How to prevent and control jujube fruit shrinkage disease? Understanding the occurrence pattern is key
Jujube fruit shrinkage disease, also known as jujube iron skin disease, jujube black rot, jujube dry waist fruit shrinkage disease, jujube wilt fruit disease, wu chao, wu luo tou, wu chao, tie jiao, etc., was first officially reported in China in the late 1970s. It is found throughout all major jujube-growing regions nationwide, and its damage is becoming increasingly severe, having already become a devastating disease in jujube production. In recent years, the disease has become increasingly serious in jujube-producing areas of Shanxi Province, becoming an important factor affecting jujube yield and commercial quality. However, current reports on the pathogen and control patterns of this disease are inconsistent, and no effective control method has been found in production. This article is based on years of investigation and research in more than 10 jujube orchards in Taigu County, Shanxi Province, and preliminarily determines the occurrence pattern of jujube fruit shrinkage disease. At the same time, based on recent jujube tree management practices, control measures suitable for the actual production conditions of Shanxi Province have been summarized.
I. Occurrence pattern of jujube fruit shrinkage disease
1. Pathogen
Opinions vary widely regarding the pathogen of jujube fruit shrinkage disease in jujube-producing areas. One theory suggests it may be caused by physiological nutrient deficiency or a virus, and there are reports of infection by Macrophoma kawatsukai. Liu Yuanrong et al. reported that it is caused by a bacterium called Erwinia jujubovora, denying that nutrient deficiency can cause fruit shrinkage disease. Qu Jianxu et al. identified the pathogen as Dothiorella gregaria, and additionally isolated an Erwinia species. Zheng Xiaolian et al.'s experiments showed that the pathogens of fruit shrinkage disease include three weakly parasitic fungi - Coniothyrium olivaceum, Alternaria tenuis, and Dothiorella gregaria - and one bacterium, and that inoculation with any of the four pathogens alone or in combination can produce typical symptoms. Among these, Dothiorella gregaria is consistent with Qu Jianxu's report, but Qu Jianxu's experiments showed that fruit shrinkage disease is caused by Dothiorella gregaria alone. Kang Shaolan et al.'s report differs from previous ones; they believe the pathogens of fruit shrinkage disease are three fungi: Alternaria tenuis, Phoma destructiva, and a fungus of the genus Fusicoccum, and pointed out that these three fungi can infect jujube fruits alone or in combination to cause disease. To date, there remains great controversy over the pathogen of fruit shrinkage disease, with a total of 6 fungi and 1 bacterium publicly reported.
2. Disease characteristics
The disease mainly damages jujube fruits, causing fruit rot and premature dropping. Affected fruits first show a pale yellow halo on the shoulder or body, with relatively clear edges, gradually expanding into concave irregular pale yellow lesions; subsequently the peel becomes water-soaked, exudate-type, sparsely covered with needle-prick-like round brown spots; the flesh turns from pale green to yellow, becoming soft and shrunken, and the outer peel becomes dark red and lusterless; healthy fruits have green stalks, while diseased fruits have brown or dark brown stalks. Anatomical observation of the stalks shows that diseased fruit stalks form an abscission layer prematurely and drop early; diseased fruits are small, shriveled, and thin; diseased tissue becomes spongy and necrotic, bitter in taste, and unfit for consumption; fruits are very likely to drop after infection, and the larger the lesion on the diseased fruit, the more likely it is to drop; within the same tree, the disease incidence rate is higher in large fruits than in small fruits.
The disease course of fruit shrinkage disease can be visually divided into 5 stages: halo, water-soaked, coloring, shrinkage, and dropping. However, the dropping stage varies greatly; early-stage diseased fruits mostly drop during the water-soaked stage, mid-stage ones mostly drop during the coloring half-red stage, and late-stage diseased fruits mostly drop at the end of the shrinkage stage.
3. Occurrence pattern
Jujube fruit shrinkage disease can invade during the flowering stage and remain latent. Lesions begin to appear in late July, but the disease onset begins in mid-to-late August. Generally, from when the jujube fruit stalk cavity turns red (red circle stage) to when 1/3 has turned red (coloring stage), when the sugar content of jujube flesh exceeds 18%, the hydrogen ion concentration is 1000-3163 nmol/L (pH 5.5-6), and the temperature is 23-26°C, this is the peak period of disease occurrence. During this period, if there is continuous rain or nighttime rain followed by daytime clearing and foggy weather, the disease often breaks out into disaster. When active accumulated temperature is high and rainfall is abundant, the disease peak occurs earlier; otherwise, it is delayed. That is, the timing of the peak occurrence period varies from year to year, which is related to the climate factors and fruit growth conditions of that year. Disease severity is closely related to factors such as jujube fruit development stage, tree age, branch age and tree vigor, meteorological factors and site conditions, insect pests, cultivation conditions, and cultivated varieties.
