Gladiolus Diseases

F.L. Pfleger and S.L. Gould
Plant Pathology

Gladiolus, commonly called "glads," are frequently planted in home gardens or field grown for cut flowers. Gladiolus plants and corms are susceptible to diseases caused by fungi, bacteria and viruses. It is best to avoid diseases whenever possible by starting with healthy stock; however, if problems do occur, learn to identify disease symptoms so the proper management steps may be taken. Integrated pest management for gladioli includes proper handling, harvesting, and storage of corms. Chemical applications may be necessary as well in some situations.

Common Gladiolus Diseases

Scab

Scab infection is caused by the bacterium Pseudomonas marginata. The infection on corm husks appears as elongated lesions with black rough margins. Early corm infections start as pale yellow, water-soaked spots. As the disease progresses, circular lesions turn brown and sunken with distinct raised margins (see Figure 1). In some cases, a gummy exudate produced by the bacteria may glue husks to the corms. The bacteria may spread from infected tissue in splashing water or by bulb mite feeding.

To minimize scab infection, remove and destroy infected corms or plants. Follow cultural management recommendations and chemical recommendations in Table 1 if needed.

Figure 1. Scab-infected corms with sunken,
circular lesions with raised margins.

Fusarium Rot and Yellows

Corms infected with the fungus Fusarium oxysporum f.sp gladioli may produce symptomatic plants or develop dry rot in storage. Plants growing from infected corms may develop arching young stalks or premature yellowing of leaves and faded flower colors. Often plants are stunted and fail to bloom. The corm rot, not always visible externally, is often restricted to the corm base. When the corm is split in half, there may be dark-colored streaks that extend from the corm base through the flesh. In storage, corms may develop dark spots on the surface or in severe cases the entire center may be black and decayed.

Management includes removal of infected plants and corms with obvious decay. Follow good harvesting and storage procedures. Fungicides may be used to dust corms before planting. The use of high nitrogen fertilizers tends to increase corm rot development.

Penicillium Storage Rot

Wounded or bruised corms in storage can be infected by the blue mold fungus, Penicillium sp. Corm surfaces may be roughened by concentric wrinkles or a rot may appear as reddish-brown sunken spots. A green-blue mold grows over lesions.

Often this rot is associated with harvesting in wet weather or improper curing of corms before storage. Fungicides may be used to dust corms before planting or storage.

Leaf Spots and Blights

Botrytis leaf spot and blight

The fungus Botrytis sp causes leaf and flower lesions, stem rot and corm rot. The leaf lesions vary in size and shape and often have brown or gray centers covered with gray masses of spores. As the disease progresses, the tips of the leaves or the stem may turn yellow and die.

Infection of the flowers usually begins near the edges of the petals and appears as water-soaked spots. The spots enlarge rapidly and the flowers become slimy, turn a brown color, and begin to droop. Under moist conditions the flowers are soon covered with a mass of gray spores.

Infection of the stem near the soil line results in a basal stem and corm rot and the outer leaves of the plant turn yellow and die. The fungus may produce small, black, hard structures (sclerotia) on dead plant tissue and corms. These sclerotia serve as survival structures for the fungus and can remain in the soil for years.

Symptoms on infected corms include sunken, round, green-brown lesions. The lesions may vary in size from pin-point to ½ inch in diameter. The inside of the corm may be partially rotted with brown strands radiating from the center or have a punky, spongy decay. Badly rotted corms are usually lightweight and may be covered with sclerotia.

Management of Botrytis sp may involve applications of protective fungicides (see Table 1). The plants should be sprayed during or prior to cool, rainy weather since these environmental conditions are conducive to disease development. Fungicide rotation is necessary to achieve good levels of control and prevent the development of fungicide resistance. Remove diseased plants. Harvest corms during dry weather as early as possible.

