Diseases Of Irises

Like other plants in your garden, irises are also prone to various diseases. However, following hygienic gardening practices will go a long way in protecting your favourite plants from diseases, especially if you are growing the tall bearded irises. As in the case of humans, the plants will be less susceptible to diseases if they are kept clean and provided with the correct amount of food and water. It is sensible to keep your plants free from weeds and also clear up all dead foliage, which would otherwise provide a good hiding place for slugs and snails.

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Some iris varieties are tough and can resist diseases to some extent. For instance, the Japanese irises and Siberian irises are considered to be among the healthiest irises, as they can resist root rot and viral or fungal diseases. However, when Siberian irises do come under fungus attack, it destroys a large part of the clump, resulting in the death of that particular part of the plant. When this happens, you should excavate the clump, cut away the affected parts and burn them immediately. As for the remaining part, immerse it in a diluted fungicide solution. Try to replant the healthy part of the Siberian iris in a new, uncontaminated soil and water the plant with the fungicidal solution. Often, bad drainage system may be responsible for the fungal attack.

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Even the Californian irises are affected by fungal diseases very much akin to the Siberian irises. In order to protect the plants from being affected, it is advisable that you soak the plants in a fungicide and water the plants properly using the fungicidal solution. In case the problem is severe, you may require fumigating the bed.

The surface of the bearded iris is somewhat waxy, which protects the plants to some extent from pests as well as diseases. On the other side, it also means that sprays will also not stick to the leaves, thereby losing much of their potency. Therefore, it would be wise to include a spreader in any spray you might be using to obtain the best possible results.

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Bacterial leaf spot

Bacterial leaf blight, also known as bacterial leaf spot (caused by the bacterium Xanthomonas tardicrescens), may affect several iris species and cultivars, counting all varieties of the bearded irises as well as the Japanese and Siberian irises. In addition, this bacterial disease may also affect other iris varieties like I. tectorum, I. tenax, I. cristata, and I. missouriensis. Therefore, it is not surprising that people often confuse this bacterial disease with the leaf spot disease caused by fungi. Both these conditions mainly occur when the weather conditions are foggy and rainy. Most often, irises get this bacterial disease when the weather is mild. On the other hand, the plants can be affected by the fungal disease any time when the temperature is higher than the freezing point.

When a plant is affected by bacterial leaf spot it results in large unequal spots that initially appear close to the leaf margins near the tips. In the beginning, the appearance of the spots is similar to small light hued areas on the leaves. When a plant is affected by Xanthomonas tardicrescens it first causes watery spots on the surface of the leaf, which change into pale brown later. This is the key to diagnose bacterial leaf spot. Gradually, these pale brown spots grow larger and have greyish or whitish centers. The infection spreads downwards following the vein of the leaf and the blemishes may occur simultaneously. In fact, the splotches of bacterial leaf spot are bigger and even more irregular compared to the fungal leaf spots.

As of now, bacterial leaf spots cannot be cured. Therefore, the disease can be avoided by adopting preventive measures. The bacterium responsible for this condition spreads very easily on tools used in gardens and also when the plants are splashed with water. Hence, one should be cautious while using any garden tool on vigorous plants, especially if the tool has earlier been used on plants infected by microbes. Moreover, you should always wash your hands meticulously after working on plants that are affected by bacterial leaf spot. In order to disinfect the garden tools, dip them in a watered down 0.5 percent sodium hypochlorite solution (also known as domestic bleach and prepared using bleach and water in proportion of one part of bleach to nine parts of water).

Other measures that can help in preventing occurrence of bacterial leaf spot including getting your garden rid of all old foliage during the fall and, subsequently, destroying it. When you do this every fall it helps to minimize the potential problems in your garden. As the bacterium does not affect the rhizome of iris plants, you can overcome the problem of bacterial leaf spot by transplanting your irises.

