Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Foliar Disease and Weakened Stalks
This year many growers experienced higher than normal levels of foliar diseases on corn, particularly Gray leaf spot and Northern corn leaf blight. One item to keep in mind as we approach harvest is the effect that foliar diseases can have on stalk health. Carbohydrates are translocated from foliage to developing kernels during grain fill. When carbohydrates are in short supply the plant uses available carbohydrates in stalks to meet demands of grain fill (Figure 1). This weakens the stalks and can predispose them to stalk rotting pathogens. Foliar diseases of corn reduce photosynthesis and therefore carbohydrate production in corn and thus, can impact stalk strength. Now is a good time to start to check stalk strength and lodging potential. This can be done by pinching the lower stalk internodes or conducting a push test. In general, you should scout 10 stalks at 10 sites for every 10 acres of field. If you note that more than 10% of your corn exhibits the potential for lodging, consider an early harvest for that field.
Figure 1. Severely blighted corn has a difficult time meeting the carbohydrate requirements for grain fill. These needs are met by moving carbon stores from the stalks to the ear. This can weaken stalks and predispose them to stalk rots.
Corn Smut
Smut is common in field and sweet corn grown throughout the world, and can be economically important in some cases, resulting in yield losses approaching 20%. This disease is caused by a fungus (Ustilago maydis), and is easily identified by the large outgrowths found most often on corn ears (Figure 2) and tassels; however, any aboveground tissue can show symptoms. Galls initially appear white, and become black over time as the fungus produces spores (teliospores). If foliage is infected, pustules develop on the midrib. These pustules mature and release spore masses. Losses are greatest when the ear becomes infected or if the gall forms directly above the ear.
Figure 2. Smutted corn ear infected with Ustilago maydis
Corn can be infected at any time during the early stages of growth and is less susceptible to infection after pollination. The fungus often enters the plant through wounds, but it can also penetrate tissues directly under the appropriate environmental conditions. Developing silks are a prime target for infection and entrance into ears, but once pollination occurs infection and gall formation is unlikely.
Teliospores are the overwintering stage of the fungus and survive on crop debris or soil for several years. In the spring and summer these spores germinate and produce basidiospores, which are carried by wind or rain to corn plants. Teliospores can survive a trip through the gut of animals. Therefore, manure from animals fed infested corn material can serve as a source of primary inoculum.
The most effective management of corn smut is through the use of resistant varieties. Rotation out of severely infested fields may help reduce inoculum levels in subsequent years, although long distance dispersal and survival of spores in soils may limit the effectiveness of this practice. Minimizing plant damage when possible (i.e. insects, mechanical damage) may help minimize smut.
On a side note, our neighbors to the south consume immature smut galls from sweet corn. I’ve tried them before and they are quite good.