Transmission of Fusarium oxysporum f. sp. fragariae Through Stolons in Strawberry Plants.

Fusarium wilt of strawberry, caused by the soilborne pathogen Fusarium oxysporum f. sp. fragariae, is a growing threat to the strawberry industry worldwide. Symptoms of the disease typically include stunting, wilting, crown discoloration, and eventual plant death. When Fusarium wilt was discovered in California, the disease was not known to occur anywhere else in North America. Long distance movement of the pathogen would most likely occur through transport of infected plants, which seems plausible if strawberry plants can sustain infections without showing symptoms of disease. The results of this study document that F. oxysporum f. sp. fragariae can move through stolons of infected mother plants and colonize first-generation daughter plants. The pathogen can also move through stolons from first to second-generation daughter plants. Daughter plants of both generations were always symptomless. The pathogen was recovered from both roots and petioles of infected daughter plants. Similar results were obtained for two cultivars known to be susceptible to Fusarium wilt, Albion and Monterey. Transmission through stolons from mother to daughter plants also occurred in the resistant cultivar, San Andreas, but less frequently than in Albion and Monterey.

The Effect of pH on Spore Germination, Growth, and Infection of Strawberry Roots by Fusarium oxysporum f. sp. fragariae, Cause of Fusarium wilt of Strawberry.

Previous work has shown that raising the pH of acidic soil to near neutrality can reduce the incidence of Fusarium wilt. The basis for this effect has not been established. The present study assessed effects of pH on spore germination, growth, and infection of strawberry roots by Fusarium oxysporum f. sp. fragariae, the cause of Fusarium wilt of strawberry. There was not a significant effect of pH (5 versus 7) on the rate of spore germination at either 20 or 25°C for any of the three tested isolates (one representative of each clonal lineage of F. oxysporum f. sp. fragariae found in California). Likewise, pH did not have a significant effect on fungal growth at 20°C. At 25°C, two isolates grew faster at pH 7 than at pH 5. Growth of the third isolate was unaffected by the difference in pH. For the strawberry cultivar Albion, the frequency of root infection was significantly higher for plants grown in acidified soil (near pH 5) than for plants grown in soil near neutrality. The higher frequency of root infection in acidified soil was associated with a lower level of microbial activity, as measured by hydrolysis of fluorescein diacetate.

The Population of Fusarium oxysporum f. sp. fragariae, Cause of Fusarium Wilt of Strawberry, in California.

The objectives of this study were to investigate the structure of the population of Fusarium oxysporum f. sp. fragariae in California and to evaluate methods for its detection. Fifty-nine isolates of F. oxysporum f. sp. fragariae were obtained from diseased strawberry plants and their identity was confirmed by pathogenicity testing. The full nuclear ribosomal intergenic spacer (IGS) and elongation factor 1-α gene (EF-1α) were amplified by polymerase chain reaction (PCR) and sequenced to elucidate phylogenetic relationships among isolates. IGS and EF-1α sequences revealed three main lineages, which corresponded to three somatic compatibility groups. Primers designed to detect F. oxysporum f. sp. fragariae in Japan amplified a 239-bp product from 55 of 59 California isolates of F. oxysporum f. sp. fragariae and from no nonpathogenic isolates of F. oxysporum. The sequence of this PCR product was identical to the sequence obtained from F. oxysporum f. sp. fragariae isolates in Japan. Intensive sampling at two locations in California showed results of tests based on PCR and somatic compatibility to be in agreement for 97% (257 of 264) of isolates tested. Our findings revealed considerable diversity in the California population of F. oxysporum f. sp. fragariae, and indications that horizontal gene transfer may have occurred.

Fusarium oxysporum f. sp. mori, a New Forma Specialis Causing Fusarium Wilt of Blackberry.

