Cotton Bunchy Top Virus
Leaves usually have short petioles and develop pale, light-green angular patterns along the margins. They appear leathery and brittle compared to the leaves on healthy plants. Subsequent growth is characterized by smaller leaves, shorter internodes and smaller bolls. If the infection takes place at very early stage (e.g. as seedlings), the growth of the whole plant is stunted and compact. Roots appear hairy and dark brown (normally light yellow-brown color) and form small knots on the secondary root branches. Affected plants have a reduced number of bolls and ultimately lower yields.
The symptoms are caused by the cotton bunchy top virus, which can only survive in living plant tissues. It is transmitted in a non-persistent manner by the cotton aphid Aphis gossypii. There is usually a time-lag of 3-8 weeks from infection until the symptoms become obvious. Fields with high aphid populations are at highest risk. Volunteer plants, regrowth or ratoons that survived from the previous season can also be a problem since they act as both a preferred host for the aphids and a reservoir for the disease, creating a source of infection in the new season. It is thus not uncommon to see patches of infected plants around a ratoon. Disease transmission is favored by climatic conditions suitable for aphid reproduction, feeding and spread.
Beneficial insects such as predatory ladybugs, lacewings, soldier beetles and parasitoid wasps are important agents to control populations of aphids. In case of mild infestation, use a simple soft insecticidal soap solution or solutions based on plant oils. Aphids are also very susceptible to fungal diseases when it is humid. A simple spray of water on affected plants can also remove them.
Always consider a integrated approach with preventive measures together with biological treatments if available. Insecticides containing cypermethrin or chlorpyrifos can be used as foliar spray against the aphids. Remember to change products between applications in order to avoid the development of resistance