What's with these leaves?

Asked June 21, 2016, 10:46 AM EDT

Hi, This is a photo of my (hardy) mum. The spots on its leaves began maybe a month ago, and seemed to coincide with my snipping them back (which is something someone told me should get done through July 4 to promote growth ?) though it makes no sense to me that these marks have anything to do with the snipping. What is going on with this plant, can you tell?

Ramsey County Minnesota chrysanthemum plant disease identification horticulture

1 Response

Thank you for the question. The damage to your mum is probably being caused by four-lined plant bug, common at this time of year. We have an Extension publication about this insect and I'm pasting some information from it here for you:
"They feed by inserting their needle-like mouthparts into leaves and removing chlorophyll. This feeding produces dark, round, sunken spots, about 1/16 to 1/8 inch wide. The spots may become clear and after several weeks the affected tissue drops out leaving small holes. If feeding occurs on new growth, wilting may result". http://www.extension.umn.edu/garden/insects/find/fourlined-plant-bugs/

Often the damage is only cosmetic and when the insect life cycle is over, the plant will be fine and no chemical control is needed. Sometimes there are many bugs and the damage is more severe. These insects move fast so you may not be able to catch and squish them. This fall, it is important to cut down and discard all plant material because adults lay eggs in the plant that will overwinter and hatch into a new crop of four-lined plant bugs next year. Read the publication above to learn more about the insect and other control measures.

As for pinching hardy mums back; this is often done to promote stockier, bushier plants but might not be necessary if you have one of the newer hardy mums. This publication explains mum care: http://www.extension.umn.edu/garden/yard-garden/flowers/garden-chrysanthemums/

Thank you for contacting Extension.