<i>Paecilomyces</i> <i>formosus</i> Causing Leaf and Stem Blight on <i>Amaranthus hybridus</i> in Korea

Amaranthus hybridus (=A. patulus) is a highly invasive weed in Korea, causing agricultural losses by competing with crops (Kim et al., 2013). In September 2019, leaf and stem blight was observed on A. hybridus plants growing in agricultural fields in Wanju (GPS 35°50'42"N, 127°16'34"E), Korea, with disease incidence averaging 10-20%, but ranging from 5% to 40% across different sites. Affected plants exhibited brown, irregular lesions (2 to 3 mm) on leaves and stems, leading to plant death. Small pieces (5 mm²) from the lesions of 10 affected samples were surface sterilized, plated on PDA with streptomycin sulphate (30 mg/liter), and incubated at 25°C for 5 days. After 3 days, fungal colonies were 3-4 cm in diameter, white to yellow-brown with a smooth texture, and gradually darkened to brown after 2 weeks. Conidiophores were irregularly branched, with one to five phialides. Chlamydospores were spherical, yellowish-brown in color, and measured 5.42 to 6.44 × 3.61 to 4.47 µm (n = 50) with a smooth, thick wall. Conidia were ellipsoidal to cylindrical, with truncate ends, formed chains, and measured 3.63 to 4.5 × 2.24 to 2.78 µm (n = 50), with a length/width ratio of 1.46 to 1.78. Identical fungal cultures were isolated from 7 of the 10 symptomatic plants, including two representative isolates (KACC49613, KACC410985) deposited in the Korean Agricultural Culture Collection. Based on morphological characteristics, the fungal isolates were identified as Paecilomyces formosus Urquhart (Samson et al., 2009; Sabernasab et al., 2019). For molecular identification, genomic DNA was extracted from mycelia, and PCR was performed to amplify the ITS region and beta-tubulin (TUB) gene with ITS1, ITS4, BT1A, and BT2B (White et al., 1990; Glass and Donaldson, 1995), respectively. Obtained sequences were deposited in GenBank (ITS: MW412748.1, PV436946.1; TUB: PQ659292.1, PV446603.1). A BLASTn search revealed 99.81% similarity for ITS and 99.1% for TUB to P. formosus CBS 990.73B in GenBank. Phylogenetic analyses of ITS and TUB sequences showed Korean isolate clustering with P. formosus sequences in a well-supported group, confirming its identity. Pathogenicity test was performed twice with two isolates (KACC49613 and KACC410985) separately by spraying 15 ml of a conidial suspension (2.8 × 104 conidia /ml) on 30 healthy A. hybridus plants, with five control potted plants sprayed with sterile water. All plants were kept at 80% relative humidity and 23 °C, with a 12-h photoperiod. Symptoms resembling to those observed in the field appeared after four weeks, while control plants remained asymptomatic. The pathogen was successfully re-isolated from symptomatic tissues, confirming Koch's postulates. This study is the first report of blight caused by P. formosus on A. hybridus in Korea, although this pathogen has been recorded to infect a broad range of host plants (Farr et. al. 2001). Field surveys suggest that P. formosus has potential as a biocontrol agent for managing this invasive weed. However, further studies are needed to evaluate its host range and potential risks to edible amaranths in Korea.