Crowding brings on disease

Q. My lily of the valley plants have large, brown spots on them. Many of them are turning brown like someone sprayed a herbicide. What is wrong with them?

Kathy from Canfield

A. Many of us love the spring blooms of lily of the valley (Convallaria majalis). The scent is amazing, and the plants spread fast, making for a great show each spring. But this plant spreads quickly through its rhizomes. For this reason, it commonly overcrowds in the garden.

When the plant becomes crowded, it is increasingly susceptible to disease. This is especially true in areas where the plant’s leaves are allowed to stay wet (no morning sun) for several hours per day. Besides overcrowding, other factors such as limited airflow through the planting creates the perfect environment for disease to flourish. Fall cleanup of the leaf and stem debris is paramount to reducing infection in next year’s growth.

As this infection continues to take hold due to this year’s cooler-than-normal temperatures and above-average rainfall, the disease can get so bad that the entire leaf from the stem out has turned brown in many gardens here in the Valley. If you look close, you will still see several rings of darker brown areas where the disease first began development. This rapid disease development can make a leaf turn brown in just a few days.

This plant already has stored root reserves for next season, thus removing the leaves will not have a detrimental effect on next year’s blooms. So you can cut the plants down to reduce the impact of infection this year. When you do this, you may want to thin the plant population by at least half to increase airflow in the area next spring. A generally accepted practice would be to thin the area by one-third in June each year to promote good plant health if the disease persists every year. This plant is aggressive, but not invasive, so it will fill back in before you know it.

For details on control options, visit: http://go.osu.edu/lily.

Eric Barrett is OSU Ext. educator for agriculture and natural resources in Mahoning County. Call the office hotline at 330-533-5538 from 9 a.m. to noon Mondays and Thursdays to submit your questions.