Pustules of leaf rust are small and circular producing a mass of orange-brown powdery spores predominantly on the upper leaf surfaces, (Figure 2.22). Later in the season pustules also develop on leaf sheaths. The pustules easily rub off on a finger. As the crop matures the pustules turn dark and produce black spores embedded in the old plant tissues. Leaf and stem rust may be confused but are distinguished by their colour and size, leaf rust being lighter coloured, smaller and rounder than stem rust.
Figure 2.22 Leaf rust symptoms on barley leaf
The primary source of inoculum for leaf rust is green volunteer barley plants that survive over summer. Development of leaf rust is most rapid during warm (15-20°C) moist (rain or dew) weather. Crops sown before May, when nights are still warm, are often more severely infected.
The ‘Star of Bethlehem’ ( Ornithogalum umbellatum ) weed, (Figure 2.23) can also be a source of inoculum and is a host for the alternate, sexual stage of teleospores. The existence of a sexual cycle means the formation of new strains of rust can occur, increasing the occurrence of new, virulent strains that can overcome current resistances. Currently the Star of Bethlehem weed occurs in South Australia with isolated occurrences through the Victorian cropping zone.
Figure 2.23 Star of Bethlehem
RUSTS OF BARLEY