7 Corn Diseases to Watch for in the Garden
Corn, in its tasty, sugary goodness, brings us the taste of summer after awaiting peak readiness. Gardening expert Katherine Rowe outlines corn diseases to keep an eye out for, as well as prevention, for the sweetest, plumpest kernels.
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While diseases in the home garden don’t usually plague corn, the crop is susceptible to a number of them that sometimes make us miss out on enjoying the juicy, sweet kernels. Environmental factors like climate and weather play a role in accelerating fungal and bacterial problems, as does cultural management and variety.
Corn sometimes falls victim to seedling, foliar, stalk, and ear diseases. Depending on the extent of the spread, they can stunt growth, cause underdeveloped cobs or low yields, or lodge the entire stalk.
To keep corn from experiencing growing problems, start with strong varieties like heirlooms and those with good disease resistance. Knowing what to look for to scout for potential diseases helps lessen the spread, and preventative measures lay the groundwork for sturdy ears.
About Corn Diseases
Diseases in corn appear when a few variables align, usually related to external factors like cool, wet conditions or hot, dry spells that prompt pests and diseases. They also relate to the presence of the pathogens themselves through nearby hosts, with pests as vectors, or with plant debris in the plot, especially where we’ve grown another corn crop recently.
Meeting corn’s best growing situations helps establish a robust crop. Opt for disease-resistant varieties to stave off problems. Optimal growing conditions for corn include:
- Full sun (six plus hours of sunlight daily)
- Well-draining, fertile soils
- Soil pH near 6.0-6.5
- Warm days and nights
- One to two inches of water per week (rainfall and irrigation)
In addition to meeting the best growing conditions for corn and choosing disease-resistant varieties suited to our growing area, there are a few key preventative measures. To minimize harboring disease-causing agents:
- Keep beds and surrounding areas weed-free. Grasses and other weeds compete for resources like moisture, sunlight, and nutrients, and can weaken corn plants. They also host pathogens and provide overwintering sites. When insects like thrips and aphids feed on infected weeds and then visit corn, they spread disease to the healthy crop.
- Practice crop rotation. Give the plot a rest from corn-growing for at least two seasons. The pathogens live in residue left behind in the soil and in plant debris until the host is present again.
- Maintain good garden sanitation. After the harvest, immediately remove stalks and debris to reduce the chance of overwintering and proliferating soil-borne diseases. Consider a cover crop if you plan to leave the bed bare for the cool season.
Seedling Rot
Seed and seedling dieback happen during initial development, early in the season. Soil-borne fungi like Pythium, Rhizoctonia, and Fusarium infect many crops and ornamentals, and corn is no exception. With infection, tender new growth begins to rot near the soil level. Seedlings will show yellowing, wilting, and dropping leaves.
To prevent seedling rot, ensure the soil is well-draining and loose. Cool, damp spring conditions foster the onset of fungal problems in young plants, especially as seeds sit in too much moisture with slow germination. Poor drainage, compacted soils, and cool temperatures are prime for weakened seedlings.
Wait until temperatures warm to sow kernels, with soils above 55°F (13°C). In addition to increased disease susceptibility, cold snaps can hinder growth and future production. The warmer the better for corn, which takes off as the soil rises near 70°F (21°C).
Smut
Smut is a fungus (Ustilago maydis) that is easy to spot. It begins with grayish white galls (swollen areas) on leaves, stems, ears, and tassels. They age to brown and hold black, sooty spores.
Smut occurs most often in high temperatures and periods of moisture. To prevent progression, pick off infected ears, taking care not to disrupt the galls or cause the spores to release. They can spread to other plants or into the soil for overwintering. Dispose of or burn the debris to reduce harboring the spores. Interestingly, the fungus is edible, high in protein, and a delicacy in Mexican cuisine.
While we can’t control hot weather, we can aim for even moisture when irrigating. Avoid overly wet conditions when possible, halting irrigation during rainy spells. During dry spells, apply extra water, especially two weeks prior to silking and as kernels develop. Avoid excess fertilizing, as too much nitrogen causes weakness and susceptibility to smut. Taking care not to damage plants during weeding or tending limits access points for the spores.
