Monday, September 29, 2014
Soybean Stem Diseases - Knowing What You May be Seeing in the Fields
Given that the leaves on most of the soybean crop in Minnesota have turned color or dropped due to maturity or frost, symptoms of stem disease may be easy to see. Root rot diseases are also widespread, but they are difficult to scout for and diagnose. This article covers the basics of how to recognize and diagnose common late-season soybean stem diseases in Minnesota.
 Anthracnose. Very common on senescing soybean stems. Irregular brown to black spots and patches develop in a random pattern on stems and pods. The infected areas are covered with tiny black fungal spines (setae) that can be seen with a 10X hand lens.
 Pod and Stem Blight. More common this year than is typical in most years. A key sign of infection is many small, black, raised dots (fungal pycnidia) arranged in rows on infected stems, pods, and fallen petioles late in the season.
 Stem Canker. Generally found in scattered fields across Minnesota Lesions usually appear at the base of branches or leaf petioles and can develop into elongated, sunken, dark brown cankers that spread up and down the stem. Tiny black dots called perithecia (spore producing fungal structures) may appear on the stem singly or in clustered groups on plants killed by stem canker. Reddish-brown discoloration may also occur inside the stem.
 White Mold. Infected stems often become soft and watery, and become covered with white moldy growth in moist conditions. Dry, dead stems can develop a bleached, white appearance. Hard, black sclerotia develop on or inside infected stems and pods.
 Brown Stem Rot. Brown pith develops in the center of the
stem, especially near the lower nodes in stems that are split; whereas the pith
is white in healthy stems.
 Charcoal rot. Typically seen where plants have been stressed by heat and drought. The inner tissues of the taproot and lower stem may be streaked with light gray. Small black specks typically form inside the lower stem and taproot to give them a charcoal-sprinkled appearance.
More information and photos of these diseases can be found on the Minnesota Crop Diseases Web Site at www.extension.umn.edu/agriculture/crop-diseases/soybean/.