Bacterial Blight of Rice and Its Control
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Bacterial blight in rice, also known as white leaf mites, terrestrial fire, or thatch grass, is a serious disease affecting the entire growing period, with seedlings and tillering stages being most vulnerable. All plant parts can be infected, but leaves are the most commonly affected. The symptoms vary depending on the infection site, variety resistance, and environmental conditions, and are typically categorized into three main types.
(1) Leaf Blight Type: This form primarily affects the leaves and leaf sheaths. The disease usually begins at the leaf tip or margin, forming dark green, water-soaked linear spots. These spots quickly develop into yellow-white lesions, which then spread along the leaf margins or midrib, turning brown and eventually drying out. Lesions have clear boundaries, while resistant varieties show irregularly curled edges. Diseased leaves appear gray-green, lose moisture rapidly, and become blue-dry inside, mostly visible on the upper part of the leaves.
(2) Acute Wilt Type: This type occurs from the seedling to tillering stage. Pathogens enter through wounds in the roots or stem base, causing the heart leaves to wilt and die. Other leaves curl, and the entire plant eventually withers, leaving only the heart and leaves. The stems contain large amounts of bacteria, and some roots may show yellow-brown discoloration. In some cases, the flag leaf midrib turns yellow, and the lesions extend upward, leading to premature death before heading. Resistant varieties often show brown necrotic bands under unfavorable conditions, slowing disease spread.
(3) Chlorotic Type: This rare form shows early chlorotic spots on the heart leaves, followed by dry yellow spots and occasional water-soaked strips at the leaf base. Under wet conditions or when morning dew is present, milky white dots appear on the leaves, later forming small yellow rubber particles that easily fall off.
Dead heart seedlings caused by bacterial blight appear during the tillering stage. The heart leaves or lower layers lose moisture, curl, and eventually die. As the diseased plants dry, other leaves shrink and the whole plant collapses. Peeling the leaves or cutting the stems reveals yellowish-white bacterial exudate, distinguishing it from damage caused by pests like stem borers.
Pathogen: The causal agent is *Xanthomonas oryzae*, which includes two pathovars: *pv. oryzae* and *pv. oryzicola*. These Gram-negative, rod-shaped bacteria measure 1.0–2.7 × 0.5–1.0 μm, are motile via a single polar flagellum, and produce a viscous extracellular polysaccharide. Colonies on artificial media are honey-yellow and produce insoluble pigments. They thrive between 17–33°C, with optimal growth at 25–30°C, and prefer pH 6.5–7.0. The bacteria infect cultivated and wild rice species, and are classified into three serotypes, with Type I widespread nationwide.
Transmission: Primary sources include infected seeds, straw, and plants. Weeds such as Lees' weeds can also serve as carriers. Bacteria overwinter in seeds, entering through leaf holes or wounds after sowing. Spread occurs via wind, rain, irrigation, insects, and human activities. Low-lying water, heavy rain, and high humidity promote disease outbreaks. Excessive nitrogen fertilizer, long-term flooding, and acidic soils increase susceptibility. Mid- and late-season rice, especially japonica varieties, are more prone to infection, particularly during panicle development.
Control Methods:
(1) Use 2–3 local-resistant rice varieties. Examples include early rice varieties like Jiayu 280, Japonica No. 61, Yuzao, and mid- and late-season resistant strains such as Shennong 514, Ningshu No. 15, and several indica types.
(2) Implement strict quarantine measures. Seeds should be soaked in 1% lime water or 80% 402 antibacterial agent (2000x), or formalin (50x) for 3 hours, followed by thorough washing. Seed soaking methods using mycobiotin or antibiotics can also help.
(3) Agricultural practices such as compost application, proper water management, and timely drainage reduce disease risk. Infected straw should be properly disposed of to prevent contamination.
(4) Chemical control involves spraying 20% Ye Kuning (Ye Qing Shuang) at 100g/667m², or mixing with phytarctin if needed. Other options include streptomycin, chloramphenicol, or chlorobromoisocyanuric acid. Applications during the 5-leaf stage and before transplanting enhance effectiveness.
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