For most of the state the overwhelming issue right now is the weather. The unusually hot and dry conditions are definitely taking a toll of corn and soybean fields throughout the state. There are a few fortunate areas which have received timely rains and thus crops are in better condition. But for most rain is the immediate and pressing need.
Although I have gotten a few questions about common rust on corn and frogeye leaf spot on soybean, weather conditions for most of the state have simply not been favorable for the development of foliar diseases on either corn or soybean. And that is true even in most irrigated fields. The low relative humidity, high evapotranspiration rate and drying winds, mean that even in fields which are overhead irrigated the foliage is drying off quickly and humidity within the canopy is low. Most of the common foliage diseases are favored by periods of moisture on the leaf surfaces. Under the current conditions, foliage diseases aren’t likely to be widespread or severe.
There have been some examples of root rots contributing to decline in soybean fields. Later planted fields in which seed germinated over several weeks to a month may show very uneven growth due to the uneven germination. Some of these plants may have rotted and discolored taproots. Fusarium and Rhizoctonia may be causing some of this taproot deterioration. A compromised root system may result in plants showing symptoms of scorch, especially on days with hot, drying winds. There are no rescue treatments for the root rot diseases.
One disease which is favored by hot, dry conditions is charcoal rot. Although we have not received any samples with charcoal rot yet, it is certainly a disease to be watching for as the season progresses. Macrophomina phaseolina, the fungus which causes charcoal rot, can infect both corn and soybean plants, although it may be more common on soybeans in Missouri.
Charcoal rot may be considered a mid to late season disease on maturing soybean plants, but it can also occur early in the season on seedlings. Infected seedlings tend to show a reddish brown discoloration from the soil line up the stem. The discolored area changes from reddish brown to dark brown to black. Foliage may appear off color or begin to dry out and turn brown. If the growing point is killed, a twin stem plant may develop. Under hot, dry conditions, infected seedlings may die. More typically symptoms begin to develop as plants move into reproductive stages of growth. Infected plants are less vigorous and have smaller leaves. Leaves may turn yellow and wilt, eventually turning brown and having a dry appearance. The taproot and lower stem develop a silvery gray to light-gray discoloration of the epidermis (outer layer of the soybean stem). The epidermis may flake or shred away from the stem, giving the stems a tattered appearance. Fine black speck or microsclerotia bay be evident in tissues below the epidermis and eventually in the epidermal tissues.In corn, charcoal rot may begin as a root rot and move into the lower internodes of the stalks. Brown, water-soaked lesions develop on the roots. As the plant matures, the fungus spreads into the lower internodes of the stalk, causing premature ripening of the plants, shredding pith tissues and breaking of the stalks at the crown. The small, survival structures or microsclerotia may be visible as small, black flecks just beneath the stalk surface or on the vascular strands remaining in the interior of the shredded stalks. The stalk and pith may have a silvery gray to gray cast from the buildup of microsclerotia in these plant tissues.
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REVISED: October 1, 2015