Throughout the growing season there are several foliar diseases that affect soybean and corn production in Arkansas. These include a few minor diseases that do not warrant a fungicide because they do not cause any significant yield loss. Currently, a few of these minor soybean and corn diseases are present in fields across the state.
It is common to find downy mildew in practically every soybean field in the mid-South. Typically, it is first observed in the lower canopy then later in the growing season on leaves in upper canopy. Early symptoms are pale green or light yellow lesions on the upper leaf surface (Fig. 1). Fuzzy tufts of spores can be observed on the lower leaf surface. Lesions are lighter than frogeye leaf spot, an important disease that also produces fuzzy tufts of spores. Currently, frogeye leaf spot is picking up in many soybean fields across the state (see earlier article on blog for frogeye identification). Though downy mildew can appear severe in some cases, commercially available fungicides have little activity on downy mildew; thus, fungicides are not economically beneficial to control this disease.
Ascochyta leaf spot is often found in the lower canopy throughout the growing season. It looks similar to frogeye; however, black fungal fruiting structures (“black dots”) can be seen on the upper leaf surface (Fig. 2). This disease often develops in old frogeye leaf spot lesions or spots caused by herbicide injury.
Bacterial leaf blight is often limited to the lower soybean canopy (Fig. 3.) Angular lesions differentiate this disease from brown spot, another minor disease of soybean that occurs later in the season. Close inspection of the lesions often reveals a “bacterial exudate” on the underside of the leaf. This pathogen can move up the canopy when conditions favor disease later in the growing season, but typically does not move up the canopy during dry growing conditions. A fungicide will not control a bacterial disease thus, fungicides provide no protection for this disease.
Of the few corn diseases that have been observed this year, common rust has been the most frequent (Fig. 4). Typically common rust occurs low in the corn canopy; however, this year in rare instances it has been observed in the mid- to upper-canopy. Common rust is a minor pathogen on corn and no fungicides are recommended for control of this disease. Pustule development on the lower leaf surface differentiates common rust from southern rust, which has yet to be reported this year in Arkansas. Contributors: Travis Faske and Terry Spurlock, Extension Plant Pathologists