Bacterial soft rot

This condition is caused by a bacterium called Erwinia carotovora and may prove to be severe for bearded irises. In fact, bacterial soft rot also occurs in Japanese and Siberian irises. The bacteria present in the soils are same across the globe. Experts are of the view that these bacteria are basically pathogens that enter the iris rhizomes as well as the parts at the base of the plants through wounds or injuries caused to the plants due to negligent gardening practices. Alternatively, such wounds may also be caused by insects. It has been found that the bacterium responsible for bacterial soft rot is carried by iris borers. Hence, if you want to protect your favourite irises from bacterial soft rot, you should necessarily initiate measures to keep iris borers under control.

If you notice the leaves of your irises turning yellow and wilting, you can be sure that the plants have been affected by bacterial soft rot. When this disease occurs, you will also notice soft and slippery rot at the plant's base and this will be accompanied with a foul smell, which is another sign that the plants have been infected by bacteria. In addition, this may also result in holes in the rhizomes of your favourite irises. In fact, this bacterium produces a pectolinic enzyme which works inside the plant tissues and digests the pectin layer that bonds the cells. As a result, the tissues not only lose their form, but also their structure. In fact, this pectin is akin to that present in fruits or added to fruits with a view to coagulate them while making jellies and jams.

A bacterial disease, soft rot is more widespread in hot and humid weathers and it is especially common when the temperature rises above 80°F and there is plenty of moisture at the plants' base. In addition, heavy soils which are very poor in oxygen content aggravate bacterial soft rot. This disease also worsens when it rains for prolonged periods.

To a great extent, undertaking some traditional cultural practices will help in protecting your irises from bacterial soft rot. If you follow these practices the possibility of the disease infecting your plants will be reduced significantly.

As in the case of many other diseases, planting the irises to close to one another makes them more susceptible to bacterial soft rot. In fact, this is a vital cultural aspect, as good ventilation is necessary for the health of your irises. In addition, you should also make it a practice to divide and replant the irises once in three years, particularly if they are already growing in a crowded manner. At the same time, you should remember not to grow irises in a shaded place, where they are likely to be more vulnerable to diseases. Allow the rhizomes to receive full sunlight, especially the bearded irises. When you follow these practices, it is unlikely that your plants will be troubled with diseases often.

Also ensure that the rhizomes of your irises are not damaged or dry when they are being planted. Clearing up and obliterating all dead leaves and foliage from other plants nearby is an excellent way to avoid bacterial soft rot. This will also help to control iris borers and keep fungal diseases away. Never use old iris foliage to prepare compost, as they may be infected or diseased or contain pathogens and pests.

You should remember that warm and damp conditions are ideal for the spread of bacteria responsible for soft rot. However, having an excellent drainage system coupled with good soil structure will go a long way in preventing this bacterial disease. Moreover, you should be careful while handling the roots and rhizomes of the plants so that you don't cause any damage to them. This is of utmost importance because the bacteria can easily go into the rhizomes and roots through such wounds. The plants are more likely to be injured by gardening activities like weeding and raking. From time to time, you should also examine the plants to see if there is any sign of iris borers or other pests that may also injure the plants, making it easier for the bacteria to go into the plants.

If you notice any rhizome being infected, pull it out and remove the affected parts/ tissues using a sharp knife. At the same time, scratch off all the squashy tissues using a spoon down to the solid parts. Subsequently, put the rhizomes in an open area under the sun to become dry and callus. Allowing the rhizomes of the tall bearded irises to remain in the sun for several days to become dry is an excellent means to cure them of the infection. Replant them once they are dry and ready for dividing.

Before replanting the rhizomes, dip them in diluted domestic bleach (sodium hypochlorite), which is an excellent antibacterial solution. Prepare a 2 percent solution of common household 5 percent sodium hypochlorite in the ratio of one part bleach and nine parts water. Soak the rhizomes in the solution, and dry them before replanting.


