First Report of Trichoderma parareesei Causing Seedling Blight on Eggplant in China

Eggplant (Solanum melongena L.) is an economically important vegetable crop in subtropics and tropics. In March 2021, a serious disease on eggplant seedlings about 20 days after transplanting was found in Rong'an County (25°28' N; 109°53' E), Guangxi, China, with an incidence of diseased plants of 35%. The initial symptom was water-soaked spots on the leaves, followed by irregular black-brown spots that gradually expanded outward, causing leaf necrosis and defoliation. Even parts of eggplant seedlings died after lesions extended to the stem and the surface of the diseased tissues was covered with white to blue mold. Four diseased eggplants were randomly collected from different fields. Small pieces of the symptomatic tissues were surface sterilized and incubated on potato dextrose agar (PDA) at 28°C for 4 days. A total of 12 strains with similar morphological characteristics were isolated, and four representative strains (FW-01 to FW-04) were characterized. The colony was initially white, changing to yellow-green after 7 days. Phialides were lageniform or ampulliform, 2.9 to 9.75 μm × 1.36 to 4.3 μm (n=50). Conidia were green, ellipsoidal to oblong, smooth, 2.1 to 3.3 μm × 1.6 to 2.33 μm (n=50). Chlamydospores were not observed on PDA. These morphological characteristics are consistent with the description of the genus Trichoderma (Samuels et al. 2012). To confirm the identification, from mycelia of the four isolates and DNA was extracted using the Fungal Genomic DNA Extraction Kit (Bioer Technology [Hangzhou] Co., Ltd.). Three gene regions (ITS, tef1 and rpb2) were amplified (Sadfi-Zouaoui et al. 2009; Atanasova et al. 2010) and sequenced (GenBank Accessions: OL677389 to OL677392 for ITS, OL743178 to OL743181 for tef1 and OL743182 to OL743185 for rpb2). ITS sequences shared 100% identity with sequences of T. reesei (MW514156) and T. parareesei (HM466668), and tef1 and rpb2 sequences showed more than 99% similarity with sequences of T. parareesei (KM263190 and HM182962). The phylogenetic tree of the concatenated sequences showed that four isolates were clustered with T. parareesei. Therefore, the isolates were identified as T. parareesei. To satisfy Koch's postulates, the pathogenicity of four strains was tested on healthy eggplant seedlings planted in a sterile potting mix. Eggplants at four leaves stage were inoculated using conidial suspensions (with a concentration of 1 × 106 conidia/ml), with two leaves of each eggplant inoculated with each isolate and the test repeated three times. The control eggplants leaves were inoculated with sterile water. All plants were placed in a greenhouse at 22 ± 3°C and 85% relative humidity, with a photoperiod of 12 h. The water-soaked spots appeared 48 h after inoculation. All inoculated leaves showed symptoms 3 days post-inoculation. The diseased leaves became brittle and abcissed, while the control leaves remained symptomless. Only T. parareesei was successfully re-isolated from the lesions. Atanasova et al. (2010) found that T. parareesei inhibited the growth of Lepidium sativum seedlings under in vitro conditions (Atanasova et al. 2010). To our knowledge, this is the first report of T. parareesei causing eggplant seedling blight in China. The pathogen can cause substantial economic losses in eggplant production. Therefore, the identification of the pathogen is of great significance for the diagnosis and control of the disease. The results of this study deepen the understanding of the pathogenicity of Trichoderma.