Some Fungi That Attack Gladioli

Robert O. Magie.

The modern gladiolus has been developed during the past 125 years by cross-breeding several gladiolus species native to Africa and by crossing many of the hybrids and varieties obtained thereby. It is adapted to a wide range of soils and climate and is now grown in many countries as a garden flower.

In the United States gladioli are grown extensively for cut flowers on hundreds of acres in States bordering the Great Lakes, in New England, in some of the Atlantic and Gulf Coast States, and along the Pacific coast. About 7,000 acres are planted in Florida each year for winter and spring production of flower spikes, which are shipped to all but the most distant sections of the country.

The growing of glads is estimated to be a multimillion-dollar industry in the United States. Bulbs properly called corms are produced principally in areas where the flowers are grown for market because most bulb growers sell flowers as well as bulbs. The corms are sold to flower growers and to thousands of gardeners. Each year millions of corms are transported from State to State and country to country. Often the traffic is two-way and makes ideal conditions for dispersal of insect and disease pests.

Diseases are not only dispersed on corms. Some are also carried from year to year in the corms and cormels. The control of disease, especially the fusarium disease, therefore is much more difficult.

The worst disease is caused by Fusarium oxysporum f. gladioli. That fungus invades the vascular tissue of roots, corms, and leaf bases. It causes a rot of the underground parts of the gladiolus, a yellowing of leaves, and distortion or modification of leaf, stem, and flower growths. In Florida the disease is estimated to cause an average loss of 200 dollars an acre. Some of the best commercial varieties are most susceptible to the corm-rot phase of the disease, which is conspicuous both in storage and in the field.

Two distinct fusarium diseases have been reported, fusarium yellows and brown rot of corms. Apparently in the Southeast only one fusarium fungus is involved. Fusarium infection of corms or roots is accompanied by yellowing or other symptoms typical of a vascular fusarium disease. All corm stocks with corm rot have shown these symptoms. Some varieties resistant to corm rot show "yellows" symptoms as a result of root or vascular infection. Corm rotting is not always a part of the fusarium yellows disease, but the vascular disease symptoms have always been associated with fusarium brown rot.

Symptoms of fusarium infection include bending of the young leaf stalk, cupping of leaf stalk in older plants, and limber flower stem, often crooked just below florets and greener than normal. Curving of the leaf growth is always away from the side of corm showing rot. There is a modification of floret shape, size, and color as a result of slight or recent corm infection. Those symptoms may appear on only one side of the spike.

A dark-pink color in the Picardy variety is a response to partial rotting of the mother corm. Some growers mistakenly refer to that color transformation as a sport or as the result of soil conditions. The commonest symptom seen on most varieties in the field is a gradual yellowing and dying of the foliage, beginning with the oldest leaves. Leaf yellowing is not very pronounced in some of the more resistant varieties.

Symptoms of corm rot may vary greatly in different varieties and in different stocks. Rotting may begin anywhere on the corm surface, but most spots are found at the base next to the core. By scraping lightly at that place, one may find a slight amount of browning. Rotting may involve the core before spreading out along the vascular tissues, or one side Of the corm may rot before the core is completely discolored. In some stocks only the core is rotted.

Rot spots may show up where the corm was cut or bruised in digging, handling, or grading. The spot, usually round or oval, becomes depressed as the rotted tissue dries. The surface often wrinkles and forms concentric rings. The rotted tissue becomes compressed and tough on drying. Rotted corms are mummified or are greatly shrunken in storage and sound like a stone when dropped. When rotting begins after corms are planted, the rotted tissue remains soft and is invaded by other organisms.

Picardy, an outstanding commercial variety, is so susceptible that it is no longer grown on some farms. Nearly Zoo million Picardy corms have been shipped into Florida since 1944, mainly to replace rotted corms. Fusarium infection was found to be carried in the corms; losses were great even though corms were treated with fungicides and planted in uncontaminated land. Few commercial stocks of Picardy have been found to be free of fusarium infection of corms.

The infection often does not show up until after the corms have been planted. Commercial stocks of other varieties usually carry fusarium infection, although losses from rotting are generally less severe than in Picardy.

Fusarium infections tend to remain dormant in the corms. They pass from one crop to the next and break out as rot when temperature and nutrition favor the growth of the fungus or when the natural resistance of the host is weakened by adverse conditions. Volunteer stands of Picardy plants, which seem healthy, are found in fields abandoned because of severe loss from fusarium disease. Some of the stands continue to grow in uncultivated fields for several years. Seeing this, growers have propagated the corms, hoping to find a resistant strain of Picardy. They learned, though, that the corms carry latent infections that are activated when the plants are fertilized for the production of flowers.

