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Prevention and treatment of susceptible diseases in the middle and later stages of corn
According to recent field monitoring, the current corn disease has entered its peak period. The outbreak began on July 14 and lasted for 20 days. Prolonged hot and humid weather conditions have created an ideal environment for the development of high-temperature diseases. Specifically, the weather conditions in the fields are particularly favorable for several common corn diseases, including leaf spot, brown spot, curvularia leaf spot, and bacterial stalk rot. These diseases are now widespread, and it is crucial to focus on their prevention and management during the mid-to-late growth stages of the crop.
First, large leaf spot primarily affects the leaves, but can also invade the leaf sheath and the cob. In severe cases, the lesions can expand into long, prismatic spots that reach up to 10–30 cm in length. Initially, the spots appear water-soaked, then turn blue-gray and eventually become brown with dead areas. When humidity is high, black mold (conidia) may develop on the lesions. This disease thrives at temperatures between 26°C and 32°C, especially in low-lying areas with high plant density. The pathogen spreads through wind and rain, and thrives when relative humidity exceeds 90% and temperatures are between 20°C and 28°C. Disease development is suppressed when temperatures go above 25°C or below 15°C, or when humidity drops below 60%. Continuous rain during the jointing to heading stage can lead to outbreaks.
Second, brown spot typically appears at the base of the leaves and on the leaf sheaths, but can also affect the stems. Lesions start as small yellowish-white spots and later develop into brown or purple, bubble-like patches. The area around the lesions becomes reddened, and those on the midrib or sheath tend to be larger. The tasseling stage is when corn is most susceptible to brown spot, and there are significant differences among varieties. This disease favors temperatures between 23°C and 30°C, and rainy weather during the middle to late growth stages promotes its spread. High humidity and temperature further increase the risk of an epidemic.
For chemical control, you can use 20% Triadimefon EC diluted 3000 times or 12.5% Diniconazole powder at a rate of 30g per 30kg of water.
Third, Curvularia leaf spot mainly affects the leaves, though it can also damage the leaf sheath and cob. It starts as small chlorotic spots that gradually expand into round, translucent areas with a white to yellow-brown center and a dark brown border. A light yellow halo often surrounds the lesion, and symptoms vary widely. Some plants may only show small white or brown spots. This disease tends to occur after the initial leaf spot outbreak, especially during the hot summer months of July and August, when 4–5 days of rain can trigger an outbreak.
To manage this, you can apply 50% Tuzet WP diluted 1000 times or 12.5% Teptopol WP diluted 4000 times. Alternatively, 70% Mancozeb WP at 1000 times dilution can be used. Spraying should be done every 7–10 days, with 2–3 applications recommended.
Fourth, bacterial stalk rot is characterized by sudden plant death during the mid-growth stage. The lower part of the stem shows yellow-brown to dark brown water-soaked lesions, which become soft and sunken, often emitting an unpleasant odor. This disease occurs during the tasseling stage, and is favored by temperatures around 30°C and humidity over 70%, especially in hot and wet conditions.
For treatment, you can use 85% Trichloroisocyanuric acid diluted 3000 times, and repeat the application after 7 days.
Note: During the mid-to-late growth stages, multiple diseases and pests often coexist on the same plant. Therefore, it's important to choose pesticides that have multiple effects and provide broad protection. For example, mixing an insecticide with a broad-spectrum fungicide like Diniconazole or Carbendazim can be effective. Additionally, foliar fertilizers such as Penshibao, Duoxin, or potassium dihydrogen phosphate can be added to the spray mixture to help the plant recover and enhance its resistance. It is generally recommended to apply these treatments once every 7 days, with 2–3 consecutive applications for best results.