Sunflowers do not need fertilizer, and overdoing the nitrogen can create a leggy plant with delayed flowering. If your soil is poor, add compost or slow-release fertilizer to the surface of the soil.  This should help strengthen the plant without overdosing it. Protecting your plants against slugs and snails is especially important. You can surround your plants with slug repellent from a garden store, or make your own "beer traps" for slugs to fall in. Downy mildew can be a major problem in clay or waterlogged soils. Check regularly for this fungus, which causes shrunken yellow leaves in addition to its namesake down. If you see these symptoms, reduce water use to keep soil dry, and apply fungicide immediately. Infected plants rarely produce flowers, so you may want to simply remove them to avoid spreading the disease.  If the leaves look normal except for a striking yellow bleached color, this is a bacterial infection instead. Keeping soil dry will help here as well, and the plant will usually be fine. If the leaf looks yellow but still has green veins, the problem is most likely a mineral deficiency. It can be tricky to pinpoint the exact problem, but diluted fertilizer may work. Be aware that it’s normal for sunflowers to lose their first set of leaves once they begin growing in earnest. Don’t worry if the bottom leaves yellow and drop, but the rest of the leaves are fine.

How would you rephrase that briefly using Vietnamese?
Bón phân nếu đất cằn cỗi. Bảo vệ cây khỏi các loài gây hại. Chú ý đến các lá vàng.