Late Blight

Initial symptoms of late blight consist of small, discrete, yellow spots on leaves and petioles. The spots are often circular but may be angular when they are delineated by leaf veins. As the disease progresses, leaf lesions enlarge and usually coalesce. Older lesions turn tan and dry out, becoming papery. A characteristic feature of these lesions is the presence of small, dark, round structures that are the reproductive bodies (pycnidia) of the fungus. Under conditions favorable to disease development, lesions may develop over a large area on both leaves and petioles, resulting in blighting of the plant.

Note that this disease is not the same as late blight of tomato and potato caused by Phytophthora infestans, which does not infect celery.

The primary inoculum source for late blight is contaminated celery seed.

When air temperatures are below 55°F, disease development ceases until temperatures increase and moisture is present. Rain, heavy dew, fog, and sprinkler irrigation when temperatures are above 70°F favor disease development. Splashing water disperses spores and aids in spore germination and infection. The pathogen can survive in undecomposed plant residue. Septoria apiicola may also be found on celeriac.

• Because this pathogen is seedborne, use Septoria-indexed seed.
• Hot water seed treatments may effectively reduce infestation levels on seed but also may reduce seed germination.
• There is some indication that storing celery seed for at least 2 years can significantly reduce pathogen viability on seed.
• Plant only disease-free celery transplants. Production of disease-free transplants involves planting of Septoria-indexed or hot water treated seed, roguing of infected plants, and using fungicides when necessary.
• Once plants are established in the field, avoid overhead sprinkler irrigation.
• Reduce movement of equipment through fields when foliage is wet because such passage may spread spores from diseased to healthy plants.
• Plow under infected celery tissue after harvest and rotate out of celery for at least 1 year.

Use cultural controls and copper sprays in a certified organic crop.

Monitor celery fields for late blight symptoms. If symptoms are detected, apply protectant fungicides, especially if sprinkler irrigation is used or rain has occurred or is forecasted. To obtain best results, apply fungicides at first appearance of disease symptoms. Fungicide applications are not necessary when temperatures remain below 55°F.