Master Gardener Help Desk emails have really been different this past two weeks. Our early spring challenges have left and along came the first of our summer concerns in the landscape and vegetable beds. The list turned into more than a column’s worth, so going to hit the big ones this week:
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Maple trees have seen two kinds of foliage damage that are obvious. We have had Tar Spot for several years, and some springs worse than others. This year, a newcomer came to play, it is called Maple Leaf Blister, which is a gall. The gall is not typical, and most would not even see this on the leaf. Both problems start out while certain maple tree species and cultivars are budding out. Tar Spot starts out as a dime- or nickel-sized spotted circle, becoming completely black, just as if someone had dropped a bit of tar on the leaf. Maple Leaf Blister causes distortion and inner veinal necrosis, leaving the areas between the veins dark and blackened. Leaves will rain down for a while. Neither one requires a treatment, just visually bothersome.
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Viburnum leaf beetle (VLB) can defoliate an entire plant in a matter of days. The insect favors the dentatum types, and lucky for us so far, its range is limited to northeastern Illinois. It is the larval stage that does the real damage by feeding until it will pupate into an adult beetle. The most effective means for controlling VLB is to prune infested branches in the fall. However, reducing the larval population will ultimately reduce the adult population that lays the eggs. Products containing carbaryl as the active ingredient or one of the pyrethroids (cyfluthrin, permethrin, resmethrin) are effective as foliar sprays. In regards to other management options, Cornell University has a very helpful management guide for homeowners. At this time of year, for homeowners who are experiencing defoliation, the best option is going to be pesticides. It is important to make sure larvae are present and to make a thorough application so the pesticide comes in direct contact with the larvae. Spraying adults or eggs is less effective. There is some information on the use of horticultural oils and insecticidal soaps. On any product, always read and follow label instructions.
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Lichens, sometimes known as Lichen Moss, is a combination of algae and fungi causing a silver-gray crusting or “growth” on trunks and branches. Totally harmless, the lichen just needs a place to exist. It does not cause a branch to die, though it can continue to grow on a dead branch. Another one that does not need any control measures.
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Tomato foliage diseases are back as well. Our common foliage diseases on tomatoes overwinter in the soil. Rain or watering will splash soil particles up and onto the lowest leaves and away it goes. From those lower and interior leaves, the fungal spores will spread upwards and outwards leaving us with a skeleton of what was a healthy tomato plant. Some precautions include using black plastic as a barrier, ditto for layers of newspaper and landscape fabric. Culturally, staking or caging gets foliage off the ground as does growing in containers using a potting soil. Once the disease is established, fungicide sprays are not really going to work.
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Hostas and slugs go together like salt and pepper. If you have hosta, eventually you will have slugs, which leave a slime trail to let you know they have been there feeding. Slugs hide beneath the foliage at or just under the soil line in leaf litter and organic debris, out of the sun. They feed at night. There are baits available to manage slug populations if they get severe. Since they hide under cover during the day, you can place flat pieces of cardboard, small boards, even a roof shingle, amongst the hosta. Then, during the day, pick up the pieces and the slugs will be gathered in numbers on the bottom side. Dispose appropriately (such as placing them in the garbage or feeding them to the birds).
To learn how to email your local Master Gardeners for gardening help, find your county website.
About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.