Smut Common Rust Southern Corn Rust Notes Systemic Diseases and Diseases involving Obligate Parasites Physoderma Brown Spot Physoderma Brown Spot occurs from Texas north to S. Dak. Outbreaks are scattered but occasionally severe. It is widespread in the tropics where it is a serious problem because of warm temperatures and abundant rainfall. Infection occurs via zoospores which are released from sporangia.
Zoospores differentiate to become amoeba by losing flagella.
Host cells are infected by fine hyphe from the amoeba. The resulting rhizomycelim occupies cells and is eventually converted to a sporangium. Stalk breakage may occur. These genera belong to the order Peronosporales in the Oomycetes . Downy mildews are systemic diseases involving obligate parasites.