Rust (fungus)

Rusts are considered among the most harmful pathogens to agriculture, horticulture and forestry. Rust fungi are major concerns and limiting factors for successful cultivation of agricultural and forest crops. White pine blister rust, wheat stem rust, soybean rust, and coffee rust are examples of notoriously damaging threats to economically important crops.[2] Climate change can have a possible impact to increase rust fungi by increase in CO2 and O3, climate warming, humidity, extreme weather changes.[5]

All rusts are obligate parasites, meaning that they require a living host to complete their life cycle. They generally do not kill the host plant but can severely reduce growth and yield.[6] Cereal crops can be devastated in one season; oak trees infected in the main stem within their first five years by the rust Cronartium quercuum often die.[7]

Germinating urediniospore of Puccinia graminis, model from the late 19th century, Botanical Museum Greifswald

Rust fungi can produce up to five spore types from corresponding fruiting body types during their life cycle, depending on the species. Roman numerals have traditionally been used to refer to these morphological types.

• 0-Pycniospores (Spermatia) from Pycnidia. These serve mainly as haploid gametes in heterothallic rusts.
• I-Aeciospores from Aecia. These serve mainly as non-repeating, dikaryotic, asexual spores, and go on to infect the primary host.
• II-Urediniospores from Uredia (Uredinia). These serve as repeating dikaryotic vegetative spores. These spores are referred to as the repeating stage because they can cause auto-infection on the primary host, re-infecting the same host on which the spores were produced. They are often profuse, red/orange, and a prominent sign of rust disease.
• III-Teliospores from Telia. These dikaryotic spores are often the survival/overwintering stage of the life cycle. They usually do not infect a plant directly; instead they germinate to produce basidia and basidiospores.
• IV-Basidiospores from Teliospores. These windborne haploid spores often infect the alternate host in Spring.[8][9] They are rarely observed outside of the laboratory.

Rust fungi are often categorized by their life cycle. Three basic types of life cycles are recognized based on the number of spore types as macrocyclic, demicyclic, and microcyclic.[2] The macrocyclic life cycle has all spore states, the demicyclic lacks the uredinial state, and the microcyclic cycle lacks the basidial, pycnial, and the aecial states, thus possess only uredinia and telia. Spermagonia may be absent from each type but especially the microcyclic life cycle. In macrocyclic and demicyclic life cycles, the rust may be either host alternating (heteroecious) (i.e., the aecial state is on one kind of plant but the telial state on a different and unrelated plant), or non-host alternating (autoecious) (i.e., the aecial and telial states on the same plant host).[2] Heteroecious rust fungi require two unrelated hosts to complete their life cycle, with the primary host being infected by aeciospores and the alternate host being infected by basidiospores. This can be contrasted with an autoecious fungus, such as Puccinia porri, which can complete all parts of its life cycle on a single host species.[8] Understanding the life cycles of rust fungi allows for proper disease management.[10]

There are definite patterns of relationship with host plant groups and the rust fungi that parasitize them. Some genera of rust fungi, especially Puccinia and Uromyces, comprise species that are capable of parasitizing plants of many families. Other rust genera appear to be restricted to certain plant groups. Host restriction may, in heteroecious species, apply to both phases of life cycle or to only one phase.[2] As with many pathogen/host pairs, rusts are often in gene-for-gene relationships with their plants. This rust-plant gene-for-gene interaction differs somewhat from other gene-for-gene situations and has its own quirks and agronomic significance.

The spores of rust fungi may be dispersed by wind, water or insect vectors.[11] When a spore encounters a susceptible plant, it can germinate and infect plant tissues. A rust spores typically germinates on a plant surface, growing a short hypha called a germ tube. This germ tube may locate a stoma by a touch responsive process known as thigmotropism. This involves orienting to ridges created by epidermal cells on the leaf surface, and growing directionally until it encounters a stoma.[12]

Rust hypha attacking stoma (1600x magnification)

Over the stoma, a hyphal tip produces an infection structure called an appressorium. From the underside of an appressorium, a slender hypha grows downward to infect plant cells[13] It is thought that the whole process is mediated by stretch-sensitive calcium ion channels located in the tip of the hypha, which produce electric currents and alter gene expression, inducing appressorium formation.[14]

Once the fungus has invaded the plant, it grows into plant mesophyll cells, producing specialized hyphae known as haustoria. The haustoria penetrate cell walls but not cell membrances: plant cell membranes invaginate around the main haustorial body forming a space known as the extra-haustorial matrix. An iron and phosphorus rich neck band bridges the plant and fungal membranes in the space between the cells for water flow, known as the apoplast, thus preventing the nutrients reaching the plant's cells. The haustorium contains amino acid and hexose sugar transporters and H⁺-ATPases which are used for active transport of nutrients from the plant, nourishing the fungus.[15] The fungus continues growing, penetrating more and more plant cells, until spore growth occurs. The process repeats every 10 – 14 days, producing numerous spores that can be spread to other parts of the same plant, or to new hosts.

[8][10][16]

• Cronartium ribicola (White pine blister rust); the primary hosts are currants, and white pines the secondary. Heterocyclic and macrocyclic
• Gymnosporangium juniperi-virginianae (Cedar-apple rust); Juniperus virginiana is the primary (telial) host and apple, pear or hawthorn is the secondary (aecial) host. Heteroecious and demicyclic
• Hemileia vastatrix (Coffee rust); Primary host is coffee plant; unknown alternate host. Heteroecious
• Phakopsora meibomiae and P. pachyrhizi (Soybean rust); Primary host is soybean and various legumes. Unknown alternate host. Heteroecious
• Puccinia coronata (Crown Rust of Oats and Ryegrass); Oats are the primary host; Rhamnus spp. (Buckthorn) is alternate host. Heteroecious and macrocyclic
• Puccinia graminis (Stem rust of wheat and Kentucky bluegrass, or black rust of cereals); Primary hosts include: Kentucky bluegrass, barley, and wheat; Common barberry is the alternate host. Heteroecious and macrocyclic
• Puccinia hemerocallidis (Daylily rust); Daylily is primary host; Patrinia sp is alternate host. Heteroecious and macrocyclic
• Puccinia triticina (Brown Wheat Rust) in grains
• Puccinia sorghi (Common Rust of Corn)[17]
• Puccinia striiformis (Yellow Rust) of cereals
• Uromyces appendiculatus (Bean Rust) in common bean (Phaseolus vulgaris)[18]
• Puccinia melanocephala (Brown Rust of Sugarcane)
• Puccinia kuehnii (Orange rust of Sugarcane)
• Puccinia porri (Leek rust); Autoecious

In the family Sphaeropsidaceae of Sphaeropsidales fungi, species of the genus Darluca are hyperparasites on rusts.[25]

• 
  Rust fungus on a leaf, under low magnification.
• 
  Urediniospores of a rust fungus.
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  Diagram representing the infection process of rust fungi
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  Rust fungus, Puccinia urticata on the surface of a nettle leaf
• 
  Rust on onions

• Stem rust
• Wheat leaf rust
• Leaf rust (barley)
• Fungus
• Smut (fungus)
• Soybean rust
• Rust (programming language) (named after the Rust fungus)

• Media related to Pucciniales at Wikimedia Commons