First Report of <i>Coleosporium plectranthi</i> Causing Rust Disease on Perilla in Northeast China

Perilla frutescens var. frutescens is an important economic crop, and is widely cultivated in Northeast China. In August 2019, rust disease on this crop plants with mild symptoms was observed at Pingfang Forest Farm and United Forest Farm in Jidong County, Jixi City, Heilongjiang Province. By 2023, the rust disease had erupted on a large scale, causing significant economic losses with an estimated 40% reduction in yield. The affected plants showd pale yellow, blister-like pustules on the lowerside (or backside,or abaxial surface) of the leaves, and the pustules broken subsequently and released orange-yellow urediniospores. The urediniospores produced in a single or multiple chains, circular to elliptical in shape, which were measured 26.28 (20.11–29.20) µm × 18.42 (15.68–22.50) µm in size (n=30), with a reticulated surface ornamentation and warty projections, and contained yellow contents. As the disease progressed, telial columns appeared around the uredinial sori, and surrounded by a transparent sheath. The teliospores were mostly cylindrical or long-clavate, with in a single layer parallel to form a palisade-like structure, being measured 64.64 (54.65–71.72) µm × 12.32 (10.55–14.22) µm (n=30). The spore walls was transparent, with orange-yellow contents. These characteristics of this rust were consistent with those of Coleosporium plectranthi as previously reportd et al (Yun et al., 2007). For molecular identification, total DNA was extracted from fungal colonies collected from three groups of infected leaves. For each DNA sample, the partial ITS region was amplified using primers ITS1/ITS4(Tian et al., 2004; Yun et al., 2007). BLASTn analysis of the sequenced amplicons showed a match with Coleosporium plectranthi sequences available in GenBank, and the sequences were submitted to GenBank. Accession numbers for these genes from strains JX2 are as follows: JX2 (PX578096). A phylogenetic tree was constructed using the neighbor-joining method in MEGA 5.0. Based on the ITS rDNA phylogenetic tree, the sequences obtained from the specimens clustered within a strongly supported clade of Coleosporium plectranthi. Based on both morphological and molecular analyses, the pathogen was identified as Coleosporium plectranthi. We used perilla seeds from the affected region as plant material. Pathogenicity was confirmed by inoculating these 45-day-old plants with fresh urediniospores brushed onto the abaxial surfaces of leaves that had been pre-humidified for 24 hours. Specifically, a total of 15 plants were inoculated, while 5 plants treated with sterile water served as controls. The inoculated leaves were immediately covered with black plastic bags for 7 days to create localized darkness, while all plants were maintained under a standard 12-hour photoperiod throughout the experiment. The plants were kept in a growth chamber at 20±1°C and >95% relative humidity. After the bagging period, the bags were removed, and the plants continued to be incubated under the same conditions for an additional 7 days. Following this incubation period, typical light yellow blister-like protrusions and orange-yellow urediniospores developed on the abaxial leaf surfaces of all inoculated plants. Microscopic examination confirmed that the spores were morphologically identical to the urediniospores observed on the originally infected plants. All control plants remained completely asymptomatic.The fungus reisolated from the inoculated leaves was morphologically identical to that observed on the original diseased leaves. Thus, Koch's postulates were fulfilled. To the best of our knowledge, this disease has been previously reported in the middle and lower reaches of the Yangtze River in China and in South Korea. This is the first report of Coleosporium plectranthi causing rust disease on perilla in Northeast China. The infection by this pathogen severely impacts perilla seed setting, leading to substantial economic losses for farmers. Field observations indicated a strong association between disease occurrence and overly dense planting coupled with poor field drainage. These findings, which highlight specific cultural practices for intervention, will assist the horticulture industry in developing targeted management strategies for this disease.