Downy Mildew on brassica (cabbage & broccoli) seedlings

Description: Scattered, irregular-shaped, sunken, gray-black lesions on a mixed sample of cabbage and broccoli seedlings. Symptoms ranged from subtle (broccoli) to more severe (cabbage).


Tests done: The diagnostician had already recognized this as downy mildew (based on experience), but I decided to place leaves in a moist chamber to induce sporulation just to double-check.

Final Diagnosis: Downy mildew caused by Peronospora parasitica. Forgot to take a picture of the diagnostic sporangiophores and sporangia! The University of Florida has a good factsheet describing symptoms, although the accompanying photos seem over-exposed.

References used: Compendium of Brassica Diseases

Abiotic stress leading to secondary pathogen invasion on Calibrachoa

Description: Calibrachoa is an ornamental plant related to the petunia but with smaller flowers. It is commonly used in hanging baskets and as a groundcover. It is propogated as cuttings and requires full sun.
Sample showed entire missing seedlings in the liners, leaf spots that began at the margins and tips, and dieback beginning at the base of the plant.

Possibilities: Rhizoctonia root rot, Phytophthora, Pythium

Tests done:
1) moisture chamber-showed extensive Botrytis
2) plating roots and leaves onto acidified water agar (AWA), Phytophthora agar and Pythium agar
3) check soil pH and soluble salts (SS)
4) float leaves in sterile distilled water to see if sporangia develop

Results:
1) moisture chamber-extensive Botrytis (common secondary pathogen on ornamentals)
2) plating roots and leaves onto acidified water agar (AWA), Phytophthora agar and Pythium agar-No Rhizoctonia, mixed secondary infections on other agars
3) check soil pH and soluble salts (SS)-within normal range
4) float leaves in sterile distilled water to see if sporangia develop-negative
5) replated roots and leaves onto Phytophthora agar and AWA-negative

Final diagnosis: This is likely abiotic stress (probably nutrient issues) which affected the lower leaves during transplant allowing secondary pathogens (Botrytis, etc) to invade.