The 2106 corn production year has proven to be a banner year first for Diplodia ear rot, and more recently Diplodia stalk rot. The disease is caused by the fungus Stenocarpella maydis, which survives in old crop debris. Diplodia ear rot is most prevalent in years where there is significant rainfall beginning at silking and then for two to three weeks after silking. Much of the corn in the eastern two-thirds of Kansas was silking in late June through mid-July. Considering all the rain that came around the time of July 4th, it is understandable why the disease is so prevalent.
Back in August, Diplodia would have been easily spotted because the husks on the ear die prematurely (Fig. 1), and the leaf attached to the ear may die prematurely as well.
Figure 1. Prematurely dried husks on a Diplodia-infected ear. Photo: Doug Jardine, K-State Research and Extension.
If you were to pull back the husks at that time, you would have seen the kernels covered by a white mold. The disease is usually worse at the butt of the ear and in worse case scenarios, the entire ear may be molded (Fig. 2). The husks often seem to be glued to the ear.
Figure 2. White, moldy growth covering a Diplodia-infected ear. Photo: American Phytopathological Society.
At this time of the season, much of that white moldy growth has disappeared and the ear will have a shrunken, grayish-brown coloration (Fig. 3).
Figure 3. Diplodia ear rot at harvest. Photo: Paul Bachi, University of Kentucky Research and Education Center, www.bugwood.org
The fungus also penetrates and begins to decompose the cob, hence this disease is sometimes referred to as “cob rot.” When infected cobs are broken in half, small, black reproductive structures, called pycnidia, are often visible (Fig. 4).
Figure 4. Black pycnidia visible when the ear is broken in half. Photo: Carl Bradley, University of Kentucky.
When the cob is rotted, it disintegrates as it passes through the combine, resulting in high levels of “foreign material” in the load. The foreign material, in addition to the shrunken discolored kernels, typically leads to large dockages at the elevator. Reduction in price from the dockage, when added to the reduction in yield from the shrunken, light-test-weight kernels, can make this a very expensive disease problem for producers.
Hybrids vary in susceptibility to Diplodia ear rot, so choosing a more resistant hybrid is a good place to begin a management plan. Since the disease resides in crop residue, using a crop rotation or some tillage, where it fits into the production system, can also be useful. Some fungicides have a label for “suppression” of Diplodia ear rot, but in university trials there have been very inconsistent results in this approach to management and we do not recommend fungicide applications solely for Diplodia management.
Also associated with Diplodia ear rot is Diplodia stalk rot. Diplodia stalk rot can enter the lower stalk from the soil or the upper stalk from the diseased ear. Diplodia stalk rot looks very much like other stalk rots. There is a shredding of the lower internodes and a general tan discoloration to the tissue.
What sets the identification of Diplodia apart from other stalk rots is the development of the pycnidia on the lower internodes (Fig. 5) brace roots, or on the collars of leaves when the disease moves into the stalk from the ear.
Figure 5. Pycnidia of Diplodia stalk rot forming on the lower stalk. Photo: Doug Jardine, K-State Research and Extension.
Like the other stalk rots, fields with high levels of Diplodia stalk rot should be targeted for early harvest to avoid the threat of lodging.
Doug Jardine, Extension Plant Pathology