Leaves Turn Yellow means Soil Too Alkaline
When acid loving plants are in soil that is too alkaline they develop a condition called chlorosis. The yellowing of their foliage is a signal that they need more acid in their soil. This sometimes happens when shrubs are planted near masonry walls and over time the rain leaches lime from the mortar into the soil. To acidify soil, add peat moss or aluminum sulfate to it. Do not use lime near mountain laurels.
Leaves Turn Brown in Winter Shows Winter Burn.
Mountain laurels that are exposed to bright winter sunlight and harsh winds often develop winter burn on their foliage. Their leaves dry out, turn brown, and will eventually drop. Minimize this danger by siting shrubs carefully when planting them. Spray foliage with anti-transpirant just before cold weather sets in and shield exposed shrubs with a burlap windscreen.
Foliage Turns Pale Or Mottled indicates Lace Bug.
Adult rhododendron lace bugs are small square-shaped bugs, 3/16 inch long or less, with elaborately reticulated wings that resemble lacework. They suck sap from the undersides of mountain laurel leaves, causing them to turn pale or mottled. This sucking insect appears in late June and in the summer on the undersides of the leaves of broadleaf evergreens like azalea, rhododendron and mountain laurel. Control light infestations by crushing the bugs. Pinch the leaves on which they are feeding between your thumb and forefinger. Treat heavier infestations by spraying the bugs on leaf undersides with insecticide.
For more information see the file on Dealing With Lace Bugs
Branches Wilted; Stems With Borer Holes means Rhododendron Borer.
The borer is a yellowish larva (worm), about 1/2 inch long. They over winter as partly grown larvae in burrows in mountain laurel stems. Adults emerge in June, and lay eggs on leaves, new twigs, or rough bark of the main stem. They sometimes girdle branches when laying their eggs, causing tips to die back and break off. They enter into the tips of stems and bore out some of the twigs soon after blooming time. As they get larger, these larvae bore into the woody part of the laurel shrub, pushing out fine sawdust as they go. When its main stems are bored, ugly scars remain and sometimes-large branches die.
To control borers examine the mountain laurel before the spring season arrives and cut and burn any dying stems below visible borer holes. In June, crush any eggs that you can find. During the summer season, check to see if fine boring dust is being pushed from small borer holes. Cut out these holes with a sharp knife. If the tunnels are fairly straight, probe in them with a flexible wire to kill the worm inside, or pull it out by means of a hooked wire to make certain it is destroyed. Borers can also be killed with nicotine sulfate. Dip a piece of cotton or soft cloth into a solution 1 part nicotine sulfate to 4 parts water, and stuff it into the borer's hole or try injecting nicotine paste into the holes.
Another approach is to introduce BT (Bacillus thuringiensis) into each hole at ten-day intervals until no more telltale sawdust appears. A special hypodermic needle for injecting BT is commercially available. Coat or seal wounds with tree paint, putty, paraffin, or chewing gum. A black light trap may prove effective against the adult borers, if used in May or June when the insect is in the moth stage. Practice garden hygiene and burn all weeds, stems and plant remains likely to harbor over wintering eggs.
For more information see file on Dealing With Borers
Leaves Turn Yellow; Weak, Dying Branches appears to be Weevil.
Black vine weevil adults are brown beetles with long snouts. They have tear-shaped and hard-shelled bodies, averaging 3/8 inch long. They feed on foliage and bark, leaving characteristic notches along the edges of mountain laurel leaves. Weevil larvae, white grubs with brown heads, also feed on laurel roots deep in the soil. These pests are hard to spot because they feed at night, living under tree bark and debris on the ground by day.
Because these weevils "play dead" when disturbed, folding their legs and dropping off plants to the ground, they can be trapped. Gently beat the branches of the infested shrub and catch the startled insects when they fall onto a cloth spread beneath the shrub. Apply tanglefoot to the trunks of the shrub to prevent the adults from climbing up and eating the leaves. As soon as weevils appear, begin spraying weekly with a pyrethroid insecticide which is a synthetic insecticide.
For more information see file on Controlling Weevils
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Dead Blotches on Leaves means Leaf Spot.
Various leaf spot fungi cause yellow, brown or black dead blotches on mountain laurel leaves. These blotches frequently run together, causing heavily infected leaves to turn yellow or brown and fall prematurely. Cool, moist weather encourages these diseases, especially when new leaves are developing. Shake out all fallen and diseased leaves from the center of each laurel shrub and destroy them. Remove all dead branches in the center of individual plants or hedges to allow better aeration. Mulching helps prevent the disease from splashing up from the ground and infecting plants. Spray at weekly to 10-day intervals with sulfur or copper fungicide, particularly in rainy weather. Dig up and discard seriously infected shrubs together with the root system and soil ball.
For more information see file on Controlling Fungal Disease.
