Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Anthracnose
Anthracnose was diagnosed on corn seedlings submitted to the University of Delaware diagnostic clinic. The fungus responsible for anthracnose, Colletotrichum graminicola, causes disease on leaves, stalks, and ears of corn, with the stalk phase being more important from a yield loss standpoint. Anthracnose is common early in the season in no-till or minimal tillage fields and is generally more severe in fields where continuous no-till corn has been grown.
Symptoms of anthracnose appear initially on lower leaves as small, oval, water-soaked spots that enlarge and turn tan to brown with yellow-brown borders. Lesions may coalesce and blight entire leaves. Older lesions will turn gray in the center with small black fungal structures. These fungal structures (acervuli) contain black spines (setae), which can be visualized in the field with a hand lens. Leaf lesions do not indicate that stalk rot will occur later in the season. Root infection by C. graminicola is more likely to cause stalk rot than foliar infections. Detection of foliar symptoms early in the season should alert scouts to keep an eye out for stalk and ear symptoms later in the season.
This fungus overwinters on residue where it grows and produces spores. Spores spread by wind or rain to leaves, where infection occurs. Severe leaf damage can result after long periods of heavy overcast and wet weather. Anthracnose management is based on the use of resistant hybrids, crop rotation, tillage, and balanced soil fertility. Fungicides that control foliar symptoms are not likely to result in a yield benefit and are not recommended in most cases.
Corn seedlings with symptoms of early season anthracnose.
Root Rot
Waterlogged soils result in nutrient and oxygen deficiencies, slowed germination, and reduced root growth, which contributes to infection by root rot pathogens. These pathogens include Pythium, Fusarium, and Rhizoctonia. Flooded conditions stress corn seedlings and allow pathogens to easily infect roots and cause root rots. Other factors, such as insect or nematode damage may facilitate infection by root rot pathogens.
Corn seedlings with symptoms of root rot.
Treated seed may not be as effective in situations where soils are waterlogged because fungicides are being asked to do more for a longer period of time. In serious cases replanting may be required, but often disease is limited to a few small areas within the field where drainage is poor. Plants with low levels of infection may be able to recover if environmental conditions favor plant growth.
Announcement
This summer I would like to help growers to scout fields for plant diseases. If anyone is interested please contact Nathan Kleczewski (phone: 302-300-6962; email: nkleczew@udel.edu).