QBARS - v2n2 Rhododendron Culture: Pests and Diseases

Rhododendron Culture

Rhododendron Pests and Diseases

Climatic conditions of the Pacific Northwest are so well suited to the great majority of rhododendron species and garden hybrids but few pests will bother them. Usually the presence of insect troubles indicates that there are cultural factors present which are not congenial to normal growth. If the situation is properly examined, the factors that promote insect pests will be discovered and may readily be altered in such a manner as to discourage the spread of insect life and promote normal growth of the plants.

Strawberry Weevil

One of the most serious insect enemies to rhododendrons is a tiny beetle which ignores weather or climate entirely and will cause serious losses when allowed to spread in rhododendron plantings. This insect is the well known Strawberry or Black Vine Weevil, (Othiorrhinchus sulcatus). Even though the insect is seldom seen, for it is a night feeding pest that hides during daytime, its presence in a planting is readily detected. This beetle when full grown is about 3/8" long, 3/16" to a quarter inch broad, is of a dirty gray color and has a pointed beak for feeding. It appears in the early part of summer or late spring when young foliage on the plants begins to expand. This beetle, of hard shelled covering, is in the habit of eating only on the edges of the young leaves by biting roundish nicks each night in a new place on the leaves.
The damage to the leaves is not really serious and would be ignored by the average plant fancier, but this insect when full grown will deposit a lot of eggs about the size of snapdragon seeds near the base of the plants. The eggs will then hatch out and produce small white grubs that live under the surface of the ground and gather at the base of the rhododendron stem. They feed on the bark during the fall and winter season unobserved. Usually by spring the maggots or weevils will succeed in girdling the entire stem of its bark. The flow of sap from the roots to the foliage of the plants is thus cut off, causing the plants to die suddenly just about flowering time. When the leaves begin to wilt there is no remedy possible. It the stems are not entirely girdled the plants will live but make only crippled growth. As this insect is deadly to many garden shrubs such as azaleas, camellias and ericas, it pays to be alert for its presence and to apply preventative measures. The practical remedies consist of applying a poison bait available in almost every seed store under various trade names. This bait is applied on the ground near the affected plants when the young foliage begins to unfold. Another application is advisable in early fall, as late batches of the insects frequently hatch out to feed on the second growth of the season.
Another remedy perhaps even more positive in results consists of spraying all affected plants as soon as insects become active with a solution of arsenate of lead. The solution should contain a full tablespoon of the poison per quart of water. This mixture is harmless to plants but kills the insect at the first feeding. If attacks are noticed in early fall the spraying should be repeated.

Other Insects

Occasionally caterpillars will feed on the leaves, but if the leaves have been treated with the arsenate of lead solution no damage occurs. When rhododendrons have been planted in warm or dry locations they frequently will be attacked by thrips and the lacewing fly. These minute insects thrive on the under surface of the leaves and cause them to drop off prematurely or to become colorless. Much harm may result from these pests in a very short time during spells of hot dry weather. Prevention is a lot easier than a cure, for they are almost invisible on the under surface of the foliage and hence difficult to eradicate.
The logical means of prevention consists of generous sprinkling of foliage from underneath, with water late in the evening. If this is repeated almost each night during the hot spells it will check the spread of the insects very quickly, as they do not thrive under moisture that is constant. Red Spider mites are also to be found when dryness prevails and will do some injury to plants. A high degree of moisture in the atmosphere is greatly appreciated by rhododendrons in general and that is why those growing along the coast are so far superior in vigor to those planted in drier regions. Lacewing fly, Thrips, and Red Spider mites are never a problem in the moist coastal climate. Observing persons will find this a clue to rhododendron welfare.

Protection From Lime

One of the rhododendron troubles very frequently noticed in garden plantings is a form of chlorosis, or yellowing of the leaf surface. When it begins the veins of the leaves turn light green and the edges of the leaves assume a sickly yellow color as they gradually die back. The entire leaf will often show a tendency to curl up slightly. This may appear in one or more of the branches to start with and gradually spread to all others until the entire plant becomes afflicted in this manner. This trouble, which may take a whole year to develop, is caused by the root system reaching into some lime content in the soil. At times it is only some of the roots that reach limy soil, as, for instance in bushes planted near the base of a concrete wall. The root system near the wall will absorb lime and transmit it to definite branches which soon begin to show by yellow veining. As the lime accumulates in the system and cannot be evaporated, the cells begin to suffer and die back gradually. Soils containing free lime will invariably cripple the growth of most garden rhododendrons. There are a few species that are lime tolerant. The only remedy is to take up the plant at once, free the original ball of earth from all new adhering soil by hosing it off with water under pressure. Excavate the hole deeper and wider, removing the limy soil and replacing it with sandy soil mixed with an equal amount of peat moss. If leaf mold is available it may be used also. The plant may be set out again and will gradually recover. It takes considerable time, however, as the lime has a paralyzing influence on cell activity and slows down normal function. Whenever this trouble is observed, it is best to lift the plants at once, except during May, June and July, when then- immature growth might be endangered.
Taken as a whole, rhododendron plants thrive with less trouble than most other garden shrubs if growing conditions are congenial and soil is loose and porous.