The fusarium disease is most destructive in areas with light sandy soils, heavy rainfall, and warm climate. To grow quality flowers on such soils, large amounts of fertilizers are used. But corm rot is increased with the use of nitrogen fertilizers and manures, especially in places where the phosphate supply is low in comparison to available nitrogen. Dried blood, tankage, fresh manure, and ammonia nitrogen are especially undesirable because of their tendency to bring about extensive rotting. These relationships between plant nutrition and disease were discovered by W. D. McClellan and Neil Stuart at the Plant Industry Station.

GROWERS USUALLY TREAT corms with chemicals to control the disease. Many fungicides have been tested in the search for good treatments.

The more effective chemicals, including mercury compounds and trichlorophenates, delay the rotting of infected corms but do not effectively reduce the percentage of corms carrying the fungus into the next crop season. Despite fungicidal treatment, annual losses of corms in the major cut-flower areas range from 5 to 40 percent in susceptible varieties.

Growers used to treat corms only before planting. Because many corms rotted during storage, I investigated the possibility of applying a treatment after harvest and, in 1948, pointed out the advantages of treating the corms immediately after they are"cleaned." Cleaning is the removal of the old corms and roots from the new corms after harvest. The best post-harvest treatment at present is a dusting of the corms immediately after the cleaning operation with a 48-percent formulation of tetrachloro-p-benzoquinone (Spergon).

Corms of susceptible varieties contaminated with the fungus are treated again, immediately before planting, by soaking them for 15 minutes in a solution of ethyl mercury phosphate, 8 ounces of a 5-percent formulation (New Improved Ceresan) in 50 gallons of water. Gardeners and small growers often soak their corms in a solution of Lysol (1 quart in 50 gallons) for 2 to 3 hours just before planting.

Some varieties are very resistant to the fusarium disease, but one or more stocks of some have developed serious cases of the disease because of unusually virulent strains of the fungus. By replacing infected stock with disease-free corms, growers have been able to continue with the same variety with little loss from disease.

Replacement of diseased stocks with healthy corms helps control the disease in susceptible varieties, too.

With fusarium-free stocks, growers have demonstrated on land used for gladiolus every third year, that susceptible varieties such as Picardy and Spotlight may be grown for 2 to 3 years without severe losses of corms. Some stocks obtained from an area in western Washington were found to be free of the gladiolus Fusarium.

C. J. Gould, at the Western Washington Agricultural Experiment Station, suggested that low temperatures of the soil may explain the disappearance of fusarium disease from corms grown in that area during several successive years. Observations in other cool climates suggest that low soil temperatures suppress disease expression but may not eradicate the fungus. Antibiotics produced by soil microorganisms may be one factor in the eradication of fusarium from corms in certain soils.

Replacement of susceptible varieties with resistant varieties is not always satisfactory in growing flowers for long-distance shipping. Those substituted for such susceptible varieties as Picardy, Leading Lady, Corona, and Spotlight are generally less suitable for cutting in tight bud. But growers catering to nearby markets have a wide choice of resistant varieties that produce satisfactory flowers if one or two florets are allowed to open in the field before the spike is cut.

Growers can exclude the fusarium disease from their plantings. We found on a few farms that disease-free corms planted in uncontaminated soil remained disease-free until the introduction of contaminated soil or diseased corms. The usual source of contamination is the latent infection of corms. In 1953 no corm stocks were certified as free of the fusarium fungus. Such stocks are greatly needed as foundation stocks for propagation of corms. Freedom from latent infection may be tested by growing corms for at least 2 years in warm, sandy soil and fertilizing for maximum flower production. Stocks of fusarium-free corms are found occasionally in gardens, and some bulb growers are trying to propagate large stocks of healthy corms of the more important varieties.

No chemical method of eradicating the fungus from corm stocks is known, nor has it been feasible to eradicate the fungus from the soil in commercial acreages. A search has been started for a systemic fungicide that is taken up by the roots so as to kill the fungus in the corms. Such a chemical which would eliminate latent infections would greatly help in disease control.

Infection from contaminated soil can be avoided by fumigating the soil with methyl bromide. That is recommended for growing seedlings, if the soil is contaminated. Some corms of each seedling stock should be tested for disease resistance by growing them in contaminated soil for at least 3 years. The propagating stock, however, should be grown only in clean soil so that latent infections may be avoided.