Diagnosis

As mentioned earlier , plants which have sustained only a mild infection have a way of remaining undetected . Unless there is great retardation of vegetative growth due to infection , the flower output is not greatly reduced . Some diseases become apparent only after flowers have opened . Generally warmth suppress virus activity . It is very often noticed that an apparently healthy plant growing under reasonably warm conditions , suddenly break out in lesions after a cold spell . Because of these characteristics it is very often difficult to say whether a plant is infected by virus or not , just by looking at it . The safest way to detect presence of a virus in a plant is to inoculate or rub the juice of the suspected plant on other susceptible hosts like Datura stramonium , Cassia occidentalis , Chenopodium amaranticolor or the common ground orchid Spathoglottis plicata , all of which show lesions within a few days . The leaves when they are held aganist light will show innumerable transluscent dots which turn black as the leaf matures. Serological methods are also available for detection of virus in plants (For details refer Thibault , 1962).