Rust in Biology: Complete Guide

Plant diseases induced by pathogenic fungi of the order Pucciniales are known as rusts (previously known as Uredinales). There are currently 168 rust genera and 7,000 species recognised, with more than half of them belonging to the genus Puccinia. Rust fungi are highly specialised plant diseases with a number of distinguishing characteristics. Rust fungi are a varied group of organisms that harm a wide range of plants. Each species, however, has a very limited host range and cannot be transmitted to non-host plants. Furthermore, most rust fungi are difficult to cultivate in pure culture.

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In different stages of its life cycle, a single species of rust fungus can infect two different plant hosts and produce up to five morphologically and cytologically diverse spore-producing structures, including spermogonia, aecia, uredinia, telia, and basidia. Each spore type has a very specialised host and can often only infect one variety of plants.

Only living plants are infected by a rust fungus, which is an obligatory plant disease. When a spore lands on the plant's surface, it germinates and infects its host. Plant parts infected include leaves, petioles, tender shoots, stems, fruits, and so forth. Plants infected with severe rust may be stunted, chlorotic (yellow), or show symptoms of infection such as rust fruiting bodies. Rust fungus develops within cells and produces spore-producing fruiting bodies within or on the surfaces of afflicted plant components. Some rust species can induce plant abnormalities such as growth retardation, witch's broom, stem canker, galls, or enlargement of susceptible plant sections by forming perennial systemic infections.

Rusts acquire their name from the deposits of powdery rust-coloured or brown spores that typically appear on plant surfaces. Robigalia (April 25) is a Roman agricultural festival with ancient origins in combating wheat rust.

Impacts 

Rusts are among the most destructive infections in agriculture, horticulture, and forestry. Rust fungi are a major source of concern and a limiting factor in the production of agricultural and forest crops. Known destructive dangers to economically important crops include white pine blister rust, wheat stem rust, soybean rust, and coffee rust.

Life Cycle 

To complete their life cycle, all rusts are obligatory parasites, meaning they require a living host. They do not usually harm the host plant, but they can significantly limit growth and yield. Crops can be destroyed in a single season, and oak trees infected with the rust Cronartium quercuum in the main stem within their first five years generally die.

Depending on the species, rust fungi can produce up to five spore types from corresponding fruiting body types during their life cycle. These morphological types have typically been referred to by Roman numerals.

• 0-Spermatia (Pycniospores) from Pycnidia. In heterothallic rusts, they mostly serve as haploid gametes.

• I-Aeciospores from Aecia. These spores are non-repeating, dikaryotic, and asexual, and they infect the primary host.

• II-Urediniospores from Uredia (Uredinia). These are dikaryotic vegetative spores that repeat themselves. Because these spores can cause auto-infection on the original host, re-infecting the same host on which the spores were formed, they are referred to as the repeating stage. They're usually abundant, red/orange, and a telltale symptom of rust sickness.

• III-Teliospores from Telia. The survival/overwintering stage of the life cycle is commonly represented by dikaryotic spores. Rather than directly infecting a plant, they germinate to produce basidia and basidiospores.

• IV-Basidiospores from Teliospores. In the spring, these windborne haploid spores frequently infect the alternate host. Outside of the laboratory, they are rarely seen.

The life cycle of rust fungi is frequently used to classify them. The number of spore types distinguishes three major types of life cycles: macrocyclic, demicyclic, and microcyclic. All spore states are present in the macrocyclic life cycle, while the demicyclic lacks the uredinial state and the microcyclic cycle lacks the basidial, pycnial, and aecial phases, leaving just uredinia and telia. Each form of the life cycle, particularly the microcyclic life cycle, may be devoid of spermatogonia. Rust can be either host alternating (heteroecious) (i.e., the aecial state is on one sort of plant but the telial state is on a different and unrelated plant) or non-host alternating (autoecious) in macrocyclic and demicyclic life cycles (i.e., the aecial and telial states on the same plant host). Heteroecious rust fungus requires two unrelated hosts to complete their life cycle, with aeciospores infecting the primary host and basidiospores infecting the alternate host. An autoecious fungus, on the other hand, can complete all stages of its life cycle on a single host species. The life cycles of rust fungi must be understood in order to effectively manage the illness.

