The recent change in the weather pattern in Indiana from excessively wet to unusually warm and dry has resulted in the development of a substantial crust in fields that were previously worked and planted in late April. Obviously, a thick crust can restrict the emergence of corn and cause underground leafing. The rapid drying of the upper soil layer is also conducive for the development of the "floppy corn" syndrome.
My accompanying article, " A Primer on Corn Root Development", briefly describes the normal development of a corn seedling's root systems. Root buds from any given node that begin to elongate in dry soil or in soil cracks may quickly cease growth if soil moisture is insufficient. Without adequate soil moisture, the root tips may dessicate and die. If the soil remains dry long enough, the whole root bud may die. At this point, the plant's survival depends on improved soil moisture conditions and the development of the next set of nodal roots.
If dry surface soil and/or hot, dry weather prevail, several sets of nodal roots may fail to form, giving rise to the "rootless corn" phenomenon. Affected plants must depend on the seminal roots and mesocotyl for nourishment, when normally this lifeline has already taken a backseat to the nodal root system.
In addition to the nutrient stress imposed on the plants by an inadequate nodal root system, the rootless phenomenon can eventually result in the floppy corn syndrome, whereby plants simply flop over at the soil surface at the slightest nudge from wind, tire traffic or even crop scouts walking down the row.
These plants are technically not root-lodged, they are simply broken over at the base of the stem near the crown area. The nodal roots will appear stubbed off but not eaten. The root tips will be dry and shriveled.
These symptoms are unlike any associated with herbicide injury or insect feeding. Because several sets of roots may not have formed below-ground, the crown may "appear" to be at or above the surface.
The important thing to remember is that roots do not grow toward moisture on purpose. If the root tips of very young roots die before soil moisture is encountered, root elongation will simply cease. If roots are already in moist soil, however, they may proliferate rapidly enough to "follow" moisture down as the soil dries. Row cultivation may encourage root development if moist soil is thrown around the bases of the plants. The ultimate solution to the problem is a good soaking rain.
Rootless corn can also be caused from extremely shallow seeding depths that result in nodal root initiation beginning at the soil surface rather than at the usual ¾ inch depth. Growers should avoid seed depths shallower than about 1 to 1½ inches.