Sweet corn is a high-value crop for the fresh market, sustaining jobs and local economies across south Florida and other production regions. This publication is intended for Florida sweet corn growers, crop consultants, and Extension professionals to support accurate identification and effective management of foliar diseases on this important crop. Foliar diseases caused by fungal pathogens consistently threaten market quality. Effective management requires an integrated approach that combines host resistance, cultural practices, and fungicide-based strategies. Host resistance comes from selecting and planting resistant hybrids, when available, that match the diseases most likely to occur in the given planting season. Cultural practices help reduce inoculum sources and make conditions less favorable for disease, thus lowering disease pressure. Fungicide-based management uses fungicides, including multi-site protectants and site-specific products. Fungicide applications are most effective before disease is observed, when timed to coincide with crop growth stages, and during periods of high disease risk. Considering that pathogen pressure, corn susceptibility, and weather conditions vary across Florida’s production regions, regular scouting, accurate disease identification, and timely decisions are essential.
Sweet Corn Production
Sweet corn is a major vegetable crop grown in the United States for both fresh and processed markets, with an annual farm-gate value near $1 billion. Fresh sales account for about 70% of its total sales (Amon et al. 2024; USDA-NASS 2025b). Florida is the leading producer of fresh sweet corn, contributing $293 million in 2024, with most acreage in Miami-Dade and Palm Beach counties (Bayabil et al. 2022; USDA-NASS 2024; USDA-NASS 2025a). In south Florida, the primary season for this crop ranges from winter to spring, with peak harvests from March to May, whereas summer is the off-season (Amon et al. 2024; Bayabil et al. 2022; Moura et al. 2026; Xavier et al. 2025). Foliar diseases caused by fungal pathogens are among the primary constraints on sweet corn production (Miller et al. 2019; Moura et al. 2026), and they are the focus of this publication.
Foliar diseases reduce leaf photosynthetic area, thereby lowering kernel set and tip fill and consequently reducing yield (Wise and Mueller 2011). Injuries to the ear leaf and the leaves above it, such as lesions and spots, that occur around the tasseling to silking stages are most harmful for crop production (Pignon et al. 2017; Mueller et al. 2017; Wise and Mueller 2011). In fresh market sweet corn, lesions and rust pustules on flag leaves (outer, leafy layers that cover the sweet corn ear), husks, and shanks blemish the appearance and downgrade pack-out (Pataky 1992; USDA-AMS 1992; Stall et al. 1989).
From Field Diagnosis to Epidemic Development
Accurate field diagnosis is the starting point for timely and effective disease management. “Symptoms” are the plant’s responses such as lesion shape, color, blighting, chlorosis, and necrosis, whereas “signs” are parts of the pathogen itself, including rust pustules and spores. Learning to distinguish between symptoms and signs caused by biotic factors and to separate them from diseases caused by abiotic factors, such as nutrient stress, windburn, or herbicide injury (see Ask IFAS SS-AGR-365, “Diagnosing Herbicide Injury in Corn”), prevents unnecessary spray applications and improves product selection and timing.
Foliar corn diseases are polycyclic, which means they can complete many infection cycles in a single season. Epidemics build quickly during humid (>90% RH) weather and prolonged leaf wetness. Disease risk can occur across a wide range of temperatures. Cool (16°C–24°C; 60°F–75°F), moderate (24°C–29°C; 75°F–85°F), and hot periods (29°C–35°C; 85°F–95°F) can each favor the development of different pathogens (Munkvold and White 2016). In Florida’s tropical/subtropical climate, these conditions occur frequently during sweet corn growing seasons, so conditions favorable for disease epidemics often persist. Spores usually originate from infected crop residue or nearby fields and spread primarily by wind over long distances (Munkvold and White 2016). In Florida, diseases that survive on crop residue are usually less of a concern because most fields are tilled and plant debris breaks down quickly (Moura et al. 2025). Understanding disease development, spore sources and movement, conducive weather conditions, and crop growth stages all help forecast outbreaks and schedule scouting and fungicide sprays.
Sweet Corn Foliar Diseases of Importance in Florida
The primary foliar diseases in Florida are northern corn leaf blight (Exserohilum turcicum) and southern corn leaf blight (Bipolaris maydis), followed by common rust (Puccinia sorghi), southern rust (Puccinia polysora), and northern corn leaf spot (Bipolaris zeicola) (K. V. Xavier, personal communication). Tar spot (Phyllachora maydis) has been one of the most important foliar diseases of field corn in the Midwest/northern United States in recent years, and it was estimated to be the greatest yield-reducing corn disease in 2021, 2022, and 2024 (Mueller et al. 2022, 2023, 2024, 2025). Although tar spot remains uncommon on sweet corn in Florida (Ferreira et al. 2024), this publication still includes information on the disease for awareness and accurate diagnosis.
Each of the following sections summarizes what is needed for field identification and situational awareness of the most important diseases for sweet corn in Florida. These factors consist of the causal agent, key symptoms/signs, and weather windows when risk increases. Disease pressure can vary by region and field, and it also shifts from season to season and year to year.
