Pests, Diseases, And Disorders Of Herbs

Herbs that are used to treat numerous health conditions are themselves susceptible to the plant diseases that inflict many other garden plants. While some care is required to grow garden plants to keep them healthy, it is essential to take special care while growing herbs since they are primarily cultivated for culinary or remedial reasons. When herbs are grown under glass, ventilation and hygiene are especially vital while cultivating herbs. In addition, you may help the herbs to avoid fungal infections by getting rid of the dead leaves and flowers as well as by using gummy traps to catch flying pests prior to the temperatures becoming extremely high to take initiatives for biological controls. At times it may be necessary to spray insecticides and in such instances ensure that you only use those insecticides that are safe and suggested by those who supply biological controls. In addition, while spraying insecticides, it is also important to follow the instructions meticulously. It is also advisable that you immerse the pots that have been brought under glass in biological solutions with a view to eliminate any slug or snail that may be present.

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Aphids are known to be one of the most disparaging pests that invade the cultivated plants in the temperate climatic regions. In effect, they have turned out to be the enemies of the farmers as well as gardeners across the world for the extent of the damage done by them to the cultivated plants. However, when you look at aphids from the zoological viewpoint, you will find them to be among the most successful groups of organisms. Partially, their success is owing to some of their species' ability to reproduce asexually.

Plants that have been damaged by aphids may have an assortment of symptoms, for instance, lower growth rates, spotted leaves, undersized development, yellowing, coiled leaves, drooping, burrowing, low yields and even untimely deaths. Taking away the plant's sap results in absence of vitality in the diseased plant. Also, the saliva of aphids is noxious to the plants. In effect, it has been found that often aphids pass on organisms that cause diseases to the plants, such as viruses to the plants which they inhabit. Especially, the green peach aphid (biological name, Myzus persicae) is known to be a vector or carrier for over 100 plant viruses. Similarly, the cotton aphids (botanical name, Aphis gossypii) generally taint the papaya, sugarcane and peanuts with viruses. It may be noted that aphids have been responsible for spreading late blight (scientific name, Phytophthora infestans) amongst potatoes during the Irish potato famine that occurred in the 1840s.

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On the other hand, the black cherry aphid or the cherry aphid (biological name, Myzus cerasi) is liable for various leaves curling of the cherry trees. This symptom can be easily differentiated from 'leaf curling' caused by the species of Taphrina fungus owing to the presence of aphids underneath the leaves.

It may be noted that the harm or diseases caused to the plants, especially the commercial crops, have led to the spending of huge amounts as well as great efforts in trying to control the actions of aphids. Therefore, it is advisable that you control the growth, spread as well as destruction using insecticidal soft soaps, or direct a potent, fine stream of water onto the part swarming with the pests to knock off the aphids from the plants.

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Gray mold (Botrytis)

Gray mold, also known as Botrytis blight, is known to be a gardener's nightmare. In fact, this is a widespread plant ailment, particularly non-woody plants that are grown under humid conditions. Gray mold may be a major problem in the greenhouses, especially when the dampness is high, while the temperatures are reasonable. This disease generally infects tomatoes and, sometimes, cucumbers too. At times, this disease is also a problem on decorative plants, especially the African violets.

It has been detected that Botrytis blight distresses the plants in several ways. This disease may result in the collapse and can weaken the seedlings, cause disfigurement or blotched to blossoms, lead to the rotting of the fruits, stem and the plant crown as well as afflict the shoots. The first symptom of this common plant disease is generally a water-soaked spot on the plant that subsequently makes the tissue soft and watered-down. The inflicted parts of the plant gradually droop and collapse. In case the humidity level stays high, you will notice that a greyish-brown outside layer or mesh of mycelium of fungus threads as well as spores developing over the surface of the bowed tissue.

The infected parts of the plant develop numerous spores that are easily blown or splattered on top of healthy foliage. In case there is a thin layer of moisture and other conditions are encouraging, germination of the microorganisms as well as contagion may occur only within a few hours' time. In addition, sclerotia (a vegetative, resting food-storage body in specific higher fungi, made up of a dense mass of toughened mycelia) may develop on the plump parts of the stems and fruits. The sclerotia are hard-wearing resting structure that allows the fungus to stay alive at times when the conditions are not conducive for its growth. The sizes of the sclerotia differ and may be up to one-fourth of an inch and are generally black and evened out. In fact, sclerotia are always not noticeable, but may be entrenched in decomposed tissue or covered with soil and other wreckage or plant remains.

The film or thin covering of moisture is essential for the germination of the spores and infecting the plants. Hence, gray mold normally has a preference for humid and comparatively cool climatic conditions. It is possible to bring down the levels of humidity by opening up plantings while cultivation, by leaving greater spaces between the rows and plants or by means of pruning. This helps more free movement of air through the plants and reduces humidity. When plants are grown in greenhouses, gray mold can be brought under control by making provisions for excellent ventilation. It may be noted that the gray mold fungus flourishes on plant remains.

In addition to proper ventilation, sanitation is another vital aspect for controlling the germination and growth of gray mold. Therefore, it is important to get rid of the fallen leaves and dead plants from the greenhouses and set ablaze, buried or leave them at some place where they will dry quickly. By doing this, you will be able to lessen the amount of contagious substances in the area.

Leaf miners

Generally, leaf miners feed in the leaves or needles that results in tunnelling or digging injuries on the leaves. In effect, numerous types of insects as well as the larvae of moths (Lepidoptera), sawflies (Hymenoptera), beetles (Coleoptera) and flies (Diptera) have assumed this habit for their survival. Majority of these insects spend their entire larval period feeding in the leaves. While a number of them also develop from the larva stage to pupa (pupate) in the leaf mine feeding on the leaves, there are others whose larvae, when full grown, cut their way out of the leaves and drop on the soil to develop into a pupa.

