Infectious Diseases of Carnation

Emil F. Guba, Ralph W. Ames.

Growing carnations under glass is an extensive industry in the United States, one with a long history of progress despite ever-present disease problems.

Control of the diseases embraces a sequence of practices associated with each of the stages of culture, beginning with propagation.

Propagation by cuttings is an important consideration because it is the source of most of the disease problems. It offers the means for diseases to spread to places where the stock is sent. The problem is not easily avoided because of the varying conditions of growing and the widespread dissemination of stock.

The mother stock must be kept healthy by rigid controls. The danger of infection must be avoided by discriminate selection and segregation. Outbreaks of systemic diseases in certain classes of stock, even after rooting, are unquestionably linked with the source of the stock. There are ways of meeting this problem.

After the cuttings are rooted, they are transplanted in soil and pinched to make plants with a potentially productive frame. Conditions having to do with water, drainage, physical and chemical properties of the soil, and insect and fungus contamination affect the survival and growth of the young plants. Soil and the insects, weeds, and pathogens it contains may be cleansed by steam or chemicals. Efforts have been made to control diseases with chemicals applied to the soil to disinfest it, to prevent spread from centers of infection, or to be absorbed into the plant to ward off infection or even to cure the plant of infection after attack.

Field growing in the early part of summer is still practiced in some places. It favors stronger growth than inside culture, but it may also expose the plants to weather that favors disease. Plants that are brought indoors may bring infection with them.

Inside culture provides better control over growing conditions, easier control of pests by fumigation, greater security from disease, and economies in labor. Therefore the trend has been toward greenhouse culture entirely.

THE LEAF SPOT diseases mainly destroy foliage. They also attack the stems and calyxes of flowers. They progress from the lowest leaves upward and cause dead spots, which join or expand across the leaf and ultimately wither and kill either the part beyond or the entire leaf. Infection is favored by syringing, overhead watering, excessive moisture in the beds, and moist, stagnant air. Strong, dense growth and close planting favor the diseases. Those conditions can be made worse by long spells of wet weather. The inoculum of the pathogens is transmitted on the surface of the cuttings, especially if the cuttings come from infested growing areas. The susceptibility of different varieties to the diseases varies markedly.

Bacterial leaf spot, caused by the bacterium Phytomonas woodsii, produces spots that are elliptic, barren, brown, and surrounded by a purple zone within .a light-green halo or margin. The spots dry out, but under moist conditions beads of bacterial ooze appear on their surfaces. When the spots are numerous, the leaves twist, wither, blight, and turn completely brown, but the outline of the spots remains clearly apparent. The disease spreads easily. Widespread epidemics have occurred in cycles, one of them between 1930 and 1940.

The disease is controlled by removing diseased leaves, careful bottom watering, and spraying the plants with phenyl mercury acetate (Tag Fungicide or Puratized Apple Spray), 1/2 pint to 100 gallons of water. A spray of zinc sulfate, 3 pounds, lime 4 pounds, and alum 1 1/2 pounds to 50 gallons of water is said to give satisfactory control. The addition of a spreader (Dreft, Penetrol, or Dupont Spreader-Sticker) helps to get better wetting of the foliage.

Fairy ring spot is caused by the fungus Heterosporium echinulatum. Its spots are usually circular, about one-fourth inch in diameter, zoned, ash gray or brown, surrounded by a pale-green halo, and beset with an olive-brown mold, which is most prominently confined to the centers of the spots. The stems break off easily at points of infection. The olive-brown spore stalks arise in tufts from the stomata and bear three- or four-celled brown spores with spiny walls.

Bottom watering to avoid wetting the plants and ventilating and heating to avoid too low temperatures and excessive dampness are advisable for satisfactory control. Spraying with ferbam or copper fungicides is necessary if infection persists. Sanitation of diseased foliage is also important.

Septoria leaf spot, caused by the fungus Septoria dianthi, attacks leaves stems, and calyxes and causes light brown, elliptic spots that have violet or purple borders and unite to form irregular areas. The spots are dotted with black, pin-point spore sacs of the fungus. Control measures are the same as those for fairy ring spot.

Carnation rust is caused by the fungus Uromyces caryophyllinus. It is prevalent wherever carnations are grown. The round or elongate fungus pustules, filled with rusty-brown spore powder, break out on both sides of the leaves and occasionally on the stems. The area about the infected tissue at first is yellowish, then brown.

