Portulaca, also called purslane or moss rose, is a genus comprising roughly 100 species of tropical and subtropical plants. Despite the common name, these are low-growing annuals, not moss, and are all edible, noted for their lemony taste. They produce five-petaled flowers in a profusion of orange, red, yellow and white. Because of their high moisture needs, portulaca plants are prone to fungal diseases, most prominently white rust, root rot and botrytis.
White rust is the common name for several possible single-celled fungi that make up the Albugo genus. It is called white rust because of the white, dusty pustules that develop on infected foliage that look similar to those created by rust fungi. As they are spread by airborne sporangia, the initial site of infection will be an upper level leaf. This leaf will develop thick bumps anywhere between five days to two weeks following infection. If the leaf is isolated from the rest of the portulaca before the bumps rupture and disseminate spores, the infection can be halted.
Root rot is a disease caused by one of four possible soil-borne fungal pathogens. Typically these fungi are not able to overcome the portulaca’s defenses except when the plant has become stressed and weakened due to overwatering, which strengthens the fungi as well. Initial signs of infection are lateral brown lines or spots along the base of the primary stem. These spread upward and are followed by a wilting and yellowing of foliage. The fungus decomposes the root system from the bottom up. There is no treatment once the infection sets in. It can only be prevented by heating or fumigating the soil to the degree that fungi cannot survive.
Botrytis blight is also known as gray mold. Its optimum growing conditions are high humidity and an average temperature of 60 degrees Fahrenheit. Symptoms include brown or spotted leaves and stems that host tiny hair-like masses of silver-gray spores. While leaves are typically the initial site of infection, botrytis can infect every part of portulaca above the soil. Though this can lead to death if left untreated long enough, it is a contact-based pathogen, meaning it will not move into the portulaca’s vascular structure. A contact fungicide and removing any infected foliage are effective forms of treatment.
- "Annuals"; Gerald Jonas, Ann Reilly, David S. Thomson; 1988
- Kansas State University: Root Rot Diseases of Flowering Potted Plants