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The uredial pustules of P. polysora are densely scattered over the surfaces of leaf blades, leaf sheaths and stalks on maize. The uredia are light cinnamon-brown, circular to oval, 0.2-2.0 mm long and generally epiphyllous. The few hypophyllus pustules were usually on or near the midrib (Ullstrup, 1965). Uredia when ruptured and sporulating abundantly produce the rust symptom. The rust was most severe on the lower leaves; severity decreased with successive leaf position (Raid et al., 1988). Severely infected leaves became dry and defoliated prematurely, and in this case stalk lodging may occur. Some resistant hosts were characterized by abortive and isolated pustules surrounded by sharply defined, necrotic or chlorotic areas (Stanton and Cammack, 1953). Susceptible hosts had consistently greater incidence of pustules, which were larger, more tumid and ruptured earlier, and sporulated more readily than those on resistant hosts (Zummo, 1988).
The minute black spots beneath the epidermis are telia (Cummins, 1941). The telia which develop in old uredia are circular to elongate, 0.2-0.5 mm diameter, chocolate-brown to black, and remain covered by the epidermis longer than in common rust (Shurtleff, 1980). The telia often appear in circles around the uredial pustules to form the unique symptom of telial rings (Sim, 1980).
The use of resistant maize hybrids is the most feasible means of controlling southern rust. A hypersensitive type of resistance, conditioned by a single dominant gene, was highly effective in preventing yield loss in crops and eliminated sporulation of secondary inoculum (Storey and Howland, 1957; 1959), however, races of P. polysora existed that limited their wider use (Futrell et al., 1975; Scott et al., 1984; Ullstrup 1965). Slow rusting, a general form of resistance that results in a reduced rate of disease development without severely affecting the rust population, effectively controlled rust in Africa in the 1950s (Bailey et al., 1987). Registered germplasm with various levels of resistance to P. polysora is available (Craig et al., 1976; Eaton et al., 1993; Kim et al., 1987; Lu et al., 1990; Scott and Davis, 1981; Scott et al., 1982; Williams and Davis, 1980; 1982; 1984; Yeh, 1984; York, 1991). Lu et al. (1990) found that resistance to P. sorghi correlated with resistance to P. polysora (r = 0.347).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Southern rust is generally considered a minor disease in the USA, but it can be destructive in late-planted maize (Futrell, 1975). Of the three rusts that occur on maize, southern rust is the most destructive. Rodriguez-Ardon et al. (1980) estimated yield losses of 45-50% caused by P. polysora; it also has the capacity to cause significant reductions in yield (17.7-39.1%) even as far north as Pennsylvania and Maryland (Raid et al., 1988).