First Report of Leaf Spot Disease Caused by Gibbago trianthemae on Sesuvium portulacastrum in China

Sea purslane (Sesuvium portulacastrum L.) , a member of the family Aizoaceae, is a sprawling perennial herb with smooth, fleshy, glossy green leaves grown in most coastal areas, and is highly tolerant to salt, drought and heavy metals, which make it a pioneer plant species for dune fixation, seawater desalination and phytoremediation in coastal areas (Lokhande et al., 2013). In addition, it can also be used as vegetable, livestock feed, ornamental and medicinal plant. During the investigation of coastal plant germplasm resources of Fujian province of China in June 2021, a severe leaf spot disease of S. portulacastrum was observed. Infected leaves showed round, sunken, dark-brown necrotic lesions with maroon to brown margins, and severely diseased plants showed considerable leaf necrosis and premature defoliation. Typical infected leaves of S. portulacastrum were collected from Houhai (25°18'11.54''N, 119°14'31.98''E), located in Putian city, Fujian province. After isolation and purification, one fungal isolate SPHH was obtained. Colonies of SPHH on PDA medium reached 4.0 cm in diameter after 7 days at 26℃. On the medium prepared with S. portulacastrum leaves (fresh leaves 200 g/L, glucose 15 g/L, agar 20 g/L, pH 6.5) at 26℃ with cycled light (12 h light/12 h dark) for 7 days, conidiophores were slightly pigmented, solitary or loosely fasciculate, erect, rarely branched, septate, and slightly swollen at the apex with sympodial proliferation; mature conidia were yellow-brown, smooth, beakless, ellipsoid, muriform, possessing 3-4 transverse septa with slight constriction at the median septum and 1-4 longitudinal septa, with average dimension of 31-41 × 15-22 μm (n=100). Conidia germinated from apical, lateral and basal cells to form multiple germ tubes. Germ tubes could develop into secondary conidiophores to produce similar conidia. These morphological characteristics were similar to those of Gibbago trianthemae (Simmons, 1986). Until now, G. trianthemae has only been reported as the pathogen on horse purslane (Trianthema portulacastrum L.) in Cuba, the USA, Venezuela, India, Pakistan and Mexico (Simmons, 1986; Aneja and Kaushal, 1999; Akhtar et al., 2013; Rubén et al., 2020). The internal transcribed spacer (ITS) region of rDNA was amplified and sequenced using primers ITS1F and ITS4 (Gardes and Bruns, 1993; White et al., 1990). The resultant amplicon sequence (NCBI accession No. OK513442) showed 99.64% identity (559 bp/561 bp) with the ITS of G. trianthemae strain GT-VM (NCBI accession No. KJ825852, Kumar et al., 2016). A phylogenetic tree was constructed using the ITS sequences of strain SPHH, G. trianthemae strains and other strains from genera Dendryphiella, Paradendryphiella, Stemphylium and Pleospora. Strain SPHH was located within the same clade with G. trianthemae strains. Healthy S. portulacastrum plants were spray inoculated with the conidial suspension (105 conidia/mL) of SPHH and were kept at 26℃ with high humidity. Round, water-soaked lesions were observed on inoculated plant leaves 3-5 days after inoculation, similar to the symptoms found in the natural environment. As the lesions expanded, infected leaves gradually became chlorotic and abscised after 7-10 days. Plants inoculated with 0.02% Tween-20 solution which was used to prepare the conidial suspension of G. trianthemae remained symptomless. All pathogenicity tests were performed in two independent biological replicates, with five technical repeats for each, and similar results were obtained. Re-isolation of G. trianthemae from the diseased tissues of inoculated S. portulacastrum fulfilled Koch's postulates. Thus, G. trianthemae was the causal agent of the leaf spot disease of S. portulacastrum. This is the first report of G. trianthemae as a pathogen on S. portulacastrum. G. trianthemae has great potential for development as a mycoherbicide against T. portulacastrum (Mitchell, 1988; Kumar et al., 2016.), however, due to the ecological and certain economic importance of S. portulacastrum, management of this fungal disease on S. portulacastrum may be necessary.