Figure 1. Different stalk rots of corn are known to occur in Pennsylvania. Photo Credit: Ayllssa Collins
We have received a few questions recently related to fields where some of the corn plants look dead in different pockets of the field. Given the different weather conditions that have occurred during the 2017 growing season, including both a combination of cool and damp conditions, followed by the most recent warm and dry conditions, it is possible that we could be seeing an increase in stalk rots as we move into harvest (Figure 1). The image in Figure 1. displays the different stalk rots of corn are known to occur in Pennsylvania. Factors that impact the development of stalk rots include carbohydrate stress due to things like pests and diseases, fertility, water and hybrid genetics. Key to differentiating corn stalk rots begins with proper identification. In Pennsylvania, several different stalks rots are known to occur, so proper diagnosis is very important, especially since several of these stalk rots have similar symptoms, including the shredding of the internal pith tissue. Do not hesitate to submit a sample for diagnosis if you are unsure of the corn stalk rot since this information is important to help with future management, especially hybrid selection. As such, the focus of this article will be on the distinguishing characteristics for five common fungal stalk rots.
Anthracnose stalk rot (Colletotrichum graminicola): One the most common stalk rots of corn, symptoms of anthracnose stalk rot often first appear around physiological maturity, although infection can also occur during vegetative growth stages. At maturity, there are three different types of symptoms that we can look for to help identify anthracnose stalk rot. One is the disintegration of the pith tissue, giving the appearance that the stalk is shredded. Another is the distinctive blackening of the stalk rind, as illustrated in Figure 2. Finally, a top dieback can also occur, whereby the flag leaf may be yellow, purple or dead, which then affects the tassel.
Figure 2. Anthracnose stalk rot of corn (Photo credit: Paul Esker).
Charcoal rot (Macrophomina phaseolina): Typically, charcoal rot in corn can been be diagnosed by noting that the pith and stalk rind tissue appear to have a silvery gray appearance, due to the development of black microsclerotia, which in turn leads to the pith tissue being disintegrated. It is the appearance of these microsclerotia that is the key characteristic to distinguish charcoal rot from other corn stalk rots. Development of charcoal rot is normally favored by hot and dry conditions, although the disease can also be seen in normal years.
Diplodia stalk rot (Stenocarpella maydis): Identification of Diplodia stalk rot is based on the identification of small, black pycnidia located on the lower stalk rind. These pycnidia are the size of a pinhead and are embedded in the rind tissue, which means that they are easily scraped away. This can be used as a method distinguish this stalk rot from other corn stalk rots, although proper identification may be require a laboratory analysis. When conditions are very wet, it may be possible to see a white mold on the stalk, with internal tissue being discolored and shredded.
Fusarium stalk rot (Fusarium spp.): Several different Fusarium spp. can cause stalk rots in corn. Fusarium stalk rot will cause the internal pith tissue to shred and may also cause a discoloration that, as the pith tissue continues to rot, becomes pinkish or salmon-colored. In the field, we may also note that plants suddenly die before maturity, whereby leaves are wilted and have a dull green or grayish color, along with a stalk that is straw-colored. The lack of pycnidia is one way to differentiate Fusarium stalk rot from Diplodia stalk rot.
Gibberella crown rot and stalk rot (Fusarium graminearum; synonym, Gibberella zeae): Favored by plant stress with warm and wet conditions shortly after silking, Gibberella stalk rot can be identified by the presence of perithecia, which are small, round, black fungal structures, found on the internodes and nodes. Very important is that these fruiting structures can be easily scraped away from the stalk using a fingernail. Internally, it is very common to see rotted pith tissue that has a light to dark pink color. Like Fusarium stalk rot, plants may die suddenly, with leaves having a similar dull green or grayish color like Fusarium stalk rot.