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First Report of Leaf Spot Caused by Alternaria alternata on Marigold (Tagetes erecta) in Beijing, China

Marigold (Tagetes erecta) is an important commercial crop and 200 ha are planted every year in the Beijing district of China. A leaf spot disease of T. erecta was observed during 2012 and 2013 in the Beijing district. The disease was widespread, with 60 to 75% of the fields affected. Leaves of the affected plants had small, brown, necrotic spots on most of the foliage. Yield losses of flowers of up to 20 to 30% were reported. The spots gradually enlarged, becoming irregular in shape, or remained circular, and with concentric rings or zones. In the later stages of infection, the spots coalesced, and the leaves withered, dried, and fell from the plants (4). A fungus was consistently isolated on potato dextrose agar (PDA) from the infected leaves of T. erecta. After 6 days of incubation at 26°C and a 12-h photoperiod, the fungus produced colonies that were flat, with a rough upper surface (2). The conidiophores were short. Conidia varied from 18 × 6 to 47 × 15 μm and were medium to dark brown or olive-brown in color, short beaked, borne in long chains, oval and bean shaped, with 1 to 5 transverse septa and 0 to 2 longitudinal septa. The rDNA of the internal transcribed spacer regions 1 and 2 and the 5.8S gene in seven isolates were amplified using primers ITS1 (5′-TCCGTAGGTGAACCTGCGG-3′) and ITS4 (5′-TCCTCCGCTTATTGATATGC-3′). The nucleotide sequence was the same as isolate No. 7, which was deposited in GenBank (Accession No. KF307207). A BLAST search showed 97% identity with the strain Alternaria alternata GNU-F10 (KC752593). Seven isolates were also confirmed as A. alternata by PCR identification performed by specific primers (C_for/C_rev) of A. alternata (1). Seven isolates were grown on PDA for 2 weeks and the conidia harvested. A 5-μl drop of spore suspension (1 × 10⁵ spores/ml) was placed on each leaflet of 140 detached, surface-sterilized T. erecta leaves. Twenty leaves were inoculated with sterile distilled water as a control. The leaves were incubated in a growth chamber at 80 to 90% relative humidity, 50 to 60 klx/m² light intensity, and a 12-h photoperiod. After 6 days, leaf spots similar to the original developed at inoculation sites for all isolates and A. alternata was consistently re-isolated. The control leaves remained symptomless. The pathogenicity test was performed three times. Leaf spot of T. erecta caused by Alternaria spp. is well known in Asian countries such as Japan (3). To our knowledge, this is the first report of A. alternata on T. erecta in the Beijing district of China.

References: (1) T. Gat. Plant Dis. 96:1513, 2012. (2) E. Mirkova. J. Phytopathol. 151:323, 2003. (3) K. Tomioka. J. Gen. Plant Pathol. 66:294, 2000. (4) T. Y. Zhang. Page 284 in: Flora Fungorum Sinicorum, Volume 16: Alternaria. Science Press, Beijing, 2003.