Infection causes constriction and curling of the foliage, stunting, and death. Varieties vary in susceptibility to infection. The same conditions contributing to the development of the leaf spot diseases apply to rust, and the same cultural practices apply to its control. Careful bottom watering and a frequent dusting with zineb, ferbam, or a mixture of ferbam and sulfur are advised. Dry lime-sulfur, 4 pounds to 50 gallons of water, applied once also gives good results. The spore inoculum is carried on the cuttings into the propagation. Protection from such infection may be obtained by dousing the cuttings in ferbam, 1 ounce to 4 gallons of water, with spreader (Dreft three-fourths to 1 level teaspoonful to each gallon) added before planting.

Gray mold rot, due to the fungus Botrytis cinerea, attacks many kinds of plants. With carnations the fungus attacks the sepals and petals, wilting and rotting them. On the decayed parts it produces a dense growth of mold and spores. The fungus is favored by cool, excessively damp conditions in the greenhouse and mild winter temperatures that are at the low range of temperature for carnation culture. Flowers that are packed for market during the incubating stage of infection show serious gray mold in transit and upon arrival. The fungus grows profusely on carnation debris in the greenhouse. Hot-water heat appears to control the disease better than steam heat. Fluctuations in temperature and rises in humidity encourage the disease. Control is obtained by a drier atmosphere by means of more pipe heat, more uniform distribution of heating pipes, more ventilation, and colder and drier outside weather. Sanitation and the removal of plant debris are helpful.

Bud rot, caused by the fungus Fusarium poae, keeps the buds from opening or causes them to open partly and imperfectly. The inside is decayed and moldy, brown or pink, and usually infested with the grayish-white grassmite, Pediculopsis graminum. Many spores of the fungus are also present on the rotted tissue. The fungus and the mites live together. The mites introduce the spores into the buds. The decay that follows infection provides favorable conditions for the mites. Both mites and fungus infest various grasses, notably Kentucky bluegrass, Poa pratensis, on which it causes the so-called silver spike disease. Excessive dampness and wide extremes of temperature under glass favor bud rot of carnations. Diseased buds should be removed and destroyed. Heating and ventilation should be managed to avoid abnormal temperature and moisture conditions. Fumigants, such as organic phosphate compounds as aerosols, will control the mites. Soil from outside grassland used in resoiling beds should be steam-sterilized as a precautionary measure. Improvements in cultural practices, more general soil steaming, and the less frequent use of new sod have reduced the number of grass mites and the incidence of bud rot.

Stem rot, caused by the fungus Corticium (Rhizoctonia) solani, is the most generally destructive disease of carnations in greenhouse and field when excessive water and high temperatures appear together. Losses from it can be serious in summer. When the fungus causes bad rotting of cuttings in the propagating bench, it is referred to as the cutting bench fungus; its spawn binds the sand into masses. The foliage loses its green color. The entire plant wilts, collapses, and turns straw-colored. The part of the stem in contact with the soil is attacked. Its bark becomes a soft shredded rot, but the underlying wood remains firm. If pulled, the plant breaks off easily at the rotted stem, and the firm root remains in the soil. Threads and brownish knots of fungus sometimes appear about the rotted stem and nearby soil. Even branches in contact with the soil may become infected and wilt independently of the rest of the plant.

The fungus occurs even in virgin soil and attacks all kinds of plants. The carnation is susceptible in all its stages of growth. Cleanliness and cautious watering and airing throughout the growing period are desirable control measures. The use of clean sand, soil steaming, shallow planting to assure that the stems are well above the soil line, shading, and cool temperatures are important. The use of plant bands helps. In the bands the plants grow compact blocks of roots and are planted three-fourths of their depth so that the stem is exposed well above the soil. Without plant bands, it is hard to plant at the proper depth in relation to the soil line.

Blight (leaf spot and canker) is caused by the fungus Alternaria dianthi. It is manifested by brown spots on leaves, flowers, and stems; blighting of foliage; and cankers in stems and branches, sometimes followed by wilting and death. The brown spots are overlaid with a black deposit of spores, which give the blighted parts and plants a blackish appearance. Infection in the blade causes constriction and twisting of the leaf. Often the tips of the leaves are killed. If weather conditions favor a progressive development of the disease, infections appear in the calyx and corolla lobes. Spores lodged in the leaf axils may infect nodes of branches and girdle them. Ordinarily the disease is restricted to the bark and to the outside of the cambium, but occasionally the fungus grows deeper into the tissue of the branch and causes a localized, dry, brown or black rot and the death of the branch above. Black crusts of spores appear on the surface of the cankers, especially if unusual moisture is present. Then, also, injuries from chemicals and pests can contribute to attacks of alternaria blight. Injuries of all kinds favor infection indirectly, although infection may also be direct.