Other fungal leaf diseases caused by Stemphylium sp and Curvularia sp occur during wet seasons. Stemphylium sp causes small light-green to yellow leaf lesions with a distinct red center. Curvularia sp causes characteristic diamond-shaped lesions with yellow borders. In either disease, infected tissue should be removed and destroyed. Fungicides may be used to protect healthy tissue from infection.

Stromatinia Corm Dry Rot

This corm disease, caused by the fungus Stromatinia gladioli, is found during periods of cool, wet weather. Leaves produced from infected corms turn yellow prematurely and die. Small, red-brown, sunken lesions develop on the corms. When an infected corm is cut in half, dark streaks can be seen radiating out from the core to the surface of the corm. The fungus produces sclerotia (over-wintering structures) in infected tissue. Often plants are infected in groups as the fungus spreads from the original infected plant.

To manage this disease, discard infected corms, plant only healthy corms in well-drained soils and in the fall harvest corms during dry weather. Treating the soil and corms with a fungicide prior to planting may be beneficial. Do not replant gladioulus corms in infected soil.

Virus and Phytoplasma Disease

There are many viral diseases that produce a wide range of symptoms. Leaves may be streaked, spotted or mottled (mosaic). Flowers may be distorted and plants may be stunted. One example, White Break Mosaic Virus, has symptoms most evident on flowers. The contrasting white or yellow streaks in normally dark colored flowers is accompanied by crinkling and other distortions. In some varieties, corms are warty.

Figure 2. Warty appearance of
corms infected with virus.

Light streaking on leaves and flowers produced by feeding of the thrips insect can be confused with a viral disease. Thrips damage usually occurs uniformly in a planting, while viral symptoms generally are on scattered plants only.

Aster yellows disease, caused by a phytoplasma organism, causes distorted, twisted flower spikes. Flowers may remain green and plants may be stunted. Aster yellows is spread by leaf hopper insects.

Most viral and phytoplasma diseases are spread by insects, vegetative propagation or nematodes. Remove and destroy infected plants and corms. Early season insect control may also help avoid introduction and spread of disease. There is no chemical control for viral or phytoplasma diseases.

Cultural Management

Many disease causing organisms are carried on the surface or interior of corms. An easy way to avoid gladiolus diseases is to purchase healthy corms each year; however if you want to save corms over the winter, they must be handled properly. Old or newly purchased corms should be inspected carefully before planting.

Spring Treatment

It is important to start with healthy corms. Examine each corm for evidence of decay or storage rot (see figure 1). Remove the husk to make inspection easier since this does not injure the corm. Destroy any corms that are badly damaged. These corms will not produce satisfactory plants and will only spread disease.

Summer Care

Diseases which originate in infected corms cannot be controlled with fungicidal sprays during the summer. Dig up and destroy abnormal, stunted or discolored plants. Since some diseases are transmitted to healthy plants by insects, an insect control program may help prevent the introduction and spread of disease.

During rainy periods which last for several days, leaf blight diseases may develop. When these conditions occur, protective fungicides may be used to prevent foliar or flower infections (see Table 1). When spraying gladioli with protective fungicides, improved coverage and thus additional protection may be obtained by adding a spreader sticker or a detergent to the sprayer.

Fall and Winter Care

Proper fall and winter corm treatment is necessary for keeping healthy corms. Dig corms about 4 to 6 weeks after flowering and preferably before the foliage normally turns yellow. Dig carefully to avoid injuring corms since wounds are often entry points for disease organisms. Remove and destroy plant tops immediately.

After digging, place corms in shallow trays or screens in a well-ventilated area. Keep at 60-70 degrees F for 2-3 weeks. Sometime during this period remove old corms and cormels. Sort out and destroy corms with signs of decay or viral symptoms (see Figures 1 and 2). Store clean corms in paper bags at a temperature of 35-40 degrees F. Inspect corms during the winter and discard any with obvious signs of storage rot. (See fact sheet "Storing Tender Bulbs and Bulblike Structures", FS-1117).

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