Fungal leaf spot

Leaf spots are caused by various different types of fungi and bacteria. For instance, the fungus called Didymellina macrospora is responsible for widespread leaf spots in irises as well as some other plants related to irises. When this fungal leaf spot occurs, it causes petite brownish spots measuring about 1/4 inch on the surface of the iris leaves. These spots usually have reddish edges and sometimes they may have borders that become yellowish later. These spots become larger when the plants are in bloom and may also run alongside to form blotches. Similar to nearly all other fungal diseases, fungal spots are more common when the weather conditions are damp.

When the weather is very humid causing the leaves to be damp, fungal leaf spots generate spores that are transferred to other plants by wind or when you splash water on the affected leaves. During the winter, this fungus lies dormant in the soil debris and old foliage that are infected from before. Therefore, it is advisable that you should regularly clear out, get rid of as well as obliterate old iris foliage as well as other debris on the soil. You should preferably undertake this task in the later part of winter just prior to the start of the warm weather. Here is a word of caution - never use the old iris debris to make compost because it may contain fungal infections and other pests as well as diseases. Also ensure that you plant the irises in sufficient distance from each other so that the fungus responsible for the leaf spots is not able to spread easily. In fact, having proper air circulation or ventilation in your garden is another way of preventing fungal leaf spots. At the same time, get rid of the foliage that has been infected by fungal leaf spots and destroy it to ensure that the disease does not spread to other plants.

Fusarium wilt

The fungus called Fusarium oxysporum is responsible for this disease, which is also referred to as fusarium basal rot. Most commonly, bulbous iris varieties are affected by fusarium wilt. When affected by this fungus, the roots of bulbous irises suffer from a condition usually known as "dry rot". The symptoms of fusarium wilt include yellow leaves whose growth has been stunted. When you notice that the leaves of your irises are becoming stunted and turning yellow, you should know that the plants are suffering from some kind of problem related to their roots.

At first, the fungus affects the roots, causing symptoms like brown depressed spots. Subsequently, the roots die and gradually the fungus travels to the bulbs, causing greyish abrasions, which eventually become light brown or reddish. Usually, most fungal infections exhibit specific borders between the infected parts and the healthy tissues. With the advancement of the fungal infection, white or reddish mats may appear on the iris bulbs.

Commonly, irises are affected by Fusarium wilt when the climatic condition is warm or the plants are grown in sandy soils. Fusarium wilt occurs all through the world and, besides infecting the iris bulbs, this fungus also attacks bulbs of other plants, such as gladiolus and crocus. For some unknown reasons, this fungal disease is most severe in bulbous irises of the yellow variety.

The most effective means to avoid this disease from spreading is to excavate the infected plants and eliminate them at the earliest. Replace the diseased plants with healthy ones planted in fresh, uncontaminated soil. It is important to change the soil, as Fusarium oxysporum has the aptitude to survive in the soil for anything between three and four years. Moreover, you should be careful while digging irises so that you do not cause any damage to their roots and bulbs. The fungus enters the plants through the wounds in the roots and bulbs. At the same time, you should always store the iris bulbs in a place having good air circulation, which ensures that the bulbs remain dry - wet/ damp bulbs are easily affected by this fungus. It is also advisable that you immerse the bulbs in a good fungicide prior to planting them. You may dip them in finely powdered elemental sulfur or even soak them in a watered down solution of domestic bleach containing bleach and water in the ratio of 1:9.

Sclerotium root rot

A deadly fungus called Sclerotium rolfsii that is borne in the soil is responsible for sclerotium root rot, which is also known by a number of other names, including crown rot, southern blight, southern wilt and mustard seed fungal disease. In fact, a crown rot may turn out to be very serious disease in bulbous irises and spuria irises. This fungal disease is particularly very common in the United States' southern states. In addition, bearded irises as well as the native irises of the Pacific Coast are also very prone to this disease. This fungal disease is also called white bulb rot and is prevalent in spuria irises. This fungus inhabits an assortment of plants and it multiplies very rapidly, provided the temperature as well as moisture is appropriate for its growth. Sclerotium rolfsii also flourishes on plant tissues that are dead and produces a cotton-like mass of strands as it continues to grow.

