First Report of Pythium myriotylum Causing Damping-Off of Amaranthus tricolor in Florida

Abstract

Amaranthus tricolor ‘Garnet Red’ is an edible ornamental often sold for its striking color as well as its culinary uses. There are several cultivated species of amaranth that vary in shape, size, and color. This particular cultivar (Garnet Red) is praised for its high nutritional value. It can be harvested at any age, making it commonly used as either microgreens, sprouts, salad leaves, or for seed and grain. Amaranth is easy to grow and reasonable yields can be produced even in poor soils (Sealy et al. 1990), but damping-off of seedlings by Pythium sp. is a problem for producers and breeders (National Research Council 1984). In November 2011, amaranth seedlings cv. Garnet Red from a local nursery were submitted to the Florida Extension Plant Diagnostic Clinic in Homestead, FL. Symptoms began as a rot and that ultimately resulted in the loss of the entire planting of seedlings, causing post-emergent damping-off in both the greenhouse and shadehouse locations where the crop was grown. A Pythium sp. was confirmed via microscopic observations from cultures on PARP medium showing aplerotic oospores, about 20 to 27 μm in diameter and walls up to 2 μm thick (n = 15) (van der Plaats-Niterink 1981). The region of rDNA, the internal transcribed spacer (ITS), was amplified with primers ITS1 and ITS4 and the PCR product was sequenced. The resulting sequence was deposited in GenBank (accession no. KF008434). A 99% nucleotide identity was found with a BLAST search in GenBank with an isolate of P. myriotylum (HQ237488) associated with bud rot of oil palm. MEGA 6.06 software was used for the phylogenetic analysis by the maximum likelihood method (Tamura-Nei model). Our isolate grouped with other P. myriotylum isolates with high support (100% bootstrap values, 1,000 replicates). To confirm pathogenicity, amaranth ‘Garnet Red’ seeds were sown in 4-inch pots with Promix BX Mycorrhizae dry mix under greenhouse conditions where temperature ranged from 25 to 32°C. Seeds were inoculated 24 h after sowing by drenching with a 5 ml sporangial suspension (1 × 10⁶ sporangia/ml) of our P. myriotylum isolate. Control plants were drenched with an equal volume of sterile water. Five plants per treatment were used and the experiment was repeated. After 72 h, seedlings had collapsed. P. myriotylum was reisolated from symptomatic tissues and was confirmed through morphological and molecular analyses. No symptoms developed on the control plants. P. myriotylum is an aggressive pathogen causing important economic losses with a very large host distribution worldwide in warm regions. In south Florida, it had previously been reported causing aerial blight of tomato, and more recently, basal rot on Echeveria (Roberts et al. 1999; Suarez et al. 2016). To our knowledge, this is the first report of P. myriotylum causing damping-off of an Amaranthus species in Florida.