Disease Corner
by Emily Gatch

In the cool, wet soils of spring, seedling diseases known collectively as damping-off can challenge the emergence and establishment of direct-seeded crops. The period between seed germination and seedling emergence is one of heightened vulnerability: the rapidly expanding new tissues of the germinating seed have few defense mechanisms in place to thwart the numerous opportunistic pathogens that are present in soils. The collapse and death of seedlings known as damping-off can occur either pre- or post-emergence. Often pre-emergence damping-off, which results in failure of the seedling to emerge from the ground, is simply attributed to poor seed viability, when in fact an attack of damping-off pathogens may be the culprit. While healthy, emerged seedlings can be affected by damping-off, stresses such as insect feeding and damage from cold temperatures make seedlings particularly susceptible to invasion by this complex of soilborne pathogens. Typically, the pathogen(s) invade at or just below the soil line, causing water-soaked and discolored lesions that collapse, resulting in a basal portion of the stem that is much thinner and weaker than the upper stem. The disease often continues to progress until the entire seedling dies.

Damping-off pathogens:

Cultural practices to prevent damping-off:

1. In the greenhouse, there is debate about whether sterile potting media and sterilization of propagation trays and equipment with dilute bleach solution is necessary to prevent or minimize the occurrence of damping-off. In the Seeds of Change greenhouse, the approach is to create a microbially-diverse community through the use of vermicompost in potting mixes and compost tea as a transplant drench, based on the body of scientific evidence which suggests that beneficial microorganisms help keep damping-off pathogens in check.
2. In the greenhouse and in the field, sufficient soil drainage is crucial to prevent damping-off. Soils that are excessively wet are havens for most damping-off pathogens, with the exception of Rhizoctonia. In gardens and fields with heavy clay soil, raised beds will address poor drainage.
3. Pull back mulch on beds in early spring and allow the sun to directly warm the soil. The dark soil will absorb solar radiation much better than a mulched surface. Mulch only after seedlings have emerged and are well-established.
4. Wait until soil temperatures reach 60 F for direct-seeded spring crops such as corn and beans.
5. Avoid overcrowding seedlings in the greenhouse and in the field.
6. Do not overfertilize, as lush tissue growth is more susceptible to attack.
7. Water from the bottom up, particularly with germinating herb seed.
8. Maintain a slightly lower potting mix pH, if necessary by adding 1 tablespoon of vinegar per gallon of tap water.
9. Avoid working with seedlings (potting up, etc) when plants are wet.
10. Do not use water from drainage ditches or ponds unless unavoidable.

by Emilly Gatch
Greenhouse and Pathology Coordinator