Pests & Diseases
The corn leaf aphid, a bluish-green or black pest with black legs, feeds on certain varieties of sweet corn. Infestation results in stunted and misshapen tassels, black mold on leaves and silk, and poor pollination. These destructive aphids also carry maize dwarf mosaic virus.
Cutworm larvae feed on leaves after dark, causing major damage to plants and often cutting of seedlings near the soil line. Corn earworms lay eggs on fresh silk, which hatch within days. Caterpillars then crawl into the silk and feed on kernels at the tip of the ear. Once embedded, they are protected from insecticide sprays.
Corn smut, also known as common smut, leads to shiny, round lesions on growing ears, transforming kernels into cysts filled with black, dusty spores.
Common rust has become widespread due to increased planting of popular sweet corn hybrids, which are highly susceptible to the disease.
Once crops are infected, oval-shaped brown blisters form on the leaves then release rusty red spores. During severe outbreaks, blisters also appear on corn tassels and ears, causing leaves to turn yellow and ragged.
Stewart’s wilt is caused by bacteria transmitted by flea beetles. Once corn is infected, the bacteria spreads rapidly through the plant. Seedlings will develop pale green or yellow stripes before wilting and dying.
When older plants are infected, white or yellowish streaks appear on leaves, causing them to dry up and die.
Storage & Packaging
Sweet corn can be harvested mechanically or by hand in the morning before temperatures climb. Mechanical harvesters may result in ears with shorter shanks and fewer husks than hand harvests. Corn for processing is mechanically harvested and hauled in bulk.
Sweet corn will begin to lose freshness and sweetness rapidly after harvest, so it’s important to pack and ship quickly. Moisture and cool temperatures are essential to slow the sugar to starch conversion.
Hydrocooling and/or vacuum-cooling can be used after harvest, with top icing to maintain moisture. If ice is applied, storage temperatures should be slightly warmer than the usual recommendation of 32 to 34°F, up to about 36°F.
If temperatures drop below 31°F, freezing injury may occur. In addition to freezing, ice crusting can inhibit adequate air circulation.
References: Clemson University Cooperative Extension, Cornell University, PennState Extension, University of Arizona, University of Florida, University of Illinois, University of Massachusetts Amherst, University of Minnesota Extension, USDA.
GRADES & GOOD ARRIVAL
Sweet corn is divided into three USDA grades: U.S. Fancy, U.S. No. 1, and U.S. No. 2. Length is a major component in grading: U.S. Fancy requires a minimum of 6 inches and should be unclipped; U.S. No. 1 requires a minimum of 5 inches and may be clipped; and U.S. No. 2 requires a minimum of 4 inches and may also be clipped.
Quality is measured by trimming, the fullness of the cob, covering, maturity, and the absence of insects, mechanical injury, or discoloration. Condition defects include freshness or husk color, maturity, kernel health and color, and the presence of decay, rust, or live insects.
Generally speaking, the percentage of defects shown on a timely government inspection certificate should not exceed the percentage of allowable defects, provided: (1) transportation conditions were normal; (2) the USDA or CFIA inspection was timely; and (3) the entire lot was inspected.
|U.S. Grade Standards||Days Since Shipment||% of Defects Allowed||Optimum Transit Temp. (°F)|
Canadian good arrival guidelines (unless otherwise noted) are broken down into five parts as follows: maximum percentage of defects, maximum percentage of permanent defects, maximum percentage for any single permanent defect, maximum percentage for any single condition defect, and maximum for decay. Canadian destination guidelines are 15-10-5-10-4.
References: DRC, PACA, USDA.