Vegetables and Ornamental Horticulture : Non-Pathogenic Sweet Corn Disorders

COMMON NON-PATHOGENIC SWEET CORN DISORDERS
Steve Reiners
Associate Professor in Horticultural Sciences

Sweet corn growers are well aware of symptoms associated with diseases like Stewart's Wilt, Common Rust or Smut, but many symptoms seen every year are not caused by a pathogen. Many are due to environmental conditions and are called physiological or abiotic problems. Some of the most common problems include;

SPIKED LEAVES - Dry fields throughout New York are showing the typical rolling with spiky looking leaves that give plants more of the appearance of pineapples than sweet corn. In older plants, dry soil will also limit nitrogen uptake and give plants a dull green color on young foliage and yellow to brown older leaves.

SUNSCALD - Temperatures exceeding 95F with low humidity and inadequate soil moisture, and dry winds, can lead to leaves yellowing then rolling and bleaching. Sunscald varies greatly by variety.

EXCESSIVE LEAF WAX - This condition has been seen in several fields this year. The plant produces greater amounts of wax than is normal in an attempt to minimize water loss through its leaves. This is normally due to high temperatures but any stress can cause some varieties to produce a waxier leaf.

POOR TIP FILL and MISSING KERNELS - Usually related to poor pollination due to excessive heat and drought. Irrigation at the optimum time, when plants are silking and ears just developing, is critical. Any insect that feeds on the silks can affect pollination and the final ear quality. High plant populations and low fertility may also cause the problem. Varies very much by variety.

BARREN STALKS - Differs from poor tip fill or missing kernels in that no ears are produced on the plant. Often related to high plant populations or very low fertility. Barley Yellow Dwarf virus may be associated with barren stalks.

BUGGY WHIP- Also known as "onion leafing", in which leaves remain wrapped in a spike. Usually associated with 2,4-D injury, but plants that have a difficult time emerging through a thick soil crust may also show this effect.

TASSEL EARS - A normal looking ear has a tassel growing from its end. Usually only a few plants will show this odd trait and its cause, although likely genetic, is not known for sure.

EXPOSED EAR TIPS - Usually variety related but may be worse in years with dry conditions at silking and kernel set followed by rainfall and optimum growing conditions.

NUTRIENT DEFICIENCIES - Nitrogen, phosphorus, magnesium and occasionally zinc may be the nutrients most likely to be deficient for sweet corn in New York soils. Symptoms include;

Nitrogen - Stunted, yellow growth. Lower leaves of older plants will show tip dieback while the margins remain green. Often seen in excessively wet soils or in dry soils after mid-season. Many fields are showing N deficiency this year, even those that have been sidedressed, manured, or planted in killed, legume cover crops. Dry soils are making N uptake difficult. In addition, bacteria that normally convert soil organic matter to plant available nitrogen are not very efficient in hot, dry soils. Nitrogen levels must be optimum no later than when plants are knee-high. Later applications beyond this stage will not be effective.

Phosphorus - Most New York soils that have been in vegetable crops test medium to high for phosphorus. Unfortunately, only a small fraction of the soil P is available when the soil is cool in the early spring. Typical symptoms are a reddening or purpling of young plants early in the growing season. Starter fertilizer containing 20 - 40 pounds per acre P2O5 at the time of seeding is usually enough.

Magnesium - Usually seen as an interveinal yellowing of upper leaves. This differs from nitrogen deficiency which is first seen on lower leaves. In addition, N deficient leaves are yellow throughout while magnesium deficient leaves show very clear green veins. Most likely to occur in low pH soils as is commonly found when sweet corn follows potatoes in rotation. Five pounds per acre in the band at planting is sufficient for fields where the pH is being kept low. Using a dolomitic limestone high in magnesium will help fields where the pH is to be adjusted upward. A foliar spray of epsom salts (magnesium sulfate) can be helpful.

Zinc - Symptoms occur early, first with a light streaking of leaves followed by a broad stripe of bleached tissue on each side of the midrib. New leaves will appear almost white while older leaves will have a characteristic green midrib, margins, and tip with a whitish area in between. Usually a problem in cool soils with pH above 7.0. Use one to two pounds of zinc per acre in the band at planting or a 0.25 pounds of actual zinc per acre as a foliar spray.