Black spot on roses; snails and slugs in the garden

QUESTION: I have a 'Pearlie Mae' rosebush that is covered with black spot. The plant is growing in a container. Should I isolate it in the garage until February? D.T., Dallas

ANSWER: Keep the plant outdoors in the sun and spray it with a solution containing an organic fungicide such as garlic, cornmeal or neem oil.  Follow label instructions when using neem oil or plant wash.

QUESTION: Would spraying orange oil in the garden this winter help prevent a squash bug infestation next year such as the one I had this year?  K.R., Mineral Wells

ANSWER: A small amount of orange oil mixed with Garrett Juice (1 ounce of orange oil per gallon of Garrett Juice) and sprayed weekly or biweekly may help.  Although it won't injure too many beneficial insects, spray the orange-oil solution only on actively growing plants such as cool-season crops. Applying beneficial nematodes to the soil during fall and early spring may work even better (follow label instructions).
 

QUESTION: My patio garden is overrun with small snails and slugs. Last year, in desperation, I spread Epsom salts generously in the flower beds. That got rid of the slugs, but it contaminated the soil. Now, there are snails with flat, circular shells about ’¼ inch in diameter and other snails in inch-long, cone-shaped shells. How can I get rid of these pests without damaging the soil?

J.R., Dallas

ANSWER: To resolve your soil problems, buy phosphate rock and fork it into the garden soil at a rate of about 4 or 5 pounds per 100 square feet of garden area. Add a 1-inch layer of compost to the area, and then cover that with a 2- to 4-inch layer of shredded tree trimmings from your yard. Don't pile mulch against tree trunks or plant stems because it holds moisture that could contribute to crown and stem rots.  To kill slugs and snails, dust natural diatomaceous earth over the site during dry weather or apply a product called Sluggo.

QUESTION: I returned from a weeklong business trip to find that the leaves on my horseradish plants looked bad. I checked closer and saw hundreds of orange, black and white insects, ’¼ inch to 1/3 inch long, chewing on them. Since then, they also have attacked cabbage and broccoli plants.  I am an organic gardener, and I have tried several remedies without success: natural diatomaceous earth, orange oil and an organic spray that contains herbs and malic acid.

D.Y., Denton

ANSWER: You have harlequin bugs, a Southern pest insect related to stink bugs. Horseradish, cabbage and broccoli are among their favorite foods. The situation that you describe is a sign that the plants are bolting and should be dug up and thrown into the compost bin. Some of the horseradish roots left in the soil will grow new plants next spring.

QUESTION: A live oak growing about 8 to 10 feet from my house has two trunks and is 25 feet tall. About five years ago, during a drought, I noticed that doors in my house weren't closing properly and cracks appeared in the exterior brick on the side of the house where the tree is growing.  Do you think the tree is sucking moisture from under the house and causing the foundation to shift? My house is 32 years old.  J.B., Rowlett

ANSWER: Unless the house's foundation beams are faulty, they create a barrier that prevents tree roots from growing under the house. And unless there is a moisture problem under the house (such as a plumbing leak), tree roots won't try to go there in the first place. Have your house checked for plumbing leaks and drainage problems, and make repairs if needed. Then, get advice on how to keep the foundation watered properly. If you keep the soil moisture around the perimeter of the house balanced, it should be fine.

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