In July and August I noticed something I’d never seen before along the trails of western Pennsylvania — scattered instances of leaves turning white.
The leaves had been green but now their tips or even whole branches were white. The plant below had advanced to the stage where some of the stems were completely white.
The condition is called chlorosis and it means the plant is not producing enough chlorophyll to look green. Since chlorophyll uses sunlight to make food for the plant, it’s a sign the plant is in distress. But why?
Causes of chlorosis are wide-ranging. Here’s the list from Wikipedia, with my [notes] added:
- a specific mineral deficiency in the soil, such as iron, magnesium or zinc
- deficient nitrogen and/or proteins
- a soil pH at which minerals become unavailable for absorption by the roots
- poor drainage (waterlogged roots) [Not likely in this case.]
- damaged and/or compacted roots [Not likely in this case.]
- pesticides and particularly herbicides may cause chlorosis, both to target weeds and occasionally to the crop being treated. [Not likely in this case due to location.]
- exposure to sulphur dioxide [Possible in Pittsburgh but not likely in this case.]
- ozone injury to sensitive plants [Not likely in this case.]
- presence of any number of bacterial pathogens, for instance Pseudomonas syringae pv. tagetis that causes complete chlorosis on Asteraceae.
Interestingly, the plants I photographed are in the Aster Family (Asteraceae) and one of them has complete chlorosis.
Was the 2017 growing season especially bad for the bacteria mentioned above? Or does chlorosis happen every year and I’ve just not noticed?
If you know more about this condition in the wild, please leave a comment. I’m really curious!
(photos by Kate St. John)
