Pests And Diseases Of Irises

Clean gardening practices are the best defence against disease but particularly so with bearded irises. As with people, where careful hygiene is observed and the right amounts of food and water are provided, disease is much less likely to strike. It makes sense to keep plants weed-free and tidy up dead foliage where snails and slugs love to lurk.

Japanese irises are resistant to rhizome rot and fungal or viral diseases and, along with Siberian irises, are the healthiest of all irises. But when a fungus does attack Siberian irises, part of the clump dies off. Dig it up, cut away and burn the affected part and soak the remainder in a fungicide solution. If possible, replant it in fresh soil and water it in with the solution. Poor drainage can be the cause.

Californian irises are often similarly affected and, to prevent the disease, dip the plants in fungicide and water them well with the solution. If it's a real problem the bed may need fumigating.

The leaves of bearded irises have a waxy surface that gives the plant some protection from both pests and diseases. However, this also means that sprays won't stick and their potency can be lost. It is wise to add a spreader to any spray to achieve maximum effect.

Pests

Aphids

Aphids of many species are common throughout the garden and the woods and fields beyond. There are over 4,000 aphid species in the world and most of them feed only on a few closely related plants. Adult aphids are tiny-no more than about 1/8 inch long-pear-shaped and equipped with long antennae. Often called plant lice, they come in several colors: green, yellow, red, brown, gray and black. The different stages of aphids may be different colors. They have two tubercles (tube like organs) projecting backward from the top rear of the abdomen. Some have wings and others don't, depending on environment and the species. When food runs short or other conditions are not right for aphids, the females produce more winged aphids to flyaway and colonize better possible sites.
Aphids overwinter as eggs in protected spots on plants. In the spring, the unfertilized females that hatched from eggs give birth to live young that are all female. This is an example of parthenogenic reproduction. Aphids have piercing, sucking mouthparts that they use to suck the plant juices from foliage. They excrete a sugary substance called honeydew that is sweet and attracts ants. In fact, honeydew is so attractive to ants that the ants may care for aphids, tending them as we would tend sheep. The honeydew makes the plants sticky, and it also supports the growth of a black sooty mold that is both unsightly and can cut back on plants' manufacture of carbohydrates if it covers large areas.
When there is extensive aphid damage on foliage, it can look like a pale rash where the aphids have sucked out the plant juices, including some of the green chloroplasts from the cells. Extreme aphid damage may also cause leaves to curl or bend in an unnatural way because the side of the leaf with the most damage has lost the most fluid, and so is less turgid than the other side. The unequal pressures cause the leaves to curl.
Primary aphid damage is seldom a big problem to iris growers. The greater danger is that some common aphids are carriers for iris mosaic virus. When they feed on the iris foliage, they can inject the virus into the plant. This is why it is a good garden practice to monitor for aphids.
Aphids are comparatively simple to control. The easiest way to get them off plants is to spray them in the morning (to avoid wet foliage at night that can encourage fungal disease) with a hard fine spray of water to knock them from the plants. Make sure the spray reaches insects underneath plant parts as well as those in sight. Repeat as needed. Often, this is the only treatment that is needed when you have just a few irises. If aphids persist, a spray of ultra fine horticultural oil or insecticidal soap should take care of the problem. These substances are safe and almost as conservative as spraying with the garden hose. Remember, though, to be sure that you spray all sides of the foliage to reach the majority of aphids.
There also are direct biological controls that involve aphid predators such as ladybugs, green lacewings and aphid-eating gall midges. Be sure to read the instructions or get information from your organic-garden supplier before you use these biological controls. Other conservative substances are pyrethrum or pyrethrins (the former is the natural substance found in a kind of composite plant and the latter are manufactured replicas of pyrethrum); use according to directions if the most conservative methods do not result in good aphid control. Be sure to aim at the pesky aphids and not broadcast insecticidal soap, pyrethrum or pyrethrins, because they also will kill beneficial insects.

