July 2007 - Indiana Yard and Garden - Purdue Consumer Horticulture

July 2007

Q. Some years ago, I had a list of plants in the garden that were compatible and those that weren’t. If you have such information, could you include it in your “question and answer” column? It’s too late for this season, but we’d have it for next.

A. The principles of “companion planting” are based on the belief that certain plants can benefit others when planted in combination or nearby. Such strategies can include using specific plants to draw pests away from the main crop, interplanting with nitrogen-fixing legumes, suppression of pests or diseases by certain plants and, in general, strength through diversity.

On the other hand, some plants have the ability to suppress, or even kill, other plants by secreting specific chemicals into the soil. This type of relationship (allelopathy) is commonly demonstrated by the black walnut tree suppressing many other plants, including tomatoes.

Specific benefits of companion planting can be difficult to document scientifically compared to other gardening techniques and is much debated among gardeners and scientists. One of the most sensible and well-documented discussions is offered by the National Sustainable Agriculture Information Service at  https://attra.ncat.org/attra-pub/summaries/summary.php?pub=72.

Q. We have hollyhocks planted near our back door. For the last couple of years, about the time they are blooming good, they start to get lesions on their leaves. The lesions are a brownish color. After a short time, these leaves drop off and eventually kill the plant for that season. Last year we got some fungicide and sprayed on them, but it did not seem to correct the problem. We were wondering if this is a fungus or a bug problem, and what we can use to eliminate it.

A. Hollyhocks are susceptible to a number of different leaf spot diseases, but, by far, the most common and destructive is known as rust. This fungus begins by causing tiny pinhead-sized brown spots on the undersides of the leaves. At the same time, the top of the leaf shows a larger yellow-to-orange-to-tan spot. Eventually, the spots get larger and join together as the disease spreads to the stems and even the green parts of flowers. The leaves then shrivel and turn brown giving the plants a blighted appearance. You can see photos and a brief discussion, courtesy of the Purdue Plant and Pest Diagnostic Lab http://www.ppdl.purdue.edu/PPDL/weeklypics/Weekly_Picture6-8-99.html.

The disease is especially favored by damp and/or humid weather. Fungal spores are produced in the brown-raised spots on the lower leaf surface and overwinter in plant debris. Removing infected leaves promptly, and cleaning up all plant residue at the end of the growing season is critical to reducing the spread of the disease.

Some fungicides, such as chlorothalonil, mancozeb and sulfur, are labeled for use in controlling hollyhock rust. But, keep in mind that fungicides are preventative, not curative. They can only protect healthy foliage from becoming infected. If the plant is heavily diseased, it is too late to apply fungicides. Always consult the label for recommended rates and safety information BEFORE you apply.

Q. Is there an animal that will eat every tomato on 18 plants and leave no trace, or should we suspect a human animal?

A. It is difficult to know which of the potential culprits is responsible, without knowing a bit more about the situation. Most animal pests will leave some sort of calling card, if you look closely for animal tracks, animal droppings, damaged fruit from beaks or teeth, etc.

If the fruits are disappearing during the day, the more likely suspects are squirrels and/or birds. Of course, the size of the fruit would also be a factor. Birds can easily pluck cherry or grape tomatoes but aren’t likely to be removing large fruits from the plant. If the culprit is on night shift, suspect raccoons, known for consuming large quantities of fruit.

You could do a bit of sleuthing by sprinkling a light dusting of flour on the surface of the garden all around the plants, then look for the tracks made by the intruder. You can compare your evidence to the chart of tracks provided by the USDA/APHIS Wildlife Serviceshttp://www.entm.purdue.edu/Wildlife/TRACKS.htm. Links from there will provide information on controlling and preventing wildlife damage to home, gardens and landscapes.

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