Diseases of plants caused by bacteria
Aster yellows
Aster yellows,caused by a phytoplasma bacterium, affecting over 300 species of herbaceous broad-leafed plants. Aster yellows is found over much of the world wherever air temperatures do not persist much above 32 °C (90 °F). As its name implies, members of the family Asteraceae are vulnerable to infection, though the disease can also affect a variety of common vegetables, cereals, garden plants, and wild species.
Typical symptoms include yellowing (chlorosis) of young shoots, stiff and erect bunchy growth, greenish and distorted or dwarfed flowers, and general stunting or dwarfing. The phytoplasma lives in the phloem of infected plants and is transmitted by leafhopper insects when they feed on an infected plant and then on a healthy one. No transmission occurs through leafhopper eggs or plant seed. The phytoplasma is perpetuated in overwintering weed and crop plants, in propagative parts (bulbs, corms, tubers), and in leafhoppers in mild climates. The phytoplasma is destroyed in plants and leafhoppers subjected to temperatures of 38 to 42 °C (100 to 108 °F) for two to three weeks; thus, aster yellows is rare or unknown in many tropical regions.
Though the disease is not lethal, control is effected chiefly by promptly removing diseased plants and all overwintering susceptible weeds. Spraying or dusting with a contact insecticide repulses the leafhopper carriers.
Bacterial Wilt
Bacterial wilt is caused by numerous species of the genera Corynebacterium, Erwinia, Pseudomonas, and Xanthomonas, induces stunting, wilting, and withering, starting usually with younger leaves. Stems, which often shrivel and wither, show discoloured water-conducting tissue. A bacterial ooze is often evident when infected stems are cut and squeezed. Rapidly expanding, dark green, water-soaked areas or streaks may develop first in leaves.
Bacterial wilt may be managed by growing resistant varieties; planting disease-free materials in well-drained, fertile soil that is clean or sterilized; observing stringent sanitation including weed- and insect-control measures; and rotating susceptible crops.
Blight
Blight, any of various plant diseases whose symptoms include sudden and severe yellowing, browning, spotting, withering, or dying of leaves, flowers, fruit, stems, or the entire plant. Most blights are caused by bacterial or fungal infestations, which usually attack the shoots and other young, rapidly growing tissues of a plant. Fungal and bacterial blights are most apt to occur under cool moist conditions, and most economically important plants are susceptible to one or more blights, including tomatoes, potatoes, and apples, as well as many ornamental species.
Measures for controlling and preventing blights typically involve the destruction of the infected plant parts; use of disease-free seed or stock and resistant varieties; crop rotation; pruning and spacing of plants for better air circulation; controlling pests that carry the fungus from plant to plant; avoidance of overhead watering and working among wet plants; and, where needed, the application of fungicide or antibiotics. Proper sanitation is key to stop the spread of the infestation. For bacterial blights (e.g., fire blight), fixed copper or streptomycin is an effective antibiotic if applied weekly during damp weather when leaves and shoots are expanding.
Canker
Canker, plant disease, caused by numerous species of fungi and bacteria, that occurs primarily on woody species. Symptoms include round-to-irregular sunken, swollen, flattened, cracked, discoloured, or dead areas on the stems (canes), twigs, limbs, or trunk. Cankers may enlarge and girdle a twig or branch, killing the foliage beyond it. They are most common on plants weakened by cold or drought stresses, insect injury, nutritional imbalances, nematodes, or root rot.
Control includes removing diseased parts in dry weather; growing adapted or resistant varieties in warm well-drained fertile soil; avoiding overcrowding, overwatering, and mechanical wounds; treating bark and wood injuries promptly; controlling insect and rodent disease carriers; wrapping young trees to prevent sunscald; and keeping plants vigorous by the use of fertilizers.
Crown gall
It’s a plant disease, caused by the bacterium Agrobacterium tumefaciens (synonym Rhizobium radiobacter). Thousands of plant species are susceptible. They include especially grape, members of the rose family (Rosaceae), shade and nut trees, many shrubs and vines, and perennial garden plants. Symptoms include roundish rough-surfaced galls (woody tumourlike growths), several centimetres or more in diameter, usually at or near the soil line, on a graft site or bud union, or on roots and lower stems. The galls are at first cream-coloured or greenish and later turn brown or black. As the disease progresses, plants lose vigour and may eventually die.
Crown gall can be avoided by using nursery stock free of suspicious bumps near the crown, former soil line, or graft union; practicing five-year rotation or avoiding replanting for that period; removing severely infected plants (including as many roots as possible); protecting against injury; keeping down weeds; controlling root-chewing insects and nematodes; cutting away large galls on trees; and disinfecting wounds.
Scab
Scab, is any of several bacterial or fungal plant diseases characterized by crustaceous lesions on fruits, tubers, leaves, or stems. The term is also used for the symptom of the disease. Scab often affects apples, crabapples, cereals, cucumbers, peaches, pecans, and potatoes.
Leaves of affected plants may wither and drop early. Potatoes are especially susceptible to common scab, caused by a bacteria (Streptomyces scabies and related species) that spreads rapidly in dry alkaline soils. It can be prevented by avoiding the use of materials such as wood ash, fresh manure, and lime that will add alkalinity to the soil. Other disease-prevention methods include planting resistant varieties or disease-free seeds, tubers, and corms; destroying diseased parts; removing weeds; rotating vegetables and flowers; and regularly spraying plants with fungicides, if appropriate.
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