Rust (fungus)

Rusts are plant diseases caused by pathogenic fungi of the order Pucciniales. About 7800 species are known. Rusts can affect a variety of plants; leaves, stems, fruits and seeds. Rust is most commonly seen as coloured powder, composed off tiny aeciospores which land on vegetation producing pustules, or uredia, that form on the lower surfaces. During late spring or early summer, yellow orange or brown, hairlike or ligulate structures called telia grow on the leaves or emerge from bark of woody hosts such as Juniperus species. These telia produce teliospores which will germinate into aerial basidiospores, spreading and causing further infection.

Taxonomy

The taxonomy of Pucciniales is complex and the darker coloured smuts can be mistaken for rust. Rusts are so named after the reddish rusty looking sori and the disease is usually noticed after rain.

The group is considered as one of the most harmful pathogens to agriculture and horticulture.

Spores

Rusts can produce up to five spores types during their life cycle

0-Pycniospores (Spermatia)-Haploid gametes in heterothallic rusts.

I-Aeciospores-non-repeating dikaryotic vegetative spores

II-Urediniospores-repeating dikaryotic vegetative spores. These spores are referred to as the repeating stage because they can cause auto-infection (re-infect the same host from which the spores were borne). These spores are red/orange and are a characteristic sign of rust fungus infection.

III-Teliospores-Diploid spores that produce basidia and are the survival stage of life cycle

IV-Basidiospores-stem from basidia. Haploid spores which infect the alternate host. although these are rarely observed outside of the laboratory.

Rust fungi can be categorized by how many types of spores are produced during the life cycle. Fungi that produce all five spores (sometimes excluding pycniospores) are termed macrocyclic. Fungi that lack pycniospores, aeciospores, and urediniospores in their life cycle are termed microcyclic and always have an autoecious life cycle. Demicyclic fungi delete the uredial (repeating) stage from the life cycle. Understanding the life cycles of rust fungi allows for proper disease management.

Life cycle

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. Cereal crops can be devastated in one season and trees that get infected in the main stem within the first five years, invariably die.

Rust fungi can also be categorized by their life cycle. 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 with basidiospores. This can be contrasted with an autoecious fungus which can complete its life cycle on a single host species.

Infection process

The fungi produce asexual spores which disperse by wind, water or by insect vectors spreading the infection. by and plants.

Rust fungi are biotrophs taking nutrients from living cells. When airborne spores settle on a plant, weak hydrophobic interactions are formed with the cutin on the plant cell surface, securing it. By a process not fully understood, the production of mucous like substances called 'adhesins', initially stick the spore to the plant surface.

Once attached, the spore germinates by growing a germ tube and eventually locates a stoma by a touch responsive process known as thigmotropism. This involves growing towards a ridge between the epidermal cells, followed by a perpendicular growth which end at the stoma. Inside the stoma, the hyphae tips flatten out to form structures known as appresorium that lock to the cell walls. It is thought that the whole process is mediated by a mechano-sensitive calcium ion channel, located within the germ tube tip, which produces electric currents that stretch the cell membranes, changing gene expression and forming the appresorium.

Then a peg grows into the plant's mesophyll cells. The peg produces specialised hyphal tips, known as haustorium. These spread around the plant cells without invading the membranes. The 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. It continues growing until spore growth occurs. The process repeats every 10 – 14 days, producing numerous spores, carried by wind to new hosts.

Common Rust Fungi in Agriculture

Gymnosporangium juniperi-virginianae (Cedar-apple rust); the Juniperus virginiana is the primary (telial) host and the apple, pear or hawthorn is the secondary (aecial) host.Heteroecious and demicyclic

Cronartium ribicola (White pine blister rust); the primary host are white pines, and currants the secondary. Heterocyclic and macrocyclic

Hemileia vastatrix (Coffee rust); Primary host is coffee plant; Unknown alternate host. Heteroecious

Puccinia graminis (Stem rust of wheat and Kentucky bluegrass); Primary hosts include: Kentucky bluegrass, barley, and wheat; Common barberry is the alternate host. Heteroecious and macrocyclic

Puccinia coronata (Crown Rust of Oats and Ryegrass); Oats are the primary host; Rhamnus spp. (Buckthorn) is alternate host. Heteroecious and macrocyclic

Phakopsora meibomiae and P. pachyrhizi (Soybean Rust); Primary host is soybean and various legumes. Unknown alternate host. Heteroecious

Uromyces phaseoli (Bean rust); Primary host:bean. Autoecious and macrocyclic

Puccinia hemerocallidis (Daylily rust); Daylily is primary host; Patrina sp is alternate host. Heteroecious and macrocyclic

Puccinia persistens subsp. triticina causes wheat rust in grains. It is also known as 'brown or red rust'.

