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How to distinguish between cucumber spot and downy mildew
Different factors can lead to various plant diseases, and environmental conditions play a crucial role in their development. For instance, low temperatures combined with high humidity often promote bacterial angular leaf spot, while normal temperatures paired with high humidity are more likely to cause fungal downy mildew. The symptoms on the leaves differ significantly between these two diseases. In the case of bacterial angular leaf spot, the affected leaves show light-colored spots, while the leaves affected by downy mildew do not exhibit any such light perception.
The location of the disease also varies. Angular leaf spot primarily affects the leaves and the tendrils of the plant, and in severe cases, it may even damage the vines. On the other hand, downy mildew mainly targets the leaves, with less impact on other parts of the plant.
The appearance of the lesions is another distinguishing feature. Lesions caused by angular leaf spot tend to be lighter in color, appearing grayish, and they may crack during the early stages, eventually forming small holes. In contrast, the lesions from downy mildew are darker, usually yellow-brown in color, and do not crack or form perforations.
Looking at the underside of the leaves, there are clear differences. The back of the leaves affected by angular leaf spot shows milky white bacterial growth, which turns into a white or powdery residue when dry. Meanwhile, the underside of leaves infected with downy mildew develops a layer of black or purple-gray mildew.
Although the lesions may appear similar in shape, their sizes and progression rates differ. Angular leaf spot lesions are generally smaller and spread more slowly, while those of downy mildew are larger and spread rapidly, often merging into larger patches. In the early morning, the back of the leaves affected by angular leaf spot may appear water-soaked, whereas the back of leaves affected by downy mildew shows irregular, fuzzy lesions.
For the prevention and treatment of angular leaf spot, several effective fungicides and bactericides can be used. These include 72% agricultural streptomycin or 72% neophytin diluted at 4000 times, 60% benzothiazole at 800 times, and 70% mancozeb at 1000 times. To control downy mildew, options include 69% dimethomorph diluted at 1000 times, 72% pyraclostrobin at 1000 times, 70% azoxystrobin at 1000 times, and 68.75% famoxadone (Fabili®) diluted at 1400 times.
Proper identification of the disease is essential for effective management. Monitoring the environment and applying the right treatments at the right time can significantly reduce the impact of these diseases on crops.