Rice downy mildew

Rice Downy Mildew

Rice downy mildew, also known as yellowing and atrophy, typically appears in the later stages of the growing season, with symptoms becoming most noticeable during the peak growth period. The first signs include yellow and white spots on the leaves, which then develop into irregular stripes and a mottled mosaic pattern. The center of the affected plant turns yellow, becomes curled, and is difficult to pull out. The lower leaves gradually wither, the roots become stunted, and the overall plant becomes dwarfed. The leaf sheaths of infected plants become soft, unevenly wrinkled or corrugated, twisted, and show reduced growth. In severe cases, the entire plant may fail to produce an ear, while less severely affected plants may form small, distorted, or box-shaped panicles that are not fully developed.

The pathogen responsible for this disease is *Sclerophthora macrospora* (Saccardo) Thirumalachar, Shaw & Narasimhan var. *oryzae* Zhang & Liu. It produces bacterial spores that are spherical, pale yellow-brown, measuring 65–95 × 4–78 μm. The male cells are 1–4 in number, lateral, and measure 45–75 × 7.5–10 μm. The oospores start off colorless and turn yellowish-brown, being oval in shape and measuring 51–75 × 51–75 μm. Sporangia are lemon-shaped, colorless, and appear at the tip of cysts. They grow from stomata, branch out, and contain multiple oval spores with two flagella. After resting, these spores become spherical.

The disease spreads through various means, including contaminated soil, weeds, and infected rice seedlings. The oospores can survive over winter in the soil and are carried by water currents. Under flooded conditions, oospores produce spores and zoospores, which then infect rice plants once they settle. The optimal temperature range for oospore germination is between 10–26°C, with 19–20°C being ideal for disease development. The infection is most likely to occur between 10–25°C, with 15–20°C being the most favorable. The seedling stage of rice is the most vulnerable period, and the disease is often introduced through infected seedlings from Putian. Heavy rainfall, continuous cloudiness, and low temperatures significantly increase the risk of disease outbreaks.

To manage rice downy mildew, several control measures can be implemented. First, select high ground for paddy fields and ensure proper drainage by constructing ditches. Second, remove all weeds and diseased seedlings to reduce the spread of the pathogen. Third, apply fungicides early in the disease cycle. Recommended treatments include spraying 25% metalaxyl WP diluted 800–1000 times, 90% mancozeb WP at 400 times, 80% chlorothalonil WP at 700 times, 64% triazoxide wettable powder at 600 times, or 58% metalaxyl-manganese zinc or 70% mancozeb-zinc manganese WP at 600 times. Another option is 72.2% propamocarb (Pric) diluted 800 times. These preventive steps help reduce the impact of the disease and protect crop yield.

Lycopene

Lycopene, a carotenoid found in plant foods, is also a red pigment. Dark red acicular crystal, soluble in chloroform, benzene and grease but insoluble in water. Unstable to light and oxygen, turning brown when met with iron. Molecular formula C40H56, relative molecular mass 536.85. The molecular structure has 11 conjugated double bonds and 2 unconjugated double bonds, constituting a straight chain type hydrocarbon. It does not have the physiological activity of vitamin A, but it has strong antioxidant function. The content is higher in the fruits of mature red plants, especially in tomato, carrot, watermelon, papaya and guava. It can be used as pigment in food processing and as raw material of antioxidant health food.

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