Shrub's needles are susceptible to edema
Q. Why are my yews turning yellow, brown and falling from the plant?
A. The symptoms of leaves or needles turning yellow, brown and falling off are the result of too much water.
Corky bumps can also be seen if you look closely at the underside of the needles that have changed color or fallen. When soil becomes oversaturated, plants can't absorb all the excess water so they develop edema, which is a drowning condition where a plant receives more water than it can use. You may also see this in houseplants that are overwatered.
Yews are susceptible to edema, or oedema as it is sometimes spelled.
If you have an irrigation system that is causing the oversaturated condition, make changes so the soil is less saturated. If the problem is being caused by grading of the soil (low area) you can try to level the grading so that pools do not occur.
If your yews are not suffering too badly, you can attempt to reverse this condition by improving drainage in the soil around your yews. Healthy soil supports plants with water, air and nutrients in a mix that allows plant roots to absorb what the plant needs to keep the plant healthy. If water can't move through the topsoil down into the earth, then your yews can't breathe or take up nutrients.
Organic matter such as compost, which you can either purchase or make yourself, can be added to the soil to improve drainage. As this organic matter breaks down, it improves the space for air and water to be correctly mixed in the topsoil around your plants. This is an ongoing treatment that must be applied every season to keep the soil healthy.
If the soil is continuously soggy, yews and other plants are susceptible to root rot pathogens that will kill them. Fungal diseases are easily spread through soils to plants when there is too much water.
Verticillium or Fusarium wilt pathogens will attack plants in this condition and can remain in the soil for many years after the affected plants have died. If your yews are severely affected, you may need to remove them and work on repairing the soil drainage before you replant the area. You will likely have to try a less susceptible plant and wait several years before planting in that location if a wilt pathogen is found to exist in your soil.
- Nancy Degnan
• Provided by Master Gardeners through the Master Gardener Answer Desk, Friendship Park Conservatory, Des Plaines, and University of Illinois Extension, North Cook Branch Office, Arlington Heights. Call (847) 298-3502 on Wednesdays or email northcookmg@gmail.com. Visit web.extension.illinois.edu/mg.