Occurrence Patterns and Control Techniques of Jujube Iron-Skin Disease

Jujube iron-skin disease, also known as fruit-shrinking disease and black rot, commonly called wu zhuo, wu liao tou, dry waist, black waist, iron scorch, fire head, etc., is a fruit disease that seriously harms jujube trees. Since the early 1980s, the damage from this disease has become increasingly severe. It begins around the coloring stage, develops rapidly and in concentrated fashion, and often breaks out in epidemics. It occurs in many jujube-producing areas including Hebei, Henan, Shandong, Shanxi, Inner Mongolia, Guangxi and Sichuan. In normal years the disease causes yield losses of 20%-50%, and in severe years losses reach 70%-80%, or even total crop failure.

1. Symptoms of iron-skin disease damage

The disease begins to show symptoms at the white-ripe stage of the fruit. Initially, water-soaked, irregular yellowish-brown lesions appear from the middle to the shoulder of the jujube fruit. The lesions continuously expand and develop deep into the flesh along the longitudinal direction; the diseased flesh turns yellowish-brown and tastes bitter. Affected fruits drop easily and lose their food value. The disease develops rapidly from mid to late August to early September, often appearing suddenly and explosively. Especially during the period from the white-ripe stage to the mature stage, within 3-5 days after rain or heavy fog, the disease suddenly worsens and the rate of diseased fruits increases rapidly.

2. Pathogens of iron-skin disease

As early as the late 1970s, the press had already reported on jujube iron-skin disease. Han Jinsheng (1979) inferred that the disease was caused by a physiological nutrient deficiency or a virus. Chen Yijin et al. (1989) considered the pathogen to be a new species of the genus Erwinia in the family Enterobacteriaceae; Qu Jianxu, Shen Ruixiang et al. considered it to be Macrophoma kawatsukai and Dothiorella gregaria; Zheng Xiaolian et al. (1995) proposed that its pathogens include three weakly parasitic fungi - Coniothyrium sp., Alternaria tenuis, and a septate, colored filamentous fungus of the order Mycelia Sterilia - and one bacterium.

Peng Shiqi, Kang Shaolan, Mao Yongmin et al. (1996) collected diseased fruits with typical symptoms of jujube iron-skin disease from Wangdu, Hejian, Xinle, Fuping and Zanhuang in Hebei and Xinzheng in Henan over three consecutive years from 1993 to 1995 for indoor isolation and culture. Through many batches of large-scale isolation over many years (a total of 6,358 fruit pieces isolated), it was found that the diseased fruits mainly contained three fungi: Alternaria alternata (Fr.) Keissler; Phoma destructiva Plowr; and a fungus of the genus Fusicoccum, Fusicoccum sp. Among them, Phoma sp. was the most numerous. From a single jujube fruit, only one fungus could be isolated, or the above three fungi or any two of them could be isolated simultaneously. The isolation results from samples collected in different years were not completely the same. In 1993, Fusicoccum sp. was not detected in Po jujube collected in Hejian and Xinle, whereas in samples collected in 1995 the detection rate of Fusicoccum sp. reached 34.09% (Hejian Po jujube) and 30.91% (Xinle Po jujube). Isolation results also differed among varieties. The frequency of isolating the three fungi from Po jujube in the three jujube areas of Wangdu, Xinle and Hejian was basically consistent, with Alternaria sp. and Phoma sp. at higher frequency and Fusicoccum sp. at lower frequency, whereas Zanhuang jujube and Henan grey jujube were just the opposite, with the occurrence frequency of Fusicoccum sp. significantly higher than that of the other two fungi. Following Koch's postulates, back-inoculation and re-isolation confirmed that the pathogens of jujube iron-skin disease are the above three fungi. These three fungi can infect alone and cause disease, or any two of them can infect together, or all three can infect together to cause disease.

3. Occurrence patterns of iron-skin disease

1. Overwintering sites

The overwintering sites of the iron-skin disease pathogens are 1- and 2-year-old branches and perennial jujube branches, jujube spurs, bark, fallen fruits, fallen fruit clusters, fallen leaves, etc. This indicates that the pathogens can overwinter and survive in these tissues and become a large source of primary infection during the growing season.

2. Infection period

In addition to infecting fruits, the three pathogenic fungi of jujube iron-skin disease also infect jujube leaves, flowers and jujube hanging branches, but show no symptoms on jujube leaves, flowers or jujube hanging stems. The times when the three fungi were first isolated from jujube leaves, jujube hanging stems, jujube flowers and jujube fruits differed. Alternaria alternata (Fr.) Keissler could be first isolated from young leaves in late April, from jujube flowers in late May, and from young fruits in mid-July; Phoma destructiva Plowr could be first isolated from jujube flowers in late June and from young fruits in mid-July; Fusicoccum sp. was first isolated only from jujube fruits in early August. No obvious peak could be seen in the infection of jujube hanging branches, jujube leaves, jujube flowers and jujube fruits by the three pathogens.

Research shows that iron-skin disease is a monocyclic infection disease, with no secondary infection.

3. Control techniques for jujube iron-skin disease

(1) Maintain orchard sanitation. In autumn and winter, clear away fallen leaves, fallen fruits and fallen fruit clusters, and burn them in a centralized manner, and scrape tree bark in early spring to reduce the source of primary infection.

(2) Scrape tree bark in early spring and burn it in a centralized manner.

(3) After scraping the bark, spray 3-5 degrees Baume lime-sulfur mixture before bud break.

(4) Starting from mid to late June, spray broad-spectrum fungicides such as mancozeb once every 10-15 days.