There are more than 4,000 species of rust fungi, found on a wide range of wild and cultivated plants in many parts of the world. All are members of the Basidiomycota (Order: Uredinales) but they are microscopic species, in contrast to the basidiomycota that produce the larger mushrooms.
The term, rust fungus , refers to the yellow- or rust-coloured uredospores , the main dispersal phase of the rust fungi. All rust fungi are obligately dependent on host plants to complete their life cycle, and all act as biotrophic parasites. In other words, they feed on the living tissues of their hosts, and they do this by producing specialised nutrient-absorbing structures termed haustoria to obtain nutrients from the living cells.
A complex life cycle, involving up to 5 different spore-producing stages uredospores , teliospores , basidiospores , pycniospores and aeciospores, termed macrocyclic.
Usually two very different types of host plants - "primary host" and "alternate host". Ex. black stem rust of cereals and grasses caused by Puccinia graminis. primary host -wheat and the alternate host - barberry.
Microbial Plant Diseases
Fig. Stages in the life cycle of black stem rust of wheat caused by Puccinia graminis . (a) Elongated stem lesions bearing uredospores. (b) Stained section of a wheat stem with a pustule of uredospores breaking though the plant epidermis. (c) Section of a pustule with teliospores. (d) Lesions containing spermogonia on the upper surface of a barberry leaf. ( e) Aecia erupting through the lower epidermis of a barberry leaf. (f) A spermogonium (pycnium) , showing the tiny spermatia (pycniospores) and receptive hyphae. (g) Cross-section of an aecium.
Life cycle of Puccinia graminis
(a) (b) (c)
Common Rust of Sweet Corn caused by Puccinia sorghi
The spots caused by Puccinia graminis increase in size over time and change color from gray, to tan, reddish-brown or black.
(c) Part of a wheat leaf infected by Puccinia striiformis (yellow rust), showing numerous pustules of uredospores erupting through the leaf epidermis.
Symptoms of Common rust are elongate to oval brown pustules on both leaf surfaces which rupture and release red spores
Use disease resistant cultivars where available.
Scout for pustules beginning at whorl stage and spray a fungicide when 80 % of leaves observed have one or more pustules.
Obligate biotrophic plant pathogens. Parasitize their host plants as an intercellular mycelium using haustoria to penetrate the host cells.
Reproduce asexually by releasing sporangia or conidia. These may collectively be referred to as conidiosporangia. Sexual reproduction is via oospores.
Host range: crops infects cabbage, Brussels sprout, cauliflower, Chinese cabbage, turnip, radish, and mustard as well as cruciferous weed species.
Infection of leaves and inflorescences results from conidia produced on living hosts. Secondary conidia are spread by wind and splashing water. Oospores, if produced, survive in crop residues and in the soil. The pathogen is favored by cool, moist conditions.
Downy Mildew in Crucifers
In more mature plants, small, angular lesions develop on leaves and inflorescences. Lesions enlarge and become irregular, yellow to orange necrotic patches, with dense sporulation on leaf undersides resulting white, downy (feathery) growth. A pale brown to gray discoloration occurs on the surface of heads or curds and black streaks may develop on the stems. Affected tissues become susceptible to attack by secondary rotting organisms. Downy mildew also attacks the taproots of turnip and radish and infected organs develop a black, epidermal blotch and an internal discoloration.
Radish leaves with downy mildew Cucumber leaf with downy mildew
Seedlings, cotyledons and hypocotyls may become infected and seedling loss can occur. sporangiophore and sporangia of downy mildew ( Peronospora parasitica ) Hyaloperonospora parasitica hyphae and haustoria Conidiospores of Hyaloperonospora parasitica
Disease management Removal of crop debris and weed hosts may reduce inoculum. Practice rotation with non-brassica crops. Manage Downy Mildew on transplants in the seedling bed by improving air circulation, irrigating early in the day, and applying fungicides. Plant resistant or tolerant cultivars. Fungicides registered for Downy Mildew control include foestyl-Al (Aliette), phosphorous acids (ProPhyt), famoxadone plus cymoxanil (Tanos), and mefenoxam plus chlorothalonil (Ridomil Gold/Bravo).
Causative agent: Fungus Claviceps purpurea
Ergot is a disease of cereal crops and grasses. The disease causes reduced yield and quality of grains. The fungus produces toxins that cause ergotism in animals, including humans.
