Fusarium oxysporum, F. proliferatum and F. redolens associated with basal rot of onion in Finland

Abstract

In recent years in Finland, Fusarium infections in onions have increased, both in the field and in storage, and Fusarium species have taken the place of Botrytis as the worst pathogens causing post‐harvest rot of onion. To study Fusarium occurrence, samples were taken from onion sets, harvested onions and also from other plants grown in the onion fields. Isolates of five Fusarium species found in the survey were tested for pathogenicity on onion. Fusarium oxysporum was frequently found in onions and other plants, and, of the isolates tested, 31% caused disease symptoms and 15% caused growth stunting in onion seedlings. Fusarium proliferatum, a species previously not reported in Finland, was also identified. Over 50% of the diseased onion crop samples were infected with F. proliferatum, and all the F. proliferatum isolates tested were pathogenic to onion. Thus, compared to F. oxysporum, F. proliferatum seems to be more aggressive on onion. Also some of the F. redolens isolates were highly virulent, killing onion seedlings. Comparison of the translation elongation factor 1α gene sequences revealed that the majority of the aggressive isolates of F. oxysporum f. sp. cepae group together and are distinct from the other isolates. Incidence and relative proportions of the different Fusarium species differed between the sets and the mature bulbs. More research is required to determine to what extent Fusarium infections spoiling onions originate from infected onion sets rather than the field soil.

Sampling and PCR method for detecting pathogenic Fusarium oxysporum strains in onion harvest

“…In Finland, FBR is mostly caused by pathogenic strains of F. oxysporum f. sp. cepae and F. proliferatum , and to a smaller extent by Fusarium redolens (Haapalainen et al ). The primary source of the pathogenic Fusarium isolates found from onion in Finland is the imported onion sets (Haapalainen et al ).…”

“…In a previous study, Fusarium strains were isolated from onion sets and mature bulbs and tested for pathogenicity on onion seedlings (Haapalainen et al ). While all the F. proliferatum strains tested were more or less virulent, the majority of the F. oxysporum isolates were non‐pathogenic.…”

“…While all the F. proliferatum strains tested were more or less virulent, the majority of the F. oxysporum isolates were non‐pathogenic. Yet, the most aggressive pathogens were F. oxysporum strains, and all the isolates from symptomatic mature bulbs were pathogenic (Haapalainen et al ). In the storage, the number of onions infected with F. proliferatum was found to be small compared to the number of onions infected with F. oxysporum (A. Hannukkala, unpubl.…”

“…Thus, the screening method should be specific for the pathogenic strains. Although the F. oxysporum strains show large variation in colony morphology, a test based on morphological features would be ineffectual, since these features showed no correlation to virulence (Haapalainen et al ). Instead, most of the highly virulent strains were found to be genetically closely related (Sasaki et al ; Haapalainen et al ), and thus a DNA‐based method would be applicable for detection of latent infections with these strains.…”

“…Although the F. oxysporum strains show large variation in colony morphology, a test based on morphological features would be ineffectual, since these features showed no correlation to virulence (Haapalainen et al ). Instead, most of the highly virulent strains were found to be genetically closely related (Sasaki et al ; Haapalainen et al ), and thus a DNA‐based method would be applicable for detection of latent infections with these strains. The onion‐pathogenic strains have also been shown to carry additional pathogenicity‐related genes, called SIX genes (for secreted in xylem), encoding small secreted proteins that are supposed to function as virulence factors (Sasaki et al ; Taylor et al ).…”