Organic Garden Guide: Pests And Diseases Of Lilacs
Diseases are always harmful. When lilacs or Syringa shrubs are affected by diseases, it may cause the plants to stop blooming or reduce their number of flowers. In many cases, diseases occurring in the previous year may result in the destruction of the flower buds. In some instances, growers may take preventive or even curative measures. It has often been seen that the lilacs in dry gardens in rural areas are perfectly healthy, while many of those in major cities are vulnerable to a condition known as leaf-roll necrosis. Usually, lilacs grown in places having dry summers and chilly winters are mostly free from diseases compared to those grown in places where the summers are humid and winters are mild.
Like diseases, even problems caused by pests like scales and borers may cause the plants to bear fewer blooms. For instance, Syringa vulgaris or the common lilac, as well as several of its cultivars, are among the lilacs that are very vulnerable to pest invasions.
- Lilac borers
- These are very niggling pests that are actually the larvae of a wasp-like moth called Podosesia syringae var. syringae, whose wings are semi-transparent and brownish. This pest lays large amounts of eggs in late spring. They usually lay their eggs on the stems of lilac shrubs and ash plants. While they are hatching, the larvae penetrate the branches and are nourished by the wood. Initially, these pests remain out of sight and one notices their presence for the first time when they find the entire leaves on a branch or stem turning yellow and wilting. This usually happens during the spring or towards the beginning of summer. Infestation by lilac borers may cause the bigger branches to become distended and eventually break.
If you take a closer look at such branches, you will notice tiny holes measuring roughly about the size of a pencil lead in diameter at a level of one or two feet (30 cm to 60 cm) higher than the ground. Just below it, you will notice some sawdust. In fact, these holes are actually exit paths of lilac borers and they suggest that the pests have already left, but still some others may be at work. These pests are very visible when you are pruning the plants. In fact, you may even find the borer tunnels breaking through the heartwood, especially of the older branches.
Precisely speaking, stresses, as well as wounded plants, are most common hunting grounds of lilac borers. They are also common on the bigger and older stems or branches, particularly on the common lilac (Syringa vulgaris). Several pesticides are available in the market to effectively deal with lilac borers.
- Lilac leaf miners
- Leaf miners are basically the larvae of a small moth species called Caloptilia syringella. These pests bore tunnels between the leaf layers, thereby giving a blotched appearance to the leaves during the start of summer. Subsequently, lilac leaf miners turn over the leaves and feed on them externally. Eventually, the color of the affected leaves become brown, thereby giving a burnt appearance to the entire plant. When the first sign of leaf miner assault becomes evident, you can soak the leaves with neem or a nicotine spray. However, when you notice the damage, it is already very late to take curative actions. Nevertheless, you should immediately remove the affected leaves and clear the area beneath the shrubs of all leaves and debris during the fall with a view to protecting the plants from being infected again. In fact, the damage caused by lilac leaf miners is more aesthetic compared to physiological. S. vulgaris or the common lilac is most vulnerable to invasion by leaf miners.
- Lilacs are often susceptible to the eriophyid mite (scientific name Aculus massalongoi). This pest creates a rust or silver color on lilac leaves and may sometimes result in leaf-rolling.
- Oyster shell scale
- These are very bizarre looking insects that appear as flat, oval-shaped and lifeless lumps. While shielding themselves under a wax-like coating and scales actually damage the plants by pulling out their fluids. These pests lay eggs on lilac shrub/ tree barks either during fall or in spring. These eggs are hatched later in spring. The young scales are mobile and have a light yellow or orange hue. They measure roughly 0.1 inches (2 mm) in length. The legs of these pests wither when they settle on a place to feed. When you are pruning the stems of lilacs, you may possibly notice that the stems have become roughened and have an unusual grey and dry appearance. When you take a closer look at the stems, you will notice minute bumps that can be skimmed using your fingernail or the pruning shears’ blade.
- Lepidosaphes ulmi
- This type of scale is known as the apple mussel scale and you can control their infestation on lilac plants by pruning the branches that have been infested heavily. In the next spring, you should apply a dormant-oil spray before the emergence of new leaves. You need to undertake this treatment prior to the bud break and essentially on an arid, sunlit, mild morning. In order to avoid harming the lilac plant, ensure that you do not apply the dormant-oil spray when the temperature is below 39°F (4°C) or 48 hours before or after a frost. Preferably, you should undertake the treatment on a calm morning when there no frost is anticipated that night and the atmospheric temperature is over 60°F (15°C).
You can also eliminate scale by painting the lilac branches with a solution of lime-sulfur using a paintbrush. Alternatively, you can directly spray a soapy water solution on the plants to kill scale. Add one teaspoon (5 ml) of any liquid dish soap that is additive free (such as Ivory) to one quart (1 liter) water and spray the solution on the plants. In fact, the soap water solution is most effectual when sprayed in the later part of spring or the beginning of summer – the time when these pests are in the crawler stage. Continue spraying the soap solution till the foliage seeps.
- Usually, you will notice the first symptoms related to bacterial blight (scientific name Pseudomonas syringae var. syringae) right in the start of the season – either when the weather is hot and humid or immediately after the spell. The beginning of this bacterial disease may often be remarkable. All of a sudden, the leaves start appearing as if they have been scorched all along their edges, and subsequently the stems become black. Bacterial blight may also result in the formation of black spots confined by a pale circle on the leaves. When this disease affects plants, their flower clusters first wilt and eventually die. This bacterium also infects other plants including cherry, pear, maple and many varieties of ornamental plants.
