Precautions for Soilless Cultivation of Flowers and Common Pests and Diseases: Black Spot

Symptoms: Leaves, petioles, young shoots and pedicels can all be affected, but the main damage is to the leaves.

There are two types of symptoms: one is that in the early stage of the disease, small reddish-brown to purplish-brown spots appear on the leaf surface, gradually expanding into round or irregular dark black lesions. There is often a yellow halo around the lesion, the edge is radial, and the lesion diameter is about 3-15 mm. In the later stage, small black particles are scattered on the lesions, namely the pathogen's acervuli. In severe cases, the lower leaves of the plant turn yellow and withered, leaves fall early, and individual branches die, such as rose black spot.

The other is that brown to dark brown nearly round or irregular concentric ring spots appear on the leaves, on which black mold-like substances grow, namely the pathogen's conidia. In severe cases, the leaves fall early, affecting growth, such as black spot of elm-leaved spirea.

Disease development pattern: Black spot is the main disease on roses, occurring commonly and causing severe damage. The pathogen overwinters as mycelium or acervuli in dead branches or soil. In mid-to-late May of the following year, infection and disease onset begin, and July to September is the peak period of the disease. Conidia are spread by wind, rain or insects, expanding and causing secondary infection.

Rainwater is the main condition for the epidemic of the disease. In years with early and abundant rainfall, the disease occurs earlier and more severely. Low-lying waterlogged areas, poor ventilation, insufficient light, and improper fertilizer and water management are conducive to the disease.

Control methods

① Select high-quality disease-resistant varieties.

② After autumn, remove dead branches and fallen leaves, and destroy them.

③ Strengthen cultivation management, pay attention to shaping and pruning, and ensure ventilation and light transmission.

④ When new leaves unfold, spray 50% carbendazim wettable powder at 500-1000 times dilution, or 75% chlorothalonil at 500 times dilution, or 80% zineb at 500 times dilution, once every 7-10 days, for 3-4 consecutive sprays.