Ridding geranium leaf spots

By George McVey, Ph.D.
Contributing Writer
Union County Post

Q. Is there anything we can do for bacterial blight on geranium? We planted about 150 plants this spring in our front flower beds, and they are looking very bad now - wilted and yellow. Please help!
A. Bacterial leaf spot and stem rot are probably the most common and the most serious diseases affecting commercial and home-grown geraniums (Pelargonium spp.). Also known as bacterial blight or wilt, this disease is a familiar problem on the common and ivy geranium. Under warm humid conditions, losses may be as high as 100-percent in a planting, depending on cultivar. The disease is difficult to manage because cool temperatures mask disease symptoms. Plants that appear healthy may later show symptoms when conditions favorable for the bacteria occur. Generally, when this blight occurs in the home landscape, infected plants were purchased at a garden center or nursery.
This disease of geranium is caused by Xanthomonas campestris pv. pelargonii. Symptoms of leaf spot and stem rot are often followed by sudden wilt and collapse of the infected plant parts.
Leaf spots may develop on the underside of infected leaves. Initially the spots are small, rounded and usually water-soaked, but within a few days the spots develop into large sunken areas. The spots could eventually reach a diameter of an eighth to three-sixteenths of an inch, then turn darkand become hard and dry. Infected leaves may drop shortly after they die or remain soft and wilted, but attached to the stem for several days. Bacteria can enter the stem through the petioles of infected leaves and spread to other plant parts, resulting in more leaf infections. If the bacteria reach the upper part of the plant, stem rots may develop; these frequently result in plant death. Stem rot is often seen at the point where diseased leaves are attached to the stem.
There are a few things you can do; buy plants from a reputable greenhouse or nursery, irrigate at soil level and don't splash water onto plants, don't crowd plants, and pick off any spotted leaves you find.
There is not effective treatment for bacterial blight once it becomes established. Diseased plants should be removed and destroyed. Since the pathogen can survive for some time in the soil, it is important to wait one to two years before planting geraniums in a home flower bed where diseased plants were previously grown. Resistant geraniums include the cultivars Lady Washington and Martha Washington (P. domesticum), Madame Layal and Marie Vogel(P. x domesticum), P. `Torento', Peppermint (P. tomentosum), and Countess of Scarborough (P. scarboroviae).

Q. The leaves on my hollyhocks are disgraceful! They look as though they have been skeletonized almost like a Japanese beetle would do, but it's too early for them. I don't see any insects attacking them. What could be causing this?
A. Several gardeners have recently commented on how bad their hollyhocks are looking this spring. Your plants may have been attacked by hollyhock sawfly. Sawfly larvae appear slug-like. All stages feed on the underside of the leaves. Young larvae skeletonize small areas of the lower leafsurface, leaving behind the upper leaf epidermis, giving the leaves a "window pane" appearance. Older larvae completely skeletonize the leaves and produce damage that may be mistaken for Japanese beetle feeding.

Q. What could be eating holes in the leaves of my hostas and other annual and perennial flowers?
A. Holes in hosta foliage, as well as in foliage of some annual flowering plants, are often caused by slug populations that build rapidly in abundant wet weather. Slugs are most active at night under lower temperatures and more humid conditions, and hide during the day beneath mulch, low-lying leaves and other debris. Populations can be curtailed somewhat by habitat modification such as reducing mulch and cleaning up garden debris. Trapping with fermenting beer or a mixture of sugar, yeast, and water in shallow pans sunk at ground level will catch some slugs, but leaves a lot to be desired. Handpicking is also possible. Slug baits are probably the most consistent and efficient method of slug control. Several commercially available baits are available that contain a molluscicide. Since these may be toxic to pets, fish, and humans, carefully follow label directions.

Q. I'm new to perennial gardening, and don't understand the term "deadheading." What is this?
A. Deadheading is the removal of spent flowers from a plant. Deadheading may encourage the plant to produce more blooms, since energy is not put into seed production. The average perennial blooms for about three to four weeks in a peak season, but some will re-bloom sporadically for the rest of the season if they are deadheaded. A few perennials that tend to rebloom if deadheaded include: butterflyweed, many of the coreopsis, purple coneflower, baby's breath, rose campion, many of the salvia, scabiosa, Stoke's aster, lavender, shasta daisies, many of the veronicas, and the repeat blooming daylilies.

Information submitted by George McVey, PhD. Union County Extension Program Assistant, Horticulture and Master Gardener Coordinator.
The article was taken from The Ohio State University Plant Facts Website.
The Master Gardener offers horticulture advice and tips, focusing on current issues, by volunteers who are with The Ohio State University Extension, Union County Master Gardener Program. For answers to gardening questions, call 937-644-8117 or e-mail mcvey.22@cfaes.osu.edu. Hours are 8 a.m. to 4:30 p.m. Monday through Friday.