern for cucurbit crops around the world. In Argentina CPM constitutes the most common and damaging disease for cucurbits, especially for squash crops (Cucurbita moschata). The present study displays initial insights into the knowledge of the disease in western Argentina, including the determination of the prevalent species causing CPM, as well as the evaluation of the resistance of squash cultivars and breeding lines. Fungal colonies were isolated from samples collected in Mendoza province, Argentina. A field trial was also performed to assess the resistance of five squash accessions, including commercial cultivars and breeding lines. The severity of CPM was analyzed and epidemiological models were built based on empirical data. The morphological determinations and analysis with specific molecular markers confirmed Podosphaera xanthi as the prevalent causal agent of CPM in Mendoza. The results od the field trial showed differences in the resistance trait among the squash accessions. The advanced breeding line BL717/1 showed promising results as source of CPM resistance for the future development of open pollinated resistant cultivars, a crucial tool for an integrative control of the disease.