Identification and Control of Black Spot, a Common Disease of Canna Lilies

Canna Lily

As residents' lifestyles change, potted flowers and garden-grown flowers are becoming increasingly popular. Even though canna lilies are relatively tall plants, people still find ways to dwarf them and grow them indoors. However, when canna lilies are improperly cared for and managed, they are prone to black spot, and infected plants greatly lose their ornamental value. So when canna lilies are infected with black spot, how should flower lovers take effective measures?

I. Symptoms

Black spot affects the leaves of the plant. In the early stage of the disease, small yellow spots appear on infected leaves. After the spots enlarge, they become round to oval and brown to dark brown. Some spots show faint concentric rings. The spots can merge with one another to form large lesions, causing the leaves to wither and die. In the late stage of the disease, a pale black mold-like substance, which is the conidia of the pathogen, appears on the lesions.

II. Disease Pattern

The pathogen overwinters as mycelium on fallen leaves. In the following spring, it produces conidia, which spread through airflow to cause infection and damage. The disease generally begins to occur from May to June, with the most severe occurrence in July and August. High temperature and high humidity favor the development of the disease.

III. Control

1. Reduce sources of infection. Remove diseased leaves promptly when found during the growing season, and clear away diseased leaves in winter for centralized destruction.

2. Strengthen cultivation management and do not plant too densely.

3. Chemical control: In the early stage of the disease, after pruning off diseased leaves, spray 65% zineb wettable powder at 500-800 times dilution, or 1:1:100 Bordeaux mixture, or 50% thiophanate wettable powder at 500-800 times dilution, once every 10-15 days, for 3-4 sprays.

Common Diseases of Canna Lilies and Their Solutions

Canna lilies have bright and eye-catching flower colors and a long flowering period, making them one of the excellent flower varieties for beautifying the environment. Some diseases also occur during canna lily care. Let us learn about them below:

Canna Lily Diseases:

Canna Lily Diseases: 1. Black Spot

Black spot is one of the common diseases of canna lilies. Under conventional cultivation, it occurs severely from June to August, producing large dead spots. On the underside of the leaves, the lesions bear a black mist-like substance. The disease first appears at the leaf margin or leaf tip and then spreads toward the center of the leaf.

Solution:Before planting the rhizomes, soak them in 50% thiophanate-methyl wettable powder at 800 times dilution for 5-10 minutes to prevent the disease. In the early stage of the disease, prune off diseased leaves and burn them, and spray 75% chlorothalonil wettable powder once every 7-10 days for 2-3 consecutive applications, which can inhibit the development of the disease.

Canna Lily Diseases: 2. Mosaic Disease

Mosaic disease is a common disease of canna lilies. When it occurs, mosaic patterns or yellow-green mottling appear on the plant's leaves, the petals become smaller and develop mixed colors, and when the plant is severely affected, the leaves become deformed and curl inward, with necrotic patches.

Solution:Spray with 40% omethoate at 2000 times dilution, or 50% malathion, 20% diazinon, and 70% menazon each at 1000 times dilution .

Canna Lily Diseases: 3. Bud Rot

Before canna lilies unfold their leaves and bloom, the bud rot bacteria invade through the stomata of young leaves and flower buds. When the leaves unfold, many small spot lesions appear on the leaves and gradually expand, connecting with one another along the leaf veins to form large patches; sometimes the lesions form stripes. The lesions initially appear grayish white and quickly turn black. Infected flower buds turn black and die before flowering. The lesions can extend downward along the petiole, causing young stems and buds to die. When old leaves are affected, the lesions expand slowly, are irregular in shape, yellow, and have a water-soaked margin.

Solution:Before planting, soak in streptomycin at 500-1000 times dilution for 30 minutes to control the bacteria. In the early stage of the disease, spray Bordeaux mixture (1:1:200), or 77% copper hydroxide wettable powder at 500 times dilution, 14% copper ammonia complex aqueous solution at 400 times dilution, etc. In severe cases, remove severely diseased plants and diseased remnants and burn them.

Canna Lily Diseases: 4. Leaf Roller Pest

Canna lilies are highly adaptable, but from May to August each year, attention should be paid to leaf roller pests to avoid damage to their young leaves and flower spikes.

Solution:Spray 50% dichlorvos at 800 times dilution or 50% fenitrothion emulsifiable concentrate at 1000 times dilution for control.

Canna lilies have large flowers and bright, eye-catching colors. Growing them in home courtyards and other places brings unexpected beautification effects. When growing canna lilies, attention should be paid to the occurrence and treatment of common canna lily diseases. We have summarized the solutions to common canna lily diseases for everyone, and hope it is helpful to you.

Canna Lily Pest and Disease Control

    Large dead spots are produced, and the lesions on the underside of the leaves bear a black mist-like substance. The disease first appears at the leaf margin or leaf tip and then spreads toward the center of the leaf. This should be canna lily black spot. Areas with relatively severe original disease include Guangdong, Fujian, Hainan, and Zhejiang. In recent years, it has also been seen in Beijing, Hubei, Tianjin, and other places. Under conventional cultivation, the disease occurs severely from June to August.
Control methods: (1) Strict quarantine should be carried out when introducing rhizomes or young seedlings to prevent further spread.
(2) Before planting the rhizomes, soak them in 50% thiophanate-methyl wettable powder at 800 times dilution for 5-10 minutes, take them out and dry them slightly; this can prevent the disease.
(3) In the early stage of the disease, prune off diseased leaves and burn them, and spray 75% chlorothalonil wettable powder once every 7-10 days for 2-3 consecutive applications, which can inhibit the development of the disease.