What Is Rice Leaf Blast? A Detailed Guide to Prevention and Control

Rice blast that occurs on rice leaves is called leaf blast. The prevention and control of rice leaf blast should run through the entire rice growth period, because leaf blast can occur throughout the whole growth period, with damage being more severe from tillering to jointing. An outbreak of leaf blast directly affects the normal growth of rice, and in severe cases the plants wither and die, causing rice yield reduction or even total crop failure.

1. Lesion types

Leaf blast occurs on seedlings after the three-leaf stage or on the leaves of mature plants, generally breaking out heavily from tillering to jointing. The lesions on the leaves often differ in shape, size and color depending on the weather and the disease resistance of the variety, and can be divided into four types: chronic, acute, white-spot and brown-spot, among which the first two are the most important in terms of damage.

Chronic-type lesions: diamond-shaped or spindle-shaped, generally 1 to 1.5 cm long and 0.3 to 0.5 cm wide, red-brown to gray-white, with necrotic lines at both ends. It begins as a small dark green spot on the leaf, gradually expanding into a spindle-shaped lesion, often with extending brown necrotic lines. The center of the lesion is gray-white, the edge is brown, with a light yellow halo outside, and a gray mold layer on the underside of the leaf. When there are many lesions, they merge into irregular large patches. This type of lesion develops relatively slowly.

Acute-type lesions: nearly round or irregular, dark green, with a dense gray mold layer on the underside of the lesion. On susceptible varieties, dark green nearly round or oval lesions form, with a brown mold layer on both sides of the leaf; when conditions are unfavorable for disease development, they transform into chronic-type lesions. The appearance of acute lesions can be regarded as a precursor to a major field epidemic.

White-spot-type lesions: after susceptible young leaves are infected, small nearly round white spots form, producing no spores; when climatic conditions favor their expansion, they can turn into acute-type lesions.

Brown-spot-type lesions: mostly found on highly resistant varieties or old leaves, producing pinhead-sized brown spots only between the leaf veins, with relatively little spore production.

2. Prevention and control methods:

First, select resistant varieties, and pay attention to variety combination and rotation.

Second, carry out rice seed treatment before sowing. Seeds can be soaked with 45% prochloraz. Before transplanting, apply a pre-transplanting fertilizer and a pre-transplanting pesticide; one week before transplanting, apply 3-5 kg of urea per mu to the seedling field to enhance the plant's disease resistance, and apply it together with the pre-transplanting pesticide for controlling leaf blast as well as stem borers and planthoppers. You can also soak the seedlings in a 20% tricyclazole wettable powder solution for 5 minutes, take them out and pile them up for 30 minutes, then transplant.

Third, manage fertilizer and water reasonably: apply sufficient base fertilizer, apply topdressing early, apply appropriate nitrogen in the middle stage to control the seedlings, and supplement fertilizer in the later stage according to the condition of the seedlings. Avoid applying too much nitrogen fertilizer or applying it too late, and focus on increasing the application of phosphorus and potassium fertilizers. Water management should follow the principles of shallow-water planting of seedlings, deep-water greening, thin-water tillering, and drying the field once enough tillers have formed.

Fourth, carry out chemical control in a timely manner. Leaf blast should be detected and controlled early. Starting from the tillering stage, if an infection center is found or acute lesions appear on the leaves, chemical control should be carried out promptly.

3. Commonly used pesticides

6% kasugamycin wettable powder 30-40 g per mu per application, or 75% tricyclazole wettable powder 30-40 g per mu per application, or 40% Fuji-one emulsifiable concentrate 100 ml per mu per application. Dilute any one of the above pesticides with 45-60 kg of water and spray for control, then apply once more after 6—8 days. If rain occurs within 4 hours after spraying, it must be re-sprayed in order to achieve the desired control effect.