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Holcus
spot is caused by the bacterium Pseudomonas
syringae. Holcus spot is not known to cause economic damage in dent corn, but the
disease can look serious when spots are numerous (see photo). Symptoms of holcus spot are light tan (sometimes almost white)
round or oval spots on the lower leaves, initially about 1/4 inch in diameter,
but sometimes growing larger and coalescing into irregular spots and streaks of
dead tissue. The spots may appear water-soaked at the margins or have a light
brown border. Holcus spot symptoms can resemble chemical injury to leaves,
similar to paraquat injury. Although the disease is caused by a bacterium, fungi
can sometimes be seen producing spores on these lesions. These fungi are
secondary invaders such as Alternaria that grow on the already-dead leaf
tissue. The bacteria
survive in plant residue and splash onto leaves where infection takes place
after a heavy rainfall. Symptoms often appear suddenly after a heavy rain, but
then do not spread to new leaves. Early in the season if plants are not growing
vigorously, they can look very poor with leaf damage due to wind, blowing soil,
cold temperatures, or holcus spot. But as the weather improves and the plants
put on new leaves, these problems will likely disappear. There are no practical
management tactics for holcus spot, mainly because of its lack of economic
impact. However, it is important not to mistake this disease for a fungal leaf
disease, as instances where unnecessary (and ineffective) fungicide
applications have been made against holcus spot.
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