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First Report of Root and Collar Rot Caused by Fusarium tricinctum and Fusarium avenaceum on Carrot in France

Le Moullec-Rieu, T., Chateau, C., Pascouau, C., Bastide, F., Hamon, B., Poupard, P., Laurent, E., Gombert, J., Morel, E., Sérandat, I., Guillermin, P.-L., Brandeis, P.-E., Mallard, S.
Plant disease 2020 v.104 no.2 pp. 591
ATP citrate synthase, DNA, DNA-directed RNA polymerase, Daucus carota, Fusarium avenaceum, Fusarium tricinctum, agar, carrots, climatic factors, color, conidia, culture media, fungi, genetic markers, greenhouses, mycelium, pathogenicity, polymerase chain reaction, seed development, seeds, sporodochia, sporulation, ultraviolet radiation, France
In 2017, carrot (Daucus carota L.) seed production represented around 22% of the area devoted to the production of vegetable fine seeds. Since 2015, symptoms of root and collar rot have been observed in carrot seed parcels located in the Central Region, one of the most important production zones in France. Diseased plants became dried prematurely, compromising seed development. Depending on the year and the climatic conditions, the disease in a same field can be considered as epidemic (rate losses between 30 and 100% of plants in 2016) or can impact plants more sporadically (less than 10% in 2017 and 2018). Sixteen diseased carrot samples (Nantaise type) were collected from five fields of seed production in the Central Region: two fields in 2016 and 2017, and one field in 2018. Seven fungal isolates, obtained from lesions, were grown on potato dextrose agar medium and incubated for 1 week at 20°C in darkness. From the colony top, fluffy mycelium pigmented in pink, red, purple, or orange was observed, with a red color at the reverse. To induce sporulation, isolates were grown on Synthetischer Nährstoffarmer agar medium during 3 weeks at 24°C with near-UV radiation under a 12-h photoperiod. Four isolates (FT001, FT003, FT007, and FT017) developed orange sporodochia with lunar or crescent-shaped macroconidia (40.3 ± 0.8 × 5.9 ± 0.1 µm; n = 90) and lime or pear-shaped microconidia (10.7 ± 0.2 × 7.7 ± 0.2 µm; n = 60), as described for Fusarium tricinctum (Leslie and Summerell 2006). Three isolates (FA001, FA002, and FA006) developed orange sporodochia with sickle-shaped macroconidia (50.5 ± 1.1 × 5.0 ± 0.1 µm; n = 60) but no microconidia, as observed in Fusarium avenaceum (Leslie and Summerell 2006). To confirm the identification, DNA was extracted from the mycelium of the seven isolates, and molecular markers (ATP citrate lyase, ACL1; RNA polymerase II, RPB2) were used for PCR amplification (Gräfenhan et al. 2011; O’Donnell et al. 2013). The ACL1 sequences from the seven field isolates (GenBank nos. MK183788 to MK183791; MK181528 to MK181530) were 99 to 100% identical to the ACL1 sequence of a reference F. tricinctum isolate (query coverages 99 to 100%; E-values of 0.0) and a reference F. avenaceum isolate (query coverages 98 to 99%; E-values of 0.0) (respectively DAOM 235630 isolate, GenBank no. JX397813, and BBA64135 isolate, GenBank no. JX397768 [Niessen et al. 2012]). Using RPB2, sequences from field isolates (GenBank nos. MK183109 to MK183115) were 98.5 to 99.9% identical to the RPB2 sequence of a reference F. tricinctum isolate (query coverages 96 to 100%; E-values of 0.0) and a reference F. avenaceum isolate (query coverages 95 to 100%; E-values of 0.0) (respectively MRC 1895 isolate, GenBank no. MH582113, and MRC 1413 isolate, GenBank no. MH582082 [O’Donnell et al. 2018]). To confirm pathogenicity, FT001 and FA002 were inoculated on collars of 10-week-old carrot plants in the greenhouse. Forty plants per isolate and 40 control plants were used. Ten microliters of a conidial suspension (10⁵ conidia/ml), or sterile water for the controls, was deposited at the collar, which was previously wounded using a scalpel blade. Necrotic lesions developed at 20 days postinoculation (dpi) (FT001) and at 30 dpi (FA002). F. tricinctum and F. avenaceum were reisolated from the lesions and identified by sequencing using ACL1 and RPB2 markers. No isolation of Fusarium was obtained from the controls. To our knowledge, this is the first report of F. tricinctum and F. avenaceum in carrot in France.