Symptoms and Control Methods of Anthracnose in Impatiens
[Distribution] Shanghai, Hangzhou.
[Symptoms] Both branches and leaves can be affected, mainly on the branches and leaves. In the early stage of the disease, small withered-yellow or brown spots appear on the leaf surface, then gradually develop into circular to nearly circular spots, light brown to brown, with a darker edge and indistinct concentric rings, and small black dots scattered on the lesions; as the disease progresses, adjacent lesions join to form irregular large spots, causing the leaves to shrivel, wilt, and drop off.
In humid weather, pink gelatinous material oozes from the small black dots; it can occur from June to September, and sometimes causes the branches and stems to turn black and rot.
[Pathogen]
The pathogen is a fungus, Gloeosporium impatientis and Vermicularia sp.; conidia: the former is oval, long oval, colorless, containing 1 to 2 oil droplets; the latter is crescent-shaped, single-celled, larger, generally light grayish-brown; antennae serrate, each segment with tufts of hair, gray to brown; forewings grayish-brown, the outer margin line composed of a row of crescent-shaped spots, the discal spot white, surrounded by a dark brown ring.
[Occurrence pattern] 4 to 5 generations occur per year; it overwinters as mature larvae pupating in the soil. In early summer of the following year, it spreads and infects via wind and rain. In April of the following spring, overwintering-generation adults appear; the adults hide in dark places or among branches and leaves during the day, and come out at night to be active and lay eggs; the eggs are laid singly on the stems, leaf undersides, and other parts of the plant; young larvae spin silk and drift with the wind, the larvae are not very active but are mimetic, often resting on the plant stems, resembling tender shoots, and are therefore not easily found. In summer, 1 generation is completed in 40 days. The egg stage is 5 days, the larval stage is 18-21 days, and the pupal stage is 9-10 days. After adult emergence, they pupate 1-3 days later. Generally, the leaves on the lower part of the plant are severely affected. The disease is most severe in hot, rainy seasons.
[Control methods]
(1) Manual control: upon finding signs of damage, carefully search for and kill them, without being misled by their mimicry.
(2) Horticultural control: in conjunction with turning the soil and preparing the land, eliminate overwintering pupae. Strengthen fertilizer and water management to enhance the plant's disease resistance. In the early stage of the disease, promptly remove diseased leaves and diseased plants. In late autumn, collect diseased leaves and diseased plants and destroy them; for potted impatiens, avoid using old soil to reduce sources of infection.
(3) Chemical control: in the early stage of the disease, alternately spray with 50% amobam 500-600 times dilution, or 50% anthracnose Fumei magnesium wettable powder 500 times dilution, or 75% chlorothalonil wettable powder 600 times dilution. During the larval damage period, spray with 50% phoxim emulsifiable concentrate 1000 times dilution, or 20% fenvalerate emulsifiable concentrate 1500-2000 times dilution.