RUSTS
SYMPTOMS
Black stem rust is characterized by the development of the ‘red rust’ or summer stage represented by the uredial stage appearing on leaves and culms, followed a little later by the ‘black rust’ or winter stage represented by the telial stage.
With the onset of the disease, elongated brown or reddish-brown pustules or sori burst through the epidermis of the host tissue (Fig. 368C). These pustules or uredia or uredosori split open in an irregular manner having an elongated crater-like opening with ruptured host epidermis clinging to one or both sides of the opening.
The size, shape and number of sori vary with environmental conditions and particularly with the inherent resistance of the host.
The uredosori may develop without any surrounding chlorotic or dead cells or they may be seated in chlorotic areas which soon become dead. They may be few in number, or they may be very numerous and may coalesce to form more or less elongated brown or rusty powdery streaks.
Within these sori are masses of reddish or rust-coloured powder consisting of thousands of minute dust-like uredospores. Later in the season the black rust stage or winter stage or telial stage appears. This consists of elongated pustules or telia or teleutosori similar in shape to the uredosori but black in colour. The black colour is due to the presence of masses of dark teleutospores they bear.
