Fungal and Insect Enemies
Rhododendron ABC's #9
Having now covered site selection, hardiness, fertilizing naming, etc., we come to the subject of keeping plants in pleasing appearance in the landscape. Fortunately, rhododendrons have relatively few natural enemies and, thus, do not require constant attention such as some other ornamental plants do.
There are, however, some conditions caused by parasitic fungi and rusts for which the grower should be on the alert. It is of utmost importance to maintain plants in good environments so that they can remain vigorous and resist the fungal infections. Despite best efforts, the infections do occasionally occur. Of these fungal ailments, there are two forms: those with airborne spores that attack stems and leaves; and, those with sod and water- spores that attack root systems. The latter being more difficult to control. For the gardener, the best procedure is to remove and burn the infected plant to prevent further spore spread.
A few fungicides are now available which can be used to sterilize infected plant sites. These are very expensive and may not be very cost effective for treating one or two plants. If care is used to have open and aerated plant sites which are well drained, the root fungi should not be too much of a problem. Sometimes, the root problem will show itself as a dieback on only some stems and branches. If this occurs, the affected part should be removed and burned, and the rest of the plant may not get into further trouble.
Airborne fungi (except for rusts) are more easily controlled. Captan, benlate, and micro-cop seem to give good results when applied as directed. Rusts are quite difficult These are fungi which have very complicated life- generally involving two different plant hosts. They ordinarily do not kill the host but can cause stunting, yellowing and distortion. The best control, if you suspect rusts (which seem to attack only certain rhododendrons) is to submit an infected sample to an expert with the local county or state extension service. If rust is confirmed, sacrifice and burn the infected specimens.
Of late, there appears to be an outbreak of powdery mildew fungus in the Pacific Northwest on rhododendrons. Leaves become dark, turn brown and defoliate. Unchecked, the fungus may kill plants. I have had good control results by spraying with "micro-cop". This product gives good results and does not harm plant times.
In addition to fungal attacks, rhododendrons can be harmed by certain insect parasites. Stems may be invaded by borers and leaves attacked by loopers and caterpillars. Backs of leaves may be attacked by white flies. These pests can be controlled by inspection and physical removal or by using any of several insecticide sprays.
The most important rhododendron parasitic agents are the group, generically called, root weevils. These pests probably do more to make rhododendrons unsightly than all other agents combined. There are several species of these pests and all seem to have similar life-cycles. The adults feed on leaves, notching them as though cut with pinking shears. Eggs are laid in the ground and these hatch to give white grubs which feed on plant roots, sapping the vigor but not necessarily killing the plant host. If (and it may occasionally happen), the bark of the underground stem is completely girdled, the plant will die much to the dismay of the owner who does not know anything is amiss! The grubs stay in the soil until they mature, then, especially during warm summer months, the adults emerge, crawl up the stems and begin to feed, starting the cycle over again.
Control is relatively easy. There are two ways to go. If you are adverse to chemical sprays, there are sprays available that contain certain nematodes (small worm-like creatures) that attach only the weevil larvae and do not harm any other material. These can be very effective. There are also some very effective chemical sprays. One such product is called "Orthene". Orthene is very effective but one must follow the application directions with care as the product is toxic to animals and humans. It should be used carefully. I have used it for many years and have never had any adverse effects from it. Its only drawback is that it does not give permanent control and must be used every three weeks or so from about May through September to give good protection. Control of root weevils willrepay itself many fold by the resulting improvement in plant appearance.
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