Symptoms of Spotted Stems in Soybeans

Diseases, mechanical damage, application of chemical products, meteorological phenomena and even pest attacks are among the possible causes of the presence of spots on soybean stems observed throughout the 2016/17 harvest. Knowing the main symptoms of problems that occur in the crop makes their diagnosis, prevention and control easier.

In the 2016/2017 harvest, the occurrence of several spots on the stems of soybean plants was observed, which, in most cases, could be confused with diseases. Other times they are caused by mechanical damage or the application of a chemical product, but this always raises doubts as to the true causes.

One of the important points for correctly diagnosing the problem is observing the entire plant: whether the root is healthy (light internal color) or diseased (dark and missing the main or secondary roots); if the leaves are yellowing and withering, with dry tissue between the veins; and what type of symptom appears on the stem (depressed? red? dark? Does the darkening persist inside the stem when the superficial layer of the lesion is removed?). All this information can be collected by superficially analyzing the plants with problems, which is important to discover the cause. Some cases:

Example 1: plants wither intensely, with necrosis of growth points, both in young leaves and in floral buds. Reddish spots on the stems, more intense in the upper and middle thirds, decreasing towards the lower third. Reddish veins on the underside of the leaves (Fig. 1). Healthy roots. Result: effect of applying ALS-inhibiting herbicide. In this case, the error occurred due to a product change (herbicide stored in insecticide packaging). Plant poisoning is also common due to the use of a sprayer with remains of ALS-inhibiting herbicide in the tank. In this case, the symptoms appear in bands, being stronger in plants located at the beginning of the application, gradually reducing as the operation progresses. 

Example 2: plants wilt slightly, with some branches more affected than others, on the same plant. Darkened lesions on the lower third of the stems, both between healthy tissue and progressive from the ground. Roots with signs of development in compacted soil, showing dead main root and considerable development of superficial secondary roots (Fig. 2). Lesions not darkened below the surface, on the stem (Fig. 3). Inside the root tissue, under an optical microscope, circular structures with double walls (oospores) are observed, typical of Phytophthora soyae (Fig. 4). Result: after finding oospores in the roots, it can be said that the symptoms were caused by phytophthora root rot. This type of injury is the result of the action of a moderate to low type of partial resistance of the affected soybean cultivar. These lesions on the stems could be confused with stem canker, but the lack of internal darkening in the pith and leaves with interveinal necrosis already indicates that this is not the problem. It is also necessary to be careful when trying to isolate the pathogen in the laboratory, as P.syiae does not grow on standard culture medium (potato-dextrose-agar). For information on techniques with P.syiae, see: http://www.infoteca.cnptia.embrapa.br/handle/doc/1058082.

To help differentiate symptoms, stem canker lesions are presented, identified in previous harvests in Brazil, in figures 5 and 6. These lesions normally originate in the insertions of lateral branches, progressing both vertically and towards the interior. of the stem, darkening the wood (Fig. 7) and the pith, causing the death of the affected branch. The roots remain healthy.

Another cause of confusion is shown in Figure 8, when spots appear after the occurrence of hail, a sporadic meteorological phenomenon during the summer harvest, in the conditions of Rio Grande do Sul. The main difference between stem canker and hail is the absence of leaves with interveinal necrosis and the sudden appearance of darkened spots along the entire length of the stem, on one side (the side facing the rain of stones), with the center of the spot split due to the death of the affected tissue.

Example 3: curved and darkened branch ends, deformed floral shoots, smaller upper leaves with a lighter green color, with blisters on the leaf blade. Attacked petioles and vegetables are brown in color. Result: it is not a soybean disease per se, but symptoms of the action of the white mite (Polyphagotarsonemus latus). Unlike other soy mites, the white mite develops better in rainy periods. Because it is tiny, it is best observed under a magnifying glass in the laboratory.

Example 4: withered plants, with damage to the stem starting at the insertion of the lateral branch, after flowering. Presence of white cottony mycelium over the lesion (Fig. 9). Result: white mold, caused by Sclerotinia sclerotiorum. This disease occurs in rainy summers, in regions with mild temperatures, on plants that are very close or bedded. It is easily identifiable by the white mycelium and the development of sclerotia, which are small black structures that develop both inside and outside the affected stem.

Knowing the main symptoms of problems that occur in soybean crops makes their diagnosis easier, in order to avoid or control them in the next harvests.

Leila Maria Costamilan, Leandro Vargas, Embrapa Trigo

Article published in issue 214 of Cultivar Grandes Culturas.