Factors Influencing Rust (Melampsora apocyni) Intensity on Cultivated and Wild Apocynum venetum in Altay Prefecture, China.

Rust (Melampsora apocyni) on Apocynum venetum is the major constraint to the commercial development of this medicinal herb. To determine the factors influencing rust intensity (maximum disease index [DImax]), rust was investigated from 2011 to 2015 in both cultivated and wild A. venetum plants. Partial least squares path modeling (PLS-PM) was used to analyze the paths and extent of the factors related to pathogen, environment, and host that affect rust intensity. DImax exhibited considerable variations across years and study sites, with variations linked to various factors fostering disease development. PLS-PM explained 80.0 and 70.1% of variations in DImax in cultivated and wild plants, respectively. Precipitation was the key factor determining DImax in both cultivated and wild plants (path coefficient [PC] = 0.313 and 0.544, respectively). In addition, the topsoil water content in cultivated plants and the total vegetation coverage in wild plants were also critical determinants of DImax via their effects on the microclimatic factor (contribution coefficients [CC] = 0.681 and 0.989, respectively; PC = 0.831 and 0.231, respectively). In both cultivated and wild plants, host factors were mainly dominated by A. venetum density (CC = 0.989 and 0.894, respectively), and their effect on DImax via the microclimatic factor (PC = 0.841 and 0.862, respectively) exceeded that via the inoculum factor (PC = 0.705 and 0.130, respectively). However, the indirect effects led to DImax variation, while the dilution effect on host (CC = 0.154) from weed in wild plants led to the indirect effect size in wild plants of 0.200, which was lower than -0.699 in cultivated plants.

First Report of Septoria apocyni Causing Spot Blight on the Species of Apocynum venetum and Poacynum pictum in China.

The species of Apocynum venetum and Poacynum pictum grow widely from the middle to northwestern regions of China. During the summers of 2011 to 2013, a spot blight was found in wild and cultivated both species in Altay Prefecture of the Xinjiang Uygur Autonomous Region, China. The spot blight caused leaf yellowing and leaf drop, and serious damage to plant phloem. Lesions were circular to irregular, and the diameter of lesions on A. venetum and P. pictum was 1.84 to 6.84 × 1.23 to 4.24 mm and 2.05 to 7.09 × 1.46 to 5.65 mm, respectively. Pycnidia were 70 to 115 × 52 to 120 µm, scattered, spherical, buried, and had a brown hard shell with a prominent ostiole. Conidia were colorless, needle-shaped, or linear. The conidia base was obtuse, containing 3 to 5 indistinct septa, 46.3 to 110.3 × 2 to 2.5 µm. Fungal cultures were obtained by cutting 1-cm-long infected leaf pieces from the margins of the lesions following routine surface sterilizing procedures. The sections were placed on potato dextrose agar (PDA) in petri dishes and incubated at 23°C for 4 weeks (4). Hyphae had septa, the aerial and base mycelium was white and rufous, and the back of the colony was sunken and cracked after 2 weeks, but no spore was observed. To verify the identity, total DNA was extracted directly from fungal mycelium with a UNIQ-10 fungal genomic DNA extraction kit (Sangon Biotech, Shanghai, China) and PCR amplification performed with primers ITS1/ITS4 (3). A 512-bp PCR product was sequenced and contrasted with GenBank sequences using BLAST, which revealed 99% identity with Septoria sp. (GenBank Accession No. KC134322.1). To confirm pathogenicity, A. venetum and P. pictum were planted in pots and grown in a greenhouse. After 6 weeks of growth, plants were inoculated by spraying a mycelial suspension onto the foliage while control plants received a similar application of sterilized distilled water. Five pots (3 plants per pot) were used for each treatment. The pots were then placed on plates filled with tap water and covered with Plexiglas hoods in the greenhouse at 20 to 25°C. Lesions began to appear 6 to 7 days after inoculation with the mycelial suspension, whereas control plants remained healthy. The average disease incidence was 19.3%. The symptoms and morphology were similar to Septoria apocyni in Teterevnikova (2). It was determined that spot blight of A. venetum and P. pictum was caused by S. apocyni based on morphological comparison. There is one relevant literature report of spot blight on A. venetum and P. pictum in China, but without any details of the pathogenicity or morphology of the pathogen (1). We believe that this is the first report of S. apocyni occurring on the species of A. venetum and P. pictum in China. References: (1) W. Sun et al. Special Economic Animal and Plant 8:23, 2005. (2) D. N. Teterevnikova. Page 79 in: Septoria sp. Fungus of USSR. Armenian Academy of Sciences Publishing, Armenia, USSR, 1987. (3) G. J. M. Verkley et al. Mycologia 96:558, 2004. (4) W. Zhang et al. Plant Dis. 96:1374, 2012.