The Rudbeckia hirta is more commonly known as the black-eyed Susan. This annual reaches 18 to 36 inches in height and produces blooms that are bicolored, yellow or orange. Rudbeckia is susceptible to fungal diseases, which can affect the health and vigor of the plant.
Powdery mildew is a fungal disease that often affects annuals such as rudbeckia. According to the University of Illinois Extension, other annuals such as the snapdragon, zinnia, wax begonia and red salvia are susceptible to this fungal disease, as well. Powdery mildew causes the development of white, powdery fungus to develop on the leaves, buds, flowers, fruit and stems of affected plants. This white fungus often changes to gray or tan and can cause rudbeckia plant leaves to curl, stunt and drop from the plant prematurely. Preventing powdery mildew in rudbeckia is best accomplished by placing the plant in a sunny area that has adequate air circulation. Damp, humid conditions help this disease prosper and spread. Removing and destroying infected plant parts will also help prevent the spread of powdery mildew.
Septoria Leaf Spot
Septoria leaf spot is a fungal disease that can affect rudbeckia. This disease is caused by the fungus Septoria rudbeckia. Septoria leaf spot rarely kills rudbeckia, but it can cause undesirable depreciation of foliage, states the Missouri Botanical Garden. The symptoms associated with septoria leaf spot in rudbeckia include lesions that measure 1/8 to 1/4 inch in diameter and tiny fruiting bodies in the center of the lesions. Lower rudbeckia leaves are usually the first portion of the plant to show symptoms of this fungal disease, which is spread to the rest of the plant through water. Since this disease overwinters in garden debris, sanitation is of utmost importance. Collecting and disposing of infected plant parts will help prevent the spread of septoria leaf spot, as well as providing adequate air circulation. Applying fungicides can help control this fungal disease in rudbeckia.
Rudbeckia can develop a fungal disease known as gray mold. This disease also affects other annuals such as the petunia, geranium and the Cape daisy. Perennials such as the common yarrow, hosta and rose mallow can be affected by gray mold, as well. The symptoms associated with gray mold include brown, dead areas on the plant and a gray, fuzzy webbing in these areas. Gray mold attacks the leaf buds, flowers, flower buds, stems and leaves of rudbeckia. Gray mold infection can eventually kill the plant in severe cases.