The pathogen mainly invades through wounds. According to field observations, in addition to wounds caused by wind and rain and natural friction between fruits transmitting the disease, during the peak disease period, the density of piercing-sucking mouthpart insects is positively correlated with the disease index. The main disease-transmitting wounds are those caused by piercing-sucking pests such as leafhoppers, stink bugs, and scale insects, as well as fruit borers. In intercropped jujube orchards with excessive nitrogen fertilizer application, dense branches and leaves, and poor ventilation, pest outbreaks are induced, increasing disease transmission vectors and resulting in more severe disease.
In addition to infecting jujube fruits, the pathogen of fruit shrinkage disease also infects jujube leaves, flowers, and bearing shoots, but does not show any symptoms on jujube leaves, flowers, or bearing shoots. The overwintering sites of the pathogen are old bark, various branches, fallen fruits, fallen bearing shoots, and fallen leaves.
II. Control measures for jujube fruit shrinkage disease
1. Select disease-resistant varieties
Selecting suitable disease-resistant jujube varieties according to local climate and soil conditions is the prerequisite for controlling jujube fruit shrinkage disease.
2. Strengthen jujube orchard and tree management to improve tree disease resistance
Comprehensive jujube orchard management is the fundamental measure to prevent and reduce the occurrence and spread of jujube fruit shrinkage disease. Effective measures include: (1) Scrape old bark in early spring, thoroughly clean up fallen leaves, fallen fruits, and fallen bearing shoots in autumn and winter, and burn them in a centralized manner to reduce disease sources. (2) Strengthen soil, fertilizer, and water management, carry out reasonable shaping and pruning, improve ventilation and light conditions, enhance tree vigor, and improve tree disease resistance; strengthen trenching and drainage to reduce soil and air temperature. (3) Do not intercrop tall crops or crops sharing the same pests and diseases with jujube trees in jujube orchards. Mulching or growing grass in jujube orchards can also reduce the occurrence of jujube fruit shrinkage disease. (4) Apply heavy organic fertilizer, improve soil structure, increase application of phosphorus, potassium, calcium, and boron fertilizers, appropriately control nitrogen fertilizer, achieve formula fertilization, reasonably supply nutrients, promote water-fertilizer coordination, enhance tree vigor, and improve disease resistance.
3. Control of disease-transmitting insects
In the early period (April-July), spray high-efficiency insecticides such as methamidophos, cypermethrin, omethoate, kelmite, and supracide to mainly control bean weevils, leafhoppers, gall mites, jujube red spiders, and Japanese wax scales; in the later period (August-September), apply insecticides such as cypermethrin and Taoxiao Yicijing to control peach fruit borers.
4. Chemical control
Adopt measures including scraping tree bark in jujube orchards in early spring, removing diseased residues, spraying 5° Baumé lime sulfur mixture; reasonable pruning to ensure tree ventilation and light penetration; spraying 0.5% borax once each during the leaf expansion stage, early flowering stage, and full flowering stage of jujube trees. In normal years, the first spray can be applied in late July or early August, with spraying every 7-10 days. The 15-20 days before jujube fruit harvest is the critical control period. Control agents include streptomycin 70-140 U/mL or kanamycin 140 U/mL, and 75% carbendazim wettable powder 600 times dilution. When applying fungicides, 20% fenpropathrin 2000 times dilution, red high-chlorine, and other insecticides can be added for combined pest control.
The following two specific spraying methods can be used:
(1) In July, before the onset of jujube fruit shrinkage disease, spray 68.75% Yibao granules 1500 times dilution and Bordeaux mixture once each during the young fruit stage. Entering the disease onset period in August, use 0.76% Wanxing emulsifiable concentrate 3000 times dilution. Mix some insecticides into the above solutions.
(2) During the middle and late stages of fruit growth, use 10% Score water-dispersible granules 800 times dilution plus 72% agricultural streptomycin sulfate crystalline powder 5000 times dilution plus 2.5% decamethrin emulsifiable concentrate 1000 times dilution for spraying.