Written by  Ken Gaudion

Be aware of possible herbicide damage during drought

Most cherry growing regions have experienced drier than normal conditions during summer and autumn but the weeds keep growing.

There is evidence in Australia and around the world that sweet cherry trees may be damaged by herbicide use under certain conditions.
These conditions may include: soil type (lighter soils in particular), strip culture, root system of the rootstock (fibrous or tap), mulch depth, irrigation type (micro-jet, drip line, sprinkler,) and more.
Dr Dave Rosenberger, retired plant pathologist at Cornell’s Hudson Valley Laboratory, hypothesises that trunks may be invaded by a canker pathogen as a result of herbicide damage.
According to Dr Rosenberger, the exact conditions that contribute to damage have not been defined. But he suspects that the potential for damage is significantly higher if tree trunks are hit with herbicides during or just prior to periods of drought stress.
"The additional desiccation from herbicide exposure combined with normal water stress during hot dry periods may predispose the trunks to invasion by Botryosphaeria dothidea, a canker pathogen that is incapable of killing the cambium in healthy functioning trees, but which becomes very pathogenic in drought-stressed trees," he said.
He also suspects that some herbicides can contribute to similar trunk damage, especially on young trees.
Purdue University Plant & Pest Diagnostic Laboratory reported that, "Weather records (such as wind direction and speed, relative humidity, and temperature at the time of application), soil type, pH, and site topography can provide valuable information about site conditions that affect the potential for herbicide injury."
Tree recovery
Whether a plant recovers from non-target herbicide injury depends on the overall vigour of the affected plant, the amount of herbicide it received, the type of herbicide used, and the growing conditions after contact.
Healthy woody plants and many herbaceous plants that receive low doses of a growth regulator herbicide will most likely recover. As the new growth develops it might appear normal.
However, if the plant absorbed a greater dose, the chemical may persist in woody plants (and symptoms may appear for the next two or three seasons) and herbaceous plants can die.
Contact herbicides may cause spotting where spray droplets drift onto and affect leaf tissue. Total tissue death from these herbicides is uncommon unless the herbicide completely covers the leaf tissue.
The timing of exposure also affects recovery.
Plants that receive an accidental herbicide exposure late in the year when they are preparing to enter dormancy will not be injured as much as plants exposed early in the growing season.
If you suspect herbicide injury, invigorate the tree or shrub with an appropriate fertiliser and irrigation program. Such action may help the plant to recover.
From all of this it could be assumed that the likelihood of damage is less as autumn draws near, and that damage may be more likely when trees (young or old) are actively growing.

See this article in Tree Fruit April 2018