Diagnosing boron toxicity in wheat
Boron toxicity is usually an inherent feature of a soil and is a particular problem when high boron levels occur in the subsoil. In Western Australia, boron toxicity occurs most commonly in the mallee regions north of Esperance, and on valley soils near lake chains in the south-eastern lakes area and the eastern wheatbelt.
What to look for
- Symptoms mostly occur in spring and are identical to those in drought affected plants.
- Symptoms appear first and most severely on the oldest leaves.
- Leaf tip death progressing from the tip and margins.
- In severe cases, yellow spotting occurs lower down on older leaves.
What else could it be
|Diagnosing spring drought in wheat and barley||Both show leaf tipping and shrivelled heads||Boron toxicity is patchier and restricted to specific soil areas|
|Diagnosing foliar nutrient damage in cereals||Both show leaf tipping||Plants recover quickly from foliar nutrient damage|
Where does it occur?
- Boron toxicity is an inherent characteristic of some soils especially those formed from marine or wind-blown sediments that have an alkaline sodic subsoil. It is more prevalent in dry seasons and in years with mid-season drought when crops rely on deep subsoil moisture.
- Boron is more likely to be a problem when it is present closer to the soil surface, where roots encounter high levels of boron in the early stages of development or as seedlings.
- Boron toxicity may be caused by adding boron-rich subsoil to non-wetting sandy topsoils as occurs with claying, but this is unlikely to remain an ongoing problem.
- It is not economically viable to treat boron toxicity in broadacre cropping systems and the constraint is best overcome by sowing crops and/or wheat varieties more tolerant of high boron levels.
How can it be monitored?
- Identifying boron toxicity using tissue testing is difficult due to the uneven distribution of boron throughout the plant. However, a boron concentration in whole shoots sampled at the boot stage of greater than 13-16 milligrams per kilogram (mg/kg) indicates boron toxicity.
- Wheat varieties differ greatly in grain boron levels that indicate toxicity on boron toxic soils. A range between 9-37 mg/kg is suggested as toxic.
- The standard laboratory method for extracting boron from soils provides an accurate reflection of the soil boron level in the sample. However, standard topsoil testing is unreliable because boron levels increase with depth, and boron levels can vary widely across the soil.