Thank you to everyone who sent questions this past year. I love reading the mailbag, and each week we publish questions having the widest appeal or interest. Thanks for sharing your successes and challenges. Following are summaries of this year’s three most-asked questions.
Q: Many homeowners sent photos of their birch trees, showing various stages of dead or dying branches, mostly in the upper half of the tree. Most wondered if a disease of some type was attacking their birch tree, and whether it would spread to other tree types.
A: A disease wasn’t the culprit. In nearly all cases, I could see signs of the insect called bronze birch borer, as the beetle leaves a visible D-shaped exit hole in the tree trunk as infestations progress.
As the boring insect works below the bark, a winding series of raised channels becomes visible, giving the trunk a “muscular” appearance. In early stages, yellow, sparse, stunted leaves develop in the upper branches, progressing to twig dieback and finally to branch dieback. Trees might decline for several years before dying.
Birches thrive in cool, moist conditions. Bronze birch borer can detect birch trees that are stressed by heat and drought, which were common in 2021 and 2022, and likely explains the rash of infestations. The insect only attacks birch, and doesn’t affect other tree species.
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The best defense is to keep birch trees well-watered during dry periods and mulch the root zone with shredded bark to moderate soil temperatures. Unfortunately, there’s little that can be done to mitigate the stress caused by prolonged hot weather.
Insecticides can be applied to the trunk to kill larvae as they enter. If borers are already inside the tree, systemic insecticides can be tried. But by the time large sections of the canopy are affected, though, it’s often too late to save the tree.
Q: A common tomato question this year, and most years, asked how to prevent the large, sunken, brown or black lesions on the bottom of tomato fruits, opposite the stem end.
A: The disorder is called blossom end rot, and it isn’t a disease. Instead, the tomato plant isn’t able to utilize calcium from the soil and it sucks it out of the developing fruit, leaving the sunken, leathery blemish.
Calcium is plentiful in most regional soils, so adding more can be counterproductive. Instead, the plant isn’t able to access the existing mineral for several reasons. Cultivating too closely can break roots, making them unable to mine soil calcium.
Soil that fluctuates between wet and dry is the most common cause, since roots need to absorb soil calcium in a water solution, so moisture must be readily available. Keeping soil uniformly moist by applying mulch in late June is the best preventative, along with tomato cultivars that indicate resistance.
Q: Spruce trees with dead or dying needles also made the list of top questions. Of spruce questions submitted, the brown, thinning needles were commonly on the evergreen’s lower half.
A: Diagnosing brown or thinning needles on spruce requires a great deal of detective work, because there are many causes with similar symptoms. Spruce growing in wet, poorly drained soil often succumb to needle disorders, but drought can also be a cause.
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Low fertility, herbicide drift, site disruption, soil fill, sapsucker injury, improper planting, insects and diseases can all cause needle dieback. Deciphering the cause of each individual case requires piecing together enough clues to make a diagnosis.
If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu . Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.
The Trust Project
