August 1997 - Indiana Yard and Garden - Purdue Consumer Horticulture

August 1997

Q: Could you please tell me why my hollyhock’s leaves turn pale green around the bottom of the stalks, then yellow, then brown and drop off? The same thing happens on my geranium leaves, and the flowers die and wilt in the middle of the clusters. My rhododendron leaves turn yellow and curl together with spots on the leaves, and they drop off also. — Kay Nickless, Clay City, Ind.

A: Yellowing leaves can indicate a number of different problems, including iron chlorosis, nitrogen deficiency, overwatering, and various insects and diseases. The hollyhocks may be infected with rust, which causes small brown spots the size of a pinhead on the undersides of the leaves. These are seen on the top of the leaf as larger, bright yellow or orange spots with reddish centers. Sometimes the spots are so numerous they run together, causing an overall yellow appearance from a distance. The leaves then turn brown and fall off.

To control rust, remove and destroy affected leaves. Spray with wettable sulfur weekly when the plant is rapidly growing. In my garden, the rust causes unsightly lower stems, then the Japanese beetles attack the upper leaves and flowers. It’s a tricky plant to use. I plant daylilies in front of them to hide the lankiness and put the whole planting far away so I can’t see the beetle damage.

Moving on to the geraniums, if weather is wet and cool, botrytis can cause the symptoms you see. Over- or underwatering can also cause yellowing leaves, but the wilting flowers make it sound more like overwatering. Make sure drainage is adequate, and water less often. Fertilize with a balanced fertilizer to rule out nitrogen deficiency since geraniums are heavy feeders.

The symptoms on the rhododendron suggest dieback, although the leaves usually turn more of a reddish-brown. Poor drainage and/or overwatering can cause the yellowing and rolling you mention, and the spots could be a secondary infection. Rhododendrons prefer rich, peaty, acidic soils. I’ll hazard a guess that the soils in Clay City are a little different than that!

Q: I was reading your column this morning and saw where Marjorie Leader, Hudson, Ind., was having trouble with a fungus on her hardy phlox. I have a big plant of hardy phlox. When we moved here in 1948, the plant was here and has come up every year since. It is white and smells so good. My mother always sprinkled it with sulfur. Sprinkle it well down in the bush. It if rains soon after, sprinkle it again. I hope this does the job for her. It has for me. — Florence Brewer, Columbus, Ind.

A: Sulfur is a fungicide used as a dust or spray to control powdery mildew, brown rot, black spot, and certain other pests, as well as spider mites on tomatoes. It can cause foliage injury if the temperature is over 90 F. Keep your eyes open for powdery-mildew-resistant cultivars of phlox like ‘David’ (white), ‘Alpha’ (rose-pink), ‘Omega’ (white with pink eyes), ‘Miss Lingard’ (white) and ‘Rosalinde’ (purple-pink).

Q: I sure hope you can help me. My tomato leaves are starting to curl, and I don’t know what I should do. Also, I live in an area that has a lot of clay soil. I have read that adding gypsum to the soil will help to loosen it. I am wondering how I can do this to an established lawn, as I was told that you have to till it into the soil. If I were to spread the gypsum on top of my lawn, would it work its way into the soil? Or, is there a liquid I can spray on the grass to help loosen the soil? — Rose Mary Buck, Oak Forest, Ill.

A: Many tomato diseases, including curly top, mosaic, and fusarium , begin with rolling leaves. But if no other symptoms appear, it’s probably (aptly named!) tomato leaf roll. It’s a temporary disorder resulting from excessively wet soil, especially after heavy rains. It doesn’t affect the plant’s growth, and a normal crop of fruit is produced. The rolling disappears in a few days when the soil dries out. ‘Big Boy’, ‘Floramerica’ and ‘Beefsteak’ are affected most often.

Gypsum is useful out West for loosening up soils that are tight due to high sodium content, but that’s not usually a problem here. Instead, our soils are tight due to high clay content or compaction. Still, grass usually grows pretty well on Indiana soils. If you’re having problems though, core aeration and/or adding organic matter (top-dressing with topsoil or peat moss) are the solutions.

Q: We have a hummingbird vine called Campsis radicans. We’ve had it for three years already, and it has never bloomed, although the vine is hardy and healthy. Can you tell me what is wrong? — Mary Knepp, Montgomery, Ind.

A: Several things can keep plants from blooming, but in this case it’s probably shade or nitrogen. Excess nitrogen encourages foliar growth at the expense of flowers. Trumpet creeper blooms profusely along country roads where it receives no care at all, so it shouldn’t require fertilizer. Regular fertilizing of the surrounding lawn can also boost nitrogen to high levels.

If your vines are growing in a shady location, move them or prune overhanging branches so they receive more sunlight. At least a half day of sunshine will promote blooming. Finally, pruning it back to several buds in the spring may promote flowering.

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