Mold commonly occurs on outdoor plants, particularly in areas that do not receive much sunlight and during humid or rainy weather and regions. Mold may appear on one plant only and then spread to other plants. Although it sometimes goes away on its own when drier weather occurs, human intervention is often required to prevent plant death and decay.
Mold on plants can appear as a fuzzy white coating (typical with powdery mildew disease), grey fuzz, black fuzz or brown, yellow or black leaf spots. Mold on the stem may be any of these colors, but is often dark brown or black. Affected areas of the plant may feel soft or slimy to the touch, although this is not always the case.
Overwatering, lack of sunlight, lack of airflow between plants, warmth and decaying plant debris left on or near the plants can cause mold growth. In the case of many molds, the spores can spread to other plants in the yard. Often the mold spores survive the winter in leftover organic yard debris and re-infect plants the following year, especially if conditions are ripe for mold growth (warm and damp).
Mold covering the foliage of plants prevents the plants from receiving adequate light required for growth and well-being. This alone may cause plant death eventually.
The fruits of plants such as tomatoes may also be effected by molds. In addition, some fungal diseases cause leaf wilting and stem rot which will kill the plant if left untreated.
Remove moldy foliage from affected plants using garden shears. All other plant debris near the moldy plants should be removed in case these contain mold spores. Spray the plants with a commercial fungicide--both organic (such as Neem oil) and chemical fungicides exist and you'll find several varieties at most garden stores. You'll need to treat plants more than once during the season, in most cases.
Keep areas around plants clean of debris. If soil is not well-draining and puddles form near plants with each watering or rainfall, add amendments, such as sand, to the soil to promote better drainage. Certain plants, such as tomato plants, are particularly susceptible to molds. You should spray these plants with tomato fungicides regularly (as indicated on the product label) through the growing season, before a problem occurs. Thin out plants in overcrowded areas to allow for more airflow between plants and inspect plants often for signs of mold.