Wheat harvest is underway or rapidly approaching in most areas of the state. Foliage diseases such as Septoria leaf blotch did occur in low levels across much of the state. Virus diseases, especially barley yellow dwarf, were present in low levels. Fusarium head blight or scab was perhaps more widespread and severe than predicted. But now the questions seem to focus on black heads on wheat plants or black discoloration on wheat heads.
There are several possible explanations for black heads on wheat plants or for a black discoloration on portions of wheat heads.
The black heads due to loose smut are most obvious as heads emerge from the boot and for several weeks after that. The kernels on infected heads are replaced with masses of powdery black spores. So the heads have a very distinct black, powdery appearance. These spores are eventually dislodged by wind and rain, so later in the season the smutted stems are less evident and only the bare rachis will be left.
Septoria leaf blotch was present in the lower canopy of many fields this year. It didn’t seemed to move up in the canopy to the flag leaf in many fields but with the continued precipitation and high humidity in some areas if may have developed on wheat heads. On the heads dark brown to black blotches may develop. Stagnospora nodorum may also cause leaf lesions but is usually more common on heads- again causing dark blotches on glumes of part or all of the head.
Bacterial stripe or black chaff is a bacterial disease that produces symptoms on both leaves and heads. Water-soaked lesions may develop on young leaves. These expand into reddish-brown to brownish-black streaks on the leaves. Glumes and awns show brown-black blotches or streaks. Fungicides are not effective against bacterial stripe or black chaff so the use of resistant or tolerant varieties and crop rotation are the main management options. Again, wind driven rains, rain in association with hail and heavy rains can spread this bacterial disease through a wheat field quite rapidly.
Another likely cause of black wheat heads or black discoloration of wheat heads and plants is sooty molds. Sooty molds are a number of saprophytic or weakly parasitic fungi which grow on senescing or dying plant tissue. Alternaria, Cladosporium, Aureobasidium and other species are frequently found on these discolored or black plants. Since the affected plants may have a sooty appearance these fungi are sometimes called sooty molds. These sooty molds or secondary fungi tend to develop on plants when wet or humid weather occurs as the crop is maturing, if harvest is delayed because of wet weather or if portions of the plants have died prematurely ( ex. portions of wheat heads damaged by Fuarium head blight). Typically these fungi come in on plants that are shaded, undersized, weakened or prematurely ripened and on senescing foliage. Plants that are lodged or that have been stressed by nutrient deficiencies, plant diseases or environmental conditions may be more severely affected. Although many of these fungi produce dark or black mold growth, the color of the mold growth can range for dark or black to olive green or even pink to white.
On wheat these secondary fungi tend to develop the heads or portions of the heads but may also occur on leaves and stems of wheat plants. These black molds also tend to produce large quantities of spores. It is not uncommon to see dark clouds of spores around combines moving through fields with high levels of black mold or sooty molds.
REVISED: June 14, 2013