3.6.4: Rusts and Smuts

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Rusts (Pucciniomycotina)

Rusts are plant pathogens in the subphylum Pucciniomycotina that infect one or more host species. Rusts have amazing and complex life cycles (as you saw in chapter 3.6.3) potentially involving multiple hosts and as many as five different spore stages! Autoecious rusts continue to infect the same host species, while heteroecious rusts must use multiple species of plant hosts to complete their life cycle. Rusts that form all five different types of propagules during their life cycle are called macrocyclic, while rusts that lack one or more of these spore stages are called microcyclic.

The underside of a leaf with raised, reddened areas. These are covered with small orange pustules. — Figure \(\PageIndex{1}\): *Puccinia lapsanae,* a rust fungus, forming small orange pustules in reddened regions on a *Lapsana communis* leaf. H. Krisp, CC BY 3.0, via Wikimedia Commons.

Several micrographs of similar looking spores: Each is a large, septate spore with two compartments at the top of a transparent stalk. — Figure \(\PageIndex{2}\): A variety of teliospores and urediniospores. These spore stages look incredibly similar and are generally easiest to distinguish based on seasonality (usually summer vs. fall formation). Image by McAlpine, Daniel, 1848-1932, No restrictions, via Wikimedia Commons.

A cluster of brown, stalked spores. Each spore has a septum dividing it in half. — Figure \(\PageIndex{3}\): Teliospores of *Puccina helianthi.* Photo by Vyzhdova V, CC BY 4.0, via Wikimedia Commons.

Fir needles with many skinny yellow columns sprouting from the underside — Figure \(\PageIndex{4}\): Heteroecious rusts alternate between different host plants. *Pucciniastrum goeppertianum*, fir-huckleberry rust, produces spermatogonia and aecia (yellowish columns on underside of needles, indicated by white arrows in first image) on firs. Aeciospores are then released and, if they are lucky, infect a huckleberry plant. On the huckleberry, infections manifest as swellings under the bark of younger branches (second image, the central branch is oddly enlarged and wrinkled), eventually bursting open to release spores. Uredinia and telia are produced on the huckleberry host. Urediniospores can reinfect the huckleberry plant or other huckleberries. Later in the season, teliospores are formed and can overwinter in the soil, germinating in the spring to release basidiospores that can then infect fir. Images by Maria Morrow, CC-BY 4.0.

Huckleberry branches, one is oddly thickened and wrinkly — Figure \(\PageIndex{4}\): Heteroecious rusts alternate between different host plants. *Pucciniastrum goeppertianum*, fir-huckleberry rust, produces spermatogonia and aecia (yellowish columns on underside of needles, indicated by white arrows in first image) on firs. Aeciospores are then released and, if they are lucky, infect a huckleberry plant. On the huckleberry, infections manifest as swellings under the bark of younger branches (second image, the central branch is oddly enlarged and wrinkled), eventually bursting open to release spores. Uredinia and telia are produced on the huckleberry host. Urediniospores can reinfect the huckleberry plant or other huckleberries. Later in the season, teliospores are formed and can overwinter in the soil, germinating in the spring to release basidiospores that can then infect fir. Images by Maria Morrow, CC-BY 4.0.

Smuts (Ustilaginomycotina)

Smuts are the third subphylum within Basidiomycota. These fungi form saprotrophic yeasts and can have a variety of ecological roles, but are most famous for their plant pathogens and perhaps the smut fungus (Malassezia) that lives on the oils produced on human skin, sometimes causing the formation of dandruff on your scalp.

A chaotic jumble of swollen grey and pink structures emerging from an ear of corn — Figure \(\PageIndex{5}\): *Ustilago maydis*, Ustilaginaceae, Corn smut. The strange growths on these ears of corn are swollen kernels, infected by a smut fungus. In the image on the right, dark brown powdery teliospores are bursting from one of the kernels. Left: H. Krisp, CC BY 3.0, via Wikimedia Commons. Right: Ustilago_maydis_002.JPG: H. Zellderivative work: Ak ccm, CC BY 3.0, via Wikimedia Commons.

Several healthy corn kernels with a few large, pink swellings below them. One has burst, showing it is filled with powdery brown spores — Figure \(\PageIndex{5}\): *Ustilago maydis*, Ustilaginaceae, Corn smut. The strange growths on these ears of corn are swollen kernels, infected by a smut fungus. In the image on the right, dark brown powdery teliospores are bursting from one of the kernels. Left: H. Krisp, CC BY 3.0, via Wikimedia Commons. Right: Ustilago_maydis_002.JPG: H. Zellderivative work: Ak ccm, CC BY 3.0, via Wikimedia Commons.

Globose, brown spores viewed through the microscope — Figure \(\PageIndex{6}\): Teliospores of *Ustilago maydis*. Image credit: 2011-08-27_Ustilago_maydis_(DC.)_Corda_183839.jpg: This image was created by user Alan Rockefeller (Alan Rockefeller) at Mushroom Observer, a source for mycological images.You can contact this user here. +/−derivative work: Ak ccm, CC BY-SA 3.0, via Wikimedia Commons.

Two different globose spores (sp) with hyphal structures emerging (pm). These hyphal structures branch to produce new cells (d). — Figure \(\PageIndex{7}\): A diagram of germinating teliospores from A, *Ustilago receptaculorum*; B, of *Tilletia caries*. The drawings are labeled as follows: d=basidiospore (AKA sporidia), pm=basidium (AKA promycelium), sp=teliospore. Image from Vine’s Students’ Text Book of Botany, Public domain, via Wikimedia Commons.

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