Fusarium oxysporum has recently been identified as the cause of a wilt disease affecting blackberry in California and Mexico. Thirty-six isolates of F. oxysporum obtained from symptomatic blackberry plants in California and Mexico were comprised of nine distinct somatic compatibility groups (SCGs). Phylogenetic analysis of a concatenated data set, consisting of sequences of the translation elongation factor 1-α and ß-tubulin genes and the intergenic spacer of the ribosomal DNA, identified nine three-locus sequence types, each of which corresponded to an SCG. Six SCGs were present only in California, two only in Mexico, and one in both California and Mexico. An isolate associated with the most common SCG in California was tested for pathogenicity on blueberry, raspberry, strawberry, and lettuce. All blueberry, raspberry, and lettuce plants that were inoculated remained healthy, but two of the five strawberry cultivars tested developed symptoms. The three strawberry cultivars that were resistant to the blackberry pathogen were also resistant to F. oxysporum f. sp. fragariae, the cause of Fusarium wilt of strawberry. We propose to designate strains of F. oxysporum that are pathogenic to blackberry as Fusarium oxysporum f. sp. mori forma specialis nov.

First Report of Fusarium solani Causing Crown and Root Rot on Strawberry Crops in Southwestern Spain.

Spain is the fourth largest strawberry (Fragaria × ananassa) producing country in the world. Since April 2010, stunted and dead strawberry plants have been detected in four strawberry fruit production fields in Huelva (southwestern Spain) affecting less than 1% of plants. Symptoms consisted of foliage wilt, plant stunting and drying, and death of older leaves. Internal vascular and cortical tissues of plant crowns showed an orange to brown discoloration. Crowns and roots of symptomatic plants were surface sterilized in 1% sodium hypochlorite for 2 min, rinsed in sterile distilled water for 2 min, and air-dried in a laminar flow cabinet. Small disinfested pieces were transferred to petri dishes containing potato dextrose agar (PDA) and incubated for 10 days at 25°C with a 12-h photoperiod. Cultures derived from single spores were obtained, and morphological characterization was performed by microscopic examination. White to pale cream colonies developed after 10 days of incubation. Unbranched monophialides with microconidia in false heads, micro- (0 to 3 septa) and macroconidia (5 to 7 septa) wide and robust in shape, and chlamydospores were consistent with descriptions of Fusarium solani (Martius) Appel & Wollenweber emend. Snyder & Hansen (2). In addition, the fungus was isolated from asymptomatic runner plants from nurseries by the same method described above, and from soil samples from six fruit-producing fields. Soil samples were analyzed by dilution plating on Fusarium-selective agar medium (1). Genomic DNA from three isolates (FPOST-81 from dead plant 'Sabrina,' TOR-11 from runner plant 'Camarosa,' and TOR-1 from soil) was obtained with a DNA extraction kit (Isolate Plant DNA MiniKit, Bioline). A portion of the translation elongation factor-1 alpha (EF-1α) gene was sequenced using EF-1/-2 primers (3) (GenBank Accession Nos. KF275032, KF275033, and KF275034). The sequence comparison revealed a 99 to 100% match with F. solani sequences in GenBank and Fusarium-ID databases. To confirm the pathogenicity of the fungi, runner strawberry plants 'Camarosa' were inoculated by dipping crowns and roots into a conidial suspension (106 to 107 conidia per ml) for 30 min (8 plants per F. solani isolate) or into sterile distilled water for the controls. Plants were potted in 13-cm diameter pots with peat and maintained at 25/18°C and 70/40% relative humidity (day/night) in a growth chamber with a daily 16-h photoperiod of fluorescent light. Three plants inoculated with isolates TOR-11 and FPOST-81, and four plants inoculated with isolate TOR-1, died within 10 days after inoculation. After 8 to 12 weeks, all of the remaining inoculated plants were stunted and developed symptoms similar to those observed in the field. Production of new feeder roots was lacking or scarce. Control plants remained healthy and formed feeder roots. All plants inoculated with isolates TOR-1 and FPOST-81, and 50% of plants inoculated with TOR-11, showed brown discoloration in the crown. F. solani was re-isolated from symptomatic plants at frequencies of 100% and 80 to 100% from root and crown tissues, respectively. Although F. solani has been reported as a pathogen in other crops, to our knowledge, this is the first report of the occurrence of F. solani causing disease in strawberry plants in Spain. References: (1) D. Bouhot and F. Rouxel. Ann. Phytopathol. 3:251, 1971. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual, Blackwell Publishing, London, 2006. (3) K. O'Donnell et al. Proc. Natl. Acad. Sci. USA 95:2044, 1998.