Kenaf anthracnose control methods and key technical points

Symptoms The disease can affect plants at the seedling and mature stages. When young buds are infected, the hypocotyl becomes yellowish-brown and rotten; some soften, constrict, or damp off and die soon after emergence. When cotyledons are infected, small purplish-red spots first appear, then become nearly round to irregular yellowish-brown or grayish-white lesions surrounded by purplish-red. Symptoms on true leaves are similar to those on cotyledons, but the disease can spread along the veins, causing local distortion of the leaves. Infection on the stem produces yellowish-brown, nearly oval, slightly sunken lesions surrounded by reddish-brown or purplish-red. When the terminal bud is infected, it turns black and rots, and the surrounding tissue becomes slightly swollen or produces horizontal branch-like "fork heads," causing the plant to stop growing. When buds and flowers are infected, they often rot and drop off, and capsules become deformed. Under high humidity, the diseased parts produce orange-red sticky masses of conidia.

Pathogen Colletotrichum hibisci Pollacci, called kenaf anthracnose fungus, a fungus in the subphylum Deuteromycotina. Synonym: C. hibisci-cannabini Sacc. Setae on the acervuli are rarely seen. Conidiophores are long cylindrical, unicellular, colorless, 15-24 × 4-5 (Um). Conidia are oval to long oval, unicellular, colorless, 22-24 × 3.5-6.0 (Um). The temperature range for spore germination and pathogen growth is 3-35°C, the optimum temperature is 25°C, and the optimum relative humidity is nearly 100%. The pathogen has two physiological races, race 1 and race 2.

Transmission routes and disease conditions The pathogen overwinters in diseased residues or inside and on seeds and becomes the primary infection source the following year. Conidia produced on diseased parts are spread by rain and wind splash or by insects for secondary infection. The degree of disease spread mainly depends on variety resistance, seed contamination rate, and climatic conditions. Resistance differs among varieties, and the disease is likely to occur in spring and autumn rainy seasons with suitable temperatures and abundant rain. In the south, it mostly occurs in May-June and September-October; in the north, spring incidence is light, and the disease easily occurs from July to September when the rainy season begins. In low-lying kenaf fields or fields with a high groundwater table, and with partial or excessive application of nitrogen fertilizer, the disease is more severe.

Control methods (1) Select anthracnose-resistant varieties, such as China's Xianghongma No. 2, the United States' Everglades 41 and 71, and Cuba's Cuba 961 and 1087. (2) Seed disinfection. Dress seeds with 50% Tuzet wettable powder or 80% anthracnose Fumei, Daojiaoqing, Duofu mixture, etc., then store sealed for about half a month. (3) When the disease season begins, spray promptly before onset or in the early stage of disease with 50% benomyl wettable powder or 50% Tuzet wettable powder, 80% Pencozeb wettable powder, 80% anthracnose Fumei wettable powder at 800 times dilution, or 25% Tanteling wettable powder at 500 times dilution. (4) Abroad, seed dressing with 0.05 active ingredient dodine (Dodine) or 0.2% of 50% ziram wettable powder has shown good results. (5) Strengthen kenaf field management. Drain promptly after rain, strictly prevent moisture retention, avoid partial or excessive nitrogen fertilizer, appropriately increase potassium fertilizer, and pay attention to improving disease resistance.