Q: I have a problem with what I believe is "whitefly" on my sweet potato plants that I use as a ground cover. I planted about five or six plants about three years ago. They have been very hearty and healthy and have spread to fill in as a perfect ground cover - very thick and rich color. About two or three months ago, I noticed very small white flies. After a few weeks of wondering what they were and where they were coming from, I noticed the white spots on the underside of the leaves. I went to Home Depot, where the salesperson told me they were whiteflies and sold me "Organocide" 3-in-1 garden spray- insecticide/fungicide/miticide. The directions said to spray the underside of the leaves, but there was just too many leaves and too large of an area to do this. I sprayed one application each week for about three weeks. It seemed to help, but the plants have deteriorated and the white spots remain.
We really like the looks of this plant and are disappointed that it seems to have been totally wiped out or close to it. At this point, is there anything that can be done to salvage what is left? If not, what would you recommend I do to make sure I don't have this problem again before replanting in the spring?
Russ Lynn, St. Petersburg
A: It appears that your bed of sweet potato has more than whitefly attacking it, based on your photos. A close-up of the white mass on the back of the leaves would be helpful in making a positive identification. There is a whitefly called "sweet potato whitefly" (actually classed as a scale) whose larvae feeds from the bottom side of the leaf. The adult looks like a whitefly 1/16 to 1/8 of an inch long and fly out en masse when the leaves are disturbed and secrete "honey dew," their excrement from sucking plant juices of which "sooty mold" likes to grow, resulting in a black sooty covering over the leaves, but not death and destruction of the entire plant bed.
The white underside of the leaves in the one picture looked more like mealybugs, another sucking scale, rather than whitefly, based on the amount of white debris under the leaf. When the population is high, they will kill plants.
To control mealybugs and whitefly, a systemic insecticide labeled for ornamentals, such as Orthene (acephate), will need to be applied with more than one application required to gain control of the plants you have left. Organocide (sesame oil) would work if the population and area were smaller. You might also consider eliminating the plants that are left, replant the bed with new plants, scouting the bed frequently for problems and have your products ready to apply at first sign of an infestation.
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Poor soil may have doomed vegetables
Q: I planted tomatoes, parsley, green onions and Chinese pea pods in pots, and they all did well, although the tomatoes did not get large. But the cucumbers wilted on the vine and had yellow streaks in the leaves. I got blooms, but nothing else. I didn't over- or underwater. What did I do wrong?
Also the melons did the same thing. This was grown in good black soil that I bought. Should I use a compost or something as a fertilizer, even if I fertilized them every other week in the water? Mari Howard
A: I do believe the soil is the problem for the failure of your veggies. Many times, nurseries sell "top soil" which is nice and black, looks like it should grow something, but it is heavy and has very poor drainage. What you want are peat-based mixes, which are very light and drain well. Stay away from "potting soil." Instead, you want a "potting mix", such as Promix, Metromix, Fafard or MiracleGro Potting Mix. Using a soluble fertilizer is a good choice for container gardening. Carroll's Nursery sells the Fafard Mix, which is one of my favorites. Give it a try!
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Don't spray to combat aphids
Q: How can I combat aphids on my milkweed plants and not harm the Monarch butterfly laying eggs and then the caterpillars eating the leaves before forming the chrysalis? The aphids are so nasty!
A: Not to worry! It just so happens that the orange, non-native milkweed, Asclepias curassavica, plays host to all kinds of critters: Yellow aphids with black legs clustered on its new growth stems sucking plant juices, closely. Ants follow and groom and milk the aphids for their precious "honeydew" (the clear, sweet excrement from many sucking insects, principally soft scales, aphids and whitefies). A few lady beetles and their larvae (babies which look like tiny black/orange alligators) feed on aphids. Lace wings - beautiful green insects with clear wings - feed on nectar and honeydew, and their larvae, which look similar to baby ladybugs, also munch on the aphids. Tiny wasps parasitize aphids, leaving empty tan carcasses which look like tiny tan eggs with holes in their backs. With all these, aphids don't stand a chance!
Then we get to the large milkweed bugs (LMB) and small milkweed bugs (SMB) which are orangey-red with black legs as a larva. As the LMB mature they look like they have a black and orange saddle on their backs contrasted to the SMB which have a black heart on their backs highlighted with orange-red which all feed on the seed pod and the seeds inside with their piercing sucking mouthpart.
Finally, the monarch caterpillars appear, white with black and yellow bands and two black whiskers on each end which devour the foliage.
There's something for everybody, an all-you-can-eat community smorgasbord. But after all that has happened both on the plant and to the plant, its seeds that are left are spread and the cycle starts anew and commonly the stick sprouts a new set of leaves. Unbelievable.
So, no spraying. Instead, keep a vigil and see how many different kinds of insects, both juvenile and adult, you can find - a great project for kids and grandkids. It's an entire ecosystem on one lonely butterfly milkweed plant. And for the monarch butterfly, higher predators like birds have learned to leave them alone because of their orangey-brown and black colors, a signal that as caterpillars they ate the milkweed leaves containing their toxic, milky sap (hence its name milkweed) giving them a most unpleasant taste, so the plant is beneficial even beyond its time. Isn't nature grand?
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Need help? Dr. Hort (Greg Charles) answers questions about garden problems. Email him at email@example.com or mail questions to HomeLink, Features Department, Tampa Bay Times, P.O. Box 1121, St. Petersburg, FL 33731. Describe problem in full, and include your name, city of residence and contact information. If possible, include a good-quality photo. Fuzzy ones won't do. Photos cannot be returned. Please do not send plant samples.