We often think of diseases and pests on our vegetables during the growing season but problems can also arise after those veggies are harvested. A cabbage was brought into the clinic this week with black spots on the outer leaves. The cabbage looked fine when it was brought home. Soon afterward the sunken spots appeared. See photos below.
Several post-harvest disorders with similar symptoms can happen to cabbage. These include black spot, necrotic spot, and gray speck.
Black spot has tiny sunken black spots that form around the stomata of the inner and outer leaves. The small spots give the impression of sprinkled pepper, which is why this disorder is also known as pepper spot.
Necrotic spot, which is what our sample cabbage had, has sunken black lesions that are larger than black spot. The spots are generally bigger than 1mm and can be round, oval, or irregular in shape. They are usually associated with the midrib of the leaf.
Gray speck has grayish spots on the outer leaves. These spots are superficial and do not cause the sunken areas associated with black and necrotic spot.
All three of these problems are not caused by a biotic (living) pathogen or pest. When inspected under a microscope, there is no fungal or bacterial growth. These diseases are physiological disorders. Although the exact causes are still unclear it may be a combination of storage and growing environmental conditions that trigger the development of the lesions. See ISU Extension & Outreach pamphlet PM0731, Harvesting and Storing Vegetables
, available on the ISU Extension Online Store
for recommendations on storage of vegetables.
Cabbage spot is a post harvest disorder triggered by storage conditions.