Corn & Soybean Diseases Update

Emmanuel Byamukama

Heavy rains in the past few weeks were conducive for bacterial disease development in corn and soybean. A few cases of Goss’s wilt and holcus spot have been found in the north central part of the state. Goss’s wilt is caused by the bacteria, Clavibacter michiganensis subsp. nebraskensis, while holcus spot is caused by the bacteria, Pseudomonas syringae pv. syringae. It is relatively easy to tell the two corn bacterial diseases apart. Goss’s wilt usually develops longitudinal tan lesions that have irregular margins along the leaf veins (Figure 1). These lesions also have dark green to black, water soaked irregular spots (freckles).

Figure 1. (Above) Typical Goss’s wilt symptoms. Note the long irregular lesion and freckles.

Holcus spot has numerous small round to elliptical spots. The spots begin as dark green, water soaked lesions but later turn creamy white to tan as they dry out (Figure 2). These do not have freckles. For both bacteria, the best way to confirm them is through bacterial streaming - the infected tissue is inserted in clear water and observed under magnification (Figure 3).

Figure 2. (Above) Holcus spot symptoms on a corn leaf. Round to elliptical lesions usually appear on the top half of the leaf.

The bacteria for both Goss’s wilt and holcus spot overwinter on infested crop residue on the soil surface and enter the plants through natural plant openings or wounds created by hail, sand blasting, high winds, and insect feeding. Goss’s wilt can be managed by selecting corn hybrids that are tolerant to this disease. If the field has a history of Goss’s wilt, selection of resistant/tolerant cultivars is the first step. Since the Goss’s wilt pathogen survives on residue, tillage and crop rotation will reduce the inoculum. Fields at high risk are corn following corn and no-till/minimum till fields. Some weeds like foxtail, shattercane, and barnyard grass are hosts of the bacteria; therefore, early weed control is important to eliminate further sources of inoculum. Some products are being marketed for Goss’s wilt control; however, there is insufficient data in the region on their efficacy. Holcus spot does not cause significant yield losses and is considered a minor disease on corn. General bacterial disease management practices including crop rotation and residue management will help keep the level of inoculum low.

Figure 3. (Above) Bacterial streaming is a confirmation test for bacterial diseases like Goss’s wilt and holcus spot.

On soybeans, bacterial blight has been observed almost on every field. The bacterial blight is characterized by small water-soaked yellow to light brown spots on leaves. These spots are surrounded by a yellowish-green halo. The brown spots may coalesce into blotches that may tear and fall out after windy weather, giving the leaves a ragged appearance (Figure 4). This disease is mostly seen during cool wet weather. Current conditions (hot and dry) will halt the bacterial blight progress.

Figure 4. (Above) Bacterial blight on soybean.

Bacterial blight causes minimal yield loss because most soybean cultivars are tolerant to the disease. Use pathogen-free seeds, crop rotation, and tillage to reduce the inoculum.

Other diseases observed include common rust on corn and brown spot on soybeans. Common rust can be found almost on every corn field but rarely does it cause significant yield loss, unless the hybrid planted is highly susceptible. Common rust does not survive in South Dakota so rotation and residue management cannot control this disease. For highly susceptible cultivars, a fungicide application may be recommended.

Brown spot, also known as Septoria leaf spot, is another disease that can be found almost in every soybean field. For soybean planted in soybean residue under no-till, this disease can cause early leaf drop. For tilled soybeans, brown spot remains on lower leaves and may not develop to levels causing significant yield loss. Crop rotation, tillage, and where necessary a fungicide application between R2 and R3 will manage brown spot.

Source : SDSU