User:MonicaSyd-30/Rust (fungus)1

Rusts are plant diseases caused by pathogenic fungi of the order Pucciniales (previously known as Uredinales).

An estimated 168 rust genera and approximately 7,000 species, more than half of which belong to the genus Puccinia, are currently accepted. Rust fungi are highly specialized plant pathogens with several unique features. Taken as a group, rust fungi are diverse to which they can thrive in high humidity environments but also not limited to desert climates. This pathogen affects many kinds of plants. Among those greatly affected are agriculture crops and forest crops . However, each species has a very narrow range of hosts and cannot be transmitted to non-host plants. This is due to their biotrophic nature of their pathosystem of pathogen-host interaction in order to propagate which makes them a difficult species to culture based off of their specific genetic factors . In addition, most rust fungi cannot be grown easily in pure culture. A few studies have been done to try and understand the complex life cycle of genetic manipulation on gene function interaction on Puccinia graminis f. sp. tritici-, Puccinia triticna, Cronartium quercuum f. sp. fusiforme, Puccinia striiformis f. sp. tritici, etc.

A single species of rust fungi may be able to infect two different plant hosts in different stages of its life cycle, and may produce up to five morphologically and cytologically distinct spore-producing structures viz., spermogonia, aecia, uredinia, telia, and basidia in successive stages of reproduction. Each spore type is very host specific, and can typically infect only one kind of plant.

Rust fungi are obligate plant pathogens that only infect living plants. Infections begin when a spore lands on the plant surface, germinates, and invades its host. Infection is limited to plant parts such as leaves, petioles, tender shoots, stem, fruits, etc. Plants with severe rust infection may appear stunted, chlorotic (yellowed), or may display signs of infection such as rust fruiting bodies. Rust fungi grow intracellularly, and make spore-producing fruiting bodies within or, more often, on the surfaces of affected plant parts. Some rust species form perennial systemic infections that may cause plant deformities such as growth retardation, witch's broom, stem canker, galls, or hypertrophy of affected plant parts. Once infected by the host, rust fungi can be easily spread by wind-dispersal which can possibly cause an epidemic, especially within agriculture species such as wheat, cereal, oats, barley, corn, beans, coffee, etc. are to name a few major food groups. Rust normally exerts itself into its' host by first infiltrating the stomate from exposure. This allows appressorium to form. Once exposed on the leaf the most important part in order for rust fungal to occur is if the host penetrates through the epidermal cell wall allowing germination to begin . From there the pathogen goes through a detailed process to continue its' host invasion for survival.

Rusts get their name because they are most commonly observed as deposits of powdery rust-coloured or brown spores on plant surfaces. The Roman agricultural festival Robigalia (April 25) has ancient origins in combating wheat rust. Copied from Rust (fungus)

Impacts

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 which can cause epidemics. Studies on rust fungi continues to be an on going research today, but the expansion to rust genomes have no doubtedly been categorized to be more complex than other fungi . Some rust genomes have been able to be sequenced but scientist still don't know the genomic base structure of rust. Due to rust fungi largely affecting agriculture crops and forestries, there are continued rust studies that mainly focus on trying to generate their genomic structures to try aid those species for better resistance from rust fungi for crop and forestry protection. Climate change can have a possible impact to rust fungi due to the increase in CO2 and O3, climate warming, humidity, extreme weather changes. Depending on the type of rust fungus we could possibly see no change, declines, or even an increase from the rust pathogen.

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. Many rust fungi have annual life cycles . 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.

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. These kind of rust is generally small, sexual spores that are dikaryotic and get mainly dispersed by insects .
• I-Aeciospores from Aecia. These type of rust are larger (25μm) than Spermatia and serve mainly as non-repeating, dikaryotic, asexual spores, and go on to infect the primary host. They tend to be more resistant to environmental changes and get dispersed by both wind currents and insects.
• 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, can be be 15-30μm in size, and are a prominent sign of rust disease. This type of rust gets produced in large numbers and also has the highest spore dispersal by which are fine spined and have thick cell walls that are able to move long distances by both wind currents and human activity .
• 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. 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. 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). 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. Understanding the life cycles of rust fungi allows for proper disease management.

Host Plant-Rust Fungus Relationship Portion

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. These two species are considered to be some of the critical drivers for their diversification through their co-evolution with their host due to the fact they are not limited to one plant group that influences heavily on plants and vegetation. 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. This pathogen relationship usually happens within short generation times (weeks to months), but depending on a host plant that generates within months or serval years this can be detrimental. For example Herbaceous plants and trees that have long generational growth times are more often slower at reproducing to reach full growth that adaptation/evolution towards the host declines the overall fitness loss to the species. 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. From current studies we know, we still don't completely know the genomic base structure of the rust chromosome base genome, but some studies on certain rust have been sequenced. The Puccinia graminis f. sp. tritici and M. larici-populina genomes have been some of the first genomes to be sequenced. It turns out that the genome sequence that were studied are actually larger than other fungi expect for mildew fungi. Some rust fungi have higher haploid chromosomes ranging from 2 to 8. While some rust have multiple repeated DNA or transposable elements within their sequencing. So due to this scientist have realized that rust fungi have mutated and evolved quite considerably throughout its time which makes up for how specie specific they are for their gene-for-gene relationship. This can also due to their dikaryotic nature and need for heteroecious species .

Infection process

The spores of rust fungi may be dispersed by wind, water or insect vectors. Dispersal usually happens by the wind to which can spread up to 300m or more depending on wind current . 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. The rust then penetrates to enter through the epidermal cell wall to germinate. This first process is very important for the rust to begin the host-parasitic relationship to which it needs to happen at the right time. Once there's a successful entry then the pathogen will begin to use its' host to reduce the hosts' carbon sequestration and metabolizing photosyntheses for their own benefit .

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 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.

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. 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.

Management of rust fungi diseases

Commercial control

In some large acreage crops, fungicides are 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. Southern corn rust disease, can be confused with common rust. Southern rust's distinguishing characteristic is that pustules form mostly on the upper leaf surface and spores are more orange in color. Southern rust spreads more quickly and has a higher economic impact when hot, humid weather conditions persist. Timely fungicide applications to control southern rust are more crucial than with common rust. Rust control to minimize infection or epidemic have also been used as a reversal strategy to fight invasive species an area as a biocontrol agent to increase native species to flourish as well .