Diseases of plants are spread by various types of micro-organisms, parasitic flowering plants, nematodes, viruses and adverse environmental conditions. Total number of plant diseases estimated to be in several tens of thousands and the estimated annual losses therefrom add up to several billion dollars. Injuries to plants life is due primarily to insects, mites or animals other than nematodes are not regarded as plant diseases.
Diseases of Wheat
Rusts are the most widespread diseases of wheat throughout the world because of their wide distribution. Three major rust diseases of wheat are: Leaf Rust, Stem Rust, and Stripe Rust. Leaf Rust is the most common and important rust from the crop production aspect. But since 2000, the increased incidence and severity of stripe rust has damaged the wheat crop worldwide. Most hard red and hard white winter wheat varieties have sufficient resistance to protect the crop from serious stem rust damage.
1. Leaf Rust: Leaf Rust is caused by the fungus Puccinia Triticina and is widely distributed in the world. The disease is most damaging when the upper leaves of plants become severely rusted. Heavy rusting causes early loss of such leaves, which reduces the grain filling period and results in small kernel size.
Leaf Rust is normally found on the leaves, but occasionally found on glumes and awns. Circular or oval, orange pustules develop on the upper surface of the infected leaves. The spores within these pustules are easily dislodged and cover hands and clothing with an orange "dust". Each pustule daily produces about 1000 spores and each of which is capable of reinfecting wheat. As wheat ripens, this orange stage turns into a new black stage of the fungus. Many a times a mixture of orange and black stages is intermingled on a leaf.
2. Stem Rust: Stem Rust, caused by Puccinia Graminis, has caused several major rust epidemics in the world since 1916. Stem Rust, as the name implies, infects primarily the stem but is not always confined to the stem. It can also infect leaves, sheaths, glumes and awns. Its major symptoms are elongate, reddish brown pustules with ragged margins.
The host epidermis is ruptured by the pustule, making a rough-textured surface of the stems and sheaths. Stem rust pustules are larger than leaf rust pustules and often erupt on both upper and lower leaf surfaces. As the wheat matures, the pustules are replaced by the black spore stage. Temperatures that favour stem rust development are about 5 degrees warmer than the temperatures favourable for leaf rust and 15 degrees warmer than those for stripe rust.
3. Stripe Rust: Stripe rust is caused by Puccinia Striiformis and is the most damaging rust. Pustules in this rust are light yellow and develop on leaves in distinct, straight-sided stripes of irregular length. Pustules may also develop on the heads. Stripe rust develops at slightly cooler temperatures than does leaf or stem rust. Once temperatures exceed 75F, stripe rust develops very slowly. When temperatures are high, only trace levels of stripe rust can be found.
Disease of Sugarcane
Sugarcane is cultivated in different parts of the world and is affected with variety of diseases. Some of these diseases are:
1. Eyespot Disease: This disease is caused by the fungus and transmitted by the air-borne spores which germinate and make leaf lesions resulting in yield lost because functional area of leaf is reduced. New sugarcane clones are being screened for resistance to the pathogen using a toxin from the fungus to protect plants from it.
2. Smut Disease: The Smut disease is caused by a fungus which infects the plants systemically, stunting the shoots and eventually killing the plant. New sugarcane varieties are screened for smut resistance by protecting seed pieces of all clones with fungus spores and eliminating those that become infected. All seed cane is treated by soaking in hot water for 20 minutes at 52 degrees to kill the fungus. Seed farms are inspected periodically.
3. Yellow Leaf Syndrome: It is now recognized virus disease of sugarcane which is transmitted by aphids and causes cane tonnage losses. All commercial varieties are surveyed on the plantations. Most infected sugarcane remains asymptomatic untill it is placed under stressed conditions. It has been found that some sugarcane varieties are resistant to the virus. No specific control measures are available.
4. Ratoon Stunting Disease: This is a systemic bacterial infection that is without any symptom but results in cane tonnage losses. The pathogen can be kept to a very low level by screening seed fields for infection using an immunological diagnostic technique and giving all seed a hot water treatment for 20 minutes at 52 degrees centigrade before planting.
5. Leafscald Disease: This disease is caused by bacterium. Its main symptoms are white leaf streaks that coalesce and turn brown, eventually killing the plant. They appear on infected plants during cooler season of the year. An immunological diagnostic technique can detect the bacterial infection in plants. However, control is maintained yet some sugarcane varieties are resistant to it.
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