Gerbera Nutritional Problems
Roberto G. Lopez, Ph.D., Assistant Professor & Floriculture Extension Specialist, Purdue University
Most Midwest growers would never see blackish-purple spots on their gerbera crop unless their acid or fertilizer injectors were not functioning properly. Low substrate pH can lead to iron and manganese toxicity in gerbera resulting in blackish-purple spots of the lower leaves (Figure 1 and 2). If you do encounter these symptoms, first conduct a substrate pH test. If the pH of your substrate is below 4.8, the symptoms are more than likely iron and manganese toxicity. A tissue analysis is the only way to confirm the diagnosis.
What to do if it is iron and manganese toxicity? The first step is to increase your substrate pH and check your acid and fertilizer injectors. Flowable lime can be added to the substrate at a rate of 1 quart per 100 gallons of water to help increase substrate pH. Perform a substrate pH test a few days after the application. Additional applications may be necessary if your substrate pH is not between the acceptable range of 5.5 to 6.2.