The differences between bacterial diseases, fungal diseases, and viral diseases.

　　1. Fungal diseases

　　Diseases caused by fungi can manifest as symptoms such as spots, wilting, and deformities. However, bacterial diseases can also produce similar symptoms. The key difference lies in the fact that fungal diseases often show spiderweb-like or cottony mycelium at the affected sites, and some may even exhibit specialized structures like sclerotia. Moreover, various mold-like growths typically appear on the surface of the infected tissues—this is an important characteristic for identifying fungal diseases. Regardless of the affected部位 or the specific symptoms, all fungal diseases will produce mycelium and spores under humid conditions. This is the primary basis for diagnosing fungal diseases. For example, in cucumber powdery mildew, white, powdery substances appear on the leaf lesions. Similarly, in gray mold affecting cucurbits and tomatoes, gray mold-like growths develop on the affected leaves, withered flowers, and fruits. Common fungal diseases include downy mildew, powdery mildew, gray mold, blight, and wilt in vegetables.

　　2. Bacterial Diseases

　　Diseases caused by bacteria mainly include types such as spots, ulcers, wilting, necrosis, and rot. Necrosis, rot, and malformations are all consequences of bacterial damage to cell walls and cellular tissues. On leaves with reticulate venation, disease lesions typically appear as acute necrotic spots with polygonal shapes, often surrounded by a yellow halo. On succulent tissues or fruits, lesions tend to be circular. In tender, juicy tissues, the affected tissue easily decays and rots. In some cases, after being infected, certain areas undergo promoting lesions that result in tumor formation—a phenomenon frequently observed on roots or stems. Wilt is the result of bacterial infection of the vascular bundles and can occur locally or systemically. Cellular diseases caused by bacteria do not produce hyphae or spores; the surface of the lesions lacks any mold-like growth, yet they may exude bacterial pus (except in the case of crown gall bacterium). The lesion surfaces are smooth, which serves as the primary diagnostic criterion for bacterial diseases. For example, bacterial angular leaf spot of cucumber shares similar symptoms with downy mildew: both diseases cause polygonal lesions on leaves, making them easy to confuse. However, when conditions are humid, downy mildew lesions develop black mold, whereas angular leaf spot does not.

　　3. Viral Diseases

　　Viral diseases are often systemic and chronic, with external symptoms typically including mosaic patterns, chlorosis, ring spots, necrotic lesions, stunting, and leaf wrinkling. Mosaic symptoms manifest as leaves that are curled and display alternating yellow and green patches. The yellow mosaic areas are particularly vivid, while the green mosaic areas appear a deep, dark green. The yellow regions tend to be sunken or depressed. Viral diseases exhibit only symptoms but no visible signs of infection—this is what often leads to confusion with fungal and bacterial diseases. The key distinguishing feature is that viral diseases spread gradually from localized points outward in a clear, progressive manner. On infected plants, symptoms usually first appear at the shoot tips and then spread to other parts of the plant. Viruses are transmitted through insect vectors, sap contact, and grafting. Under conditions of high temperature and drought, viral diseases tend to become more severe. In contrast, non-infectious diseases occur more uniformly and do not show the gradual progression from localized points to widespread areas. Affected plants typically exhibit systemic symptoms throughout the entire plant; however, these diseases cannot be transmitted between individual plants. The occurrence of such diseases is closely linked to adverse environmental factors.