Iris borer

The iris borer (Macronoctua onusta) is without doubt the most prevalent pest that afflicts irises. Its normal range is from Maine and eastern Canada, south to the District of Columbia and west as far as Iowa. Its southern limit is not usually south of Tennessee. The iris borer is rare outside of these limits.
This serious iris pest is the larva of a small moth. The larva's body is white shading toward pale pink to light red on the dorsal side, and its head is brown. The moth is less than an inch long with a wingspan of 1.6 to 2.25 inches. The body of this moth is dark at the thorax, grading to lighter brown on the abdomen. The front pair of wings have fine black lines on a dull brown background. The hind wings are tan at the outer edges, shading to off-white near the body. Overall, the borer moth has a plump and furry appearance and, like most moths, generally rests with its wings folded back along the body. The sexes are similar in appearance.
There may be as many as three to four different hatches of iris borers each year, beginning in early spring when the tall bearded iris foliage is only 6 to 8 inches tall. These hatches appear to be a function of the weather. If spring is cold and wet, borers will hatch later than they would in a warm spring.
You will more likely see the signs of the iris borer on iris plants beginning in late spring, usually May and June. At first, look for tiny pinholes made in the leaves near the base of the leaves or wherever the eggs were laid. Then look for young leaves that are notched or have ragged edges. Borers may start at the bottom of the foliage or on the outer leaves of the foliage fans, but then they always go toward the top of the fan and to the central, most tender and freshest leaf. They may leave a little silver trail that you can see in the right light.
You may find one or more borers in the creases of the leaves just outside of the central leaf. Although the larvae are cannibalistic, when there is plenty of tender foliage, you may find several on one fan. They will work their way down through the leaves and into the rhizomes. They may eat their way through several rhizomes before they pupate.
Later in the season, during the summer months of July and August, you are likely to find small piles of frass (insect excrement) or "sawdust" around the base of the iris plants. By this time, the borers may have hollowed out the rhizome.
In mid- to late summer, the larva is about 1 to an 1 1/2 inches in length. If the iris plant is heavily infested, you may also see that the foliage at the base of the plant looks watery and slimy, or you may see plant juices oozing from the edges of young leaves. Iris experts explain that iris borers are omnivorous and eat not only iris tissues but also each other. For that reason, you usually will find only one borer per rhizome.
If you are going to control the iris borer, it is important to be able to identify it and equally important to know its life cycle. The eggs are usually laid by the moth from mid-September through October. The parent moths mate in the leaves and ground debris soon after they hatch from the pupae that developed in the soil. The females lay as many as two hundred eggs in groups of twenty-five to thirty in sheltered places on old flower stalks or leaves and in rhizome cracks or ground debris. The moths are weak flyers that may be found on dark gloomy days or at dusk hovering around irises. Iris borer moths evidently are not attracted to light and so are seldom seen even when they are most active: at night during late September to mid-November. There will be more moths and thus more iris borers in beds where the irises are thickly planted.
Some of the shiny brown pupae may overwinter as well, in which case the adults will emerge, mate and lay eggs that hatch later than those from overwintering eggs. Once the larvae hatch, they climb up the iris foliage and chew small holes in the leaves. Then in a few days, they will begin tunneling into the leaf. They work their way toward the base of the leaf sheathes for about a week. When they gnaw the edges of the new leaves, the leaves bleed, and this is what causes the watery, slimy appearance at the base of the plant.
The larvae grow larger as they feed and soon begin to eat larger pieces from the edges of center leaves, giving them a jagged look. As they feed and grow, the larvae work their way to the base of the iris plant, where they tunnel into the rhizomes from within the leaf sheaths. They eat the rhizome tissue, often hollowing the rhizome out so that it is nothing but a shell.
Borers will also eat their way into flower stalks and even into the flowers. They are especially likely to do that in Louisiana irises. Louisiana irises are their irises of choice, with tall bearded irises second and Siberian irises third. When borers get into Siberian irises, it can be a mess resulting in the loss of the whole clump. Siberian iris foliage is tough and it is more difficult to find the borers because those tender central leaves are more hidden.
In some cases, where there are densely growing rhizomes, the larvae will work their way underground from rhizome to rhizome. That makes diagnosis difficult since you cannot detect the borers aboveground. When the larvae are mature, they pupate in the soil near the irises that have fed them. If you suspect severe borer damage in your iris rhizomes, the evidence will be clear with a little practice. Search for and destroy the larvae, using a pocket knife to get into the rhizomes if necessary. If they have eaten a lot of the rhizome, the plant probably wouldn't have lived anyway. Clearly, one control is not to let iris rhizomes become so crowded that they are touching.
If you are taking care of your irises pretty well and removing the dead foliage in very early spring, that will remove most of the iris borer eggs and thus prevent most of the larvae from hatching in your garden. Monitor your irises well during the spring months to find the larvae that remain. When the borers have gone into the central leaf, you often can run your fingers down that center lead and pinch out borers. Finding the grubs and destroying them will keep plant damage to a minimum. Tear the tip off a leaf where you found a borer so you can keep track of the places you found larvae.
Removing old iris foliage and other debris from the garden before spring arrives is a good way to eliminate iris borer eggs before they hatch. Do not compost iris foliage because of the danger of nurturing iris borers and other pests. In addition to keeping your garden clean, carefully inspect all new plants. A good idea is to soak new plants in a dilute solution of household chlorine bleach (one part bleach to nine parts water), then let them dry out. That should get rid of borers, fungal diseases and other problems.
Some iris growers do nothing at all about borers that get into their irises, but this can be risky since borers can destroy entire iris beds if ignored. In the past, many irisarians used heavy-duty pesticides in their efforts to eradicate iris borers, but the current move is far more conservative and aims to avoid the use of chemicals that may have undesirable side effects.

