Black spot disease is a global plant disease caused by various fungi (mainly *Alternaria*), posing a serious threat to agricultural production. Chitosan oligosaccharide, a highly effective plant immune inducer derived from marine organisms, has been extensively studied and proven to effectively control black spot disease through a dual pathway of “direct inhibition” and “resistance induction.”
Harmful Effects of Black Spot Disease on Plants
Black spot disease has a very wide host range, infecting various economic crops such as citrus, pear, rose, broccoli, and cauliflower. Its damage is mainly manifested in the following aspects:
1.Direct damage to economic organs: The pathogen mainly attacks the leaves, fruits, and new shoots of plants. Taking fruits as an example, small black or brown spots initially appear, gradually expanding into round or irregular lesions. In severe cases, the lesions merge, leading to fruit rot, decreased quality, and even loss of commercial value. For example, black spot disease in pears forms dark brown lesions on the fruit, promoting premature ripening and yellowing, affecting storage and sales.
2.Impact on photosynthesis and tree vigor: Infected leaves develop numerous lesions, leading to leaf scorch and drop in severe cases, weakening the plant’s photosynthetic capacity, resulting in weakened tree vigor and affecting flowering and fruiting the following year.
3.Latent infection, difficult to prevent: Many black spot pathogens have latent infection characteristics, meaning they invade during the fruit’s tender stage but only manifest symptoms during fruit ripening or post-harvest storage. This makes them difficult to detect in the field, yet causes significant losses during post-harvest transportation and sales.
4.Multiple transmission routes, difficult control: Pathogens can be spread through multiple routes, including seeds, diseased plant debris, wind, rain, and insects. Warm and humid environments are particularly conducive to disease spread, making control efforts very reactive.
Dual Mechanism of Chitosan Oligosaccharide in Controlling Black Spot Disease
Targeting the characteristics of black spot disease, chitosan oligosaccharide exhibits unique control value. Its core mechanism of action can be summarized as “direct inhibition of bacteria” and “induction of resistance.”
1. Direct Antibacterial Effect: Physical Attack on Pathogens
Studies have confirmed that chitosan oligosaccharides can directly inhibit the activity of Alternaria alternata, the main pathogen of black spot disease.
- Strong Inhibition of Growth and Germination: In vitro experiments show that chitosan oligosaccharides can significantly inhibit the colony growth and spore germination of Alternaria alternata. Studies have shown that specific concentrations of chitosan oligosaccharide complexes (such as COS-OGA) at 1% and 0.5% concentrations can completely inhibit the hyphal growth of Alternaria alternata.
- Disruption of Pathogen Ultrastructure: Scanning electron microscopy reveals that the surface of Alternaria alternata hyphae treated with chitosan oligosaccharides is rough, uneven in thickness, and partially collapsed, indicating significant disruption to the intact structure of the hyphae. This physical damage directly leads to the loss of pathogen activity.
2. Induction of Plant Systemic Resistance: Awakening Plant’s Own Immunity
This is the core function of chitosan oligosaccharides as a “biostimulant.” It acts like a biological signal, proactively “notifying” the plant to enter a state of full defense. This mechanism has been systematically validated in studies of apricots and plums.
(1)Activation of key defense enzyme systems: Pre-harvest application of chitosan oligosaccharides significantly increases the activity of a series of enzymes directly related to disease resistance in the fruit, including:
- Phenylalanine ammonia-lyase (PAL), 4-coumarate-CoA ligase (4CL), and cinnamic acid-4-hydroxylase (C4H): These are key enzymes in the plant phenylpropanoid metabolic pathway, responsible for synthesizing antibacterial substances such as lignin, total phenols, and flavonoids.
- Chitinase (CHT) and β-1,3-glucanase (GLU): As pathogenesis-related proteins, they can directly degrade the main components of the cell walls of various pathogenic fungi, thereby inhibiting pathogen invasion and spread.
- Superoxide dismutase (SOD) and catalase (CAT): These antioxidant enzymes effectively scavenge excess reactive oxygen species produced by plants under stress, reducing oxidative damage to cells caused by diseases.
(2)Strengthening Physical and Chemical Defense Barriers: The activation of defense enzymes ultimately manifests in changes in plant tissue structure.
- Chitosan oligosaccharide treatment promotes the accumulation of lignin, total phenols, and flavonoids. Lignin thickens cell walls, forming a physical barrier against pathogen penetration; total phenols and flavonoids possess direct antibacterial activity.
- Studies on apricot fruits have also found that chitosan oligosaccharide significantly increases the content of hydroxyproline-rich glycoproteins (HRGP). HRGP is a structural protein of plant cell walls; increased HRGP content contributes to cell wall stability and is one of the important mechanisms for resisting pathogens.
(3)Optimizing the Timing of Reactive Oxygen Explosions: The rapid production of reactive oxygen species (oxidative burst) is one of the early defense responses in plants. Chitosan oligosaccharide treatment promotes the rapid accumulation of H₂O₂ in the early stages of storage, quickly putting plants into a state of alert; simultaneously, it maintains low levels in the later stages, avoiding excessive reactive oxygen species from damaging plant cells themselves.
Significant Effects in Field Application
The above mechanisms ultimately translate into tangible disease control effects.
1.Prunes: Spraying with a 0.10% concentration of chitosan oligosaccharide during the four stages of fruit setting, fruit enlargement, color change, and ripening significantly inhibited the incidence of disease and the expansion of lesion diameter after post-harvest inoculation of plums with Alternaria alternata.（1）
2.Apricots: Using a similar regimen (0.05% concentration, sprayed during the four key growth stages), pre-harvest treatment with chitosan oligosaccharide significantly reduced the incidence of disease and lesion diameter in apricots by 16.37% and 17.57%, respectively, compared to the control group.（2）
Conclusion
Black spot disease is a serious and difficult-to-control disease. Chitosan oligosaccharide, through a dual mechanism of directly inhibiting pathogens and comprehensively activating the plant’s own immune system, provides effective protection for plants from the outside in, representing a green and promising disease control strategy.