Bacterial Brown Spot of Rice
Bacterial brown spot is a significant rice disease that primarily affects the northeastern region of China, though it has also been reported in southern regions and Zhejiang. Internationally, Hungary was the first country to identify this disease. It tends to be more prevalent in northern rice-growing areas.
Symptoms (Figure 8): The disease affects various parts of rice plants, including leaves, leaf sheaths, stems, panicles, and branches, causing brown lesions. On the leaves, symptoms initially appear as small, water-soaked spots that turn reddish-brown. These spots gradually expand into oblong or irregular stripes with a yellow halo. Over time, the central part of the lesion becomes necrotic and turns grayish-brown, often merging into larger streaks. Lesions can occur anywhere on the foliage, appearing slightly water-soaked and forming irregular, large spots. On the leaf sheaths, especially around the panicle, the lesions start as small, water-soaked, auburn spots that may fuse into larger irregular patches. The central tissue eventually becomes necrotic and dark brown. On the panicles, the lesions are usually round and small, appearing as brown spots. If the flag leaf sheath is severely infected before heading, the panicle may not emerge properly, significantly affecting yield.
Pathogen (Figure 8): Pseudomonas oryzicola Klement. This Gram-negative bacterium is rod-shaped with blunt ends, sometimes curved. It occurs singly or in pairs, does not form spores or capsules, and possesses 1–3 polar flagella.
Pathogenesis: The pathogen overwinters in damaged rice plants, upland rice, weed tissues, and seeds. Infected seeds can cause seedling disease the following year, and diseased plant debris serves as an infection source. The host range of the pathogen is broad, affecting not only rice and upland rice but also crops like wheat, millet, sorghum, and various weeds such as barren wild rice, leech grass, green foxtail, and Northeastern goosegrass. In the northeast, these weeds grow earlier than rice, and under natural conditions, the disease begins on them in mid-April. After rice and upland rice emerge in mid-to-late May, the disease spreads rapidly, making weeds a major source of infection. The bacteria can survive in paddy fields for about 20–30 days, and contaminated irrigation water can also transmit the disease. Therefore, irrigation water is a key vector for spread.
The disease typically appears in late May during the seedling stage and becomes visible on leaves in mid-July. By early August, the severity on the leaves decreases. However, before and after heading, the disease mainly affects the panicles and the flag leaf sheath.
Control Measures: There are significant differences in resistance among rice varieties, so selecting locally adapted resistant varieties is an effective strategy. Since the primary source of the pathogen comes from weeds, controlling weeds in the field reduces the infection risk. Maintaining field hygiene and preventing the movement of infected material to healthy areas is also crucial. According to reports from Northeast China, using Dragon Fungus (thiaca copper) has shown excellent results in controlling bacterial brown spot. Application method: Mix 100 grams of 20% Thiaca Copper SC with 50 liters of water (or spray 10 liters per acre), applying once at the onset of the disease. A second application should be made 7–10 days later, depending on disease progression and weather conditions.
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