Iris weevil

Those who breed beardless irises may be troubled by the iris weevil (Mononychus vulpeculus). The adult female of this insect, sometimes called the iris snout beetle because of its long-nosed appearance, pierces the ovary of the apogon iris flower and lays its eggs within. It prefers to lay its eggs in Siberian irises and Iris versicolor; the large blue flag that is native to the northeastern quadrant of the United States. Like other insects, the life cycle of the iris weevil consists of four stages-egg, larva, pupa and adult. There is a single generation each year.
The egg hatches into a short, fat, legless larva that is slightly curved. The larva pupates within the seedpods and, when the seedpod ripens and splits open, the adult emerges. The adult beetle is about 1/5 inch long. It is black on the dorsal surface and the body underneath the wing covers is covered with whitish to yellowish scales.
The adult weevil feeds on flower blossoms, making holes as it goes, and also feeds on seeds and other seedpod tissue by pushing its snout into seedpods. This naturally causes the pod to have irregular corky scars. For some strange reason, this insect pest prefers blue varieties of late tall bearded irises and will avoid white Siberian irises. This insect overwinters in the ground debris near iris plants as an adult.
The easiest way to control iris weevils is to remove and destroy the seed capsules of Siberian irises and I. versicolor. Those who breed these irises can prevent weevil damage by bagging the seedpods with cheesecloth or the equivalent. Old nylon stockings provide a good fabric for bagging seedpods. Again, it is important to regularly inspect your irises for signs of pest damage.

Scale

Scales are a minor pest of irises. There are many species of scale insects and some are serious pests of both ornamental and useful plants. Scale insects are small, about the same size as aphids, and have piercing/sucking mouthparts so that they can feed on plant sap. When these pests occur in large numbers, they can cause the host plants to decline or even die.
There are two main groups of scale insects, the soft scales and the armored scales. Female soft scales move around as immature crawlers, then settle in one feeding spot and are covered by cottony or smooth covers, retaining their legs and antennae. Soft scales, like aphids, excrete honeydew and often attract ants and sooty mold fungus.
Female armored scales lose their legs one to two days after hatching, settle in permanent feeding spots and develop a hard shell that usually is separate from the insect's body. The females breed and lay eggs under the shell. Protected by their shells, the scales are protected from the environment and from sprays. Only in the crawling stage are these insect pests vulnerable to spray controls.
The male scale insects resemble tiny wasps and usually live only long enough to mate. Some scale species reproduce by parthenogenesis, not needing the male insect to reproduce. The offspring of these are, of course, all female.
Fortunately, there are usually enough natural controls, predators of the scale insects, to prevent unusual population buildups. Predators of scale insects include ladybugs, also known as ladybird beetles, and parasitic wasps. These can be bought in increasing numbers of garden centers. Using an insecticide is not recommended for controlling scale since the insecticide will also kill the scale predators and the result will be a buildup in scale population.
Ultra fine horticultural oils applied according to instructions are increasingly being targeted toward scale control. If only a plant or two are severely infested with scale, the easiest thing to do is to destroy those plants. If only a few scales are present, carefully inspect all the foliage and destroy the scale insects by hand, firmly rubbing them off the leaves. Some prefer to use a cotton swab dipped in alcohol to kill scale insects.

