Scouting for Stalk Rots in Corn
Different stalk rots of corn are known to occur in Pennsylvania. The key to differentiating corn stalk rots begins with proper identification. Image Source: A. Collins, Penn State Extension
Factors that impact the development of stalk rots include carbohydrate stress from 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 and one less common bacterial rot.
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 of a shredded stalk. 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, dry conditions, although the disease can also occur 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 pinhead-sized fruiting structures are embedded in the rind tissue, making them difficult to remove by scraping. This can distinguish this stalk rot from other corn stalk rots, although proper identification may require laboratory analysis. When conditions are very wet, white mold may appear 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 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, with wilted leaves that are dull green or grayish and 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 off the stalk with a fingernail. Internally, it is common to see rotted pith tissue that ranges from light to dark pink (Figure 5). As with Fusarium stalk rot, plants may die suddenly, with leaves taking on a similar dull green or grayish color.
Bacterial stalk rot (Erwinia spp.): While less common than the fungal stalk rots, bacterial stalk rot may be first noticed in mid-season when sudden twisting and lodging of stalks is observed. The pith may become discolored, water-soaked, and soft, and a distinct odor may be present (Figure 6). Bacterial ooze may be visible when stalks are broken open (Figure 7). Conditions that favor this infection include high temperatures following heavy rainfall or overhead irrigation with surface water, which may carry the pathogen.
