Early steps in the infection cycle of the wheat leaf rust fungus, Puccinia triticina, on wheat, Triticum aestivum, leaves

Early infection structure. A germinated urediospore has formed an appressorium over a leaf stoma in a penetration attempt. Note the cytoplasm has moved into the appressorium leaving an empty collapsed spore behind (scanning electron micrograph; courtesy of Dr. Guanggan Hu)

(cartoon by K. Mendgen, From 'Introductory Mycology', 
with permission from John Wiley & Sons, Inc.)

Successful infection. Arrows indicate rust haustoria inside leaf cells (fluorescence light microscopic picture).

Successful infection by one rust spore on a wheat leaf. Mycelium is spreading from point of invasion (stoma; left panel).

Active resistant reaction of wheat leaf cells against a rust invasion (arrow). Plant cells produce anti-microbial compound which fluoresce under the conditions used for this microscopic picture (right panel). Pictures courtesy of Dr. Guanggan Hu