Thrips

Iris thrips (Bregmatothrips iridis) are small insects with the usual four-stage insect life cycle. They sometimes will attack irises and then, most often, the Japanese irises. Less than 1/20 inch in length, they are nonetheless easily recognized by the fringe that grows on the margins of both sets of wings. While some thrips are carnivorous and feed on insects and mites, others feed on plants, using their rasping mouthparts to scrape plant tissue from leaf and flower surfaces. While this is unsightly, thrips will not kill irises. The scars they leave look like windburn.
Thrips usually are fairly easy to control. In most cases, you can knock them down with a hard fine spray of water just as you would aphids. Insecticidal soap and ultra fine horticultural oil are two conservative controls for thrips. There also are predatory mites that are commercially available and recommended to control thrips.

Verbena bud moth

Next to the iris borer, the verbena bud moth (Endothenia hebesana) is the most serious iris pest. This pest destroys iris seeds while they are in the ripening pods and so are a particular threat to iris breeders. They are likely to be found on any iris, as they have many host plants including verbenas, goldenrods, snapdragons, penstemons and physostegias. You can differentiate their signs from those of iris weevils by the silken coatings they make on their tunnels between seed capsules. Look for a small hole in the seedpod. There likely will be frass below the hole and a piece of the pupal skin protruding from the hole.
Egg, larva, pupa and adult are the typical four stages that the verbena bud moth, like all insects, goes through. The entire cycle takes forty-five days. Unfortunately, there are four or more broods of verbena bud moths each year and the moths can overwinter as either larvae or pupae. The eggs, laid on the surface of the seedpod, are very tiny and white in color, turning a little darker just before hatching. The small larva, found on or in the seedpod, is pale, hairy and almost transparent with a dark head. The larva is only 1/2 inch in length when it pupates within a shiny brown silk cocoon. After the adult moth emerges, it crawls out of the seedpod, leaving the pupal skin sticking out of the hole.
The adult verbena bud moth is only 1/2 inch long and triangular in shape when at rest. It has a buff-colored head, dark eyes and short antennae. The abdomen is gray and the forewings are gray and brown with purple tinted metallic markings. The hind wings are paler brown, becoming darker toward the rear.
Removing and destroying seedpods is the best control for iris growers. Since there are many host plants, also watch for signs of damage on other flowering garden plants. Deadheading flowers when they fade to prevent seed production will remove places for the verbena bud moth to breed and, in many garden perennials, also encourage reblooming. Breeders who are hoping for seed production will have to bag the desired seedpods with cheesecloth, nylon from stockings or the equivalent.

Nematodes

Nematodes, also known as roundworms or eelworms, are microscopic organisms that live in the soil. There are many species of both good and bad nematodes from a gardener's point of view. There are beneficial nematodes that are used effectively to get rid of lawn and garden grubs. There also are bad nematodes that attack plants, sucking the nutrient-laden plant juices from small plant roots. Nematodes, because of their microscopic size, are often considered a disease rather than a pest.
Dig such a plant up and study the roots, looking for swollen nodules near the root tips.
Nematode control is difficult at best. Since it is well known that water-stressed plants are more likely to have nematode damage, increasing the organic content of sandy soils is one good way to avoid water stress since that will increase the water-holding capacity of the soil. Regular irrigation is another way to avoid drought stress. Avoid the drought-flood cycles of watering and you will lessen the chances of nematode damage.

