Rust and Smut
Rust and Smut, both diseases of plants, are caused by pathogenic fungi belonging to the class Basidiomycetes. It is caused by the pathogenic fungi of the order Pucciniales or also known as Uredinales. Rust and Smut, both are the limiting factors for successful cultivation for forest crops and agriculture.
Rust: Rusts are known to be the most dangerous pathogens to agriculture, forestry, and horticulture. Some of the examples of rust are wheat stem rust, white pine blister rust, soybean rust, coffee rust, etc. These are economically important crops that are affected by rust. All of the rusts are parasites and they require a living host for completion of the life cycle and here, they can severely rescue the yield and growth of the host plant.
Some of the characteristic features of rusts are:
The dikaryotic mycelium produces three kinds of binucleated spores: (i) uredospores and (ii) teleutospores on the primary host, and (iii) aeciospores on the alternate host.
The terminal cells of the mycelium produce the teleutospores arc.
Smut: Smuts belong to the group of multicellular fungi that are characterized by the presence of numerous teliospores. Smut got its name from a Germanic word for dirt, meaning dark, dust-like, and thick-walled teliospores. Smuts are usually Ustilaginomycetes belonging to the phylum Basidiomycota. Smuts mostly affect cereal and crop pathogens, which are the members of the grass family (Poaceas) and sedges (Cyperaceae). Some of the other host plants affected by Smut are Maize, Barley, Wheat, Oats, Forage Grasses, and Sugarcane.
Some of the characteristic features of smuts are:
All smuts are autoecious, i.e., they complete their entire life cycle on a single host.
They may be intercellular or intracellular (U. maydis). Haustoria are present.
Smut spores are created from the intercalary cells.
The teleutospores, i.e Brand spores, are unicellular and binucleate.
How do Rust and Smut affect plants?
Mechanism of Rusts
Rusts have definite patterns of association with host plant groups, where these parasitize them. Such host plant examples are Uromyces and Puccinia. Other rust genera are restricted to specific plant groups belonging to heteroecious species and host restriction may apply to both life cycle phases or to only one phase.
The spores of the fungi (rust) can be dispersed by wind, insect vectors, or water. When this spore encounters the host, it germinates and infects plant tissues.
The rust spores are germinated on a plant surface via a germ tube, which is a short hypha. This germ tube locates a stoma and the hyphal tip produces an infectious structure known as appressorium. Below this structure, a slender hypha starts growing downward to infect plant cells.
There are calcium ion channels located in the hypha tip that mediate the whole process and produce electric currents to alter gene expression that also includes appressorium formation.
Once the invasion of fungus into the plant happens, it starts growing into plant mesophyll cells and produces specialized hyphae called haustoria.
Haustoria can only penetrate cell walls, not cell membranes, and it contains amino acids and other favorable energy products that help in actively transporting nutrients to the fungus from the plant.
This way, the fungus starts growing and penetrates more plant cells till the spore growth takes place. This whole process repeats every 10-14 days, and thus, produces multiple spores that spread to other parts of the same plant as well as the new hosts.
Mechanism of Smut
Smuts are known to be cereal and crop pathogens that have teliospores.
Smuts eventually hijack the host plants’ reproductive systems that form galls, which darken and burst, thus releasing teliospores.
The teliospores infect the other plants nearby. Before occurrence of an infection, the smuts have to perform a successful mating, resulting in the formation of a dikaryotic hyphae, meaning two haploid cells fusing to form a dikaryon.
In the case of sugarcane, the fungi makes use of its smut-whip that ensures the disease spreads to other plants, and it usually occurs over a time period of 3 months.
Corn smut infecting maize is called huitlacoche in Mexico and they use it for several recipes, such as stews, steak, soups, etc. Corn smuts basically turn the kernels into powdery and black fungal tissues.
Learning the above mentioned points, students must have been much clearer about rust and smut and the differences between them. To explain further, we have put down a tabular chart that distinguishes rust and smut.
Difference between Rust and Smut
FAQs on Difference Between Rust and Smut
1. What is the main difference between rust and smut plant diseases?
The primary difference lies in their symptoms and the spores they produce. Rust diseases, caused by fungi like Puccinia, typically appear as reddish-brown, orange, or yellow pustules (sori) on the leaves and stems of plants. Smut diseases, caused by fungi like Ustilago, manifest as black, powdery, tumour-like galls that replace host tissues, such as the kernels of cereals, and contain masses of dark spores.
2. Which specific fungi are responsible for causing rust and smut diseases?
Rust and smut diseases are caused by different orders of fungi. Rust diseases are caused by fungi belonging to the order Pucciniales; a common example is Puccinia graminis, which causes wheat stem rust. Smut diseases are caused by fungi from the order Ustilaginales, with Ustilago maydis (corn smut) being a well-known example. You can find more details in the Important Questions for CBSE Class 11 Biology Chapter 2.
3. What are some common examples of plants affected by rust and smut?
These diseases affect many economically important plants. Common examples include:
- Rust: Wheat stem rust (on wheat), white pine blister rust (on pine trees), and cedar-apple rust (on apple and crabapple trees).
- Smut: Corn smut (on maize/corn), loose smut of wheat (on wheat), and covered smut of barley (on barley).
4. To which phylum of fungi do rusts and smuts belong?
Both rust fungi (Pucciniales) and smut fungi (Ustilaginales) belong to the phylum Basidiomycetes. This phylum is characterised by the production of spores, called basidiospores, on a club-shaped structure known as a basidium. More on this can be found in our Biological Classification NEET Notes.
5. Why are rust and smut fungi called obligate parasites?
Rust and smut fungi are called obligate parasites because they can only grow and reproduce on a living host plant. They cannot survive independently or be cultured on an artificial nutrient medium in a lab. They have evolved to rely completely on the living cells of the host for their nutrition and to complete their complex life cycles.
6. How do rust diseases differ from other fungal infections like blight?
The key difference is in the appearance and progression of the disease. Rusts typically form distinct, localised pustules (sori) that contain powdery spores, without immediately killing the entire plant tissue. In contrast, blight causes rapid, widespread discolouration, wilting, and death of plant parts like leaves and stems, often giving them a scorched appearance.
7. Do rust and smut fungi produce mushrooms or basidiocarps?
No, rust and smut fungi do not produce mushrooms or other large, fleshy fruiting bodies, which are technically known as basidiocarps. While they are Basidiomycetes, their spore-producing structures are microscopic. The visible signs of infection, such as rust pustules or smut galls, are simply masses of spores (teliospores) called sori, not organised fruiting bodies.
8. What is the economic significance of rust and smut diseases in agriculture?
The economic significance is substantial and almost always negative. These diseases are major agricultural pests that cause severe crop losses worldwide. Rusts can reduce photosynthetic area and deplete nutrients, leading to shrivelled grains and dramatically reduced yields in crops like wheat and coffee. Smuts often destroy the most valuable part of the crop, the grains or kernels, rendering them unmarketable and leading to significant financial losses for farmers. The economic importance of fungi highlights their role as both beneficial organisms and destructive pathogens.