You can know that your iris plants have been infected by Sclerotium rolfsii when you notice a slimy rot developing at the bottom of the foliage fans as well as the rhizome's growing ends. The fungus discharges an acid that causes this rot, which destroys the living tissues on which it grows. As a result, the living tissue becomes dead. When the Sclerotium rolfsii fungus infects a plant, you can also notice mycelial webs of filaments or white threads on the iris rhizome. Soon these mycelial webbings cover or take over the whole rhizome. You can diagnose the problem by the small oval-shaped "mustard seeds", which are basically fruiting bodies, whose color varies from tan to brownish, come to view in a dispersed manner all over the mycelium.

It has been found that the fungus called mustard seed succeeds in areas that have hot and humid summer, especially mild weather conditions. However, this fungus does not create any problem in places where the temperature drops to 10°F or even less for a prolonged period during the winter months. However, there are some exceptions to this too. For instance, fungus originating from warmer climatic regions may infect the plants in such places even before the arrival of winter.

In case you can just see a rot and webbing and are not certain as to which fungus has infected your plant, you can undertake a small test. Dig out the infected rhizome which is full with mycelium and rot and place it in a plastic bag with some water. Seal the bag and examine the rhizome after some days. See if the mustard seed fruiting bodies are present on the mycelium. Find out the precise place of the Sclerotium root rot. In fact, you may possibly be confused while diagnosing the problem, as there may be secondary infections like bacterial root rot (caused by the bacterium Erwinia carotovora). If this happens, the smell of the rot will help you in diagnosing. For instance, the bacterial rot does not have a potent smell like that of the fungal infection - actually, the smell may even be somewhat pleasing.

Fungal diseases can be avoided, but they cannot be cured. In fact, fungicides are just preventives and just a few of them actually are effective in eliminating a disease that already exists. Digging up the infected plants and destroying them immediately is the most conservative and effective means to control Sclerotium root rot. You may replace the infected plants by growing new irises in a fresh and uncontaminated soil. While preparing the bed for the new plants, ensure that the drainage is good and you will not be required to over water the plants.

Prior to purchasing iris rhizomes, it is important to inspect them carefully. Even if you are sure that the rhizomes are clean and healthy, as a preventive means, soak them in a watered down household bleach solution (containing bleach and water in the ratio of 1:9). Dipping the rhizomes in the solution will destroy fungi and bacteria, if there are any, and also other pests.


Pathogenic fungi Puccinia sessilis and Puccinia iridis are responsible for the condition known as iris rust. Puccinia iridis is a rusty red-hued fungus that commonly infects the bulbous and bearded irises. This fungus also affects other iris varieties such as Iris missouriensis, Iris fulva, Iris versicolor, and Iris tenax. Possessing the aptitude to lie dormant during the winter months, Puccinia iridis spreads from one leaf of the infected plant to another. On the other hand, basically black rust, Puccinia sessilis appears during the later part of the growing season of irises and mainly infects Iris versicolor as well as some other varieties.

Generally, these two pathogenic fungi first appear in the form of small oblong or oval-shaped spots on iris leaves and stems, particularly the ones whose color vary from dark brown to red. They may also occur as black crumbly spots, subject to the type of pathogen that is contaminating the plants. In fact, the dusty parts of these spots are spores which are easily noticeable under a microscope. Often, yellow margins may encircle these fungal abrasions. In case, the rust spots are in large number, it will result in the death of the leaves and stems.

Similar to several other fungi, rusts have a preference for humid climatic conditions as well as moderate temperatures. In some parts of the United States, especially the south-eastern regions and the foggy areas of the West Coast, rusts pose a grave problem for iris growers. On the other hand, you will rarely find them in the Pacific Northwest. Rain, fog, dew and high humidity as well as watering the irises from top all promote growth of various types of fungi, counting rusts.

Adopting preventive measures is the best means to control rust. Make it a habit to get rid of as well as destroy all old iris foliage from your garden during the fall. At the same time, avoid planting new irises in the areas which were earlier affected by rust problems. It appears that different varieties of irises have different levels of vulnerability to rust.

