Diseases are rare among bonsai, but when they occur, they need to be treated fast. Also isolate your tree from the others to avoid the spread of the disease.

Powdery Mildew

Easily recognized, powdery mildew is a fungal disease. Low soil moisture combined with high humidity levels at the plant surface favors this disease. Powdery mildew will cause the leaves to turn yellow, then brown, dry up, and drop off. Infected leaves become covered with a white to gray powder, usually on the upper surface. To prevent powdery mildew, avoid watering the leaves at the end of the day. To treat powdery mildew, apply an organic fungicide.

Sooty mold

Sooty mold is another fungal disease. It usually appears after a massive attack of aphids or mealybugs. These pests exude a thick substance that favorizes the development of the fungi. Scooty mold can usually be treated by getting rid of the pests and cleaning the infected leaves and branches. If necessary, you can apply an organic fungicide.

Phytophthora

Phytophthora is a microscopic fungus that mainly affects roots. You will notice when your bonsai tree is infected with phytophthora because the leaves and trunk will start to get dry and eventually fall off or break. Azaleas are particularly subject to this disease.

Fire blight

Fire blight is a contagious disease affecting orchard trees, and some other members of the family Rosaceae. It is due to a bacterium and will cause buds to turn black and a white liquid to leak from the tissues. Repot immediately your affected trees, remove the affected areas, burn the wastes, and disinfect properly your tools.