Molecular Pathogens, 2025, Vol.16, No.5, 236-245 http://microbescipublisher.com/index.php/mp 237 2 Overview of Major Fungal and Bacterial Diseases of Rice 2.1 Fungal diseases There are many types of fungal diseases in rice, among which rice blast and sheath blight pose the greatest threat to yields. Rice blast, caused by infection by the filamentous fungus Magnaporthe oryzae, is recognized as one of the most devastating rice diseases worldwide. Rice blast can occur at various stages of the rice growth period, including seedling blast, leaf blast and panicle neck blast. Typical symptoms are the formation of diamond-shaped necrotic spots on leaves and ear necks, which in severe cases can lead to premature senescence and death of the entire plant. In highly susceptible varieties and under suitable environmental conditions, rice blast can cause 20%~50% yield reduction, or even no harvest. Sheath blight is a soil-borne disease caused by the semi-known fungus Rhizoctonia solani. Sheath blight mainly infects the base of rice stems and leaf sheaths, forming irregular moire-like patches. In severe cases, it can lead to withering and lodging of stems and leaves (Sathe et al., 2021). Sheath blight is prone to spread over large areas under high temperature and high humidity conditions, and is one of the important threats to the stable rice production in China. In addition, fungal diseases such as rice bakanae, rice smut and brown spot also occur from time to time in certain areas and years, affecting rice yield and quality. 2.2 Bacterial diseases Among bacterial diseases of rice, bacterial blight and bacterial streak are the two most harmful. Bacterial blight is caused by Xanthomonas oryzae pv. oryzae (Xoo) and is one of the most prevalent bacterial diseases in the world. Bacterial blight mainly invades through wounds or water holes and spreads along the veins of rice leaves, causing white spots on the leaves, hence the name "white leaf blight". When the disease is severe, the entire leaf dries up, eventually causing premature senescence and death of the seedlings or adult plants. In most rice regions in Asia, bacterial blight occurs almost every year. Moderate epidemics can cause 20%~30% yield reduction, and severe epidemics can cause losses of more than 50% (Hoang et al., 2018). Since the resistance of rice varieties to bacterial blight is significantly affected by environmental factors such as temperature, high temperature and humidity often aggravate the occurrence of the disease, posing greater challenges to disease-resistant breeding (Zhao et al., 2020). Bacterial leaf streak is caused by another variant of Xanthomonas oryzae pv. oryzicola (Xoc for short). Its symptoms include the appearance of brown to yellow elongated water-soaked stripes on the leaves. Stripe disease is more common in parts of South China and Southeast Asia, and is more likely to be prevalent in hot and rainy seasons. 2.3 Disease monitoring and genetic diversity analysis of pathogenic populations Disease monitoring and pathogen population structure analysis are important foundations for formulating the layout of disease-resistant varieties and continuously controlling diseases. For rice blast, countries have long established monitoring systems for physiological races (small populations) of M. oryzae (Sheoran et al., 2021). In the rice areas of southern China, the common race types in the rice blast fungus population are significantly different from those in the northeastern rice areas, resulting in the same blast resistance gene being inconsistent in the north and the south. This geographical differentiation phenomenon has also been verified internationally. Indica and Japonica rice varieties often show different disease resistance spectrums to blast fungus originating from each other's ecological zones. Some studies have used genome-wide association analysis to reveal that the interaction between rice and Magnaporthe oryzae has formed a gene-genotype correspondence and adaptation relationship in the long-term co-evolution. For the bacterial blight pathogen Xoo, there are also abundant physiological race variations. 3 Genetic Basis and Molecular Mechanism of Rice Disease Resistance 3.1 Identification and functional classification of disease resistance genes (Rgenes) Plants sense and resist pathogenic damage through their own resistance genes (Rgenes). In rice, most of the major disease resistance genes cloned so far belong to the classic nucleotide-binding-leucine-rich repeat receptor protein-coding genes. This type of R protein is usually located within cells, has a conserved NB-ARC domain and LRR repeat sequences, and can directly or indirectly recognize pathogenic bacterial effector proteins and trigger an immune response triggered by the effector (Wang et al., 2025). Most rice blast resistance genes (Pi genes)
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