Northern Corn Leaf Blight (NCLB)
Northern corn leaf blight is caused by the fungus Exserohilum turcicum. The symptoms start as pale gray-green, elliptical (“cigar-shaped”) spots less than about 1.0 cm (0.4 in) long, and, with time, the lesions can enlarge to roughly 3.0–15.0 cm (1.2–6.0 in) (Figure 1). As they mature, lesions turn tan and often show distinct dark zones of fungal sporulation (fungal spore production). The disease can also severely affect flag leaves and husk when disease pressure is high. This pathogen is favored by cool to moderate temperatures of 18°C–27°C (64°F–81°F), combined with prolonged dew periods (Munkvold and White 2016; Miller et al. 2019; Pataky 1992). It most often occurs in spring in south Florida (Frey et al. 2022–2026; Moura et al. 2026).
Credit: Vitor A. S. Moura, UF/IFAS.
Southern Corn Leaf Blight (SCLB)
Southern corn leaf blight is caused by the fungus Bipolaris maydis. Early lesions appear as small, tan spots that expand into rectangular lesions bounded by leaf veins. Lesions typically enlarge to about 0.3–2.3 cm (0.12–0.9 in) and may show a darker brown border as disease progresses (Figure 2). The disease is favored by moderate to warm temperatures of 20°C–32°C (68°F–90°F) and humid conditions. Long, dry periods of sunny weather between rains are unfavorable for disease development (Munkvold and White 2016). This disease is most common in late spring and fall in south Florida (Frey et al. 2022–2026; Moura et al. 2026).
Credit: Vitor A. S. Moura, UF/IFAS.
Common Rust
Common rust is caused by Puccinia sorghi. The pathogen breaks through the leaf surface and produces small yellow flecks on both the upper and lower leaf surfaces (Figure 3). These lesions develop into round-to-elongated, reddish-brown pustules, containing cinnamon-brown spores. Heavy infections can leave brown, necrotic areas around pustules that merge into bands or patches, leading to yellowing and, in severe cases, death of whole leaves or leaf sheaths. The disease is favored by cool to moderate temperatures of 16°C–25°C (60°F–77°F), high humidity, and frequent rain or prolonged leaf wetness. It is most aggressive on newly expanding leaf tissue (Munkvold and White 2016). In south Florida, common rust is most often observed in spring, although pressure can be low in some years (Frey et al. 2022–2026).
Credit: Larry Newsom, BASF Agricultural Solutions North America.
Southern Rust
Southern rust is caused by Puccinia polysora. Its pustules are small, circular to oval, and light cinnamon-brown to orange in color. They form densely on the upper leaf surface and, unlike common rust, do not usually appear on the lower surface. Severe infections can also affect husks, ear shanks, stalks, and leaf sheaths. During severe outbreaks, the large number of pustules can cause extensive leaf blighting and premature death, especially in the upper canopy (Figure 4). This disease is more aggressive than common rust. Southern rust is favored by warm temperatures of 24°C–27°C (75°F–81°F) and high humidity (Munkvold and White 2016; Sun et al. 2021). In south Florida, it typically appears in late spring and fall, although pressure can be very low in some years.
Credit: Larry Newsom, BASF Agricultural Solutions North America.
Northern Corn Leaf Spot
Northern corn leaf spot is caused by Bipolaris zeicola. Lesions begin as small, tan spots and develop into long, narrow, light-tan streaks with a darker brown border. Individual lesions are typically up to about 2.5 cm (1.0 in) long and may merge into elongated patches as they expand. The disease tends to appear during periods of high humidity and moderate temperatures. It is occasionally seen in the lower canopy (Crop Protection Network 2019; Munkvold and White 2016). In Florida, it has been observed mostly during the winter (Frey et al. 2022–2026).
Credit: Vitor A. S. Moura and Katia Viana Xavier, UF/IFAS.
Tar Spot
Tar spot is caused by Phyllachora maydis. It is characterized by black, raised, “tar-like” spots called stromata that are embedded in leaf tissue and cannot be scraped off (Figure 6). The black spots can extend along the leaf veins and assume an oblong shape. Lesions mostly occur singly, but with time, they can coalesce into clusters. Another leaf symptom, called “fisheye,” occurs when a pale yellow to tan halo surrounds the black spot, as tissue around the stromata dies. Look-alikes such as insect frass, soil specks, or sooty molds can be wiped away, whereas tar spot stromata remain fixed in the tissue. Tar spot development is associated with prolonged periods of moderate temperatures, 18°C–23°C (64°F–73°F), coupled with short periods of high relative humidity (>90%). It typically arrives late in the spring season in south Florida, around April through June (Ferreira et al. 2024). If similar spots appear earlier than expected, contact your local UF/IFAS plant diagnostic clinic, plant pathologist, or Extension agent for confirmation.
Credit: Katia Viana Xavier, UF/IFAS.
