• Fungi cause more plant diseases than other microorganisms, according to the University of Missouri Extension. Although fungal diseases cause a range of symptoms in plants, some diseases cause the formation of visible mold on the leaves, fruits and other plant parts. Mold-forming diseases range from harmless but unsightly to potentially deadly. To prevent the spread of the disease to your other plants, destroy all infected plant material.
  
  

Powdery Mildew

• Many species of fungus cause powdery mildew, and each species infects one or several species, making powdery mildew a disease that afflicts many types of plants. Squash plants become frequent victims in the vegetable garden, while ornamentals affected include zinnia, phlox, rose, rhododendron, ivy and lilac. Powdery mildew overwinters in leaf debris and disperses spores in the spring. Spores stick to plants and await optimal conditions to begin germinating. Powdery mildew grows best under high humidity and low temperatures and tends to appear during cool spells in the summer when nights remain humid. Powdery mildew first appears as irregular-shaped chalky gray patches on the tops of leaves. As mold patches expand, they grow together. As the fungus feeds on plant tissue, leaves may become twisted and discolored, depending on the species. To prevent powdery mildew, avoid wetting the foliage with overhead watering or close spacing of susceptible plants. Fungicides can sometimes treat the disease, if applied as soon as you notice symptoms.
  
  

Downy Mildew

• Like powdery mildew, many pathogens cause downy mildew, allowing the disease to affect numerous plant hosts, primarily ornamentals such as butterfly bush, snapdragon, rose, pansy, rosemary and violets. Downy mildew forms as a grayish, hairy mold on the undersides of leaves. On the tops of leaves, you may notice brown blotches confined between the leaf veins that grow larger and merge as the disease progresses, until foliage falls from the plant. Downy mildew also overwinters in plant tissue and, during cool, humid conditions, spores travel in a film of water to infect new plant tissue. Removing infected plants and keeping plants dry stops the spread of infection. Fungicide application can prevent the emergence of the mold but, once you see symptoms, it's too late to treat the disease.
  
  

Sooty Mold

• Sooty mold can afflict nearly any plant in the garden because it forms as a secondary infection. Sooty mold grows on the honeydew secreted by insects like aphids and scale that infest plants. The mold develops as dark fuzz over areas sticky with honeydew. Occasionally, sooty mold develops on tree sap as well. Luckily, sooty mold is harmless to plants, although the same can't be said for some of the insects that cause it. Identifying and treating the insect infestation should be your priority. Once the insects are gone, the sooty mold infection will clear up as well.