Rice Seedling Disease
Rice seedling disease primarily affects young and mid-season rice plants, especially in the Yangtze River Basin. The symptoms typically appear on the leaves, starting with small, yellowish-white spots that gradually develop into irregular, water-soaked gray-green lesions. As the disease progresses, these lesions merge and may cause the leaves to collapse or die. Under high humidity conditions, a thin white mold layer can be observed on the affected areas, which consists of bacterial cysts and sporangia. Over time, the lesions turn brown, with a grayish-brown center, and the white mold changes to a grayish-white color. This often leads to the death of middle and lower leaves, and in severe cases, entire plants may be affected.
The pathogen responsible for this disease is *Phytophthora fragariae* var. *oryzo-bladis*, a member of the oomycete group. It produces 2–5 cysts that emerge from the stomata of the host, either singly or in clusters, sometimes with 1–2 branches. These cysts measure approximately 6.5–40.5 × 2.9–5.7 μm. The sporangia are terminal and oblong, ranging from 36–98.3 × 22.1–66.5 μm. They do not detach after maturing but release zoospores from their top. Zoospores are kidney-shaped, rest on the surface, and germinate to form secondary vesicles. The oospores are nearly round, brown, and range from 31.2–72 μm in diameter. They have a periwinkle color and an oblate shape, measuring 12.2–25.9 × 15–30 μm. Oospores are round, yellow, and vary between 21.7–57 μm in size. (left)
The disease spreads through oospores present in the soil, which germinate in the presence of water and produce zoospores that infect the plant. In conditions of high humidity, septate cysts can form on infected tissues, and the sporangia require moisture to develop. After resting, zoospores generate germ tubes that penetrate the host through leaf stomata. Under saturated humidity, infected seedlings show clear symptoms, while relative humidity between 60% and 90% may only result in light brown spots. The optimal temperature for disease development is 16–21°C, and temperatures above 25°C inhibit the disease. Rainy weather favors disease progression, and seedlings are most vulnerable before and after the three-leaf stage. Flooding or deep irrigation increases the risk of infection, as does continuous water irrigation. Overcrowded planting and excessive nitrogen application also increase susceptibility.
To manage this disease, several preventive measures are recommended. First, choose high ground for planting in Putian and practice crop rotation annually. Second, improve fertilizer and water management by using shallow irrigation instead of continuous watering and applying appropriate amounts of phosphorus and potassium to enhance plant resistance. Third, chemical control can be applied during the seedling stage. Spraying with 72.25% Propiconazole (Prike) at 800 times dilution, 64% Oxadixyl WP at 600 times, or 40% Metalaxyl wettable powder at 800 times, or a Bordeaux mixture of 1:2:240, can effectively reduce disease incidence.
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