We were only a few pitches into the first inning and everyone's feet were completely orange. No one could figure out why the grass had been spray painted.
Luckily, there was a plant pathologist in center field.
The softball field was infected with a rust fungus, most likely Puccinia graminis. This fungus grows invisibly inside the grass blades until it's ready to reproduce, when it bursts through the epidermis with structures called uredinia, covered in the appropriately named (and bright orange) urediniospores.
These fungi are famous for their very complicated lifecycles, which often require different host plants at different spore stages. The stages of P. graminis are shown in the figure: Urediniospores are an asexual spore stage that allows the fungus to spread quickly through fields during the summer. Later in the season, this infection produces teliospores (on telia), which are a thick-walled survival structure that remain attached to the host. Teliospores eventually produce basidia, which eject wind-blown sexual spores, known as basidiospores. The basidiospores of P. graminis can only infect barberry plants, on which they produce spermagonia on the upper leaf surface.* Spermagonia produce sticky male gamete cells, which are tracked onto receptive hyphae by rain or insects. Once fertilized, the hyphae of the spermagonium grow down through the leaf to produce another spore structure, an aecium. Aecia produce aeciospores, which in P. graminis can only infect certain grasses.
Rust fungi often don't hurt their hosts very much, at least until they burst through the epidermis to produce spores, which interrupts photosynthesis and makes the plant susceptible to dehydration. It's pretty much just a cosmetic problem on lawns, but causes major diseases on crops like wheat. It was such a problem for the ancient Romans that they invented a special god who presided strictly over rust epidemics, Robigo. Every spring, they celebrated the holiday of Robigalia, when they sacrificed red dogs and sheep in hope of appeasing him!**
According to Purdue Extension, lawn rust fungi rarely survive the cold winters of Indiana, but blow up north from warmer climates in the late summer and early fall. The same thing happens with wheat rust - urediniospores are carried on the wind along the "puccinia pathway," from the southern U.S., where it successfully overwinters, north into Canada, infecting progressively later and more northerly wheat crops along the way each summer.
Recently ag scientists have been scrambling to find wheat and barley genotyes that are resistant to a newly discovered strain of wheat stem rust, P. graminis f.sp. tritici Ug99. Previously, they had worked to prepare for the inevitable U.S. invasion of asiatic soybean rust, Phakospora pachyrizi. One of my old plant path professors liked to tell a story about how epidemiologists had predicted the exact location and time when they expected soybean rust spores to blow up into the Southern U.S. They were proved wrong when a hurricane brought the spores in early. Others (who hoped the invading rust would wipe out kudzu) were also proved wrong as this noxious weed just provided another host for the fungus to sporulate on.
"Alternate hosts" of rust diseases simply refer to the required host that we don't care about. In the devastating forest disease, white pine blister rust, the alternate hosts include gooseberries and currents (Ribes spp.). Unlike the previous example though, this rust attacks our host of interest as basidiospores and severely damages trees with the repeated production of bark-rupturing aecia. Although the aeciospores can travel for hundreds of kilometers to infect Ribes plants, the basidiospores that cycle back to attack pines are very fragile and short-lived. So, unlike these grass rusts, an infected pine can only sicken its neighbors if there are Ribes plants within range. For awhile, the New Deal's Works Progress Administration tried to take advantage of this by eradicating this alternate host from mountain forests of North America. To some extent, eradication of alternate hosts is still used today. And in fact, this tactic was one of the first recorded descriptions of people dealing with plant pathogens.***
Agrios, G.N. 1997. Plant Pathology. Academic Press.
*I'm impressed that Wikipedia correctly names these spores "urediniospores" and not "uredospores." This common misspelling has been tracked back to a single typo in a scientific article from many years ago. Unfortunately, they go on to state that this asexual spore stage produces basidia...
**Not to be confused with Robannukah.
***As early as the 1600 and 1700's, farmers in France and Connecticut independently noticed that wheat rust appeared to be worse near barberry bushes. Correctly deducing that the rust somehow fed on barberry, they convinced their towns to eradicate the bush as a way of controlling the disease.