Basal Rot of Hylocereus undatus Caused by Fusarium oxysporum in Buenos Aires, Argentina.

Night-blooming cereus (Hylocereus undatus A. Berger) is generally used as rootstock of ornamental cactus because of its rapid growth and tolerance to humid substrates. Since 2002, in Gran Buenos Aires, a new disease has been observed in grafted crops in all production stages. Incidence was as much as 30% in many greenhouses. Symptoms consisted of soft rot that started near the soil line and developed upward until it affected all the rootstock. The scion was not rotten, but died as a consequence of rootstock infection. All the roots were symptomless. For pathogen isolation, symptomatic tissues were surface disinfected by a 1-min immersion in 0.2% NaOCl, placed on potato dextrose agar (PDA), and incubated at 22 ± 2°C. Only a Fusarium spp. was consistently isolated in pure culture. Twenty healthy potted night-blooming cereus plants, 10 of them previously injured with needles on the rootstock near the soil line, were gently removed from the substrate and inoculated by a 1-min immersion of their base in a suspension of 1.4 × 106 conidia per ml of sterile distilled water, prepared from 15-day-old cultures. Ten control plants, five of them previously injured, were immersed in sterile distilled water. Inoculated and noninoculated plants were replanted in the original substrate, placed in a climatic chamber at 22 ± 2°C, and monitored for disease expression. Basal rot was observed on all injured inoculated plants 12 days after inoculation. Symptoms on undamaged plants appeared 22 days after inoculation. After 72 days of incubation, all inoculated plants were totally rotten. Control plants remained symptomless. The same pathogen was reisolated to fulfill Koch's postulates. For species identification, single-spore cultures were grown on PDA and carnation leaf agar in a climatic chamber at 23 ± 2°C with a 12-h darkness/near ultraviolet light regimen. The micromorphology and culture features, mainly conidial ontogeny, were consistent with descriptions of Fusarium oxysporum Schlechtend.:Fr. (1). The pathogen was able to penetrate undamaged tissues. Needle injuries accelerated infection. To our knowledge, this is the first report of Fusarium oxysporum on H. undatus in Gran Buenos Aires, Argentina. A culture of the pathogen was deposited at the fungal collection of PRHIDEB-CONICET (University of Buenos Aires) as BAFCult 3158. Reference: (1) P. E. Nelson et al. Fusarium species. An Illustrated Manual for Identification. The Pennsylvania State University Press, University Park, PA, 1983.