Ask a Master Gardener: From pesticide explanations to leaf spot identifications

Q. Explain the term pesticide.

A. Pesticide is a general term that includes insecticides, herbicides, miticides and fungicides, among others. These are the main pesticides gardeners would be concerned with. Below is a general description of the three most commonly used by gardeners.

Insecticides are the most common, developed to reduce populations of damaging insects. When selecting an insecticide, it is crucial to know what kind of insect you have, as the way they feed affects how best to control them. Insects have four main eating styles: chewing like a caterpillar, sucking like most butterflies, piercing-sucking like a mosquito and sponging like the house fly.

Each chemical is formulated to best control insects according to the way they eat. Know what you are treating, read the label, and follow all directions.

Herbicides are formulated to control growing plants. The most commonly known is Roundup, a general kill-all herbicide. If you spray a plant, you either kill it or damage it.

Next is a bare-ground herbicide. They come in granular or liquid form. They are intended to kill what is there and keep anything from growing in the immediate future. Do not use these under or around trees or around desirable plants, as the chemicals are taken up by the roots and can kill those plants as well.

Broad-leaf herbicides are formulated to kill only plants that have a wide leaf (dicots) and are generally used to kill weeds in lawns, as they will not kill grasses (monocots).

There are many more herbicides out there. Know what you are wanting to do and read the label and follow all directions.

A pesticide can be purchased in two styles: RTU or "ready to use," which means there is no mixing required, usually in a spray bottle, or "concentrate," which you mix according to the directions for your pest and spray with your own equipment. Which one you buy is strictly a personal choice.

Fungicides are also commonly used by gardeners. These are applied to prevent a disease or control its spread. Most times they are applied as a spray, but there are some formulations applied as a drench, with roses notable for having some products available. Plant diseases fall into three main groups: fungal, bacterial and viral. Fungal and bacterial diseases can often be controlled with products, but viral diseases often can't (remove and destroy virus infected plants). For fungal and bacterial diseases, different "chemistries" are generally needed. This is why it is very important to correctly identify the disease before applying a fungicide to make sure the one used is effective.

Q. What are the black and brown spots on my rudbeckia plants?

A. There are two common types of leaf spot that affect the rudbeckia and coneflower plants common in area gardens. The constant rains this year have given both types their favorite conditions.

The first is Septoria leaf spot. This fungus causes small black spots on the leaves of up to inch. Small fruiting bodies grow in the center of the spots. This disease develops on lower leaves first then travels up the plant by water splashing on the leaves.

The second is Angular leaf spot. This bacterium starts on lower leaves as a small, angular black spot. These spots can expand to encompass the whole leaf.

Prevention is best for both of these. Clean up leaf debris as both overwinter on affected leaves. Give good air circulation, and use soakers for watering, as moisture on leaves aggravates the problem.

If chemicals are required use a copper-based for either, or a chlorothalonil-based chemical for the Septoria.

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