Infection Process

Rust fungi spores can be spread by wind, water, or insect vectors. When a spore comes into contact with a plant that is susceptible, it can germinate and infect plant tissues. Rust spores usually germinate on the surface of a plant, producing a small hypha known as a germ tube. Thigmotropism, a touch-responsive mechanism, could help this germ tube find a stoma. This entails it aligning itself to ridges on the leaf surface formed by epidermal cells and growing in a direction until it reaches a stoma.

A hyphal tip forms an infectious structure termed an appressorium over the stoma. A slender hypha extends downward from the underside of an appressorium to infect plant cells. Stretch-sensitive calcium ion channels in the tip of the hypha are hypothesised to be responsible for promoting appressorium development by producing electric currents and altering gene expression.

Once the fungus has infected the plant, it develops into the mesophyll cells and produces haustoria, which are specialised hyphae. Plant cell membranes invaginate around the primary haustorial body, generating the extra-haustorial matrix. The haustoria penetrate cell walls but not cell membranes. An iron and phosphorus-rich neckband connect the plant and fungal membranes in the apoplast, the area between cells for water flow, blocking nutrients from accessing the plant's cells. Amino acid and hexose sugar transporters, as well as H+-ATPases, are found in the haustorium and are responsible for the active transport of nutrients from the plant to the fungus. The fungus continues to spread, penetrating more and more plant cells until it reaches spore formation. Every 10–14 days, the process repeats itself, resulting in a large number of spores that can be transmitted to other sections of the same plant or to new hosts.

Common Rust Fungi in Agriculture 

• Cronartium ribicola (White pine blister rust)

• Gymnosporangium juniperi-virginianae (Cedar-apple rust)

• Hemileia vastatrix (Coffee rust)

• Phakopsora meibomiae and P. pachyrhizi (Soybean rust)

• Puccinia coronata (Crown Rust of Oats and Ryegrass)

• Puccinia graminis (Stem rust of wheat and Kentucky bluegrass, or black rust of cereals)

• Puccinia hemerocallidis (Daylily rust)

• Puccinia triticina (Brown Wheat Rust) in grains

• Puccinia sorghi (Common Rust of Corn)

• Puccinia striiformis (Yellow Rust) of cereals

• Uromyces appendiculatus (Bean Rust) in common bean (Phaseolus vulgaris)

• Puccinia melanocephala (Brown Rust of Sugarcane)

• Puccinia kuehnii (Orange rust of Sugarcane)

Do you know?

How to treat plant rust disease?

• When possible, use rust-resistant plant cultivars.

• Pick off sick leaves and destroy them, and rake under plants frequently to eliminate all fallen debris.

• Water plants early in the morning, avoiding overhead sprinklers, to allow them to dry out over the day. To keep leaves dry, drip watering and soaker hoses can be employed.

• Avoid using too much nitrogen on your crops by using a slow-release organic fertiliser. New growth that is soft and leafy is the most vulnerable.

• Copper sprays or sulphur powders can be used to keep vulnerable plants from being infected. Apply early, or at the first indication of sickness, for the best outcomes. Spray all areas of the plant thoroughly and repeat every 7-10 days till harvest.

• With SERENADE Garden, you can effectively cure fungal problems. This broad-spectrum bio-fungicide is certified for organic gardening and uses a proprietary Bacillus subtilis strain. Most importantly, it's completely safe to use – you may treat and harvest crops on the same day!

• Bonide Orchard Spray, which contains sulphur and pyrethrins, is a safe, one-hit concentration for insect attacks and fungal problems. Apply early in the season as a protective spray (2.5 oz/gallon) for optimum results. Mix 5 ounces in one gallon of water if sickness, insects, or damp weather are present. Spray all sections of the plant, especially young shoots, thoroughly.

• To increase air circulation, prune or stake plants and remove weeds. After each cut, sterilise your pruning tools with a solution of one part bleach to four parts water.

• After you've raked and cleansed the soil well, cover it with a thick layer of mulch or organic compost. The disease spores will not splash back up onto the leaves if the leaves are mulched.

• After the growing season, burn or bag affected plants (see Fall Garden Cleanup). Composting is not an option.