Powdery mildew and downy mildew are two common fungal diseases that affect bitter gourd, but they can be easily confused due to similar symptoms. However, there are key differences in their appearance and the conditions under which they thrive. Powdery mildew typically appears as white, powdery spots on both sides of the leaves, while downy mildew is characterized by white mold growing mainly on the underside of the leaves, often restricted by leaf veins. In real field or greenhouse settings, these classic symptoms may not always be visible. Often, only pale yellow spots appear on the upper surface of the leaves, with little to no visible mildew layer. This makes accurate diagnosis challenging.
Many farmers mistakenly treat downy mildew as powdery mildew, leading to improper use of fungicides. This not only fails to control the disease but also delays treatment, allowing the infection to worsen. To help growers better identify and manage these diseases, here's a detailed guide from **The Chinese Pesticide Network**:
**1. Diagnosis of Bitter Gourd Downy Mildew**
Downy mildew is caused by a fungus belonging to the genus *Pseudoperonospora* (formerly classified under *Peronospora*). It thrives in cool, moist conditions, with optimal germination temperatures between 15°C and 19°C. The disease commonly occurs in Guangxi during May-June and September-October, especially after prolonged rain and in poorly drained soils. High humidity, dense planting, and poor ventilation increase the risk of infection.
Symptoms begin as small, light yellow spots on the upper leaf surface. As the disease progresses, the lesions become polygonal or irregular, following the leaf veins. These spots gradually turn yellowish-brown. Under high humidity, a white mold layer may appear on the underside of the leaves, and sometimes white hyphae are visible on the surface. When dry, the mold is rarely seen, making it difficult to distinguish from powdery mildew. Early detection is crucial, focusing on the initial signs on the leaves.
**2. Diagnosis of Bitter Gourd Powdery Mildew**
Powdery mildew is caused by fungi such as *Podosphaera xanthii* and *Erysiphe cichoracearum*, both belonging to the Ascomycota phylum. It prefers warm, humid conditions, with germination occurring between 10°C and 30°C, particularly around 20°C to 25°C. High humidity, fluctuating temperatures, and excessive nitrogen fertilizer can all contribute to its spread.
This disease primarily affects the leaves and stems, rarely attacking the fruit. Initial symptoms include small, round, yellowish spots that may grow larger and cover entire leaves. In severe cases, the leaves turn yellow, curl, and eventually die, reducing yield and shortening the growth period. In late autumn, black dots may appear on the white mold, which is a distinguishing feature from downy mildew.
**3. Prevention and Control Measures**
**For Powdery Mildew:**
- Select resistant varieties such as Fengcheng Big Meat No. 1, 2, and 3.
- Choose well-drained fields and avoid overuse of nitrogen fertilizer.
- Apply organic fertilizers and reduce watering in early spring. Improve ventilation and increase temperature in greenhouses.
- In the early stages, use fungicides like 15% Triadimefon WP (1500 times), 45% Sulfur Suspension (150–300 times), 73% Pyraclostrobin WP (1000–1500 times), and 10% Myclobutanil WP (1200–1600 times) alternately.
- Spray thoroughly on both leaf surfaces, using sufficient water volume. Repeat every 4 days for 2–3 applications.
**For Downy Mildew:**
- Follow similar agricultural practices as for powdery mildew.
- In early stages, apply 64% Mancozeb WP (500–600 times), 72.2% Kocide Water Dispersible Concentrate (500–700 times), 72% Chlorothalonil WP (500–700 times), 80% Benomyl WP (600 times), and 58% Metalaxyl-Manganese Zinc (500–800 times).
- Ensure timely, thorough, and continuous spraying.
In general, managing both diseases involves maintaining moderate temperatures (around 20°C) and controlling humidity above 80%. Proper ventilation, drainage, and crop management play a vital role in preventing disease outbreaks. Early detection and timely intervention are key to effective control.
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