Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
In my career, I have seen boron (B) toxicity in strawberries several times. This has always been related to over-application of boron containing fertilizers from foliar applications.
Boron has an important role to play in crops. It is needed for protein synthesis, development of cell walls, carbohydrate metabolism, sugar translocation, hormone regulation, pollen grain germination and pollen tube growth, fruit set, and seed development. Boron is mobile and readily leached in sandy soils and regular additions are necessary for many crops, but only in small amounts.
Boron (B) is a micronutrient required in very small amounts and there is a narrow range of safety when applying boron as toxicities can occur if too much is applied. Strawberries are considered a low boron requirement crop and are sensitive to foliar applications.
Boron toxicity is common in western states where boron levels in soils or irrigation water are high. In the east, we do not have high boron soils or high levels in irrigation water. In addition, boron leaches readily from soils. Boron toxicities therefore occur only when excess boron is applied in fertilizers. The margin of safety for boron application is small and excess application or improper blending in fertilizers may lead to toxicities – deficiencies show up at 1 ppm and toxicities appear at 5 ppm of available boron in the soil (leaf tissue levels between 20 and 100 ppm are sufficient with tissue levels over 200 ppm being excessive leading to toxicities).
Symptoms of boron toxicity in strawberries include necrotic spotting on lower leaves, followed by the development dead areas on leaf margins. Under severe conditions, symptoms can spread over the entire leaf and develop across the whole plant. Plant growth can be stunted and plants may collapse and die.
It is difficult to correct toxicity problems in strawberries because damage to the plants cannot be reversed from foliar applications. If excessive soil applications were made, overhead irrigation can leach some of the boron out of the root zone but it is difficult to do in plasticulture to get enough leaching.
Strawberries have a low B requirement. Test soil for B level prior to planting. Typical recommendations, based on if the soil test values indicate less than 1 ppm B, is an application of 1 pound of actual boron per acre prior to planting. Over application of boron can lead to toxicities and foliar applications of B should be avoided.
Boron toxicity in strawberries from a foliar fertilizer application. Note dead spots on leaves and necrotic leaf edges.