Lower leaf necrosis can occur as a result of excessive micronutrient applications. To confirm the diagnosis of a toxic condition, a tissue test is necessary.
Basil is a relatively easy plant to grow with a low fertilization requirement of 100 to 150 ppm nitrogen (N) or in hydroponic culture with an electrical conductivity level of 0.5 to 1.5 mS/cm. It grows well within a pH range of 5.8 to 6.2 (Owen et al., 2018). Typically, basil experiences N deficiency as the most common nutritional issue. Iron (Fe) deficiency may also arise but is often associated with overwatering, root rot, or elevated substrate pH.
While micronutrients such as boron (B), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) are essential elements, they are required in smaller quantities compared to macronutrients such as N, phosphorus (P), and potassium (K). Excessive application of macronutrients can lead to toxicities.
In this particular case, the grower provided macronutrients at a rate six times higher than normal, resulting in lower leaf chlorosis and subsequent necrosis (Figs. 1-4). A leaf tissue nutrient test confirmed the diagnosis (Table 1). Boron levels were excessively high at 289 ppm, four times the upper sufficiency limit. Elevated concentrations of 354 ppm Fe (above the 200 ppm Fe upper limit), 390 ppm Mn (above the 150 ppm Mn upper limit), 107 ppm Zn, and 21.5 ppm Cu were also observed. Our research studies at NC State University over the past 20 years have consistently demonstrated that lower leaf necrosis is a common symptom when any of the micronutrients are supplied excessively. There are no distinguishing characteristics that identify a specific micronutrient causing the burn; therefore, submitting a tissue sample to a diagnostic lab is necessary.
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