Integrated Disease Management on Sweet Corn
Integrated disease management relies on three complementary pillars that protect the plant against diseases: (1) host resistance, the use of resistant hybrids to provide built-in protection; (2) cultural practices, production tactics that lower inoculum and create conditions unfavorable to disease; and (3) fungicide programs, foliar applications that prevent new infections on leaves, husks, and ear shanks and lower inoculum levels. Accurate disease identification enables timely and effective fungicide applications. Scout early and throughout the season to ensure fungicide applications occur before disease becomes widespread. Be aware of surrounding fields as symptomatic neighboring fields can be a source of inoculum (Mueller et al. 2017; Wise and Mueller 2011). Consult the UF/IFAS Commercial Vegetable Production Hotline Archive for regional disease alerts and updates.
Host Resistance
- There are hybrids with adequate resistance against common rust (P. sorghi), NCLB (E. turcicum), and SCLB (B. maydis) in Florida (Table 1).
- When possible, hybrid choice should match resistance with the season. Prioritize NCLB and common rust resistance during the cooler winter through spring. During the hotter late spring and fall seasons, prioritize SCLB resistance.
Cultural Practices
- Control volunteer plants and manage infected residue (Moura et al. 2025). Abandoned fields can become major sources of spores for many foliar diseases. Early postharvest tillage buries residue, speeds decomposition, and reduces inoculum survival.
- In fields with overhead irrigation, schedule irrigation during the morning to prevent prolonged nighttime leaf wetness.
- Improve airflow within the crop canopy by maintaining proper row spacing, considering appropriate plant population, and keeping weeds under control. Sweet corn plant populations commonly range from 24,000 to 32,000 plants per acre (Xavier et al. 2025). When foliar disease risk is high, consider using the lower end of this range if it fits the hybrid and local production recommendations.
- Limit long periods of overlapping susceptible crops. Extensive staggering of planting dates can increase disease pressure by allowing pathogens to persist and move into younger fields.
Fungicide-Based Management
Fungicide application is recommended when disease risk is high, typically in susceptible hybrids. Risk is high when plants approach the tasseling and silking stages (VT–R1); when humid, rainy weather is forecast or ongoing; or when lesions are detected in pre-tasseling stages (Mueller et al. 2017). Fungicides used on sweet corn fall into two broad groups: contact protectants and systemic products.
Contact (multi-site) protectants stay on the leaf surface, act where deposited, and have short residual activity. These products provide no post-infection (curative) activity, can be easily washed off by rain or irrigation, and offer a low risk of resistance development (Mueller et al. 2017; FRAC 2024). Typical retreatment intervals on sweet corn vary from 4 to 10 days, depending on the product and active ingredient. Some multi-site contact protectants from FRAC M provide broad-spectrum protection against rusts, spots, and blights. Examples include sulfur (M2), mancozeb (M3), and chlorothalonil (M5).
Systemic/translaminar (single-site) products penetrate the leaf and protect from within, moving locally (translaminar) and/or upward through the plant (xylem-mobile). These products work best when applied preventively but may offer some activity soon after infection. They carry a higher resistance risk and require a rotation of mode of action (FRAC rotation) (Mueller et al. 2017; FRAC 2024). They are also available in premixtures. Product labels indicate retreatment intervals of 7–14 days on sweet corn. The following list provides examples of these fungicides:
- Triazoles (DMI), FRAC 3: This includes propiconazole, tebuconazole, metconazole, prothioconazole, mefentrifluconazole, and flutriafol. These products have a curative window and medium resistance risk (Mueller et al. 2017; FRAC 2024).
- Strobilurins (QoI), FRAC 11: This includes azoxystrobin, pyraclostrobin, picoxystrobin, trifloxystrobin, and fluoxastrobin. These products are most effective when applied preventively. They have limited curative activity and a high resistance risk (Mueller et al. 2017; FRAC 2024).
- Carboxamides (SDHI), FRAC 7: This includes penthiopyrad, fluopyram, benzovindiflupyr, fluxapyroxad, and pydiflumetofen. These products have some early curative activity, are commonly sold as premixes, and have a medium-to-high resistance risk depending on the pathogen (Mueller et al. 2017; FRAC 2024).
For a list of available foliar fungicides, consult chapter 17 of the Vegetable Production Handbook of Florida, “Sweet Corn Production.”
To avoid fungicide resistance, always follow label instructions, observe the seasonal maximum rate, rotate modes of action, and consider rotating premixtures with multi-site contact fungicides such as mancozeb and chlorothalonil (Miller et al. 2019). Always follow label retreatment intervals and postharvest intervals (PHI). Recent in vitro assays with Florida isolates indicate reduced sensitivity to the DMI fungicides propiconazole and flutriafol in populations of the pathogen causing northern corn leaf blight, reinforcing the need for fungicide resistance management (Anderson et al. 2024; V. S. Moura, unpublished).
For confirmation and management, contact the nearest UF/IFAS Plant Diagnostic Clinic, your appropriate UF/IFAS Research and Education Center, or your county Extension office.
Table 1. Published foliar fungal disease resistance in supersweet (shrunken-2) sweet corn hybrids.
References
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