Study conducted on white rust on Portulaca

Jean Williams-Woodward and Jeff Cook from the University of Georgia conducted a research on white rust on Portucala. White rust is not a true rust nor a fungal disease. It is an oomycete disease, and it was recently seen infecting moss-rose purslane (Portulaca grandiflora) in hanging baskets.

Cooler, wet conditions are conducive for Oomycete diseases
such as downy mildews and Phytophthora and Pythium root
and foliage blights. Recently, hanging baskets of moss-rose
purslane (Portulaca grandiflora) were declining within a
greenhouse. About half of the plants within each basket were
thinning, wilting, and the leaves were light-colored with raised
white blotches.

At first, it was assumed that the raised spots might be oedema, a physiological disorder that causes raised, scab-like growths due to plant cells bursting due to high internal water pressure during wet,
humid, overcast weather. However, microscopic examination
of the raised blotches identified the growth to be rupturing
pustules filled with white spores. It looked like a leaf rust disease, but it was white. The plant problem was identified as white rust caused by the pathogen, Wilsoniana portulacae.

Click here to read the full study.