First Report of Fusarium oxysporum Causing Stem Dry Rot in Astrophytum ornatum in Mexico

Astrophytum ornatum (DC.) Britton & Rose, family Cactaceae, is native to Mexico and is categorized as a threatened species. It has a spherical stem at the beginning of its development, but during growth it becomes columnar. Wild specimens can reach up to 1 m high and 30 cm in diameter, although in cultivation they rarely exceed 30 cm in height. During September 2013, in Texcoco State, Mexico, dry rot symptoms were observed in 8-year-old A. ornatum plants grown in pots and in green spaces. Incidence was ∼30%. Symptoms began as small dots, olive in color, in the younger tissues of the aerial part of the plant; subsequently, they developed semicircular dry spots 3 to 5 cm in diameter, light brown, delimited by a light green margin. In some cases, lesions increased in size and coalesced, and after 240 days, 100% of the plant area was invaded. Eventually, affected tissues collapsed and the plants died. Inner tissue pieces were cut from the healthy-ill transition area, placed in potato dextrose agar (PDA), and incubated for 48 h at 22°C. Mycelium was grown out of infected plant tissue and transferred to PDA dishes. The developed colonies were purified by monoconidial cultures on PDA to determine their cultural and morphological characteristics (†). The isolates were identified as Fusarium oxysporum due to the production of sickle shaped macroconidia 30.4 μm long × 3.28 μm wide with three septa, foot-shaped basal cell and a short apical cell, in branched conidiophores or on the surface of sporodochia; microconidia developed in simple lateral phialides, oval to ellipsoid shaped, straight to curved, and without septa. The colonies were purple (†;†). Molecular analysis was performed on two isolates to validate morphological characterization. DNA was extracted from monoconidial cultures and partial ITS1-4 region EF1-α subunit fragments were amplified by PCR with universal primer sequences. BLASTn analysis performed to sequences showed 98 to 99% similarity with the corresponding F. oxysporum sequences deposited in the nr-nucleotide database (†), as well as with the Fusarium-ID database (†); the sequences were deposited as KX398010 (ITS1-4) and KY379852 (EF1-α), respectively. For pathogenicity tests, 4-year-old A. ornatum plants were inoculated with the KX398010 isolate using three methods: i) spraying spores, ii) wound + spraying spores, and iii) subepidermal injection of spores. For control plants, sterile distilled water was used. Wounding was done by causing lesions with a sterile needle. For each method of inoculation, five plants were used, and the pathogenicity test was repeated two times. The inoculation in the aerial part (ribs) of the plant was carried out because the symptoms begin in this zone. For spraying spores, the first symptoms of dry rot were observed 15 days after inoculation as olive colored spots, which developed semicircular dry light brown spots; in the spores’ injection method, the tissues collapsed and died, causing perforations; lastly, by wound plus spraying spores method, they developed circular light brown spots from the damage done. The symptoms were similar to those observed in the first plants with dry rot, which originated this research. F. oxysporum was recovered from infected tissues by reisolation. To our knowledge, this is the first report of F. oxysporum in A. ornatum plants in Mexico. This work has significance in nurseries as well as for the conservation of this cactus, which is a threatened species. © 2017, American Phytopathological Society. All rights reserved.