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PESTS AND DISEASES OF PELARGONIUMS

Wayne Handlos, Ph.D.

    aphids                             ^   budworms   ^                    mealybugs  on  ^ leaf            &     ^ root                        white flies                  Geranium rust

There are a few pests and diseases that may attack the garden geranium/pelargonium. Most are easy to recognize and control, but a few may be more troublesome.

Aphids, sometimes referred to as plant lice or green fly, may attack geraniums. These small insects have sucking mouth parts and tap into the plantís vascular system and suck out the nutrients that they need. They tend to be found on the youngest, most succulent tissue (actively growing stems and leaves including flowers). The aphidís reproductive rate may be high (it is higher at higher temperatures and males are not essential to the reproductive process) so you should be vigilant. They excrete the excess sugar in the sap they suck from the plant. This sticky material provides a base/food source for fungi and is the origin of black deposits (called sooty mold) on the leaves of the plants. A simple control method for aphids is to spray the plant with a dilute soap solution (1 tablespoon of soap/detergent in 1 gallon of water). Commercial soap solutions are available for use on plants.

Mealy bugs are small, crawling insects that are covered by a cottony mass which they secrete to protect themselves. They also are sucking insects and do best where the tissues are soft. They are more common where the stem and leaf join and resemble little tufts of cotton. They can be killed by wiping them off with a cotton swab dipped in alcohol.

Whiteflies are also tiny insects with sucking mouth parts. They live on the undersides of leaves and are whitish in color as their name suggests. Whiteflies are most troublesome on regal geraniums and flourish in moist areas with little wind.

Caterpillars of several types attack zonal geraniums in particular. The bud worm flourishes on flower buds but will also attack young shoots. Frequently they will enter the flower bud and eat all of the petals so the plant does not appear to bloom. If you examine the buds closely you will find a small hole indicating the presence (or former presence) of the tiny caterpillar. They tend to be somewhat seasonal and squashing them is a very effective control method. An organic spray, specific for caterpillars, contains a bacterium referred to as BT or Bacillus thuringiensis will kill the caterpillars but does not harm people or pets.

Spider mites, very tiny spider or mites, may become a problem on plants grown under dry conditions. With mild infestations they tend to be found on the undersides of the leaves and look like tiny, moving dark specks. In heavy infestations they cover the leaves with a fine web.

Various sprays may be used to control some of these pests. Check with local agricultural experts or Master Gardeners for recommendations. Neem oil, a natural extract from a tropical tree, controls many problems including aphids, mealy bugs and rusts. Other fragrant essential oils may also be effective against various insect pests. Read the labels and buy something with which you are comfortable.  Effective control may require repeated applications of the spray.

There are a few diseases that may affect Pelargoniums. Rust, a fungus (Puccinia pelargonii zonalis), is a common pest of zonal geraniums. It looks like rusty, brown spots on the undersides of the leaves. It is more common when the leaves are wet as this provides conditions for the germination of the spores. It is most easily controlled by prevention. Do not allow the leaves to remain wet for long periods of time. In other words only water plants in the morning so that they will dry off quickly. Neem oil may provide an element of control during rainy periods of the year, otherwise pick off and destroy infected leaves.

Botrytis is a fungus that affects primarily dead and dying tissue. It becomes troublesome during rainy periods and is most commonly found in flower clusters where there are old petals. The fungus feeds on the dead tissue and then may attack living tissue and kill the developing flower buds. Control this problem by dead-heading old flowers and flower stalks or keeping the plants dry.

Sooty mold, fungi growing on the excrement of aphids and whiteflies, was mentioned above and is directly related to infestations of aphids and whiteflies. Eliminate the insects and sooty mold will not develop. Unsightly leaves may be washed off or removed.

Various stem and root rots develop in plants in poorly drained soil or pots in standing water. Control is through prevention. Once the fungus is established in the roots, the plants are doomed. Good soil drainage is the best method of prevention.

Oedema or edema may be a problem with ivy geraniums. It is characterized by watery bumps on the leaves of ivy geraniums. It is related to an excess of water in the soil. It is not caused by any disease organism. Some cultivars are more resistant to edema than others and you can avoid the problem by not over-watering your plants.

There are many chemicals available to control insects. In general we would recommend that you use the "greenest", most environmentally friendly solution available. Cultural techniques that produce the healthiest plants are the first line of defense. Beyond that, use mechanical methods if you can (pick off the creatures). After that, use organic, naturally occurring products like BT or neem oil. If you must use a chemical spray, use the most specific type available (one that attacks only the pest that is causing a problem). Again consult with experts or Master Gardeners before using a chemical spray and follow the directions precisely.

© 2010, Central Coast Geranium Society (CCGS )