Causes and Control Methods of Bacterial Leaf Spot in Zinnia

Zinnia generally refers to Zinnia elegans, Zinnia elegans (scientific name: Zinnia elegans Jacq.): annual herb. Stem erect, 30-100 cm tall, covered with strigose or long hirsute hairs. Leaves broadly ovate or oblong-elliptic, rough on both surfaces, densely short-strigose beneath, with three basal veins. Capitulum solitary at branch tips, involucre broadly campanulate; involucral bracts in multiple layers, broadly ovate or ovate-elliptic. Ligulate flowers dark red, rose, violet or white, ligules obovate, apex 2-3-toothed or entire, short-hairy above, long-villous beneath. Tubular flowers yellow or orange, apex lobes ovate-lanceolate, densely yellowish-brown tomentose above. Achenes of female flowers obovate, achenes of tubular flowers obovoid-cuneate. Flowering period June-September, fruiting period July-October. Native to Mexico, it is a famous ornamental plant, with single-petaled, double-petaled, curled-leaf, wrinkled-leaf and various differently colored horticultural varieties. It is widely cultivated throughout China, sometimes becoming wild.

Symptoms: The disease can infect the leaves, stems and flowers of the plant, mainly the leaves. Infected leaves produce polygonal lesions, the centers of which are brown to black, with a yellow-green halo at the edges, 1-4mm in diameter. When the disease is severe, small brown spots also appear on the petals.

Pathogen: The lesion is caused by Xanthomonas campestris pv. zinniae (Hopkins et Dowson) Dye, a bacterium.

Disease pattern: The bacterium overwinters as bacterial cells in diseased tissues and seeds. In the spring of the following year, when environmental conditions are suitable, it spreads via rainwater, and seeds can also transmit the disease. The optimum growth temperature for this bacterium is 28°C; it cannot grow above 40°C or below 4°C. The host range of the bacterium is narrow.

Control: 1. Establish disease-free seed-production fields to obtain disease-free seeds. 2. If seeds come from healthy plants in a diseased field, seed disinfection should be carried out. Seeds can be soaked in sodium hypochlorite solution before sowing. 3. During the disease period, streptomycin can be sprayed.