Diseases of Field Crops: General Seed Rots and Seedling Blights

| Corn | Soybeans | Forages | Cereals | Dry Edible Beans |
| Spring and Winter Canola | Other Crops | Soil Management |
| Soil Fertility and Nutrient Use | Field Scouting |
| On-Farm Stored Grain Management | Weed Control |
| Insects and Pests of Field Crops | Diseases of Field Crops | Appendices |

Pub 811: Agronomy Guide >Diseases of Field Crops> General Seed Rots and Seedling Blights

Excerpt from Agronomy Guide for Field Crops
Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Incidence: Cool or wet conditions that delay seed germination or seedling development can lead to early-season seed rots, seedling blights and/or root rots. Poor stand establishment, non-uniform emergence, "gaps" or missing plants are obvious signs of seed or seedling infection. There are many different fungal pathogens responsible for these diseases; some infect many crops and others are specific to one crop.

What makes these pathogens difficult to control is their ability to survive in many soil types. Depending on the year and field conditions, their impact ranges from minor to severe (replanting). Low-lying or poorly drained areas of the field are often the first to show disease problems. Seed rots and seedling blights can be more severe in no-till or reduced tillage fields since heavy residue keeps the soil cooler and wetter than in conventionally tilled fields. Damping-off occurs when the crop is planted early into conditions that favour disease development or when environmental conditions cause the seed to sit in the ground for a prolonged period of time. Other factors that delay germination and emergence, such as compaction, crusting or deep planting, can also cause poor stand establishment. It is important to distinguish between seedling diseases and other potential problems, such as insects, herbicide injury and soil compaction.

Appearance: It can be difficult to distinguish between pathogens since symptoms are not always clearly distinguishable. Seed rots are diseases that affect seeds prior to or shortly after germination. With seed rots, seeds decay and die. Seeds that have been damaged or have poor seedling vigour are the most susceptible to seed rots. High risk conditions include cool (10°C-13°C) and wet soil for an extended period of time after planting. Seedlings that take a long time to emerge are most susceptible to fungal infection.

Seedling blights (damping-off) are characterized into two groups, namely pre-emergence and post-emergence seedling blights. Pre-emergence seedling blights affect young seedlings prior to emergence. Affected seedlings may die or grow slower than healthy unaffected seedlings resulting in "gaps" or stand problems. Seedlings that do emerge can also be diseased. Post-emergence seedling blights affect the roots or lower stems of young seedlings from emergence to the second- or third-leaf stage. Symptoms include delayed growth, wilting, dieback and/or death. In most cases, an infected seedling will have a "pinching" or "girdling" of the stem near the soil line.

Organisms that cause root rot infect the seedling's root system, including lateral roots and root hairs. Affected plants may be stunted, off-colour or lack vigour. Infection can result in seedling death when disease infection is severe. Infected plants may be more susceptible to stalk rots later in the season.

Disease Cycle: See the individual crop sections for further details.

Management Strategies: Planting seed with good germination and seed vigour will substantially reduce the risk of seed rots and seedling blights. Remove seeds cracked or damaged during harvest or handling, as they are most prone to infection. Management tactics that favour quick germination, such as minimizing soil compaction, removal of excess soil moisture through improved drainage and removal of excessive crop residues, can also help reduce the severity of root rots and seedling blights.

Fungicidal seed treatments will provide some protection to vulnerable seedlings. It is recommended that all seed be treated with a fungicide seed treatment to minimize early-season pre-emergence and post-emergence disease problems. The average fungicide seed treatment provides up to 2 weeks of protection. There is no replacement for timely planting into a good seedbed.