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To Scout For Stalk Rots in Corn

On October 21, 2017

Extension Plant Pathologist Paul Esker reports having 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. Factors that impact the development of stalk rots include carbohydrate stress due to things like pests and diseases, fertility, water and hybrid genetics.

The 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.

First lets consider Anthracnose stalk rot (Colletotrichum graminicola). This is 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. Finally, a top dieback can also occur, whereby the flag leaf may be yellow, purple or dead, which then affects the tassel.
Charcoal rot (Macrophomina phaseolina) 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.

Indentification ofDiplodia stalk rot (Stenocarpella maydis 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.

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)is 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.

To Learn About Slugs

Extension Agronomist Liz Bosak explains over twenty species of slugs are found in Pennsylvania. The gray garden slug (Deroceras reticulatum) is typically the pest species found in field and forage crop fields. The two most common life stages present in fields now are eggs and adults. Typically, eggs are the overwintering stage but in warm winters it is thought that the adults will also overwinter and can begin laying eggs in early spring. The weather this past winter, spring and summer has been ideal for optimum slug growth and reproduction. Warm winters, and cool wet spring and summer days drastically favor slug development.

What are slug management options? There are a lot of unknowns when it comes to slug management. However, an integration of management tactics is probably the best approach. Residue management, correct planting procedures, natural predator populations, molluscicide application, and warm, dry weather can all help to limit slug populations.

Tillage in some form can reduce slug populations but for many Pennsylvania farmers dedicated to maintaining a 100% no-till system this is not an option. Researchers from Ohio found that slug populations were lowest in conventionally tilled and reduced tilled fields compared to no-till fields. The reduced tillage fields had a single disking pass in the spring. Based on the life cycle, a single disk or vertical till pass may be more effective in the fall compared to the spring. In annual ryegrass seed production, fall tillage passes have reduced slug populations but there was no comparison with spring tillage. Perhaps of more interest, in their system, alternate year tillage or tillage every three years significantly reduced slugs.

Good agronomic practices can help to limit the worst slug damage. The most important tactic is to make sure that the seed furrow is completely closed to avoid the slug super highway. An open seed furrow is the perfect environment for slugs to happily avoid predators and cool off while feeding a row on newly germinated corn and soybean seedlings. In fields with high residue, more than 75%, clumps of stalks can be pushed into the seed furrow making a perfect nest for juvenile slugs. Trash wheels push the residue away from the row and help with furrow closure. Selecting a variety with fast emergence in cooler planting situations can help to get the crop growing and outpace the slugs.

Slugs may have seemed invincible this spring but there are many natural predators including birds, frogs, toads, snakes, spiders, centipedes, and many insect predators such as firefly larvae, ground beetles, and rove beetles. Using insecticides in crop fields only when an insect pest reaches an economically damaging level will help to conserve natural insect predators.
Rescue treatments are often a last resort and vary in their effectiveness. Baited pellets (e.g. Deadline M-Ps) containing metaldehyde, a molluscicide, can effectively reduce slug populations. However, this can be an expensive option because re-application is necessary after rainfall and typically multiple applications are needed during cool, wet weather. At-planting application is only effective if eggs have already hatched and juveniles are present in the field. Researchers in Ohio evaluated bait applications at 1, 2, 3, 4, and 5, weeks after planting and the most effective applications coincided with presence of juveniles in the field. A home remedy of urea-based nitrogen applied at night has been used with variable success.

What can you do this fall? Scouting newly seeded cover crops for slug damage or for slug eggs beneath residue or in combine piles can help to predict problem fields for next spring. This requires close monitoring because slugs prefer to feed on seedlings and it is easy to miss the infestation if more than a week passes between field checks. This will help to start building a slug field history for your farm, in particular, whether infested fields in the fall are an issue for seeding corn and soybean in the next spring.

Quote Of The Week: “A gossip can’t be telling the truth all the time, there isn’t that much truth.” Amish Proverb