12.2 Major Field Crop Diseases & Infection Cycles

Understanding the specific characteristics and infection cycles of major field crop diseases is essential for effective management. We will explore four significant diseases that commonly impact crop production: Tar Spot in corn, Gray Leaf Spot in corn, Sudden Death Syndrome in soybeans, and Sclerotinia White Mold in soybeans. Each of these diseases has a unique lifecycle and set of environmental conditions that favor its development.

Tar Spot (Phyllachora maydis)

Tar spot is a relatively new and highly destructive fungal disease of corn in the United States, first confirmed in 2015. It is caused by the fungus Phyllachora maydis. The disease is named for the raised, black, irregular-shaped fungal fruiting bodies (stromata) that develop on the leaves, which resemble small spots of tar. These stromata can also appear on husks and leaf sheaths.

Infection Cycle: The fungus overwinters in infested corn residue left on the soil surface. In the spring and early summer, under favorable conditions, the stromata release ascospores that are carried by wind and rain splash to lower corn leaves. Infection requires prolonged periods of leaf wetness (high relative humidity, frequent rainfall, or heavy dew) and moderate temperatures (60-70°F).

Progression and Impact: Once infection occurs, tar spot can progress rapidly. The disease causes premature leaf senescence, reducing the plant's photosynthetic capacity during critical grain fill stages. This leads to reduced kernel weight, poor grain quality, and significant yield losses, which can exceed 50 bushels per acre in severe cases. Furthermore, the early death of leaf tissue can compromise stalk integrity, leading to lodging issues later in the season. Management relies on a combination of tolerant hybrids, crop rotation to reduce inoculum, and timely fungicide applications based on scouting and disease forecasting models.

Gray Leaf Spot (Cercospora zeae-maydis)

Gray leaf spot (GLS) is a major foliar fungal disease of corn, prevalent in areas with high humidity and reduced tillage practices. The causal agent, Cercospora zeae-maydis, primarily affects the leaves, causing distinct rectangular, necrotic lesions that are restricted by the leaf veins.

Infection Cycle: Like tar spot, the GLS fungus overwinters in corn residue. As temperatures rise in the spring, the fungus produces conidia (spores), which are dispersed by wind and rain to the lower leaves of the new corn crop. Infection is highly dependent on extended periods of high relative humidity (over 90%) and warm temperatures (75-85°F).

Development: Initial symptoms appear as small, tan spots that eventually elongate into the characteristic rectangular lesions. These lesions can coalesce, blighting entire leaves. The disease typically starts on the lower leaves and progresses up the plant canopy. If the disease reaches the ear leaf and above prior to or during the grain-fill period, significant yield reductions can occur due to loss of photosynthetic area. Management strategies include planting resistant or tolerant hybrids, implementing crop rotation (at least one year away from corn), burying residue through tillage (where appropriate), and applying foliar fungicides during the tasseling to early silking stages if disease pressure is high.

Sudden Death Syndrome (Fusarium virguliforme)

Sudden Death Syndrome (SDS) is a soil-borne fungal disease of soybeans caused by Fusarium virguliforme. SDS is uniquely challenging because the infection occurs early in the season, but the most severe symptoms do not appear until later, during the reproductive stages.

Infection Cycle: The fungus survives in the soil and soybean residue as chlamydospores. Infection of soybean roots occurs early in the spring, often shortly after planting, particularly in cool, wet soils. The fungus colonizes the root system, causing root rot and reducing root mass.

Toxin Production and Symptoms: The hallmark of SDS is the production of a phytotoxin by the fungus in the roots. This toxin is translocated upward through the xylem into the foliage. Foliar symptoms typically manifest during the reproductive stages (R3-R6). Initial symptoms include scattered interveinal chlorotic spots, which rapidly expand and become necrotic, leaving only the main veins green. The leaves eventually drop off, while the petioles remain attached to the stem. The internal stem tissue (pith) remains white, distinguishing SDS from brown stem rot. SDS causes significant yield loss through reduced pod set, fewer seeds per pod, and smaller seed size. Management involves planting SDS-tolerant varieties, improving field drainage, delaying planting until soils warm up, utilizing seed treatments with efficacy against SDS, and minimizing soybean cyst nematode (SCN) populations, as SCN can exacerbate SDS severity.

Sclerotinia White Mold (Sclerotinia sclerotiorum)

Sclerotinia white mold is a destructive fungal disease affecting soybeans and numerous other broadleaf crops. The causal agent, Sclerotinia sclerotiorum, is notorious for its wide host range and its ability to survive for long periods in the soil.

Infection Cycle: The fungus overwinters in the soil as hard, black, irregular resting structures called sclerotia, which can remain viable for many years. When conditions are cool and moist, and the crop canopy closes (creating a shaded, humid microclimate), sclerotia within the top two inches of soil germinate to produce small, mushroom-like structures called apothecia.

Infection Process: Apothecia forcefully eject ascospores into the air. These spores primarily land on senescing soybean blossoms, which provide the initial nutrient source required for the fungus to establish infection. Once established in the blossom, the fungus invades the stem, cutting off the flow of water and nutrients.

Symptoms and Impact: Symptoms typically appear during the reproductive stages. Infected stems become bleached and develop a white, cottony mycelial growth, especially under humid conditions. The fungus eventually forms new sclerotia inside and on the surface of the infected stems. The disease causes premature plant death, leading to poor pod development, reduced seed size, and significant yield loss. Management is complex and requires an integrated approach, including planting partially resistant varieties, wider row spacing to increase canopy airflow, avoiding excessive irrigation during flowering, utilizing crop rotation with non-hosts (like corn or wheat), and applying foliar fungicides specifically targeted at the flowering period (R1-R3) to protect the blossoms from infection.

Disease Comparison Summary