Horse Chestnut (Aesculus hippocastanum)

Phyllosticta is a genus of fungi responsible for leaf spot diseases in a wide range of garden and indoor plants. These pathogens thrive in warm, humid conditions, spreading primarily through water splash and prolonged leaf wetness. Symptoms typically include small, circular lesions that transition from tan or light brown to having distinct dark brown or black margins. As the disease progresses, these spots may coalesce, leading to chlorosis (yellowing) and premature abscission (leaf drop). Characteristic pycnidia—tiny, black, pinhead-sized fruiting bodies—often develop within the lesions. Phyllosticta commonly affects ornamentals like hydrangeas, irises, peonies, lilacs, maples, and roses, as well as citrus and various fruit trees.

Pseudomonas is a group of bacteria that causes leaf spots, blights, cankers, and soft rots on many garden and houseplants. It thrives in cool, wet, or humid weather and spreads with splashing water, rain, handling, and contaminated tools. Look for small, water‑soaked spots that turn dark brown or black, often with a yellow halo; on tomatoes, these can appear as tiny dark specks on leaves and fruit. On woody plants such as cherry, plum, and lilac, it can cause sunken cankers, oozing, dieback, or blossoms that fail after a cool, wet spring. Succulents and fleshy tissues may develop soft, slimy rot. Commonly affected plants include tomatoes, peppers, beans, brassicas, roses, lilacs, stone fruits, and various houseplants.

Leafminers are the larvae of various insects including flies, moths, sawflies, and beetles that tunnel between leaf surfaces, creating distinctive serpentine trails or blotchy patches. While primarily cosmetic, severe infestations can weaken plants by reducing photosynthesis capacity and making them susceptible to secondary infections.

Powdery mildew is a common fungal disease caused by various fungi species including Erysiphe, Podosphaera, Oïdium, and Leveillula. It affects over 10,000 plant species worldwide. The disease thrives in warm, dry climates with high humidity and appears as a distinctive white powdery coating on plant surfaces.