First Report of Cercospora Leaf Spot Caused by Cercospora apii (= C. molucellae) on Bells-of-Ireland (Molucella laevis) in Mexico

Abstract

In late 2007, a new disease was found in commercial cutflower fields of bells-of-Ireland (Molucella laevis L.) in Texcoco, Mexico. Four plantings surveyed during this time had 100% incidence. A few spots on cutflowers make them unmarketable. Symptoms consisted of gray-green spots on leaves, calyxes, and stems, which turned brown with age. Spots were initially circular to oval, delimited by major leaf veins, and were visible on both adaxial and abaxial sides of the leaves. A Cercospora species was consistently associated with the spots. The fungus was isolated on V8 agar medium. Three single-spore cultures were obtained from isolation cultures. Cultures were incubated at 24°C under near-UV light for 7 days. Pathogenicity was confirmed by spraying a conidial suspension (1 × 104 condia/ml) on leaves of 16 potted M. laevis plants, incubating the plants in a dew chamber for 48 h, and maintaining them in a greenhouse (20 to 24°C). Identical symptoms to those observed in the field appeared on all inoculated plants after 2 weeks. No symptoms developed on control plants treated with autoclaved distilled water. The pathogenicity test was repeated twice with similar results. The fungus produced erumpent stromata, which were dark brown, spherical to irregular, 10 to 26 μm diameter, and giving rise to fascicles of five to nine divergent conidiophores, which were clear brown, paler near the subtruncate apex, straight to curved, not branched, rarely geniculate with two to four septa, and 57 × 3.4 μm. The conidia were formed singly, hyaline, acicular, base truncate, tip acute, straight to curved with 11 to 19 septa, and 172 × 3.5 μm. Fungal DNA from single-spore cultures was obtained with a commercial extraction kit (Qiagen, Hilden, Germany), amplified with ITS5 and ITS4 primers, and sequenced. The sequence, deposited at the National Center for Biotechnology Information Database (GenBank Accession No. EU564808), aligned almost perfectly (99% identity) to the bells-of-Ireland isolates from California (GenBank Accession Nos. AY156918 and AY156919) and New Zealand (Accession No. DQ233321). A 176-bp species-specific fragment was amplified with CercoCal-apii primers but not with CercoCal-beta or CercoCal-sp primers. These results, coupled with the morphological characteristics (1) and pathogenicity test, confirm the identity of the fungus as Cercospora apii sensu lato (including C. molucellae) (2,3,4). Although C. apii sensu lato has been reported on other hosts in Mexico (1,2), to our knowledge, this is the first report of this disease on M. laevis plants in this country. References: (1) C. Chupp. A Monograph of the Fungus Cercospora. Cornell University Press, Ithaca, NY, 1954. (2) P. W. Crous and U. Braun. CBS Biodiversity Series 1:1, 2003. (3) M. Groenewald et al. Phytopathology 95:951, 2005. (4) S. T Koike et al. Plant Dis. 87:203, 2003.