This fungus, an ascomycete, forms sclerotia, ascospores and conidia. The sclerotia vary in size (2-20 millimeters long), depending on the host (the host's seed size, which the sclerotia replace) and are purple-black, elongate (spur-like), with white centers. Ascospores are long, thin, and septate; conidia are small, round and one-celled.
Host range: The disease is reported in rye, spring wheat, durum, barley, and other small grains.
Figure: Disease cycle of ergot.
Symptoms The most common sign of ergot is the dark purple to black sclerotia (ergot bodies) found replacing the grain in the heads of cereals and grasses just prior to harvest. The ergot bodies consist of a mass of vegetative strands of the fungus. The interior of the sclerotia is white or tannish-white. Prior to development of the sclerotia bodies, the fungus develops a stage in the open floret (flowering head part) commonly called "honey dew." The "honey dew" consists of sticky, yellowish, sugary excretions of the fungus which form droplets on the infected flower parts.
Ergot sclerotia and barley grain.
Rotate cereals and grasses with nonsusceptible crops for one year or longer. The ergot sclerotia usually do not survive in the soil for more than one year. Therefore, summer fallow or crop rotation to a non-cereal crop for at least one year will help reduce ergot. Deep-plow fields which have a severe ergot infestation to bury the sclerotia. The ergot sclerotia will not germinate if buried more than 1 inch deep. Plant only ergot-free seed to avoid introducing or re-introducing the fungus into the field. Eradicate or prevent wild grasses from reaching heading in fields. Resistant commercial varieties of wheat, barley, rye or cultivated grasses are not available. However, some differences among varieties may occur, and those with long flowering periods may be more frequently infected.
Causative agent: Bacteria Ex. Bacterial blight of bean ( Xanthomonas phaseoli ) Bacterial blight of cotton; Angular leaf spot of cotton ( X. malvacearum ) Bacterial leaf blight of rice ( X. oryzae ) Bacterial leaf blight of tomato and pepper ( X. campestris ) Bacterial blight of soybean ( Pseudomonas syringae)
BACTERIAL LEAF BLIGHT
Beans, cucurbits, cotton, rice, tomato, pepper, plantain etc.
Water-soaked to yellowish stripes on leaf blades or starting at leaf tips then later increase in length and width with a wavy margin
Appearance of bacterial ooze that looks like a milky or opaque dewdrop on young lesions early in the morning
Lesions turn yellow to white as the disease advances & severely infected leaves tend to dry quickly. Lesions later become grayish from growth of various saprophytic fungi
Entire plant wilt completely
Youngest leaf is uniform pale yellow or has broad yellow stripe
Bacterial leaf blight of rice The bacterium causing the disease is rod, single, occasionally in pairs but not in chains. Gram negative, non-spore-forming, and devoid of capsules.
Rice seedling wilt
Practicing field sanitation such as removing weed hosts, rice straws, ratoons, and volunteer seedlings. Carefully remove and discard any symptomatic plants or those that test positive for the bacteria. Also remove and discard adjacent plants.
Maintaining shallow water in nursery beds, providing good drainage during severe flooding
Proper application of fertilizer, especially nitrogen, and proper plant spacing are recommended for the management of bacterial leaf blight.
Use of resistant varieties.
Seed treatment with bleaching powder (100µg/ml) and zinc sulfate (2%).
TOBACCO MOSAIC VIRUS (TMV) DISEASE TMV is an ss RNA virus that infect plants, especially tobacco and other members of the family Solanaceae.
Light and dark green mottled areas on the leaves. The dark green areas tend to be somewhat thicker than the lighter portions of the leaf. The leaf mottling is seen more easily if the affected plant surface is partially shaded. Stunting of young plants is common, and often is accompanied by a distortion and fern-like appearance of the leaves. Older leaves curl downward and may be slightly distorted. Certain strains of the virus can cause a mottling, streaking, and necrosis and discoloration of fruits. TMV infected tomato leaf (mottled) & fruit (non-uniform coloration)
Control of insects, especially aphids and thrips
Virus diseases cannot be controlled once the plant is infected. Therefore, every effort should be made to prevent introduction of virus diseases into the plantation. Sanitation is the primary means of controlling the diseases. Infected plants should be removed immediately to prevent spread of the pathogens. Perennial weeds, which may serve as alternate hosts, should be controlled in and adjacent to the land.