With corn now beyond the R4 growth state in most fields, there is really nothing you can do about southern rust in terms of fungicide application. However, correct diagnosis of this disease is still very important from the standpoint of identifying the hybrids that were most severely affected. Although our growing conditions generally do not favor this disease and we may go for another several years without seeing as much southern rust as we did in 2017, we still need to identify those highly susceptible hybrids. Late-season rust symptoms have some very characteristic features that are extremely useful for diagnosis.
Late-season rust symptoms are somewhat different from those observed early in the season, adding to the confusion of trying to tell the difference between southern and common rusts. Figure 1, (click here) shows early southern rust pustules on the surface of a leaf. Notice the bright orange color that is considered typical of this disease. These look considerably different from the pale to almost yellowish-colored pustules in Figure 2 (click here). This latter picture also shows symptoms of southern rust, but these are older pustules on the leaf of an R5 plant. Southern rust symptoms often begin as bright reddish-orange pustules, but as the pustules and the leaf age, they take on the pale appearance seen in Figure 2.
Taking a closer look at aging leaves with the pale pustules, you will also notice some very distinct black specks (insert in Figure 2). There are telia, a different stage of the same disease. Yes, we are still talking about southern rust. As the leaves age and begin to die, the fungus kicks into a type of “survival mode”, producing black instead of orange pustules. These black pustules may also develop on the stems and leaf sheaths as shown in Figure 3 (click here), and their arrangement relative to the orange or pale pustules is very important for the diagnosis of southern rust. As highlighted in Figure 3, the black pustules do not replace the orange pustules, but develop around them. This can be seen with the naked eyes by carefully examining the leaves, but are better detected with a hand lens or microscope as shown in Figure 4 (click here).
Common rust produces large cinnamon-brown pustules early-on, that also become pale and later dark, but the dark pustules usually replace the cinnamon-brown or pale-colored pustules instead of surrounding them.