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Yellow leaf blight typically progresses from the lower leaves upward, causing them to turn yellow and eventually dry out into a white color. Initially, dark green spots appear at the tip or edge of the leaf, which then develop into yellow, elongated stripe-like lesions. The affected areas are clearly separated from healthy tissue and have a wavy, corrugated appearance. In later stages, small, gelatinous spheres may appear on the leaf surface. If diseased leaves are cut and placed between two glass slides, and clear water is dripped on them, bacterial exudation can be observed.
Yellow dwarf disease usually starts in the parietal lobe and the next one or two leaves, spreading both upward and downward. It leads to yellow or mottled mosaic patterns on the leaves. Infected plants become stunted, with shortened internodes and an abundance of black roots. New root growth is reduced, and the leaves remain flat, swaying, often turning yellow and drying out.
Physiological premature aging and fertilizer damage can also cause yellowing of rice leaves, and these conditions must be distinguished. In physiological premature aging, yellowing starts from the lower leaves and spreads upward. The leaves take on an orange-yellow hue with a metallic sheen and may appear in patches or across entire fields. No lesions or bacterial ooze are present.
Excessive use of ammonium bicarbonate, ammonia, or pesticides can cause yellowing or golden discoloration of rice plants, sometimes accompanied by burning-like yellow spots.
In acute rice blast, the lesions begin as small dark brown spots that later become oval with grayish-green mold on the back. In the chronic type, the lesions are fusiform, with reddish-brown edges and a gray center. A brown necrotic thread runs through the middle of the lesion, and when wet, a gray-green mold layer becomes visible.
The flax patch disease begins as small, needle-like spots that gradually develop into oval-shaped lesions resembling sesame seeds. The thickened lesions are dark brown and darker than those of rice blast. Upon closer inspection, the color shows three distinct layers: an outer yellow halo, a wide dark brown edge, and a gray-yellow center. The dark brown part often appears translucent and hides the fine vein network. There are no necrotic lines along the sides of the lesion.
Bacterial brown stripe disease initially appears on the midrib of the leaf base, spreading upward and downward as brown stripes. When cut, the affected area exudes a milky white liquid, and the plant develops rot with a foul odor. Dead leaves remain attached, and the heart leaves may die and collapse.
Pediococcus pneumoniae commonly occurs in clusters and affects multiple strains. The basal tissues of the rice plant soften, and dark brown spots appear. The most distinctive feature is the peeling of the leaf sheaths and stalks, with many small black sclerotia, smaller than leek seeds.
Bacterial base rot occurs sporadically, with one to three affected plants per spot. The base of the infected plant shows gray, rotten tissue. Affected plants are easily dislodged from the soil, and milky white, cloudy bacterial fluid oozes out when pressed. There is a noticeable odor, and the internal parts are aseptic.
Physiological blue-green stem disease causes the stems to shrink, become dry when pinched, and are prone to lodging. The base of the stems shows no insect damage or lesions, and aseptic nuclei may appear in the sheaths and stalks, especially in late rice near maturity.
Dry heart locust infestation is characterized by wormholes or insect feces in the lower parts of the rice plant, with dead hearts that are easy to pull up.
In indica rice, there are no wormholes or insects in the stems. The dead heart strains are easy to remove, and the base of the stem and roots are shredded.
Diseased stripe plants show yellow streaks on their leaves, which curl into a paper-like shape and droop, forming a "false dead heart." The base has no wormholes or rot, and the dead heart is not easily pulled up.
In white panicle rice, insects and stems are found in the upper parts of the plant, and the white panicles are easy to pull up. The panicles and necks are not affected, and the white panicles are clearly visible in the field.
In japonica rice, there are no wormholes in the stems, and the white panicles are not easily pulled up. Dark brown spots appear on the neck, cob, and branches. The neck or branches break easily, and when wet, gray-green spores are visible.
Rhizoctonia solani appears on leaf tips, leaves, or heads, starting as dark green spots that later expand into oval, cloud-like lesions with brown edges and a light brown to grayish-white center. The base becomes soft, and the white panicles fall flat.