RESUMEN
Spain is the fourth largest lettuce-producing country in the world and the leading European producer. Much of the production, mainly grow in open field, is dedicated to export with a value of 887 million U.S. dollars per year. In summer 2021 wilting symptoms were observed in a commercial field crop on butterhead lettuce 'Amible' in Albacete (Castilla-La Mancha, Spain). Approximately 15% of plants were affected, but losses were even more severe on subsequent crops. Vascular tissue of affected plants showed a brown to red discoloration. Sections of infected vascular tissue (3 to 5 mm long) were surface sterilized in 70% ethanol for 30 s, washed three times with sterile water, and plated on potato dextrose agar (PDA) amended with streptomycin sulfate (100 mg/liter). From 5-day-old cultures typical pale cream to purplish mycelia with microconidia, macroconidia, and chlamydospores of Fusarium oxysporum were observed. Microconidia were abundant on carnation leaf agar and measured 6.1 to 9.2 µm (mean 7.1 ± 0.7 µm; n=50)). Macroconidia were sparse, three-septate, straight to slightly curved, 23.3 to 34.8 × 4 to 5.2 µm (mean 31.5 ± 2.8 × 4.2 ± 0.3 µm; n=50). Chlamydospores were terminal and intercalary, rough walled, and measured 7.2 to 10.1 µm (mean 9.5 ± 0.6 µm; n=50) µm. DNA was extracted from three single-spore isolates using the protocol of Querol et al. (1992) and the translation elongation factor 1-α gene (TEF) was sequenced with exTEF-F/FUexTEF-R primers as described by Taylor et al. (2016). All TEF sequences (GenBank accession no. OP903519) were identical. In BLAST analyses, the isolates showed 100% identity to the corresponding region of Fusarium oxysporum f. sp. lactucae (FOL) race 4 (MK059958). All Spanish isolates were identified as FOL race 4 using a race-specific polymerase chain reaction (PCR) with the primers FPUF/FPUR (Gilardi et al. 2017), and a previously identified FOL race 4 isolate Fus 1.01 as a positive control. Pathogenicity tests were conducted to confirm the positive result of the race 4-specific PCR and to complete Koch's postulates. Three differential lettuce cultivars ('Costa Rica No. 4', 'Banchu Red Fire', and 'Romana Romabella 30 CN') provided by Rijk Zwaan (The Netherlands) were inoculated with three Spanish isolates (Al1A1, Al1D, Al2B) and the Fus 1.01 isolate used as FOL race 4 positive control (Claerbout et al., 2018). Roots of 3-week-old plants (five replicates per treatment) were dipped in a spore suspension (1 × 106 conidia/ml) for 10 min before transplanting into 250-ml pots with sterile substrate. Non-inoculated control plants were dipped in sterile water for 10 min. The experiment was carried out twice. Inoculated lettuce seedlings were planted and maintained in a growth chamber (25°C day, 18°C night). Plants were slightly watered every other day. After 21 days, wilting was observed in the cultivars 'Costa Rica No. 4' and 'Romana Romabella 30 CN'. Moreover, taproots were cut longitudinally, and vascular browning was observed in the taproot. No discoloration could be observed in the taproot of 'Banchu Red Fire' plants, coinciding with the result of isolate Fus 1.01 used as FOL race 4 positive control. Non-inoculated control plants remained healthy and vascular browning was not observed. In both experiments, F. oxysporum f.sp. lactucae was consistently reisolated using PDA medium. These results confirmed that the isolates Al1A1, Al1D and Al2B were FOL race 4. This race has recently been identified in The Netherlands (Gilardi et al. 2017), Belgium (Claerbout et al. 2018), United Kingdom, Ireland (Taylor et al. 2019) and Italy (Gilardi et al. 2019) and could become a serious threat to Spain lettuce production.
