Rust (fungus)

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Rusts are diseases caused by fungal pathogens of the order Pucciniales. About 7800 species are known. The taxonomy of Pucciniales is complex and the darker coloured smut is often mistaken for rust. Rusts are so named after the reddish rusty looking sori and the disease is usually noticed after the first rains. The group is considered as one of the most dangerous pathogens to agriculture and horticulture. 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.[1] Cereal crops can be devastated in one season and trees that get infected in the main stem within the first five years, invariably die.[2]

These types of fungi can produce up to five spores 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 reapeating 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.

[3] [4]

Rust fungi can 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 cylce on a single host species. [5]

Rust fungi can be further categorized by how many 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.[6]

Common Rust Fungi in Agriculture[7] [8] [9]

  • Gymnosporangium (Cedar-apple rust); the juniper 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

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