![]() ![]() It is held that some of the pathogenic effects of the disease such as chlorosis are due to the secretion of a toxic material by the fungus. Flees, beetles and Colorado bettles are reported to assist infection by carrying conidia on their bodies and facilitate penetration of germ tubes by inflicting wounds on the host surface. The mature conidia are readily detached and dispersed through the agency of air currents, water and insects. Rand (1917) reported that heavy dew with rains now and then promote abundant sporulation. Conidia start forming when the spots are about 3 mm in diameter. Secondary spread immediately follows through conidia produced on the primary spots within 5-7 days after infection. The primary infection usually appears on the foliage as leaf spots within 2 or 3 days under favourable conditions of temperature and moisture (D). The incubation period varies from 48 to 72 hours. Walker (1969) reported that the fungus penetrates the host leaf and stem directly through the epidermis (C). The germ tubes gain entry into the lower leaves of the host plant through stomata. Primary infection may be brought about by conidia (A) or mycelium from the infected debris in the soil.Īccording to Walker (1969), the conidia germinate (B) at the optimum temperature of 28° to 30☌ within 35 to 45 minutes. The source of primary inoculum is the infected plant debris such as the dried leaves, stems, potato tubers and contaminated tomato seeds. The mycelium and conidia of the pathogen remain viable for a considerable time, the former for about a year or more in the infected dry leaves and the latter for 17 months at room temperature. On a suitable host they germinate readily in moist weather each by putting out 5-10 germ tubes (Fig. The mature conidia are detached readily and dispersed chiefly by air currents, water and insects. Each conidium develops from a bud formed on the terminal cell of the conidiophore. The conidia are borne singly but in pure cultures, in short chains of two. The beak is long, septate and rarely branched. There are 5-10 transverse septa and a few longitudinal ones. The conidia which measure 120 to 296 µ in length and 12-20 µ in breadth are dark-coloured, beaked, muriform and multiseptate (Fig 22 23 E). The conidiophores which are relatively short (50- 90 µ long and 9 µ broad) and dark-coloured arise from the older diseased tissue of the host and emerge through the stomata. The hyphae ramify in the intercellular spaces but later penetrate the cells of the invaded tissues (Fig. The mycelium consists of light brown, slender, septate sparsely branched hyphae which become dark-coloured with age. Stems and petioles may also develop brown to dark lesions which may finally lead to either worthless plants or collapse of the entire over-ground portion of the plant. Falling of leaves starts with the older (lower) ones until a few remain at the top. In severe cases of infection the leaves dry up, shrivel and drop off. Under humid conditions, the diseased areas coalesce and big rotting patches appear on the leaf surface. Singh (1968), the spots become hard in dry weather and the leaflets curl. In the advanced stage when the number of spots is numerous, the leaf shows signs of old age and droops. The number of spots on the leaflets may be a few but if the conditions are favourable the spots increase in number and size involving the entire leaf surface. The necrotic tissue of the spot often shows a series of concentric ridges which produce a target-board effect, a symptom characteristic of this disease (Fig. As a rule the oldest (lowest) leaves are affected first and the disease progresses upwards. The chlorotic zone increases with the increase in size of the spot. Each spot is usually delimited by a narrow chlorotic marginal zone which fades into the normal green. The disease appears on the leaflets, 3-4 weeks after the crop is sown as small, isolated, scattered pale brown to dark spots, oval or angular in shape mostly up to 3 or 4 mm in diameter. ![]()
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