Diseases of Cotton and Control measures
Cotton Leaf Curl Virus (geminivirus) CLCV
Cotton leaf curl (CLCV) geminivirus causes a major disease
of cotton in Asia and Africa. Leaves of infected cotton curl upward and bear
leaf-like enations on the underside along with vein thickening. Plants infected
early in the season are stunted and yield is reduced drastically. Severe
epidemics of CLCV have occurred in Pakistan in the past few years, with yield
losses as high as 100% in fields where infection occurred early in the growing
season.
The virus of disease is transmitted by whitefly. The first
symptoms of infection in cotton appear within 2-3 weeks of inoculation and are
initially characterised by deep downward cupping of the youngest leaves. This is followed by either upward or downward
curling of the leaf margins and swelling, darkening and formation of enations
on the veins, which frequently (depending on cultivar) develop into cup-shaped,
leaf-like structures.
Boll Rot Disease
Cause:
Aspergillus Niger, Fusarium oxysporium, Rhizopus oryzae (belongs to fungi) and
a bacterium Xanthomonas malvacearum reported as predominant causes of boll rot.
Symptoms:
The research studies revealed that four different types of symptoms may occur,
which can be distinguished on the basis of their specific causal agent, as
below:
Black
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.
Root Rot (Rhizoctonia bataticola and R. solani)
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 occurs 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 from soil and it appears in patches. Roots and root-lets are show rotting, yellowing, disintegration and shredding.
Control Measures for all Above Diseases
Following disease management practices may help to save the crop from all above major and minor diseases of cotton.
- Cultivation of disease resistant variety is only safe measure of all different diseases.
- Eradication including collection and burning of plant debris may help to control seedling, root and boll rots as well as bacterial blight, because disease inoculum may also survive through plant debris.
- Deep plowing with short duration at least two months before sowing help to control seedling and root rot.
- Proper land leveling is a preventive measure against seedling and root rot. Use of healthy seed, acid delinting and chemical seed treatment minimize the disease incidence of seedling, root and boll rots as well as bacterial blight. Crop rotation with non-host i.e. sowing of sorghum for 3 to 4 years is useful for control of seedling and root rot.
- Mixed cropping with kidney bean or fodder and leguminous crops saves the cotton crop from root rot.
- Proper use of irrigation and chemical fertilizers improves the disease resistant power in cotton plants.
- Early sowing of crop is preventive measure for control of boll rot.
- White fly transmits cotton leaf curl virus from diseased plant to healthy one, whereas, different cotton boll worms may play a role to transmit the boll rot diseases, hence white fly and boll worms must be controlled.