Slugs and snails

Slugs are closely related to snails and both are gastropods in the phylum Mollusca. Both slugs and snails may eat the foliage of a wide variety of plant species, sometimes including irises.
Both slugs and snails lay masses of eggs in debris, soil cracks and in moist, shady places under plants. They thrive in moist places and are most active at night and sometimes on dark gloomy days. They avoid hot, sunny situations and other drying conditions. During daylight hours, you can find them in sheltered places, under debris or in any place that is dark and moist. They are not active when the temperature is under 50 degrees F.
An easy and successful control is to handpick snails and slugs from your irises at night when they are most active. Take a flashlight and a container with a weak alcohol solution (3 to 5 percent) to anesthetize the pests. Begin a couple of hours after sunset when they are at their most active. Remove them from other nearby plants as well. Combine handpicking with some other controls to successfully protect your irises.
Amphibians and reptiles are fond of these pests. Be sure to protect those that come to your garden. Barriers of copper foil are excellent repellents since slugs and snails are sensitive to some metal ions. The copper foil technique is especially effective in raised beds. Diatomaceous earth, wood ashes and boric acid crystals strewn around plants will protect plants from snails and slugs, but will have to be replaced after rains.
Snail and slug traps are another good way to control snails and slugs. Place inverted grapefruit rinds or moist trap-boards of old wood on the ground in the garden where you see signs of the pests, such as holes in foliage. Be sure to check underneath these traps every morning to remove and destroy the pests. The traditional beer-baited saucers are fine but not reliable controls when used alone. The problem is that, if young tender foliage is nearby; the pests prefer it.
There are snail baits but they may be hazardous. You should be very careful about using baits where children and pets may be around and can get exposed to the poison.

Voles

Meadow mice or voles (Microtus species) can injure irises by chewing on the foliage or the roots. When this happens, it seems to be most common on Japanese and Louisiana irises. Voles seem to like to eat anything green, including plants and also bulbs, rhizomes and tubers. They are rodents that are related to mice; they look like short-tailed mice.
They actively reproduce throughout the year and each litter typically has three to six young. They mature in a little over six weeks and have a short life span, usually under a year.
Voles often travel in mole runs, which fools gardeners into thinking that the moles have done the damage. Although moles are totally carnivorous and eat only grubs, earthworms and other animal life, not plants, they may disturb iris growth by tunneling under plants and disturbing the roots. Voles also make their own system of underground runs and surface paths that they clear and mow. Several voles usually run in one "highway" system.
One of the best controls for voles is the household cat. Owls, weasels and the larger snakes also are fond of eating voles. Controls are otherwise difficult. A sure control is to use wire meshing with spaces no bigger than 1/4 inch to surround garden beds, sinking it 6 inches into the soil and leaving it several inches high on top of the soil surface.
Mechanical mousetraps placed lengthwise along runways and aimed away from the garden can be effective. If you place them correctly, you won't even have to use bait. Hot pepper sauce diluted with water and sprayed on the plants will keep any animals from eating anything, but it washes off in rain and must be replaced.
If voles are running in mole runs, you can discourage or get rid of both by putting dry ice into the tunnels and sealing up every opening you can find. Dry ice reverts to carbon dioxide and, according to some iris experts, both moles and voles will go to sleep peacefully.
There also are poison rodent baits. If you choose to use them, read the directions carefully and use as directed.

