A Detailed Explanation of Tomato Powdery Mildew Symptoms and Key Control Techniques

Tomato is a nutrient-rich vegetable and is grown everywhere. During tomato cultivation, diseases can strike due to various reasons or climatic conditions, and powdery mildew is a common one. So what symptoms does tomato powdery mildew have? What are the key control techniques?

1. What symptoms does tomato powdery mildew have?

Leaf infection: In the early stage of the disease, when you turn over a leaf, you can see a layer of white mycelium growing on the underside. At this time, the leaf itself shows no other obvious changes. As the disease progresses, the leaf begins to curl upward like a spoon, and at the same time dark spots of various sizes appear on the leaf. Afterward, the lesions gradually expand, and a layer of white powder also appears on the underside of the leaf; that is the pathogen's conidiophores and conidia. The lesions generally exist separately, and only when the outbreak is severe do multiple lesions join together and cover the entire leaf. In the late stage of the disease, the color of the lesions changes, appearing reddish-brown, and the leaf margins become shrunken and withered.

Flower buds and flowers infected: After flowers are infected, the petals lose their previous color and turn pink, and compared with normal flower buds, infected flower buds cannot open normally.

Fruit infection: After fruit is infected, its shape changes, appearing slender and elongated, and it cannot expand normally, so the whole fruit looks shriveled. When green fruit is affected, it will not expand, and the fruit also loses its luster, like a mummified fruit; after fruit that has already colored is affected, the coloring rate decreases, and the fruit surface is covered with white powder, which affects fruit quality and gradually causes it to lose its commercial value.

2. The occurrence pattern of tomato powdery mildew

In northern China, the pathogen mainly overwinters on winter-grown tomatoes as asexual spores; in addition, cleistothecia can also overwinter with diseased plant residues on the ground. When conditions are suitable, the ascospores released from inside the cleistothecia spread with airflow, and afterward conidia are produced on the diseased parts. After the mature conidia detach, they cause re-infection through airflow. In areas of southern China where tomatoes are grown year-round, the pathogen has no obvious overwintering phenomenon, and conidia are continuously produced, causing damage in successive cycles. The optimum temperature for germination of Oidium lycopersici conidia is 20-25 degrees C, and for Leveillula taurica it is 15-30 degrees C. In open fields it mostly occurs in June-July or September-October, while in greenhouses or plastic tunnels it is more often seen in March-June or October-November.

3. What are the key control techniques for tomato powdery mildew?

1. Select and breed powdery mildew-resistant varieties, and strengthen temperature and humidity management in greenhouses and sheds. After harvest, promptly remove diseased plant residues to reduce overwintering sources of inoculum. In the early stage of the disease, greenhouses and sheds can use the dust method or smoke method, spraying 10% Duobai dust agent in the evening, 1 kilogram per mu each time, or applying 45% chlorothalonil smoke agent, 250 grams per mu each time, ignited with a slow fire and fumigated overnight.

2. In open fields or greenhouses and sheds, you can choose 15% triadimefon (Fenxiuning) wettable powder at 500 times dilution, 40% Fuxing emulsifiable concentrate at 10000-8000 times dilution, 10% Shigao water-soluble granules at 1500 times dilution, 30% Tefulin wettable powder at 1500-2000 times dilution, 50% sulfur suspension concentrate at 200-300 times dilution, or 50% qinamine emulsifiable concentrate at 500-600 times dilution, 2% Wuyi mycin aqueous solution or 2% Nongkang 120 aqueous solution at 150 times dilution, 25% diliketo emulsifiable concentrate at 3000 times dilution, 15% triadimefon (Fenxiuning) wettable powder at 2000 times dilution plus 25% diliketo emulsifiable concentrate at 4000 times dilution, once every 7-15 d, for 2-3 consecutive treatments.