Late Blight

Early symptoms of late blight consist of small, discrete, yellow spots on leaves and petioles. The spots often are circular in shape but may be angular when they are delineated by leaf veins. As the disease progresses, leaf lesions enlarge and usually grow together. Mature 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 favorable conditions (rainy weather, heavy dew or fog, or sprinkler irrigation during temperatures above 70°F), lesion development may be extensive on both leaves and petioles, resulting in blighting of the plant. Do not confuse this disease with 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 temperatures do not exceed 55°F, disease development ceases until temperatures increase and moisture is present. Rain, heavy dew or fog, and sprinkler irrigation when temperatures are above 70°F encourage 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 the use of fungicides when necessary. Plow under infected celery tissue after harvest and rotate out of celery for at least one year. Once plants are established in the field, avoid overhead sprinkler irrigation if possible. Reduce movement of equipment through fields when foliage is wet because such passage may spread spores from diseased to healthy plants.

Cultural controls and some copper sprays are acceptable for use on organically grown produce.

Monitor celery fields for late blight symptoms. If symptoms are detected, protectant fungicides may be required for disease control, especially if sprinkler irrigation is used or rain has occurred. To obtain best results, apply materials at first appearance of disease symptoms. Chemical treatment is not necessary when field temperatures remain below 55°F.