Bacteria

Bacterial leaf spot

Bacterial leaf spot (Xanthomonas tardicrescens), also called bacterial leaf blight, can affect many irises, including all of the bearded irises and Japanese and Siberian irises. It also can infect Iris cristata, I. tectorum, I. missouriensis and I. tenax. Not surprisingly, this bacterial disease is easily confused with fungal leaf spot disease. Both occur most of all during foggy and rainy weather. Irises will get bacterial leaf spot most often during mild weather, while fungal leaf spot can occur anytime the temperature is above freezing.
Bacterial leaf spot causes large irregular spots that first appear near the margins on the leaf tips. At first, the spots are just small pale areas. The key to diagnosing bacterial infections is that the spots appear watery at first, then soon turn light brown. These brown spots become larger and develop whitish or grayish centers. The bacterial infection follows the leaf veins down the leaves and the splotches may run together. Bacterial leaf spot splotches are larger and more irregular than fungal leaf spots.
Since there is no cure, prevention is the only thing that will help control this disease. The bacterium is easily spread on garden tools as well as by water splashing on the plants, so beware of using any tools on healthy plants that have been used on infected plants. Wash your hands thoroughly after working on plants infected by bacterial leaf spot. Disinfect tools with a dilute solution (1 part bleach to 9 parts water) of 0.5 percent sodium hypochlorite (household bleach).
Cultural measures to prevent bacterial leaf spot include removing old foliage from the garden in the fall and destroying it. This will minimize a number of potential iris problems. Since the bacteria do not infect the rhizome, transplanting can be helpful.

Bacterial soft rot

Bacterial soft rot (Erwinia carotovora) can be a serious problem in bearded irises. It also has been found in Siberian and Japanese irises. The bacteria are common in soils throughout the world. Experts believe that these bacteria are pathogens that get into iris rhizomes and basal plant parts only through injuries-wounds caused by careless gardening practices or by insects. Iris borers are vectors of bacterial soft rot, so try to keep them under control.
You can tell when an iris has bacterial soft rot by the yellow wilting leaves. If you pull gently on the leaves they will come off the plant. There is soft and slimy rot at the base of the plant that has a distinctly foul odor. The foul odor is the clue that tells you this is a bacterial disease. The rhizomes may have holes in them. The bacteria produce a pectolinic enzyme that acts in the plant tissues by digesting the cementing layer of pectin between cells, causing the tissues to lose both form arid structure. That is the same pectin found in fruit or added to fruits to make them gel as jams and jellies.
Bacterial soft rot is a disease of hot, humid weather and seems particularly prevalent when the temperature is above 80 degrees F and there is lots of moisture at the base of the plant. Heavy soils with low oxygen content and extended periods of rain exacerbate bacterial soft rot problems.
A number of conservative cultural practices will go far in preventing bacterial soft rot infections. If these are followed, it is unlikely that you will have severe problems with this disease.
Irises that are crowded are more prone to bacterial soft rot - this is a very important cultural factor, as good air circulation is very important in keeping irises healthy. Divide and replant irises every three years, especially if they are crowded. Don't grow irises in the shade where they are more likely to be prone to diseases. Let the sun shine on the rhizomes of bearded irises and you will have fewer disease problems.
Be sure that rhizomes are uninjured and dry when planted. Cleaning up and destroying dead iris leaves and the foliage from nearby plants is a good preventive for bacterial soft rot, as well as for iris borers and fungal diseases. Do not compost iris foliage since that might harbor pests and diseases.
Warm, moist conditions encourage soft rot bacteria. Good drainage and good soil structure will help prevent this disease. Handle rhizomes and other roots carefully to avoid injuries by which the bacteria can enter the plants. Raking and weeding are the garden chores most likely to injure irises. Inspect plants often for signs of borers or other insects that can cause injuries through which the bacteria can enter the plants.
Lift any infected rhizomes and cut away all affected tissue. Scrape out all the mushy parts down to solid tissue with a spoon, then place the rhizomes in the sun to dry and callus. Letting tall bearded iris rhizomes set in the sun and dry for several days is a good way to treat any that are divided and ready for replanting.
Household bleach (sodium hypochlorite) makes a good antibacterial solution for dipping rhizomes before replanting them. Make a 2 percent drench of ordinary household 5 percent sodium hypochlorite (1 part bleach to 9 parts water) and dip rhizomes; let them dry and then replant.