In fact, occasionally the leaf miners are categorized depending on the type of mines they create in the leaves. For instance, the serpentine leaf mines meander like a snake across the leaf slowly broadening while the insect grows. Blotch leaf miners are more widespread and they are normally erratically curved. The tentiform leaf miner is a sub-group of the insects that create swelling stained type of mines that bend upwards something akin to a tent as the tissue of the damaged leaf dries up. Usually the mining prototypes are generally a mishmash of what has been described above. For instance, the insect species initially create serpentine mines, but in the end they enlarge the leaf cavity of a splotched mine.

Normally, the areas of the leaves that the insects have mined gradually dry out and die. While the damages caused by the leaf mining insects may appear to be ugly, these insects are seldom known to affect the health of the entire plant. In addition, majority of the leaf miners have significant natural controls that usually put a check on their population prior to any excessive damage is done to the plants.

Damages done by insects that engage in leaf and needle mining may sometimes apparently look like the symptoms caused by leaf spotting fungi or different abiotic problems. These can be differentiated by separating the mottled part. In case the leaves have been harmed by insects, there will be a hollow area in the leaf or the needle, which may reveal the insect and/ or its droppings. On the other hand, the leaf spotting fungi result in the collapse of the affected areas and there is no tunnelling whatsoever.

Before concluding, it may be mentioned that the larvae of an assortment of moths, flies and beetles survive by mining through leaves of various plants. In order to check this menace, it is advisable to get rid of the affected leaves and destroy them immediately. In addition, you should spray the herbs regularly with water to keep these insects away.

Spider mites

Spider mites belong to the Acari mite family Tetranychidae and include as many as 1600 species. In fact, spider mites are quite widespread pest problems affecting several plants grown in the yards and gardens. These mites damage the plants by feeding on the plants, especially ingesting the sap, with their mouthparts that resemble whips causing bruised cells. Generally, the damaged parts of the plants are noticeable as they are marked with several minute, pale spots making the plant appear speckled to some extent.

The leaves are discoloured when there is an acute swarming of the insects, making the plant look deep gray or bronze. Eventually, the leaves as well as the needles may turn out to be seared and drop from the plant. Very often, spider mites kill the plants they invade or inflict grave stress on them.

Spider mites are included in the class of arachnid, closely related to insects, including spiders, scorpions, ticks and daddy-longlegs. The spider mites are so minute that it is normally difficult to see them with the naked eye. Depending on the species of the spider mites as well as the seasonal changes in their look, the colors of the spider mites vary from red and brown to yellow and green.

Several species of spider mites create webs, especially when their population is very high. Such webbings provide the spider mites as well as their eggs with protection from their natural enemies as well as the changes in the environment.

Hot and dry climatic conditions are ideal for the spider mites to thrive. In order to control the menace caused by spider mites, it is advisable to get rid of the webs created by them on the leaves as soon as you notice them. In addition, you also need to spray the plants with insecticidal soap and water or, alternately, use biological control by introducing a predatory mite called Phytoseiulus.


Rusts are basically a type of plant disease spread by disease-bearing fungi belonging to the class of Pucciniales. Thus far, as may as 7,800 different species of such fungi have been identified. In fact, rusts have the ability to damage an assortment of plants, including their stems, leaves, fruits and even seeds. When any plant is affected by rusts, it can be seen as a colored powder made up of minute aeciospores that land on vegetation pustules or uredia that develops on the surfaces underneath. Yellowish-orange or brownish bristles or ligulate formations known as telia grow on the leaves or surface from the bark of woody hosts, for instance, the Juniperus species, during the later part of spring or in the early part of summer. These telia result in the development of teliospores that subsequently germinate into aerial basidiospores, which spread all over the plant infecting it further.

When the humidity level is high and the air circulation is poor, blotches of rust appear on the affected parts. In order to control this problem, it is essential to provide the herbs with adequate space and sufficient ventilation. At the same time, remove the herbs that have already been infected and never replant the same species for a while.


Whiteflies are basically tiny hemipterans that are the only member of the family Aleyrodidae. Thus far, over 1,550 species of whiteflies have been identified. These small insects generally feed on the underneath of the plant leaves.

In greenhouses, whiteflies can be controlled best by making use of biological control - i.e. utilizing another insect that eliminates them and, thereby, does the job for you. In addition, if the temperatures are low, it is advisable to enhance the circulation of air, dangle sticky traps and also spray the plants with insecticidal soap. When the plants grown under glass are affected by whiteflies, use a biological control by introducing a parasitic wasp called encarsia.

It may be noted that encarsia is a tiny parasitic wasp that lays its eggs in the deposits of whiteflies where the larvae develop and, thereby, eliminates the invading insects. The larvae grow into adults that find additional scales to parasitise. It is interesting to note that when the larvae have done their work (in other words, eliminated the whiteflies) there is no food left for them and, hence, their life cycle ends. In case the invasion of whiteflies reoccurs, you will be required to introduce encarsia once again to combat the menace.

It is an excellent plan to once again use some covering over the ventilators and doors if you wish to keep the whitefly eliminator (encarsia) unharmed inside the greenhouse where they will continue to deal with the whitefly menace. In addition, you may also use insecticides even when the biological controls are in place. However, it is advisable that you use the soft insecticidal soap prior to introducing the biological controls, lest they too are harmed by the insecticides.


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