Rust
Rust is caused by species of the fungus Puccinia, and corn diseases include southern and common strains. Rust presents with raised reddish-brown spots on leaf surfaces and travels on the wind. Southern rust (Puccinia polysora) often flares up later in the season as temperatures rise, and with high humidity and rainfall. Common rust (Puccinia sorghi) thrives in cool, humid conditions (60-70°F or 16-21°C).
Fortunately, rust seldom does significant damage in the home landscape. If you have a history of rust arriving early and impacting yields, try planting an early-season crop to skip the most active time of proliferation. Avoid drought stress, which makes less vigorous plants more likely to develop problems.
Leaf Blights
Leaf blight results from several fungal and bacterial pathogens that affect foliage. These include:
Southern Corn Leaf Blight
The fungus Bipolaris maydis shows up as light brown spots on lower leaves. They enlarge and spread and can penetrate the ear, causing cob rot. Outer leaves on the ear will have bleached, whitish spots where the fungus enters.
Northern Leaf Blight
Exserohilum turcicum prefers cool, humid conditions and, depending on the timing, does little damage late in the season but can hinder yields in the pre-silk phase. Large, grayish-green lesions develop on leaves and mature to tan with dark spores.
Stewart’s Wilt
This bacterial leaf blight, caused by Erwinia stewartii, impacts both seedlings and mature stalks. You’ll see water-soaked lesions and dieback in seedlings. On mature foliage, long, rectangular streaks appear before turning brown. Flea beetles are a vector, which spores as they travel and feed.
Gray Leaf Spot
A common foliar issue in warm, humid areas, Cercospora zeae-maydis, appears as circular spots with a yellowish halo. Mature spots are tan or gray and blocky as they spread linearly.
Anthracnose
This fungal infection (Colletotrichum graminicola) is not only a leaf blight, it also causes stalk and top dieback. In leaves, it produces small, watery brown lesions with a yellow to deep red border. As the lesions spread, they cause tissues to die.
To prevent leaf blight, remove debris at harvest’s end for a clean plot. Crop rotation with a non-grass cover helps avoid overwintering flea beetles, bacteria, and fungi. As with other corn problems, disease-resistant varieties are beneficial against leaf blight.
Stalk Rots
Stalk rot happens when pathogens invade the stalks, often through the roots, and cause tissue decay. Anthracnose, Fusarium, Diplodia, and charcoal rot are corn stalk diseases. The fungi survive in the soils and in corn residue from earlier rounds of growing. They enter roots or splash up on foliage from rain and irrigation, or travel on the wind. Corn stalks may fall over (lodge), leading to ear loss or underdevelopment.
Stressors that lead to stalk rot include pest damage, a lack of nutrients, drought, and dense, heavy soils. Cloudy, wet weather can accelerate the spread. Ensure proper spacing to allow ample air flow and to reduce damp conditions. A six-inch spacing between stalks provides air circulation while reducing competition.
Most of these rots are corn-specific, and reducing residue collection by cleanup and planting rotation for at least one year can help.
Ear Rots
Common ear rots include Fusarium, Diplodia, and Gibberella in seasons with heavy rainfall between silking and harvesting. In hot, dry conditions, Aspergillus can set in. Ears suffer damage and decay as the fungi spreads.
The fungal pathogens spread through wind, rain, and insects. They may present as necrotic brown kernels (Fusarium), brown, mushy bands (Gibberella), or a pale whitish-gray coating (Diplodia).
Follow prevention protocols for stalk rots and other corn problems to boost health and provide a strong foundation. Digging in residues through tilling may help in plots.
Viruses
Maize dwarf mosaic virus (MDMV) and maize chlorotic dwarf virus (MCDV) cause stunted growth, weak cobs, or no ears at all. Infected leaves show green and yellow blotching and mottling (MDMV) or striations (MCDV).
MDMV is spread by several species of aphids beginning in spring. In impacted plants, yields are low or non-existent in severe cases, and weakened plants are susceptible to secondary diseases like root rot.
In addition to disease-resistant varieties, avoid late-season plantings if you have a history of the viruses. Keep the area surrounding the bed weed-free, especially of Johnsongrass, to prevent the virus from overwintering in nearby hosts.
