COTTON LEAF CURL VIRUS
Cause : The disease-causing virus belongs to Gemini group.
Symptoms: Upward and downward curling of leaves accompanied by small as well as main vein thickenings (SVT as well as MVT) on leaves, pronounced on underside. If a diseased leaf is viewed from beneath against the light, thickened vein found darker green and opaque than the normal. In extreme but not in frequent cases, formation of the cup shaped or leaf laminar (veins) out growth called “enation” appears on the back or underside of the leaf. The newly produced leaves are small, excessively crinkled and curled at the edge. The primary stem often tends to grow taller than normal. The enter-nodes being elongated and irregularly curved but sometimes the whole plant is stunted. The flowers checked in growth and become abortive. Bolls remained small in size and failed to open. All parts of badly hit plants are very brittle and ready broken.
Symptoms : The research studies revealed that four different types of symptoms may occur, which can be distinguished on the basis of their specific casual agent, as below: lack boll or Aspergillus rot: Affected bolls start losing green color altogether, become pinkish brown and finally sooty black due to over growth of fungus.
Rhizopus rot : Infected portions become grayish along with softening of internal tissues. The fungus grows abundantly and covers many bolls under moist conditions.
Fusarium rot : The bolls become dried with color assuming reddish and brownish tinge and showing dry and white fluffy fungal growth inside the bolls on opening.
Bacterial or Xanthomonas rot : Water soaked areas developed on the bolls giving out gummy substance and foul smell.
Perpetuation : Diseased plant debris and seeds carry over the disease causing organisms. Humid conditions after rain and speedy wind favor the spread and severity of boll rot. Sometimes, different boll worms also play a role, to transmit disease, from infected to healthy bolls/ plants.
ANGULAR LEAF SPOT OR BACTERIAL BLIGHT
Cause : A bacterium Xanthomonas malvacearum cause angular leaf spot or bacterial blight or boll rot of cotton.
Symptoms : The disease attacks all parts of the plant above ground level, at all stages, causing seedling rot, angular spots on leaves and stems (sometimes called black arm disease) and boll rot (as already mentioned). In initial stages, water soaked lesions (spots) appears on lower surface of the leaves. Later on, these spots increase in size, turn brown to black, becoming necrotic, angular and are visible on the upper surface. These spots vary in size and then coalesce, forming irregular patches and giving rise to gummy bacterial exudates. Heavily infected leaves turn yellow and drop down. The disease produces elongated black lesions on the stem, branches and petioles. Stems show cracking and gummosis (gummy substance) and are easily broken, even by wind. Bolls are marked by the appearance of water soaked lesions, which are dark brown to black, invariably sunken, results in reduced boll size, poor production of lint and loss in viability in seed.
Perpetuation : Infected seeds and plant debris act as a source of infection, but can also spread through irrigation water, rain splashes, wind, insects and contaminated field implements.
Cause : The fungi Macrophomina phaseoli, Rhizoctonia spp., Fusarium spp. etc. are predominantly isolated from diseased roots.
Symptoms : The disease affects the roots exclusively, causing pre-wilt shedding of leaves, yellowing of foliage, disintegration of root tips, discoloration and shredding of roots, exudation of drops of smelly liquid from the rotted plant parts. Mostly, wilting of shoots occur in only few diseased plants, which ultimately results in the death of entire plant. This disease generally appears, when plants are about 4-6 weeks old and continue up to boll formation. Diseased plants can be easily pulled out of the soil, appears in patches. Roots and root-lets show rotting, yellowing, disintegration and shredding.
Perpetuation : Disease causing fungi are soil borne, hence it may be claimed that both (fungi and soil) factors are responsible.