First Report of Phytophthora hydropathica Causing Wilting and Shoot Blight on Bixa orellana in China

Annatto (Bixa orellana Linn.) is a shrub or small tree that is best known as the source of achiote, an organic, red-orange extract from the seeds that is commonly used as a condiment, food colorant, and cosmetics dye. The extract is also used for its medicinal properties. In summer 2017, a severe wilting of leaves, shoot blight, and root rot on all four growing annatto trees was observed in a private garden in Foshan, Guangdong Province. About 30 to 40% of the branches were infected on each tree. Soil and root samples were collected and baited with annatto leaf disks (Ferguson and Jeffers 1999). Baits were blotted dry on paper towels and placed onto PARP-V8 medium (Hong et al. 2010). Three single-zoospore isolates (YZ-1, YZ-2, and YZ-3) were randomly selected and were subcultured on V8 agar in the dark at 25°C. All isolates had a petaloid colony pattern of growth. All isolates grew well at the examined temperature ranging from 25 to 35°C, and the optimum temperature was 30°C with growth rates ranging from 7.0 to 8.3 mm per day. The sporangia were almost spherical, ovoid or obpyriform, and 45.4 to 59.5 × 36.8 to 42.4 µm (average 53.2 × 37.6 µm, n = 30) in diameter. Chlamydospores with thin walls were terminal and approximately 39.5 µm in diameter. All tested isolates were the A1 mating type. These morphological characteristics were consistent with those described for Phytophthora hydropathica (Hong et al. 2010). Morphological identification was confirmed by sequencing the internal transcribed spacer region (ITS) with the primers ITS1/ITS4 (Vitale et al. 2014), as well as both the translation elongation factor 1a (EF-1α) with the primers ELONGF1/ELONGR1 and NADH dehydrogenase subunit 1 (NAD1) with the primers NADHF1/NADHR1 (Kroon et al. 2004). BLAST analysis revealed that consensus sequences (GenBank accessions MH321577 for ITS, MH460822 for EF-1α, and MH460823 for NAD1) shared 98 to 99% identity with sequences of P. hydropathica (KF444067, KX252303, and GQ260065, respectively). Pathogenicity assessment was performed by inoculating one representative isolate (YZ-1) of P. hydropathica on 10 surface-sterilized leaves that were detached from a 4-year-old annatto plant. Mycelial plugs of a 4-day-old YZ-1 culture were placed at a distance of 3 cm on the upper part of the same leaf, one plug on intact epidermis and the other on epidermis previously wounded with a sterile needle. Sterile V8 plugs were used as controls. Inoculated leaves were kept at 28 ± 2°C with high (∼95%) humidity and in the dark. After 3 days, all the inoculated points showed blight and rot symptoms consistent with those observed in field infection, whereas controls remained disease-free. An oomycete was reisolated from symptomatic leaves and exhibited identical morphological characteristics described for P. hydropathica, fulfilling Koch’s postulates. P. hydropathica can also attack several other plants, such as azaleas, mountain laurel, cucumber (Hong et al. 2010), alder (Pintos et al. 2016), and Laurustinus (Vitale et al. 2014). To the best of our knowledge, this is the first report of P. hydropathica causing disease on B. orellana in China. Our findings draw attention to developing effective management strategies before it causes widespread damage to annatto in China.