RESUMEN
Among the key diseases affecting the asparagus crop (Asparagus officinalis L.), vascular wilting of asparagus caused by Fusarium oxysporum f. sp. asparagi stands out worldwide. This disease significantly shortens the longevity of the crop and limits economic production. Traditional control measures have been largely ineffective, and chemical control methods are difficult to apply, making biological control approaches, specifically the use of Trichoderma, an economical, effective, and risk-free alternative. This study aimed to identify the main factors that affect the efficacy of biopesticides studied as Biological Control Agents (BCAs) against Fusarium wilt in asparagus and to assess the efficacy of Trichoderma-based biopesticides under greenhouse and semi-field conditions. We evaluated the response of three Trichoderma spp. (T. atroviride, T. asperellum, and T. saturnisporum) to environmental variables, such as temperature and water activity, and their antagonistic capacity against Fusarium oxysporum f. sp. asparagi. All three Trichoderma species inhibited the growth of the pathogen in vitro. A decrease in water activity led to a greater reduction in the growth rate. The efficacy of the three biological control agents decreased with higher temperatures, resulting in minimal inhibition, particularly under conditions of restricted available water in the environment. The effect of the fungal inoculum density was also analyzed at two different temperatures. A direct correlation between the amount of inoculum and the score on the Disease Severity Index (DSI) was observed. A notable reduction in DSI was evident in treatments with high inoculum density (106 conidium/mL) for all three species of Trichoderma tested at both temperatures. In greenhouse and semi-field tests, we observed less disease control than expected, although T. asperellum and T. atroviride showed lower disease severity indices and increased the dry weight of seedlings and crowns, whereas T. saturnisporum resulted in the highest disease rate and lowest dry weight. This work highlights that the efficacy of Trichoderma as BCAs is influenced by various factors, including the quantity of soil inocula, and environmental conditions. The study findings have strong implications for selecting appropriate Trichoderma species for controlling specific pathogens under specific environmental conditions.
RESUMEN
Asparagus crop is distributed worldwide, covering very different climatic regions. Among the different diseases that affect asparagus, vascular Fusarium wilt, caused by Fusarium oxysporum f. sp. aparagi (Foa), stands out. It is not only the cause of large economic losses due to a decrease in yield and shortened longevity of the plantation, but also prevents replanting. This work aimed to determine if F. oxysporum isolates associated with vascular wilt on asparagus have adapted differentially to the different agro-environmental conditions. The potential correlation between origin and mycelial growth under different temperatures and humidity conditions was analysed for isolates from asparagus fields cultivated in northern and southern Europe. The genetic and pathogenic variability were also analysed. While a clear effect of water activity on mycelial growth was observed, all isolates responded in a similar way to changes in water activity in the medium, regardless of their geographical origin. The results revealed a low genetic variability of F. oxysporum isolates associated with vascular wilt on asparagus without signs of differentiation correlated to geographical origin. The southernmost isolates of the two cultivated varieties inoculated did not express more pathogenicity than those isolated from the colder region.
RESUMEN
Asparagus Decline Syndrome (ADS) is one of the main phytosanitary problems of asparagus crop worldwide. Diseased plants and soil samples from 41 fields from three main production areas of Spain were surveyed. Eight Fusarium species belonging to seven species complexes were identified in soils: F. oxysporum, F. proliferatum, F. redolens, F. solanisensu stricto, F. equiseti, F. culmorum, F. compactum and F. acuminatum. Fusarium oxysporum was the most prevalent species. Statistical correlation (R2 = 88%) was established between F. oxysporum inoculum density and the average temperature of the warmest month. A relationship was also established between three crop factors (average temperature, crop age and F. oxysporum inoculum density) and field disease indices. Significant differences were observed between the distribution of F. oxysporum propagules in white and green asparagus fields. Thirteen Fusarium species belonging to seven species complexes were identified from roots of diseased plants, being F. oxysporum the most prevalent. F. proliferatum, F. oxysporum and F. redolens showed pathogenicity to asparagus and were the main species associated to ADS. Fusarium oxysporum was the species with the highest genetic diversity displaying 14 sequence-based haplotypes with no geographic differentiation. This work contributes to understanding the Fusarium complex associated to ADS for developing accurate integrated disease management strategies.