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Volume xviii, Issue 29 ~ July 22 - July 28, 2010

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The Results Are In

The Bay Gardener

by Dr. Francis Gouin

Carol Allen scores with Francis R. Gouin Undergraduate Research Grant

Botanical gardens have always had difficulty keeping plant viruses out of their orchid collections. While working at the U.S. Botanical Garden in Washington, D.C., Carol Allen often noticed small indentations or notches on the aerial roots of orchids in the conservatory. One evening, she saw cockroaches crawling over the roots. Could they be the culprits spreading viruses between contaminated plants and newly arrived virus-free plants? Always before, virus transmission between plants had been attributed to pruning tools or sucking insects such as aphids.

Allen, the 2009 recipient of the Francis R. Gouin Undergraduate Research Grant at the University of Maryland in College Park, has discovered that cockroaches can spread viruses from infected plants to healthy virus-free plants. With the Undergraduate Research Grant funded by my writing, she initiated a rather large experiment in cages using virus-free orchid plants with domestic and Australian cockroaches that had been allowed to feed on infected orchids. Four separate cages guaranteed accuracy.

With the Francis R. Gouin Undergraduate Research Grant, Carol Allen discovered that cockroaches spread disease among orchids.

The cockroaches were allowed to feed on the plants for five weeks. Evidence of feeding could be seen in injury to the roots and flowers. The virus-free plants were tested at monthly intervals. After 18 weeks, they were contaminated with viruses. 

Allen’s findings were so spectacular that she was invited to present the results at the International Orchid Symposium in Taiwan in January. Her recommendations are that it is just as important to control cockroaches in greenhouses as it is to control plant-sucking insects in order to keep orchids free of viruses.


More on Green Tomatoes

Q We planted Roma’s, Big Boys and Beefsteaks, all Burpees. Garden is in full sun until late afternoon. I get a soil test done each year from Penn State. In addition, I added about four bags of manure when I tilled. When we planted, we used a mix of mostly dirt with a bit of manure and peat moss. I added the soil recommendation of 10-10-10, and just about two weeks ago I added maybe a tablespoon and a half in a two-foot circle around the plants.

Up until one week ago, the plants were growing beautifully, and small fruit were appearing.

Several of my plants are three to three and a half feet tall and bushy. We pruned them a bit this year, cutting off the little stems so that air and sun could get through.

Our plants always thrived. The problem was rock-hard green tomatoes that never even began to ripen on the vine. 

–Peter Brooks, Chesapeake Beach

A I suspect you added dehydrated cow manure. How many square feet did you cover? From your soil test results, it is evident that the pH of the soil is excessive and should be lowered using two pounds per 100 square feet. Flowers of sulfur, wetable sulfur or flowable sulfur can all be used at the same rate of application.

Your soil test results do not identify the type of soil. If the soil is a sandy loam, it could be low in boron, which is important for fruit production and ripening. Have your soil tested for boron.


Ask Dr. Gouin your questions at frgouin@erols.com. All questions will appear in Bay Weekly. Please include your name and address.


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