Posted 26 August 2011. PMN Crop News.
Extension Observing Diplodia in Area Fields
Source: University of Illinois Press Release. aces.illinois.edu
Urbana, Illinois (August 22, 2011)--U of I Extension in Fulton, Mason, Peoria, and Tazewell Counties has begun to notice evidence of ear rot in area fields. From the roadside, the tips of ear husks appear to be bleached white or straw-brown. Shucking the ear, one can already find white fungal "hair" growing between the kernels. Our best guess at this point would be that Diplodia ear rot has begun to emerge in area corn. The fact that the disease can be found now is disconcerting. At the very least, the appearance of this disease raises the specter of past grain quality issues.
Diplodia maydis overwinters as spores or pycnidia (small black spots/fruiting structures embedded just beneath the surface of crop debris). In the spring, these pycnidia produce a large chain of spores – a process that tends to happen in proximity to wet weather. Spores are then splashed by rain, blown by wind, or vectored by insects to the plant. It is interesting to note that much of the ear rot currently observed by Extension tends to appear where Japanese beetles have fed. This may indicate that Japanese beetles have transported the fungus into the field in 2011.
Diplodia can appear as either a stalk rot or ear rot. Stalk rot symptoms usually develop several weeks after silking. The lower nodes become brown or straw colored and very soft. Splitting the stalk reveals that much of the pith has disintegrated and only the vascular bundles remain. This gives the interior of the stalk a "Shredded Wheat Cereal" appearance. The pith has none of the pink discoloration associated with Fusarium/Gibberella stalk rots and small black spots can be found on the rind. These small black spots cannot be removed when rubbed by one's thumb. Ear rot tends to occur within a couple weeks of silking as white, bleached out regions appear on the ear husk. Eventually a white mold appears between the kernels and it usually starts at the base of the ear.
Can the location of mold growth determine which rot is plaguing the ear? In other words, does mold on the tip indicate that the disease is not Diplodia? University of Illinois Report on Plant Diseases No. 205 states that Diplodia often starts at the base of the ear working its way toward the tip. Other ear rots such as Gibberella, Penicillium, and Aspergillus tend to start at the tip and work their way toward the base. However, the colors associated with these molds seem to be inconsistent with our current observations. Gibberella is typically "reddish" similar to the stalk rot version of that disease. Penicillium is green to blue-green in color, and Aspergillus can be tan, somewhat black, green or yellow green. The ear rot currently observed in fields is white, can be found between the kernels, and can be found on the ear tip. Fusarium ear rot, a relative to Gibberella, can also be found at the ear tip where birds and worms have caused damage, but the salmon-pink to reddish-brown color associated with that disease also seems inconsistent with our observations. Report No. 205 goes on to state that Diplodia "usually begins either at the base of the ear…or an exposed ear tip." This would indicate that Diplodia is a very strong candidate for our current observations and that proximity of mold to the ear tip or ear base is probably not the most reliable method by which to differentiate between ear rots.
Resistance, balanced fertility, lower plant populations, and timing harvest to remove the crop a little earlier all play a role in reducing problems/reducing consequences associated with Diplodia stalk rot and Diplodia ear rot. The University of Illinois does not believe that foliar fungicide applications will ward off ear rot and most fungicides are not even labeled for ear rot control. Resistance issues, increased amounts of residue, and more corn on corn account for the increase in Diplodia over the last four years.