Scouting for Stalk Rots in Corn

Factors that impact the development of stalk rots include carbohydrate stress due to factors such as pests and diseases, fertility, water, and hybrid genetics. When different abiotic or biotic factors stress corn during the growing season, this could increase the occurrence of stalk rots as harvest approaches. In Pennsylvania, several different stalk rots are known to occur, so proper diagnosis is essential, 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 unsure of the corn stalk rot. This information is crucial for future management, particularly in hybrid selection. As such, this article focuses on comparing the distinguishing characteristics of five common fungal stalk rots. 

Anthracnose stalk rot (Colletotrichum graminicola): One of 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, we can look for three different types of symptoms 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 1. Finally, top dieback can also occur, whereby the flag leaf may be yellow, purple, or dead, affecting the tassel.

Charcoal rot (Macrophomina phaseolina): Typically, charcoal rot in corn can be diagnosed by noting that the pith and stalk rind tissue has a silvery gray appearance due to the development of black microsclerotia (Figure 2), which in turn leads to the pith tissue being disintegrated.  The appearance of these microsclerotia is the crucial characteristic of distinguishing charcoal rot from other corn stalk rots. The development of charcoal rot is usually favored by hot and dry conditions, although the disease can also be seen in average years.

Diplodia stalk rot (Stenocarpella maydis): Proper identification of Diplodia stalk rot is based on observing the small, black pycnidia on the lower stalk rind (Figure 3). 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 distinguish this stalk rot from other corn stalk rots, although proper identification may require laboratory analysis. When conditions are very wet, it may be possible to see a white mold on the stalk, with internal tissue 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 becomes pinkish or salmon colored as the pith tissue continues to rot (Figure 4). 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 straw-colored stalks. 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 at silking with warm and wet conditions shortly after, Gibberella stalk rot can be identified by the presence of perithecia, which are small, round, black fungal structures found on the stalks. One very important point is that these fruiting structures can be easily scraped away from the stalk using a fingernail. Internally, it is common to see rotted pith tissue with a light to dark pink color (Figure 5). Like Fusarium stalk rot, plants may die suddenly, with leaves having a similar dull green or grayish color.