A comprehensive guide to rhododendron leaf spot disease

Round lesions, or irregular polygonal lesions, dark brown, 1mm-5mm in diameter. In the later stage, the central tissue of the lesions turns grayish white. When the disease is severe, the lesions merge with each other, causing the leaves to turn yellow and drop early. Under moist environmental conditions, many small brown mold spots grow on the underside of the leaf spots, namely the conidia and conidiophores of the pathogenic fungus.

Pathogen and current disease situation: it is Cercospora rhododendri Fer., Deuteromycotina, Hyphomycetes, Moniliales, Cercospora. The pathogen overwinters on plant residues, and the following year produces conidia that serve as the primary infection source. The conidia are spread by wind and rain and invade through wounds. In Jiangxi, the disease begins to occur in mid-May, and August is the peak period of the disease. In the Guangzhou area, the peak period of the disease is from April to July, and rhododendrons cultivated under greenhouse conditions can develop the disease year-round. Abundant rain, heavy fog, and heavy dew favor the disease, because conidia can only germinate in water droplets. Poor ventilation and light transmittance, and poor plant growth, can aggravate the occurrence of the disease. Control methods:

1. In autumn, thoroughly remove fallen leaves and dispose of them, and promptly remove diseased leaves during the growing season.

2. The planting density of potted flowers should be appropriate, so as to allow ventilation and light transmittance and reduce leaf surface humidity. In summer, place potted flowers in a shade shed outdoors to reduce wounds caused by sunscald and mechanical damage.

3. Immediately after flowering, spray 65% zineb wettable powder at 500-600 times dilution, or 50% carbendazim wettable powder at 500-800 times dilution, or 70% thiophanate-methyl wettable powder at 1000 times dilution. Spray once every 10-14 days, for 2-3 consecutive sprays.