The decline and death of strawberry plants in Spanish fruit production fields have mainly been attributed to the soilborne pathogens Macrophomina phaseolina, Phytophthora cactorum, and Fusarium spp. Inoculum sources of M. phaseolina and P. cactorum, and the incidence all three genera, were investigated in nurseries and fruit production fields over three consecutive seasons. M. phaseolina inoculum sources consisted of fumigated preplant fruit production soils (50%) and fumigated nursery soils (47%), although the pathogen could not be detected in nursery mother and runner plants. P. cactorum inoculum sources included nursery (20%) and preplant fruit production (17%) fumigated soils, and nursery runner plants (up to 15%). In fruit production plants, the average incidence of M. phaseolina and P. cactorum were 4.2 and 3.7%, respectively. Fusarium spp. inoculum sources could not be accessed extensively due to the lack of effective quantitative real-time PCR assays. Limited testing of nursery plants showed that Fusarium oxysporum f. sp. fragariae (Fof) was absent. In field production plants and soil, F. solani was the main pathogenic Fusarium spp., with Fof only identified once in a fruit production plant. Ineffectively fumigated soils in nurseries and production fields, along with infected runner plants, can be inoculum sources of soilborne strawberry pathogens in 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.
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.