Identification and Control of Powdery Mildew and Brown Spot on Impatiens

Powdery mildew

Symptoms: It mainly damages the leaves, but flowers and stems can also be affected. On impatiens leaves, round white mold spots first appear, generally starting in May; after June, the spots expand or several spots merge into patches, each layer thickening to form a white powdery mold layer. The mold layer can form on both the front and back of the leaves. By early autumn, small yellow round dots form within the white powdery layer, then gradually darken and finally turn dark brown. In severe cases, the leaves wither and fall early.

Pathogen and occurrence pattern: The pathogen belongs to the Ascomycotina, and powdery mildew is caused by Sphaerotheca. The ascocarps are clustered or scattered, brown, spherical or nearly spherical, with relatively large, distinct, irregularly shaped wall cells. There is one ascus, elliptical or oval; immature asci often have a short stalk. Ascospores are elliptical, with a few round. The pathogen overwinters as cleistothecia on diseased plant debris. The following summer, ascospores are released for primary infection and spread by wind and rain. After the plant becomes diseased, conidia form on the diseased parts; the conidia spread by wind and rain for secondary infection. In autumn, ascocarps form and enter overwintering sites. The disease is severe in hot, humid weather with poor ventilation and light penetration. It can occur from May and June to September and October, with more severe occurrence in September and October.

Control:

① In the early stage of the disease, spray 25% triadimefon wettable powder at 2000 times dilution, or 20% triadimefon emulsifiable concentrate at 4000 times dilution, or 70% thiophanate-methyl wettable powder at 1500 times dilution, once every 7 to 10 days, for 3 to 4 consecutive sprays, which can achieve good control results. Effective fungicides used abroad for controlling powdery mildew, such as pyracarbolid and anilazine, can also be used.

② Select disease-resistant varieties, apply more phosphorus and potassium fertilizer, and avoid crop rotation with or proximity to other ornamental flowers affected by this pathogen, so as to prevent mutual infection and worsening of the disease. At the end of autumn, thoroughly collect and dispose of diseased plants and diseased debris in a centralized manner to eliminate the source of infection.

Brown spot

Symptoms: Leaf spots are round or nearly round, 1-5 mm in diameter, initially brown, later grayish brown in the center with a dark green edge, with inconspicuous concentric rings. When moist, dense olive-brown mold-like growth covers both sides of the spots.

Pathogen and disease characteristics: The pathogen belongs to the Basidiomycotina, genus Fusicladium. The stroma is not well developed. Conidiophores are clustered, pale green, rarely septate or branched, straight or wavy. The fungus overwinters as mycelium on diseased fallen leaves. The following year, it produces conidia, which spread through airflow and infect the plant. The disease generally occurs in August and September. If there is abundant rain and moisture at this time, the disease is more severe.

Control:

① Before the disease occurs, spray with 0.5% Bordeaux mixture.

② After the disease occurs, spray 50% carbendazim wettable powder at 1000 times dilution, or 50% amobam at 1000 times dilution, or 70% chlorothalonil wettable powder at 800 times dilution.

③ Remove diseased debris and dispose of it in a centralized manner.