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.

Fusarium solani is an emerging pathogen reported on Spanish strawberry crops both in nurseries and in fruit production fields, causing wilt and root rot. Pathogenicity, morphocultural characteristics, and sensitivity to biocides of 103 F. solani isolates recovered from symptomatic strawberry plants and soils from both Spanish strawberry areas were determined. The differences of isolates within and between nurseries and field crops in relation to these parameters were analyzed. Considerable variability in morphological and pathogenic characteristics was observed among the isolates in both areas. The majority of isolates were not pathogenic (62%), and only 38 F. solani isolates (37.62%) caused disease on strawberry plants under controlled conditions; 52.63% of pathogenic isolates induced low severity symptoms. Almost 70% of pathogenic isolates caused stunting on plants. The morphological characters that best explain the F. solani variability (86.85%) were colony color and the presence of macroconidia on culture medium. The sensitivity to the fumigants tested was similar between the isolates from nurseries and fruit production fields, showing greater sensitivity to the field doses of dazomet and chloropicrin. However, the isolates were less sensitive to metam sodium and poorly sensitive to 1,3-dichloropropene. This work can contribute to the advancement of sustainable production of strawberry.

To avoid the use of agrochemicals in agriculture, alternative methods are emerging to control plant pathogens. Some plant growth-promoting rhizobacteria (PGPR) can be used as biocontrol agents since they can induce protection against pathogens. The aim of this study was to evaluate the protective effect of several PGPR strains against the main Fusarium species involved in asparagus decline syndrome (ADS): F. proliferatum, F. oxysporum f. sp. asparagi and F. redolens. In vitro antagonism assays showed that all the bacteria inhibited the mycelium growth of the three Fusarium species. The most effective strains (Streptomyces fradiae Hvs6, Bacillus paralicheniformis Hvs2 and Bacillus velezensis FC37) were tested to evaluate their protective effect on asparagus plants inoculated with pathogenic Fusarium isolates. Strains FC37 and Hvs2 were the most effective in controlling pathogenic F. proliferatum and F. oxysporum f. sp. asparagi, but neither could protect against F. redolens isolates. The production of hydrolytic enzymes such as b-glucosidase, amylase and protease by these bacterial strains could be involved in the structural degradation of the fungal cell wall. In addition, the production of toxic volatile compounds, such as hydrogen cyanide, may inhibit the fungal growth, and the production of phosphate solubilizers could be related to the plant growth promotion. These results suggest that strains FC37 and Hvs2 could be used as potential biocontrol agents as a sustainable and environmentally friendly control strategy for ADS-affected fields.

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Fusarium solani is a soilborne fungus that is a pathogen to >100 plant species. It is the causal agent of crown and root rot in strawberry. We collected 100 F. solani isolates from diseased plants and soils from two distinct geographic areas of strawberry production in Spain: plant nurseries located in the north-central region of the country and fruit production fields located in the southwestern region. The aims of this study were to accurately identify the isolates within the Fusarium solani species complex (FSSC) based on multilocus sequence typing, determine the genetic diversity and population structure of strawberry-associated FSSC based on phylogenetic analysis, and determine the vegetative compatibility among isolates in both strawberry production areas. Seven phylogenetic species, restricted to clade 3 of FSSC, were defined in the Spanish strawberry crops, showing a regional variation of species composition. Isolates from nurseries were composed of four phylogenetic species (i.e., FSSC 2, FSSC 5, FSSC 9, and an unknown FSSC species) that matched with five vegetative compatibility groups (VCGs). Isolates from fruit production fields included five phylogenetic species (i.e., FSSC 2, FSSC 3 + 4, FSSC 5, FSSC 6, and FSSC 11) distributed into 29 VCGs not correlated with phylogenetic groups. FSSC 5 and FSSC 2 were the most abundant species in nurseries and fruit production fields, respectively, and they were the only species present in both production areas. Of the 47 sequence-based haplotypes defined, no haplotypes were shared between nurseries and fruit production fields. Pathogenic isolates were present in all but FSSC 6 and FSSC 9 species, and FSSC 3 + 4 contained the higher percentage of pathogenic isolates. No relationship was observed between pathogenicity and the source of isolates (plant or soil). Generally, species present in fruit production fields showed higher genetic diversity than those present in nurseries. This work can contribute to understanding the diversity of this species complex in Spanish strawberry production areas, which will be useful for developing integrated disease management strategies.

BACKGROUND Asparagus decline syndrome (ADS), one of the most important diseases affecting asparagus crops, causes important yield losses worldwide. Fusarium proliferatum, F. oxysporum and F. redolens are among the main species associated with ADS. To explore their potential inoculum sources and the effectiveness of soil disinfestation practices for ADS management, molecular methods based on a quantitative real‐time polymerase chain reaction (qPCR) were developed. qPCR‐based molecular tools demonstrated advantages in the sensitive and specific detection and quantification of fungal pathogens in comparison with less‐accurate and time‐consuming traditional culture methods. RESULTS F. proliferatum, F. oxysporum and F. redolens could be specifically detected and accurately quantified in asparagus plants, soil and irrigation water collected from asparagus fields with ADS symptoms by means of the designed TaqMan qPCR protocols. Furthermore, these molecular tools were successfully applied for evaluation of the efficacy of diverse soil disinfestation treatments. Chemical fumigation with dazomet and biosolarization with pellets of Brassica carinata contributed to a significant reduction in the inoculum densities of the three Fusarium species in treated soils, which was correlated with production increases. CONCLUSIONS The capability to accurately detect and quantify the main Fusarium species involved in ADS in plants, soil and water samples by means of qPCR will allow identification of high‐risk fields that can be avoided or managed to reduce yield losses. Quantification of pathogen densities in the soil may also provide essential insights into the effectiveness of soil disinfestation methods for ADS management. © 2021 Society of Chemical Industry.