When hardy marigolds succumb to disease, the situation is serious

Backyard Q&A

July 21, 2002|By Dennis Bishop | Dennis Bishop,Special to the Sun

Q. My French marigolds grew well early in the summer, but since then their growth has become stunted and they look somewhat deformed and spindly. I gave them extra fertilizer, but that did not help. Do you know what could cause this problem?

A. Marigolds are only affected by a few problems, but they can be quite serious. It sounds like your plants are infected with a disease called aster yellows. Aster yellows infect more than two hundred plant species, which includes some local perennial weeds.

The disease is caused by a bacteria-like organism called a phytoplasma and is transmitted to plants by leafhopper insects. Leafhoppers suck plant sap for food and move from one plant to the next. While feeding they transmit the disease.

Because the disease infects several common perennial weeds, it can survive the winter here and then be transmitted to your marigolds the following summer.

It would help if you tried to control the leafhoppers and the host weeds, but here is no direct control for the disease. Infected plants should be destroyed and replaced with a different kind of plant.

Q. The recent dry weather has slowed the growth of our lawn, but we still have patches of grass and weeds growing. We would like to keep the lawn neat by mowing. Is this good for the lawn?

A. During periods of excessive heat and drought, many plants go into a dormant or resting state. This is a natural response of plants through which they try to limit growth and conserve their resources. However, when plants are cut, there is another natural response that tells them to start growing again.

This occurs when shrubs and trees are cut with pruning shears, and it also occurs when grasses are cut with a lawn mower.

Cutting lawns during droughts is not good because it encourages growth during a stressful period. I would leave the lawn rest during periods of drought and only mow when it is essential.

You can begin mowing more often when rainfall returns to normal and your grass begins to grow again.

Dennis Bishop is an urban horticulture educator for the Baltimore office of the Maryland Cooperative Extension Services. If you have a gardening or pest problem, you can call the Home and Garden Information Center hot line (Monday through Friday, 8 a.m. to 1 p.m.) at 800-342-2507. You can also e-mail questions, order publications and diagnose plant problems by visiting the Web site, www.hgic.umd.edu.

Checklist

1. Mulching will help most plants during drought, but be judicious. Plants such as iris and daylily will tolerate dry conditions, but do not like heavy mulch.

2. Mosquitoes breed in very small bodies of water. To prevent their spread, keep gutters clean and empty all small containers that fill with rainwater.

3. Weeds compete with desirable plants for water and nutrients. Pulling them will increase the growth of your flowers and vegetables.

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