What disease causes black spots on watermelon leaves?

When you grow watermelons, do the lush green leaves suddenly sprout lots of black spots, some with yellow halos, some that keep spreading, making you feel really anxious? Actually, black spots on watermelon leaves are mostly caused by diseases. Today we will teach you step by step how to tell apart 3 common diseases and how to treat them.

Anthracnose

Anthracnose is one of the most common diseases in watermelon growing, and it breaks out especially easily in hot, humid weather! In the early stage, round or nearly round black spots appear on the leaves, with the center slightly sunken, and if you look closely you can see small light-colored dots surrounded by an obvious yellow halo. The key feature is that the lesions can be seen on both the front and back of the leaf. Later the lesions expand and merge into patches, causing the leaves to turn yellow and drop off, and in severe cases the stems, vines and fruit also become infected.

Treatment:
 
Chemical control: at the early stage of the disease, promptly spray a mixture of "prochloraz+benzyl azoxystrobin+thiophanate-methyl". The combination of the three agents can effectively inhibit the spread of the pathogen. Spray once every 7 days, for 2-3 times in a row;

Biological control: those who are wary of chemical pesticides can choose microbial agents, such as "Jundeqing" containing Bacillus kuriensis and Bacillus licheniformis. It can both be sprayed on the leaves to kill pathogens and be watered into the roots to improve soil aeration, reducing pathogen breeding from the source;

Emergency handling: once diseased leaves are found, remove them promptly (do not crush the lesions with your hands!), and carry them out of the field to destroy them, to prevent the pathogen from spreading through rain and wind.

Black spot disease

At first, pinhead-sized water-soaked spots appear at the leaf edges or between the veins, then slowly expand into dark brown or black round lesions with clear edges and a darker center. When humidity is high (such as after rain or in the early morning), they spread quickly, and a single leaf can develop a dozen or more lesions. Eventually the leaves turn yellow and wither, greatly affecting yield.

Treatment:
 
Spray as soon as symptoms appear: use 50% procymidone wettable powder1500-fold solution, or 75% chlorothalonil wettable powder600-fold solution, focusing the spray on the undersides of the leaves and the affected areas, once every 7-10 days, for 3-4 sprays in a row;

Advanced option: if the black spots spread quickly, switch to a compound agent of "difenoconazole+pyraclostrobin-type" agents, which have a broader fungicidal spectrum and can effectively bring the disease under control;

Environmental management: after spraying, keep good ventilation and reduce the humidity on the leaf surface, to prevent the pathogen from continuing to multiply in a damp environment.

Bacterial leaf spot
 
Bacterial leaf spot is a disease caused by bacteria, and its symptoms are quite different from fungal diseases! In the early stage, light green water-soaked spots appear on the leaves, slightly raised to the touch, and after expanding they become grayish white or yellowish white lesions, possibly with slightly yellow edges. The most typical feature is that when humidity is high (such as in the early morning), milky white bacterial ooze seeps from the lesions on the underside of the leaf, like pus, and after drying it becomes a white film. It is extremely contagious.

Treatment:
 
Use targeted bacterial agents: first choice is a mix of "kasugamycin+copper hydroxide" for spraying. Kasugamycin kills the bacteria, while copper hydroxide enhances adhesion. When spraying, cover both the front and back of the leaves, especially the undersides;

Rotate the agents: bacteria easily develop resistance, so after using one for 2 applications, switch to another agent to prevent the pathogen from becoming"resistant"

Tool disinfection: after removing diseased leaves, wash your hands promptly with soapy water, and disinfect pruning tools with alcohol, to prevent spreading the bacteria yourself.

In short, do not panic when you see black spots on watermelon leaves. First observe the shape of the lesions, whether there is bacterial ooze, and whether they spread quickly. Once you have determined which disease it is, use the right treatment, and at the same time manage the environment well, and your watermelon vines will slowly regain their vitality.