P.sriiformis causes 'Yellow rust' and P. graminis causes 'black rust' in cereals.

Phakopsora pachyrhizi or P. meibomiae cause serious rust damage to soyabean.

Uromyces appendeculatus is a problem on beans

Hemileia vastatrix affects Coffea arabica - Coffee plants

Management of Rust Fungi Diseases

The control methods of rust fungus diseases depend largely on the life cycle of the particular pathogen. The following are examples of disease management plans used to control macrocyclic and demicyclic diseases:-

Macrocyclic Disease: Developing a management plan for this type of disease depends largely on whether the repeating stage (urediniospores) occur on the economically important host plant or the alternate host. For example, the repeating stage in white pine blister rust disease does not occur on white pines but on the alternate host, Ribes spp. During August and September Ribes spp. give rise to teliospores which infect white pines. Removal of the alternate host disrupts the life cycle of the rust fungi Cronartium ribicola, preventing the formation basidiospores which infect the primary host. Although spores from white pines cannot infect other white pines, survival spores may overwinter on infected pines and reinfect Ribes spp. the following season. Infected tissue is removed from white pines and strict quarantines of Ribes spp. are maintained in high risk areas.

Puccinia graminis is a macrocyclic heteroecious fungus that causes wheat stem rust disease. The repeating stage in this fungus occurs on wheat and not the alternate host, barberry. The repeating stage allows the disease to persist in wheat even though the alternate host may be removed. Planting resistant crops is the ideal form of disease prevention, however, mutations can give rise to new strains of fungi that can overcome plant resistance. Although the disease cannot be stopped by removal of the alternate host, the life cycle is disrupted and the rate of mutation is decreased because of reduced genetic recombination. This allows resistance bred crops to remain effective for a longer period of time.

Demicyclic Disease: Because there is no repeating stage in the life cycle of demicyclic fungi, removal of the primary or the alternate host will disrupt the disease cycle. This method, however, is not highly effective in managing all demicyclic diseases. Cedar-apple rust disease, for example, can persist despite removal of one of the hosts because spores can be disseminated from long distances. The severity of Cedar-apple rust disease can be managed by removal of basidiospore producing galls from junipers or the application of protective fungicides to junipers.

Home control

Rust is very hard to treat. Fugicides such as Mancozeb or Triforine may help but may never eradicate the disease. Some organic preventative solutions are available and Sulphur powder is known to stop germination. High standards of hygiene and good soil drainage and careful watering may minimise problems. Any appearance of rust must be immediately dealt with by removing and burning all affected leaves. Composting, or leaving infected vegetation on the ground spread the disease.

Commercial control

In large plantations in USA fungicide is applied by air. The process is expensive and fungicide application is best reserved for seasons when foliar diseases are severe. Research indicates, the higher the foliar disease severity, the greater the return from the use of fungicides. There are a variety of preventative methods that can also be employed.

Symptoms of rust disease are correlated to relatively high moisture. The avoidance of overhead watering at night, using drip irrigation, reducing crop density, and using fans to circulate air flow will lower the relative moisture and decrease the severity of rust infection.

The use of rust resistant plants

Crop rotation can break the disease cycle because many rusts are host specific.

Inspect all imported plants and cuttings for symptoms. It is important to continuously observe these plants because rust diseases have a latent period (plant has the disease but shows no symptoms).

Many crops, such as wheat, are replanted with disease-free seed.

Host plants affected

Rusts are often named after the host species that they infect. For example; Puccinia xanthii infects the flowering plant Cocklebur. It is probable that most plant species are affected by some species of rust. Some of the better known hosts include

Arisaema triphyllum, Jack-in-the-pulpit

Avena sativa, Oats

Barberis vulgaris, Barberry

Broad beans

Crataegus monogyna - Hathorn

Chrysanthemum

Cydonia - Quince

Euphorbia maculata, Spotted Spurge

Fuchsia spp, Fuchsia

Garlic

Hordeum vulgare, Barley

Juniperus virginianae, Red Cedar (Juniper apple disease)

Juniperus communis - Juniper

Allium ampeloprasum - Leek

Malus – Apple

Mespilus - Medlar

Onion

Pelargonium

Primula veris

Primula vulgaris

Pyrus - Pear

Rosa spp, Roses

Triticum spp., Wheat

Oxalis spp., Oxalis

Secale cereale, Rye

Senecio vulgaris -Common groundsel

Xanthium canadenseCocklebur

See also

Wheat leaf rust

Leaf rust (barley)

Fungus

Smut (fungus)

soybean rust

Gallery

References

Category:Basidiomycota Category:Plant pathogens and diseases