Fundamentals of Plant Pathology

EXERCISE 9

EXERCISE 9: Diseases Caused by Basidiomycota- Rusts

Objective: To Study the symptoms, signs and host-parasite relationship of diseases caused by Basidiomycota- Rusts

The rust fungi mostly attack leaves and stems of various crop plants. Rust infections usually appear as numerous rusty, orange, yellow or even white coloured pustules that rupture the epidermis. Some cause swellings and even galls. Most rust infections are strictly local spots but some may spread internally to some extent.

Causal organisms

Puccinia graminis tritici, causing stem rust of wheat
P. dianthi, causing rust of carnation
Gymnosporangium juniperi-virginianae, causing cedar apple rust
Phragmidium mucronatum, causing rose rust
Uromyces appendiculatus, causing rust of legumes
M. ciliata, causing poplar rust
Tranzschelia discolor, causing rust of almond, peach and other stone fruits

Classification:

Kingdom: Fungi, Division: Basidiomycota, Class: Urediniomycetes, Order: Uredinales, Family: Pucciniaceae

i) Stem rust of wheat (P. graminis tritici)

Symptoms: The symptoms appear as elliptical blisters or pustules, known as uredia, that develop parallel with the long axis of the stem, leaf or leaf sheath. The epidermis covering the pustules is later ruptured irregularly and pushed back, revealing a powdery mass of brick red coloured uredospores. Later in season, the rusty colour of the pustules turns black and the fungus produces teliospores instead of uredospores and uredia are transformed into black coloured telia.
Disease cycle: Puccinia graminis is a macrocyclic, heteroecious rust fungus producing spermogonia and aecia on barberry and uredia and telia on wheat and other cereals and grasses. In cooler regions, the fungus overwinters as teliospores on infected wheat straw debris. The stem rust pathogen is an obligate parasite and produces five distinct fruiting structures with five different spore forms that appear in a definite sequence. Basidiospores, aeciospores and uredospores can attack and infect host plants. The teliospores represent the only sexual, overwintering stage, which on germination produce the basidia. The basidium, following meiosis, produces four haploid basidiospores. The basidiospores, on infection, produce haploid mycelium that forms spermogonia, containing haploid spermatia and receptive hyphae on the barberry leaves. Spermatia act as male gametes and are unable to infect plants. Their function is the fertilization of receptive hyphae of the compatible mating type and subsequent production of dikaryotic mycelium and dikaryotic spores. This mycelium forms aecia producing aeciospores on barberry leaves itself, which on infection of wheat further form more dikaryotic mycelium and finally uredia containing uredospores. Uredospores also infect and produce either more uredia and uredospores or near host maturity, telia and teliospores. The rust cycle is thus completed. However, in India, there is no role of barberry in the survival and yearly recurrence of this disease. The black rust pathogen is believed to survive on the ‘off season’ wheat, volunteer and stubbles plants in the hills in the form of uredospore, from where the fresh infections start and travel to the plains in the North and South India.
Microscopic observations: The teliospores are brown in colour, bicelled and stalked. The uredospores are oval, yellowish, spiny and pedicellate.

ii). Bean rust (Uromyces appendiculatus)

Symptoms: The fungus produces characteristic rust pustules on the hosts. The pustules are mostly found on the leaf blade, though often they are found on the petiole and stem of some hosts. The sori are small, round, open, powdery and brown coloured. They appear in groups, several sori coalescing to cover a large area of the leaf blade. In severe infections, the leaves wither away, resulting in considerable damage to the crop.
Disease Cycle: Since the fungus is an obligate pathogen and autoecious in nature, it completes its life cycle on beans. The uredial stage repeats several times and it is often noticed on such hosts as species of Dolichus and Vigna. The urediospores build up in masses and are spread by wind to cause secondary infection. The fungus can perpetuate through the uredial stage as well as through the telial stage. The teliospores germinate to produce basidiospores, which after infection, produce pycnia, aecia and uredia. Pycnia appear in yellowish spots on the upper surface of leaves. Orange coloured aecia are formed on the lower surface of the leaf around the pycnia on the opposite side.
Microscopic observations: The uredospores are echinulate, oval, and yellowish brown in colour, and measure 20-33 × 16-23 µ. The dark brown teliospores are elliptical to ovate, pedicellate, smooth walled, single-celled, with warty papillae at the top, and measure 24-33 × 20-26 µ in size.

Examination and record

1. Study and record the symptoms of rust on different hosts.
2. Observe the teliospores and uredospores under compound microscope.
3. Draw diagrams of different stages of the rust fungi.