Fruit rot causing by Athelia rolfsii (Sclerotium rolfsii) on jackfruit (Artocarpus heterophyllus) in China.

Artocarpus heterophyllus, known as jackfruit, was a tropical fruit and cultivated extensively as nutritional and medicinal properties in southern China in recent year. During July 2022, fruit rot was observed on the fruits at the bottom of jackfruit trees in an orchard in Zhanjiang, Guangdong (N21°9' 27" E110°17' 54") 3-4 days after typhoon. The incidence rate of fruit was about 0.3%. The initial symptom was white mycelia appearing on the surface of fruits. Mycelia with rhizomorphs spread rapidly over the fruits, formed white, often fan-shaped mats with the rapeseed size sclerotia. The infected fruits were water-soaked, quickly became rotten, and fell off. Sclerotia from disease fruits were incubated on PDA with 50 mg/L ampicillin at 25-28℃ in the dark for 2 days. Hyphae tips were transferred to get the purified isolates. Colonies with a radial growth rate of 23.2 mm/day had abundant aerial mycelia and profuse sclerotia on PDA. Hyphae of the isolates were transparent, branched, with clamp connections at septa, usually 2.9-8.3 µm (Ave. 5.8 µm) (n>30) wide. Aerial mycelia were whitish-cottony, with many narrow rhizomorphs. Spherical sclerotia developed at about 10 days after incubation, and gradually changed from white to tan-to-dark brown, and mature sclerotia were about 1.7 mm in size. The morphological characteristics was similar to those of Sclerotium rolfsii (teleomorph: Athelia rolfsii). To accurately identify the fungus, the internal transcribed spacer gene (ITS) and large subunit rRNA gene (LSU) of isolate CASS-BLM-1 were PCR amplified with primer pairs ITS1/ITS4 (White et al 1990) and V9G/LR5 (Klaubauf et al 2014). The amplicons were sequenced and deposited in GenBank with accession number OP535473 (ITS) and OP535474 (LSU). BLASTn results showed that the nucleotide sequences of ITS and LSU had high identity with corresponding sequences of A. rolfsii isolates CBS 191.62 (ITS: MH858139, 472/474(99.58%); LSU: MH869724, 882/885(99.66%)) (Vu et al 2019). Phylogenetic analysis based on ITS sequence data was obtained according to maximum likelihood method using MEGA analysis software, CASS-BLM1 was grouped in A. rolfsii clade with 100% bootstrap support value. Based on morphology and DNA sequences, the fungus was identified as A. rolfsii (anamorph: S. rolfsii). To fulfil Koch's postulates, healthy fruits on the tree and detached fruits were inoculated with 7-day-old sclerotia of isolate CASS-BLM1. Five unwound sites and five wound sites with a sterile needle were tested on each fruit and a sclerotium was put at each site. Fruits not inoculated with sclerotia were used as control the test was repeated three times. All fruit were enclosed in transparent plastic bags with sterile absorbent cotton moistened with sterile distilled water. The indoor and outdoor temperatures ranged from 25 to 30 ℃. Three days later, white mycelia were observed on all inoculation sites, and 5 days later, the inoculated fruits began to rot, while control fruits remained healthy. The same fungus with identical morphology and DNA sequences was re-isolated from the inoculated sites. Previously, A. rolfsii was reported to cause fruit rot disease on jackfruit in Bangladesh (Elahi et al 2021), this is the first report of A. rolfsii causing fruit rot on jackfruit in China. A. rolfsii is suitable for high temperature and humidity environment (Punja 1985), this report will help farmers to diagnose this disease, especially to strengthen the disease prevention during the typhoon season.

Erysiphe elevata causing powdery mildew on Eucalyptus urophylla × E. camaldulensis in China.

Eucalyptus urophylla × E. camaldulensis, named Chiwei eucalypt, is a hybrid species widely used in China. Many of its clones are cultivated for afforestation due to cold tolerance, high yield, high strength and disease resistance. Clone LH1 is planted extensively for its high stability and machinability in South China. In December 2021, severe powdery mildew signs were observed on clone LH1 in Zhanjiang, Guangdong (N28°8'29"; E110°17'5"). Whitish powder principally appeared on both abaxial and adaxial leaf surfaces. All plants were infected within about a week and above 90% leaves were diseased, which resulted in abnormal growth and shrinkage of leaves. Hyphae with single, lobed appressoria were hyaline, septate, branched, 3.3-6.8 µm (ave. 4.9 µm, n>50) wide. Conidiophores with a straight to flexuous foot-cell (14.7-46.1×5.4-9.7 µm, ave. 25.8×7.9 µm, n>30) were erect, hyaline, 2-septate, unbranched, 35.4-81.8 × 5.7-10.7 (ave. 56.7×8.7 µm, n>50). Conidia were solitary, hyaline, cylindrical to elliptical, 27.7-46.6 ×11.2-19.0 (ave. 35.7×16.6 µm, n>50). Chamothecia were not found on infected trees. The further identification was confirmed by partial sequences of internal transcribed spacer (ITS), large submit rRNA gene (LSU), Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), glutamine synthetase (GS), and RNA polymerase II second largest subunit (RPB2) gene. A very small amount of mycelia and spores from voucher specimens CCAS-ASBF-1 and CCAS-ASBF-2 were deposited in the herbarium of Guangdong Ocean University. Specimens were PCR amplified and sequenced with primer pairs ITS1/ITS4 (White et al 1990), LROR/LR7 (Moncalvo et al 1995), PMGAPDH1/PMGAPDH3R, GSPM2/GSPM3R and PmRpb2_4/ PMRpb2_6R (Bradshaw, et al. 2022), respectively. BLASTn results showed that ITS (OP270019 and OQ380937), LSU (OP270018 and OQ380938), GAPDH, GS and RPB2 (OQ414445- OQ414450) were above 99% identical with those of E. elevata on Catalpa bignonioides (ITS: AY587013) (Cook et al 2004), Plumeria rubra (ITS: MH985631) (Yeh et al 2019), Cerbera manghas (ITS: MZ379159; LSU: MZ379160) (Mukhtar et al 2022), and Eucalyptus camaldulensis (LSU: LC177375-6) (Meebon et al. 2017), and above 99% identical with those of Erysiphe vaccinii FH00941201 on Vaccinium corymbosum (ITS: ON073869; RPB2: ON119159; GS: ON075687) and FH00112205 on V. vacillans (ITS: ON073870; GAPDH: ON075646) (Bradshaw et al 2022). This is the first sequence data for non rDNA of E. elevata. In an ITS tree phylogenetic analysis with Maximum likelihood (ML) method showed the fungus clustered in a highly supported clade with E. elevata and E. vaccinii. In a multi-locus tree, E. elevata grouped in a sister position to E. vaccinii FH00941201. Thus, the pathogen was identified as E. elevata based on morphology, DNA BLASTn and phylogenetic analysis (Braun and Cook 2012). Pathogenicity tests were conducted on healthy leaves of 1-year-old potted plants. Ten leaves were cleaned with sterile water, inoculated by gently dusting conidia from single lesion on the naturally infected leaves, and then covered with plastic bags containing wet absorbent cotton. Non-inoculated leaves served as controls. Symptoms developed on all inoculated leaves 3 to 5 days after inoculation, and the fungus was identical to the original fungus on the infected leaves, whereas control plants remained symptomless. This is the first report of powdery mildew caused by E. elevata on Eucalyptus sp. from China. This finding is helpful for land managers to diagnose and control the disease.