Pythium
(Seed Decay, Seedling Blight, & Root Rot)

Other Information:

Seed rotted in soil, commonly soft, wet, and overgrown by other fungi are usually killed before emergence by this fungus. Rapid death prevents accurate diagnosis. Roots are brown, watery, soft and often completely decayed. Limited infection may produce brown lesions on roots, hypocotyl, or cotyledons. Death of meristem tissue may result in a swollen hypocotyl. These species are often called “Water Molds.” They survive in oil and in plant residue. Cool (50ºF to 60ºF) and wet soils favor release of swimming” spores and infection develops rapidly. Younger seedlings are most susceptible because soybeans become more resistant as they age.

Rhizoctonia
(Root Rot & Lower Stem Decay)

Other Information:

Post-emergence damage is a bigger problem than is pre-emergence death. Root symptoms are confined to lateral root decay and outer root surface damage only. A red-brown discoloration of the hypocotyl and lower stem does not extend above the soil line. Slow growing plants are damaged most, and symptoms, wilting, and death develop following warm, dry weather early in the season. This is another soil inhabitant that survives in soil as sclerotia or as “resting” mycelium in crop residue. Infection is favored by wet and cool conditions followed by warm and dry periods that stress the plant. Young plants are most susceptible, but stressed older plants may die if moisture is limited.

Fusarium Root Rot

Other Information:

A problem on seedlings and young plant roots that develop in wet, cool Soils, below 58ºF. Seedling growth can be slowed and plants usually are stunted and weak. Infection is often limited to lower taproots, and lower lateral roots which may be destroyed. New roots can develop from the upper taproot providing a shallow fibrous root system that is prone to fail in dry soils. The vascular system can be affected, turns brown or black, and this increases late season plant wilt under moisture limiting conditions. Stress from soybean cyst nematodes, or other nematodes, and DNA herbicides predispose plants to infection. These soil born-fungi survive as chlamydospores and as mycelium in plant residues. Fusarium solani, reported to be the cause of Sudden Death Syndrome (SDS), has been reported to be near Minnesota, but isolates of this “Blue Strain” from Minnesota soils have not been confirmed to be like the isolates from Iowa or Illinois. Symptoms of SDS are interveinal chlorosis, necrosis, and leaf defoliation. Petioles remain firmly attached. The central pith (when stems are split) should be white with no discoloration or decay. It is believed that certain isolates produce a toxin that translocates to the upper leaves causing the above symptoms. Others report this fungus can be isolated from cyst nematodes.

Cyst Nematode

Other Information:

Low levels of infection can remain undetected for some time as no diagnostic above ground symptom exists. Stunted, chlorotic, vigorless beans with reduced root systems and few nitrogen nodules are typical of low infection levels in high fertility sites. Egg numbers do increase and symptom severity increases, reducing soybean yields. Carefully dig and examine roots for white to tan females containing eggs any time after late June through September. The cysts are spread by soil movement from equipment, or with water or wind. Birds can carry the cysts considerable distances. Most farms have some level of infestation.

Lesion Nematode

Other Information:

Lesion nematodes are found worldwide. They attack the root cortex. Roots develop dark lesions and an overall brown color. Loss of the epidermis and cortex decreases root growth nutrient and water uptake. Under stress, plants yellow, become stunted, and have reduced yields.

Sting Nematode

Other Information:

Seedlings can be killed, reducing stands. Larger plants are not killed, but appear stunted, chlorotic, and gray-green as if moisture deficient. Small, dark, sunken lesions are present on roots to the tip. Terminal root growth stops and roots appear stubby or have abnormal root proliferation. Damage is usually found in sandy soils. Sting nematodes feed on many host plants, especially grasses, and can also attack corn.