First Report of Phytophthora infestans Causing Late Blight on Pepino (Solanum muricatum) in China

Solanum muricatum, known as pepino or melon pear, is a species of evergreen shrub grown for its sweet edible fruits, which was introduced in Yunnan about 20 years ago. Since 2019 to now, serious blight disease was observed on foliages, haulms and fruits of pepino plants in Shilin (25°N, 103°E), which is the biggest pepino production area in China. The symptomatic blighted plants showed water-soaked and brown foliar lesions, haulm brown necrosis, black-brown and rotting fruits, and overall plant decline. The samples with typical disease symptoms were collected for pathogen isolation. After surface sterilization, disease samples were cut into small pieces and placed on rye sucrose agar medium amended with both 25 mg/liter rifampin and 50 mg/liter ampicillin, then incubated in the dark at 25℃ for 3 to 5 days. White fluffy colonies of mycelia that grew from the edge of diseased tissues were further purified and subcultured on rye agar plates. All purified isolates were identified as Phytophthora spp. based on morphological characteristics (Fry 2008). Sporangiophores were nodular and sympodial branches with swellings at the points where sporangia were attached. Sporangia that were hyaline and the average size were 22×40 μm were formed on the tip of sporangiophores and appeared as subspherical, ovoid, ellipsoid or lemon shaped with half-papillate on the spire. Mature sporangia were easilly detached from sporangiophores. For pathogenicity tests, healthy leaves, haulms and fruits of pepino were inoculated with 1×104 cfu/ml zoospore suspension of Phytophthora islolate (RSG2101), and controls were treated with sterile distilled water, respectively. After 5 to 7 days postinoculation, all Phytophthora-inoculated leaves and haulms showed water-soaked and brown lesions with white mold layer, fruits showed dark-brown firm lesions which got expanded and rotted the entire fruit. The symptoms were same as those occurred in natural fields. In contrast, no disease symptoms appeared in the control tissues. Phytophthora isolates could be reisolated and showed same morphological characteristics from the infected tissues of leaves, haulms, and fruits, sufficing Koch's postulates. Two common molecular targets of the internal transcribed spacer (ITS) region of ribosomal DNA and partial cytochrome c oxidase subunit II (CoxII) of the Phytophthora isolate (RSG2101) were amplified and sequenced with primers ITS1/ITS4 and FM75F/FM78R (Kroon et al. 2004). The ITS and CoxII sequence data were deposited in GenBank under accession numbers OM671258 and OM687527, respectively. Blastn analysis of both ITS and CoxII sequences showed 100% identity with isolates of P. infestans (MG865512, MG845685, AY770731 and DQ365743, respectively). Phylogenetic analysis also indicated that RSG2101 isolate and known P. infestans isolates localized in the same evolutionary branch based on sequences of ITS and CoxII, respectively. Based on these results, the pathogen was identified as P. infestans. It is known that P. infestans infection of pepino occurrs in Latin America and then it was recorded in other parts of the world such as New Zeeland and India (Hill, 1982; Abad and Abad,1997; Mohan et al. 2000).To our knowledge, this is the first report of late blight on pepino caused by P. infestans in China, which will be helpful to develop efficient blight management strategies on pepino.