Fungi

Fungal leaf spot

There are many fungal and bacterial organisms that cause leaf spots. Didymellina macrospora is a common leaf spot fungus that affects only irises and some of their relatives. It causes small brown spots on the leaves up to 1/4 inch in size. The spots have reddish borders and may have margins that turn yellow. After the iris blooms, the spots enlarge and may run together to form blotches. Fungal leaf spot, like most fungal diseases, is most prevalent in wet weather.
When the humidity is high and when leaves are wet, the fungal spots produce spores that spread by splashing and by wind to other plants. This fungus spends the winter months on old infected foliage and ground debris.
Clean up, remove and destroy old iris foliage and other ground debris in the late winter before warm weather begins. This will remove many sources of fungal infection as well as other pests and diseases. Do not compost this debris. Grow irises in sunny sites and do not let them get crowded. Place plants far enough apart so that the leaf spot fungus cannot readily spread. Good air circulation is another fungal preventive that works very well. Remove badly spotted foliage and destroy it to prevent further spreading of the leaf spot spores.

Fusarium wilt

Fusarium wilt, also known as fusarium basal rot, is caused by the fungus Fusarium oxysporum and affects bulbous irises most of all. This fungus causes what is often called "dry rot" of iris bulbs. Symptoms of this disease include stunted yellow leaves. This symptom reflects the fact that the plant has root problems.
The fungus first infects the roots, producing symptomatic brown sunken spots. Then the roots die and the fungus moves on to the bulbs, where gray lesions occur, then turn pale brown or reddish. Fungal infections typically show definite margins between the healthy tissue and the infected spots. As the infection progresses, there may be mats of white or reddish fungus on the bulbs.
Fusarium wilt is most common in warm climates and in sandy soils. This fungal disease is found throughout the world and attacks other bulbous plants as well, including crocus and gladiolus. Oddly, the problem is most serious on the yellow varieties of bulbous irises.
The best way to avoid fusarium wilt is to dig and get rid of diseased plants, replacing them with healthy stock in fresh soil, since this fungus will live for three to four years in the soil. Avoid damaging bulbs and roots when digging irises. When storing bulbs, make sure they are in a well-ventilated place and stored so that each bulb can be dry and have good air circulation around it. Note that acid soils are more hospitable to fusarium wilt than ate alkaline soils. Before planting, you can dip the bulbs in a fungicide such as finely ground elemental sulfur or in a dilute solution of household bleach (1 part bleach to 9 parts water).

Sclerotium root rot

Sclerotium root rot, southern wilt, southern blight, crown rot and mustard seed fungus disease are all names for a serious disease that is caused by a fungus (Sclerotium rolfsii) carried in the soil. This disease, a crown rot that can be very serious in spuria irises and bulbous irises, is especially prevalent in the southern states of United Sates. It is also found on bearded irises and Pacific Coast native irises. Also known as white bulb rot, it is a major fungal disease for spuria irises. A wide range of plants serve as hosts for this fungus, which grows very fast when it has the right temperatures and moisture. It will thrive on living or dead plant tissue, producing a cottony mass of filaments as it grows.
The first sign of this soil borne fungus probably will be the appearance of slimy rot at the base of the foliage fans and on the growing end of the rhizomes. This rot is caused by an acid emitted by the fungus that kills the living tissue upon which it is growing. Then it grows into the dead tissue. You will also see mycelial webbing of white threads or filaments on the rhizome that will soon cover the entire rhizome. The diagnostic keys are the small round "mustard seeds," fruiting bodies that are tan to brown and appear scattered throughout the mycelium.
Mustard seed fungus thrives in regions noted for mild weather and hot humid summers. Where winter temperatures go to 10 degrees F or below for extended periods of time, it should not be a problem. The exception to this is when the fungus comes in on plants from a warmer climate and does its damage before winter sets in.
If you see only rot and webbing and thus aren't sure which fungus you see, place an affected rhizome complete with rot and mycelium into a plastic bag with a bit of water. Close the bag and look again in a few days. Check for mustard seed fruiting bodies on the mycelium. Note that where there is sclerotium root rot, there also may be secondary infections of bacterial soft rot (Erwinia carotovora) that might confuse the diagnosis. When that occurs, oddly the stink of bacterial rot is not as strong as when it occurs alone - in fact, the odor may be faintly pleasant.
You can prevent fungal diseases, but you can't cure them. Fungicides are preventives and only a few will have any appreciable effect on existing fungal diseases. The most conservative control for sclerotium root rot is to dig and destroy any diseased plants, then replace them with clean plants in new soil. Be sure that the soil drains well and that you do not overwater.
When you buy iris rhizomes, inspect them carefully and, as a preventive measure, dip them in a dilute solution of household bleach (1 part bleach to 9 parts water) and let them dry before planting. This will kill fungal and bacterial organisms as well as insects and other pests.