There is a different kind of blight, which is attributed to a fungal infection. This disease is caused by Ascochyta syringae and the symptoms of this plant disease are same as those caused by bacterial blight. The disease starts from the soil and spreads up when the soil is damp during the start of spring. While these diseases do not essentially kill the plant, but they make the lilac appear unattractive and ugly. Moreover, these blights can also damage the new growth. These also affect the blooms adversely and the blooms may be lost in the year of the infection or the subsequent, subject to the time of the bacterial or fungal infection.
Usually, it is very difficult to control blights. In fact, plants that have been damaged in some way and the new growths are most vulnerable to these diseases. In order to reduce damage caused by frost keep the foliage dry, ensure that the plants are well ventilated. If it has been found that maximum damage caused by blights is in the low-lying areas – places where cold, moist air collects easily. If you notice blight soon after the plants are infected, cut down all the infected parts and remove them as soon as possible so that they do not spread to the healthy areas. Moreover, you always need to sterilize the saw or pruning shears between cutting different parts by wiping their edges with Lysol, alcohol or a home-made solution of one part domestic bleach and 10 parts of water.
In the following year, spray the plants with a copper fungicide while the weather during spring is still warm – a favorable time for infections. Do not undertake heavy pruning or feed the plants with too many fertilizers, as these promote fast, but weak growth. It has been found that the common lilac (S. vulgaris) and other lilacs like S. x hyacinthiflora that bloom early are most vulnerable to blights. Compared to the doubles, the singles are more susceptible. Similarly, lilacs that bear purple, magenta and blue blooms are more vulnerable to blights compared to those that produce lilac hued flowers. In addition, lilac species like S. villosa and S. x prestoniae are very susceptible to these diseases. However, lilac species, as well as cultivars that bloom late in the season, are able to resist blights to some extent.
- This prokaryotic microorganism causes a disease known as witches’-broom, which is widespread in large varieties of lilacs compared to the shrubs grown in the backyard. This disease is manifest in the form of a clump on very thick growth on any common lilac shrub or tree. This disease was noticed for the first time in 1951 and till date remains to be rather mysterious. However, scientists have come to know that a pathogen similar to viruses and called MLO (mollicutes or mycoplasma-like organisms) witches’-broom. It seems that these microorganisms form colonies in the phloem (sap) and disturb the flow of the sap, thereby killing a new growth point. This, in turn, leads to a thick growth of shoots on the side of the plant – generally in the lower part of the plant.
It has been found that S x. prestoniae and S. x josiflexa, lilacs that flower later in the season, are more vulnerable to this disease. Initially, the affected plant may appear unhealthy, its growth may be twiggy (fleshless) and it may produce unusual bloom out-of-season or have growth flushes. Eventually, the plant dies. Ash trees are affected by a comparable disease that is called ash yellows. This disease may also affect vigorous lilac plants and may also spread to ash trees possibly through an insect. In order to prevent the plants from being infected by this microorganism and also treating the affected plants, it is important to prune them with tools after sterilizing them properly. For instance, you may wipe or dip the blades of the pruning shears, knives and cutters in Lysol, alcohol or a home made sterilizing solution prepared by adding one part domestic bleach to 10 parts water before pruning or cutting a plant. As of now, there is no cure for this disease. As soon as you notice a plant affected by this disease, remove the plant and destroy it immediately.
- Powdery mildew
This is a fungal plant disease caused by Microsphaera alni, M. syringae. This disease attacks lilacs when the atmospheric condition is warm and humid. Generally, this fungus first attacks the older leaves that are towards the plant’s base some time in July and then spreads all over the foliage between the period mid-August and October, when the leaves fall. First, the leaves develop ugly white or pale grey blotches, which gradually change to yellowish and eventually the leaves drop from the branches. When this disease affects plants, the growth of new leaves is also stunted. In most cases, the harm done to the plant is more aesthetic compared to physical. This is because the plant looks ugly. On the other hand, the leaves generally drop off at a time when they have already accomplished their tasks.
- In fact, you can provide the plants with the best protection right at the time of planting them. Provide your favorite lilacs with total sunlight and proper ventilation. In addition, you should preferably select species and cultivars that are hardy. Water the plants deeply by supplying the water right on the ground and not the plant. Majority of the cultivars as well as hybrids of S. vulgaris or the common lilac are vulnerable to this disease. Lilacs that are worst affected by powdery mildew include “Henri Martin”, “Mrs. W. E. Marshall”, “Buffon”, and “Marlyensis”. In addition, the variety called S. x chinensis “Metensis” is also very vulnerable to this disease. If you notice a tendency of your lilacs being infected by Microsphaera alni, you should grow the plants at the back of perennially growing plants like lilies as well as ornamental grasses, whose height conceal the disfigurement of the lilacs. Some of the lilac species that possess the aptitude to resist this fungal disease include S. emodi, S. meyeri, S. x diversifolia, S. yunnanensis, and S. x persica.
- Ring spot is a viral disease that causes yellow or target marks on lilac leaves, which drop from the plants untimely. Eventually, the affected plants die. Since there is no cure for this disease, you need to destroy the diseased plants.
- While lilacs are seldom affected by verticillium wilt, this disease may affect plants of this genus when they are growing in soils where people earlier grew potatoes, tomatoes or eggplants. When grown in such soils, lilacs will die sooner or later. Hence, you need to remove the plants to a suitable location before it is too late. In fact, it is difficult to control wilt. Therefore, decide on a suitable place before planting your favorite lilacs.