Research progress on apple bitter pit: Symptoms, pathogenesis, and control measures

Bitter pit (ABP) is a prevalent physiological disorder in apple, which can reduce fruit quality and storage life. This disorder is mainly characterized by epidermal depression and browning of the flesh. The affected fruits often exhibit a spongy, necrotic tissue beneath the skin, accompanied by a bitter taste. The pathogenesis of ABP is a complex, multi-factor process. The calcium imbalance is a core factor for inducing ABP. Apples rely on roots to absorb calcium from the soil. Due to the weak transpiration of fruits, calcium absorption and transportation to fruits are difficult, resulting in physiological calcium deficiency. This deficiency weakens the cell structure and function, making fruits more susceptible to ABP. Moreover, the interactions of mineral elements play a crucial role in the formation of ABP. The imbalance in the ratios of elements like nitrogen, phosphorus, potassium, and calcium can disrupt the normal absorption, transportation, and utilization of calcium and increase the incidence of ABP. The reactive oxygen species (ROS) accumulation and oxidative damage also contribute to ABP development. Under stress conditions such as calcium deficiency, the balance between ROS production and scavenging in apple fruits is disrupted. The ROS accumulate in cells, can cause membrane lipid peroxidation and cell death, and affect normal fruit physiological functions. The hormonal regulation imbalance is another factor. The changes in the levels of hormones like auxin (IAA), gibberellin (GA3), and abscisic acid (ABA) can influence the incidence of ABP. In addition, abnormal molecular regulation is involved in the formation of ABP. Through techniques like transcriptome sequencing, numerous differentially expressed genes related to ABP have been identified. The genes related to programmed cell death and flavonoid biosynthesis are significantly changed during the occurrence of ABP. The oc-curence of ABP is also related to cultivars of scion and rootstock. Some cultivars, such as Honeycrisp and Qincui, are highly susceptible to ABP, while Pink Lady and Nagafu 2 show relatively strong resistance to ABP. Different rootstocks can affect the absorption and distribution of minerals, thereby influencing ABP incidence. Soil and environmental factors also play important roles. The soil acidity affects the availability of calcium. Excessive nitrogen, phosphorus, and potassium in the soil, along with insufficient calcium and magnesium, can exacerbate ABP. Adverse climate conditions like high-temperature, drought, or continuous rainfall can inhibit calcium absorption and transportation. Cultivation practices and storage conditions are also key factors. Unreasonable fertilization, improper pruning, and incorrect irrigation can all increase the risk of ABP. Fruit bagging may also affect calcium absorption and increase the incidence of ABP. In storage, unsuitable temperature, humidity, and gas composition can accelerate fruit senescence and promote ABP development. Currently, the prevention and control of ABP mainly focus on regulating calcium metabolism and integrating multiple approaches. Agronomic measures for reducing ABP include rational fertilization, such as increasing the application of organic fertilizers to improve soil structure and calcium availability, controlling nitrogen input, and intercropping with legumes like alfalfa to enhance soil microecology. Physical methods for reducing ABP involve optimizing orchard light conditions, improving bagging techniques to reduce calcium absorption barriers, and using trunk infusion of calcium fertilizers to increase tree calcium content. Chemical control mainly includes calcium applications at different stages, such as soil dressing, foliar spraying (e.g., using sorbitol calcium and humic acid- calcium), and postharvest immersion. Biotechnological applications, although still in the initial stage, show great potential for reducing ABP. Marker-assisted breeding can be used to screen for the genes related to ABP resistance, and genetic engineering can be employed to regulate the expression of calcium-transport-related genes and antioxidant enzyme genes. In conclusion, although current control methods can reduce the incidence of ABP to some extent, they still face problems such as high cost, poor environmental adaptability, and unstable effects. Future research on ABP should focus on several aspects. First, further exploration of the pathogenesis using advanced molecular biology techniques like transcriptomics, proteomics, and metabolomics can help uncover the complex molecular regulatory network. Second, more efforts should be put into breeding resistant apple varieties by combining traditional breeding with modern biotechnology. Third, the development of efficient and sustainable control technologies, such as new biological agents and environment-friendly chemical pesticides, is needed. Finally, establishing an ABP early- warning system based on orchard environment monitoring, tree nutrient diagnosis, and fruit physiological index detection can enable timely prevention and control measures. Through multi-disciplinary integration and continuous innovation, it is expected to achieve greater breakthroughs in the prevention and control of ABP, providing strong support for the healthy and sustainable development of the apple industry.