Rust fungi
Rust fungi

Rust fungi belong to the taxonomic class Teliomycetes within the basidiosporic phylum and include some seven thousand known species, all plant parasites, with many being extremely important plant pathogens.

Rust diseases have caused serious problems for centuries and continue to cost billions of dollars annually worldwide.

Important examples include: leaf, stem, and crown rusts of wheat, barley, and oats; the blister, fusiform, and gall rusts of pines; coffee rust; many field crop rusts, such as soybean, peanut, sunflower, and flax rusts; and rusts of many horticultural crops and ornamental plants, such as cedar-apple rust, bean rust, and rose rust.

Symptoms and Signs

Symptoms, the visible effects on a host caused by rust diseases, are often less noticeable than are the signs of infection, that is, the visible presence of a pathogen, such as reproductive spores. Symptoms such as bushy overgrowths, witches’ brooms (dense clusters of shoots), and large galls are more common in some tree hosts than in herbaceous hosts.

In most herbaceous rusts and the foliar rusts of some trees, such as coffee and poplar, little, if any, distortion of the host occurs. Most noticeable are the rust’s reproductive spores borne in various types of pustules (or sori) that rupture leaf and stemepidermis.

Loss of water vapor through ruptured epidermis can cause wilt symptoms during hot, dry periods. Several kinds of spores and pustules may develop during different stages of the rust’s life cycle. Combined results of parasitism, water loss, cell death, and other effects on the host can result in significant reductions in yield.

Life Cycle and Related Spore Stages

Rust fungi have a complex life cycle that may contain up to five different spore types, or stages, each performing specific functions in the cycle. Some rust species lack one or more spore stage or stages. Autoecious forms of rusts, such as bean rust, complete their life cycle on one host species.

Heteroecious rusts require two unrelated hosts on which to complete their life cycle. Puccinia graminis, the wheat stem rust pathogen, which produces all five spore stages on two unrelated hosts, is a good example of a rust with the heteroecious life cycle.

Distribution and Dissemination

Puccinia monoica 'Rust Fungus' infecting a Boechera sp
Puccinia monoica 'Rust Fungus' infecting a Boechera sp

Initial distribution of rusts between continents, especially if separated by oceans, is often the result of human activity, such as the inadvertent introduction of the pathogen on seeds, cuttings, or other plant parts. Once established in a region, rusts are most often spread locally and long distance by wind-blown urediniospores.

In North America, south-to-north dispersal of urediniospores of wheat stem and leaf rust fungi occurs annually from Mexico and south Texas through the Great Plains into Canada. Coffee rust, long absent in the Western Hemisphere, rapidly spread across South and Central America by wind dispersal following its 1970 introduction into Brazil.

Obligate Parasitism

Although a few rusts can be grown on chemically defined, nonliving nutrients, most rusts require a living host on which to grow and reproduce. Consequently, rusts are termed obligate parasites or biotrophs, in contrast to the majority of plant pathogens, which are facultative parasites or heterotrophs.

Those can grow and reproduce on many nutritional media in culture, in dead plant tissue, or as parasites of living plants. Within infected host tissue, rust fungi grow primarily as filamentous hyphae in intercellular spaces between host cells.

Rusts obtain nutrition through specialized branches, haustoria, that extend into and absorb nutrients from living host cells. Those absorbed nutrients are then transported through intercellular hyphae to expanding or spore-producing portions of the rust colony.

Disease Control

Typically, total control of rust diseases is impossible, both biologically and economically; hence, the usual goal is management of rust diseases, not control.

Several different management methods are commonly pursued, depending on the host, the type of rust involved, its major means of dissemination, and economics or aesthetic value of the host plant or crop. The ideal and most cost-effective management tool is host resistance, either naturally present in the host population or resulting from plant breeding.

Breeding for rust resistance has been especially important in rusts of cereals and other major agronomic plants. Unfortunately, many resistant varieties, or cultivars, may eventually succumb to new, more virulent or aggressive strains or races of the pathogen that arise by natural selection from populations of the pathogen in nature. Foliar fungicide sprays can be cost-effective management tools for rusts of some cereal crops, beans, coffee, and roses.

In some instances, removing or eradicating the alternate host, such as common barberry, in heteroecious rusts has controlled such diseases but has usually proven ineffective economically.