Rust

Iris rust is caused by the fungus pathogens Puccinia iridis and Puccinia sessilis. The first, a rusty red fungus, occurs commonly on bearded and bulbous irises and also on the species irises Iris fulva, I. missouriensis, I. tenax and I. versicolor. This fungus will spread from leaf to leaf and will overwinter in mild climates. Puccinia sessilis, a black rust that appears late in the growing season, also will infect I. versicolor and some other plants.
These fungi appear as small oval to oblong spots on leaves and stems that are red to dark brown, or as black powdery spots, depending on which pathogen is infecting the plants. The powdery parts of the spots are the spores that you can easily see with a microscope. The fungal lesions may be surrounded by yellow margins. If there are lots of rust spots, the leaves and stems will die.
Like many fungi, rusts are favored by humid climates and moderate temperatures. Rust is a common and serious problem in the southeastern United States and the foggy coastal areas of the West Coast. Rust is uncommon in the Pacific Northwest. Dew, rain, fog, high humidity and overhead watering all encourage the development of fungi, including rust.
The best way to control rust is to take measures to prevent it. Remove and destroy old foliage in the fall. Don't plant new healthy irises in a spot where you previously have had rust problems. Apparently, there is a lot of variation in different iris varieties' susceptibility to rust.

Viruses and scorch

Iris mosaics virus

The worst thing about aphids is that they carry the iris mosaic virus. They are what are called "vectors," the organisms that harbor and transmit the disease. You can easily recognize this viral disease by the streaking or mottling that occurs on the foliage, and also by the stunted growth and distorted flowers that are definitely not up to the quality one expects from irises.
The foliage is mottled or streaked with pale yellow to green areas alternating with the normal green color. Mosaic virus is common throughout the world. Iris mosaic virus is more severe in warm climates and in any places or conditions that favor high aphid populations.
There is no cure or control other than to control the aphids and be aware of what iris mosaic virus symptoms look like. Commercial growers rogue out any plants that begin to show viral signs and destroy them, and you should do the same. You can be sure that commercial growers do everything they can to get rid of any plants that develop mosaic virus. You know you will get healthy, disease-free plants from conscientious growers because they don't want any hint of virus in their plants.
Note that if a garden has some iris varieties that are diseased and you don't remove and destroy them, all the irises will eventually get the disease. But, once the diseased plants are destroyed, you don't have to worry that the virus will be in the garden soil because it can only be vectored by the aphids. This virus is carried in the rhizomes or bulbs and therefore any good control measures call for removing any infected plants as soon as they appear.

Scorch

The cause of scorch in irises is not known, although many horticulturists have studied the problem. Scorch is a pathological condition of bearded irises, but no one knows whether similar conditions in beardless irises are caused by the same pathogen. Scorch is one of the diseases that affects Louisiana irises. Aril and arilbred irises seem to be most susceptible to scorch.
Scorch does not appear to be particularly contagious since it will occur only in scattered spots of large plantings. Scorch begins with withering of the central leaves and within a few days the leaves turn a characteristic rusty red-brown, beginning at the tip and spreading down toward the base. Soon all the leaves are affected. At the same time, the roots rot and die, but the rhizome remains firm for a while after the first signs are noticed. The roots become mushy inside, then dry and hollow. Diagnose the plant at this stage by pulling it out of the ground to inspect the rhizome and roots.
To avoid iris scorch dig up the affected plants and place them in a sunny spot on asphalt paving for a week or two and then replant.
Other iris growers have had good luck if they catch the scorch early and dig the affected plants up, then dry them for about a month until they are ready to grow new roots. If you do this, it may take them two full seasons to bloom once again. In many cases, it will make better sense to discard the diseased plants. Treating the soil with calcium nitrate has helped reduce the occurrence of a similar disease in tulips and gladioli.