Russian Thistle (Tumbleweed)

The term ‘Anthracnose’ is used as a general term for a variety of diseases that affect plants in similar ways. Most of the focus is on its impact on the leaves, stems, and stalks of horticultural crops, vegetables (especially curcurbits), and field crops.

In field crops, Anthracnose is most often identified as an economic pest in corn, caused by the fungus Colletotrichum graminicola. In soybeans, the causal fungus is Colletotrichum truncatum.

Russian thistle can be found anywhere the soil has been disturbed--across vacant lots, along highways and fence lines, and in other non-crop areas. In turf crops where the soil is tilled or mowed often, it will rarely become significant.

Anthracnose in Corn

Corn is an important and economic host for this disease. In field corn, there are two phases of Anthracnose:

Anthracnose Leaf Blight impacts corn at an increasing rate as tillage systems move more toward no-till and reduced tillage. The fungi overwinter on and in trash from the previous crop. During the early development stages of the crop, the lesions from Anthracnose are small, oval-shaped, and water-soaked in appearance. They are semi-transparent and will enlarge to nearly an inch long, becoming tan in the center and characteristic borders that range from orange to yellowish, and/or red to brown. During wet weather, the fruiting bodies may develop in the center of the lesions and appear as black spots. Later in the season, as the plant develops, these lesions will coalesce and continue to consume leaf tissue. If severe, the leaves will eventually die. The inhibition of photosynthetic processes within the plant can lead to yield loss, and lower test weight. With ideal weather conditions for their survival, the symptoms may appear on the upper leaves as well.

Anthracnose Stalk Rot is a phase of Colletotrichum graminicola that can be of concern at an even greater level. Symptoms of anthracnose stalk rot are usually observed later in the season after tasseling. Water splashing transports the fungi to the stalk, where it can set up and develop as reddish-brown, water soaked lesions in the rind of the plant. These lesions, like in the leaf blight phase, coalesce, enlarge, and darken to very dark brown or shiny black color. Cross sectioning the pith reveals a similar discoloration. Further, this phase typically occurs below the ear leaf, and can cause severe lodging if the infestation is heavy.

What complicates the stalk rot phase even more is the interaction with European Corn Borer (ECB). The initial tissue damage caused by the stalk rot becomes greatly exacerbated with the presence of corn borer. The pith and rind are softened, making for more desirable pathways for tunneling by the borer, and ultimately, more damage to stalk turgidity, causing significantly more extensive lodging when both pests are prevalent.

A modest infestation of stalk rot may not result in economic yield loss, but when combined with high ECB populations, it can be severe.

Management of Anthracnose in Corn

Anthracnose resistant hybrids are available, as are ECB resistant seeds. In conditions where a field is at high risk (historical infestations, minimum or no-till, high residue), using these seeds is advisable.

Integrating your pest management strategies through scouting, rotation, trash management, and seed selection is the appropriate way to first become aware of this disease. Following this, proactively managing for anthracnose is necessary it because it’s extremely difficult to prevent loss once initial damage is done.

Anthracnose in Soybeans

Anthracnose in soybeans is far less common than in corn, and less of an economic threat--but it can occur. Anthracnose Stem Blight is the resulting disease. Periods of moderate temperatures combined with extended wet periods or high humidity are favorable to infection from Anthracnose in soybeans. Symptoms include ‘Shepard’s Crooking’ of the foliage and irregularly shaped, dark brown or reddish blotches on the stem.

Anthracnose stem blight can be present in seed that has been previously infected. A companion disease often found when anthracnose infected seeds carry over is pod and seed blight (causal fungi Diaporthe and Phomopsis). They are often confused for one another, and because they both favor the same environmental conditions for development, they can have a synergistic presence on pods and seeds of soybean.

The other way that Anthracnose can be introduced into soybeans is in the overwintering trash and residues. This is a common issue with virtually all infections of anthracnose.

Rotation, tillage that gets rid of crop residue, and fungicides can all be helpful in control.

Read more about it:

• Cornell College of Agriculture and Life Sciences - Bergstrom - Anthracnose Leaf Blight
• Cornell College of Agriculture and Life Sciences - Bergstrom - Anthracnose Stalk Rot
• Pioneer Seed - Anthracnose in Soybean
• Iowa State University Extension - Soybean Pod and Seed Blight
• Iowa State University Extension - Anthracnose in Soybeans
• Iowa State University Extension and Outreach - Blog - Rebecca Vittetoe - Anthracnose, in soybeans?
• University of Delaware Cooperative Extension - Anthracnose: foliar symptoms vs stalk infections