Viruses and scorch

Iris mosaics virus

Aphids not only damage irises but they are also the bearers of the pathogenic iris mosaic virus. In fact, aphids are basically known as "vectors" - organisms that not only host, but also transmit diseases. This viral disease can be detected rather easily from the mottling or streaking, which occurs on iris foliage. In addition, the disease can also be identified if a plant has stunted growth and bears deformed flowers, which are inferior in quality compared to the usual striking iris blooms.

When an iris plant is affected by iris mosaic virus, its foliage becomes streaked or mottled having light yellow or green patches that alternate with its usual green hue. In fact, this virus is widespread all over the globe. Infection by iris mosaic virus becomes worse when the climatic condition is warm. Moreover, this disease also becomes severe in conditions and places that promote growth of aphids.

What is worse is that iris mosaic virus cannot be cured or controlled. You can only control this disease by controlling aphids and keeping a watchful eye on your irises and see if they develop symptoms related to iris mosaic. People who grow irises commercially pull out all plants that start showing signs of viral infection and destroy them immediately. Therefore, it is advisable that you too should adopt the same method, because the commercial iris growers will do anything and everything that is necessary to eliminate the plants that are infected by mosaic virus. Moreover, while buying new plants, you should preferably acquire them from conscientious growers, because you can be sure that they will provide you with plants that are not only healthy, but also free from diseases. Such iris growers do not want their plants to have even the slightest hint of any virus.

It is important to note that in case some irises in your garden are affected by diseases and you do not take any initiative to get rid of them, soon all the other healthy irises will also become diseased. On the other hand, if you remove and destroy the diseased plants immediately when you notice them, you need not be worried about the iris mosaic virus spreading to the soil in your garden, as only aphids can carry the pathogen. As the iris mosaic virus is transmitted through bulbs and rhizomes, it is important to adopt effective control measures, which involve removing and destroying the infected plants immediately when they appear.


Despite the fact that several horticulturists have undertaken studies concerning scorch in irises, they are yet to ascertain what is precisely responsible for this problem. Plainly speaking, scorch is a pathological condition related to the bearded irises. However, thus far no one is aware if the same pathogen is responsible for a similar condition that affects beardless irises. In fact, scorch is a major disease that distresses Louisiana irises. It has been found that arilbred and aril irises are most vulnerable to scorch.

Apparently, scorch is not a very infective condition, as this disease generally occurs in dispersed spots in big iris plantings. In the initial stages, scorch causes the central leaves of iris to wither and in a span of some days, these leaves become rusty reddish-brown. The changes start at the tip of the leaves and gradually spread downwards to their base. The entire leaves of the plant become affected rapidly. Simultaneously, the roots also rot and eventually die. However, the rhizome continues to remain firm for some time even after the initial signs of the disease become visible. At first, the roots turn soggy inside and subsequently dry out and become hollow. Make a diagnosis of the plants during this stage. You can do this by excavating the affected plants from the ground for inspecting the roots and rhizomes.

If you want to prevent iris scorch from damaging your favourite plants, excavate the plants that have already been affected and place them on asphalt paving in a spot receiving bright sunlight. Allow the plants to dry for a couple of weeks and replant them again.

Iris growers usually consider themselves lucky if they detect the scorch quite early on the plants in their garden. This enables them to pull out the affected iris plants, dry them in the sun for a month or so till they are all set to grow fresh roots. When one takes this measure, the new plants may take as long as two full season to flower again. Therefore, in many instances, it is sensible to get rid of the diseased plants and grow new plants in a fresh, uncontaminated soil. However, before planting new irises you need to treat the soil using calcium nitrate, as doing this helps to lessen the recurrence of the same disease. In fact, this practice has been found to be effective in the case of gladioli and tulips.

History of irises
The anatomy of irises
Irises in the garden
Propagation of irises
Landscaping with irises
Pests of irises


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