Distinguishing disease symptoms from those of nutrient deficiencies

Tackling disease and nutrition problems usually requires different courses of treatment. As a result, it makes sense to get the diagnosis right the first time.

According to Gordon Johnson, Extension vegetable specialist at the University of Delaware, in the case of sweet corn, in particular, there are times when symptoms of plant stand issues, vigor, or disease can be confused with a nutrient deficiency.

For example, early in the season when the weather is cool, the plant leaves turn purple and growers sometimes associate that coloring with a phosphorus deficiency. Rather, says Johnson, it is that the roots aren’t functioning as well as they should, thanks to the cooler temperatures, and they aren’t taking up phosphorus.

“Some varieties are more prone to purpling genetically,” he explains. “The plants usually grow out of it, but it can be a concern for growers.”

Johnson says, in general, soil borne diseases are not common in sweet corn in his area of the country, but when growers produce multiple sweet corn crops on the same land, that can encourage soil borne diseases. One of the most common is the seedling disease Penicillium.

This disease, he says, typically shows up under warm conditions. Penicillium will produce uneven, stunted stands that sometimes can be attributed to a fertilizer issue. “This is one of the seed rots that likes warm soil,” he adds.

Understand the Symptoms

A nitrogen deficiency, on the other hand, is not an uncommon problem for sweet corn as wet weather often stops growers from getting back in the field in a timely manner to sidedress. A symptom of too little nitrogen, says Johnson, is the yellowing of lower leaves in an inverted V pattern.

“The yellowing can be the result of wet weather and denitrification in heavy soils or excess rain and nitrogen leaching in sandy soils,” he explains. “However, corn plant yellowing can also be due to sulfur deficiencies in sandy soils or damage to plant roots due to soil insect feeding or root diseases.”

Leaf scorch is another problem that can be misdiagnosed, he adds. It usually occurs as a result of water stress or soil moisture deficits but can also be caused by disease. “Leaf scorch will occur on certain varieties once the temperature goes above 100 degrees,” says Johnson. “The leaves literally look scalded. We’ve seen that two years in a row and it was misdiagnosed as a disease. Leaf scorch is variety related.” Diseases that cause similar symptoms in sweet corn include Stewart’s wilt and Goss’s wilt, he adds.

Know Your Field

In general, Johnson says it is important for growers to know their crops and know what a normal sweet corn plant looks like. “When it doesn’t look normal you need to be able to understand what the signs and symptoms are,” he says.

As there can be confusion, the only way to definitely determine what the problem is, however, is to analyze tissue and conduct soil tests. “Testing is the only way to correctly assess the situation,” he concludes.