Diseases of The Aster
( Originally Published 1920 )
Although considered a hardy plant, asters are subject to some important diseases when grown under greenhouse conditions.
Symptoms. This is a very obscure disease, the cause of which is little understood. It has been investigated by Smith, who, however, reached no definite conclusions. The roots of affected plants are apparently normal in every respect. The stems and branches, however, become pale yellow, slender and spindly, and in extreme cases stunted. The leaves too are often stunted and poorly developed. The flower bracts show no change, the calyx (sepals) has a tendency to revert to leaf-like lobes. The color of the corolla changes to a uniform light greenish yellow irrespective of the original color of the variety. In form, the florets of the corolla be-come elongated, tubular, with short lobes at the ends. The stamens have a tendency to abort, the anthers are undersized, producing little or no pollen. The pistil tends to elongate, the stigma too becomes much elongated and enlarged, protruding abnormally from the corolla tube. The ovary and ovules too are elongated and enlarged (fig. 42, a and b). Affected plants produce no seeds. The same disease also attacks the Marguerite, the Calendula, and the African Marigold. The cause of the trouble is unknown. Practically all varieties of asters are equally susceptible. The source of the seed, its storage conditions, transplanting, the physical proper-ties of the soil, are not apparently concerned in the development of this malady.
Control. It is very likely that yellows may have an origin similar to that of mosaic. In the latter case, insects are likely to carry and to spread the virus. The control of all insect pests is therefore recommended. Diseased plants should be pulled out and destroyed by fire. Spraying will be of no benefit.
Caused by Bacillus asteracearum Pava.
The disease is known to occur in Italy where it was described by Pavarino. The trouble is apparently confined to the foliage, the lower leaves usually becoming infected first, then dry and shrivel. Leaf blight is as yet of no importance in this country.
WILT OR STEM ROT
Caused by Fusarium sp.
Symptoms. This disease appears as soon as the plants are set out and persists throughout the growing season. It is, however, most noticeable during planting time and at blossoming. The trouble usually becomes apparent first on the lower leaves. Here the normal color disappears, turning to a dull yellowish green, followed by wilting. This seems to spread throughout the length of the stem although the disease is usually confined to one side of the plant (fig. 42, e). This gives it a very characteristic appearance, since one side of the plant has a dull-green, wilted, blighted appearance and only one half of some of the leaves and flowers are affected at first. When pulled up, the roots and stems of a diseased plant appear perfectly healthy. However, if one splits open lengthwise the stem of an infected plant, he will find that the seat of the trouble is localized in the interior of the woody or vascular tissue, the latter of which will be darkened. Infection in this case no doubt takes place in the seed, at the seedling stage. Although some plants are able to make a little headway in spite of the dis-ease they too finally succumb. The cause of the trouble is a Fusarium fungus, of which little is now known.
Control. Since the Fusarium fungus is a soil inhabiting organism, steam sterilization of the soil at once suggests itself. The seed should always be started on a sterilized soil, and this trouble will be entirely eliminated. Diseased plants should be pulled out and burned, and by no means allowed to find their way into the manure pile. Spraying in this case will be of no value, since the seat of the trouble is confined to the interior of the roots and stems.
Other Troubles Mistaken for Wilt. An injury inflicted by the common white grub (Lachnosterna) is often mistaken for wilt. The latter feeds on the roots, and the result is a general wilting. When the affected plant is pulled up, the grubs will be found in the act of feeding. By careful watching, they may be destroyed before serious damage results to the plants.
Another cause of apparent wilt and stunted growth may be due to the sucking of the root lice. The latter are of a bluish color, and are usually found in large number on a single plant. This pest usually is harbored in the soil, especially where asters are continually grown in the same beds. Changing the soil or sterilizing it with steam will effect a cure.
In Europe, Fusarium incarnatum (Desna.) Sacc. is believed to be the cause of an aster wilt there.
Caused by Rhizoctonia solani Kuhn.
Symptoms. The trouble is at first manifested as brownish spots on one side of the seedlings at the surface of the soil. The lesions increase in size until the seedlings are girdled and topple over. In time, the Rhizoctonia fungus spreads over the fallen plants and forms a mat of mycelia over them.
On older aster plants, a damping off is not produced, but instead the typical Rhizoctonia lesions appear on the stem end and on the roots. For a description of the causal organism and methods of control, see p. 20.
Caused by Heterodera radicicola Muhler.
Symptoms. The disease manifests itself when the plants are about three inches high. The younger portions of the plant produce spindly shoots with dwarfed, disfigured leaves. The color of this growth is yellowish pale to white, the flowers are small and stunted. Such plants are known to florists as "white legs." For a description of the organism and methods of control see Nematode, p. 28.
AZALEA (Azalea Indica)
Cultural Considerations. Azaleas are very sensitive as regards water. They require plenty of moisture, but not so much as to make the soil sogg- They 'demand a cool, shady house and a rather close atmosphere. The varieties forced for the Ea ter market should be kept in a temperature of 45 to 50 degrees F., and those forced for Christmas should be grown under a temperature of 5o to 55 degrees. Six to eight weeks before Christmas the plants 1 re given a temperature of 6o to 65 degrees F. In providing ventilation, cold drafts should be avoided. The best time to re-pot azaleas is after blooming. Neglect in this direction may seriously interfere with next year's bloom. Azaleas are very sensitive and may be injured even by the presence of organic matter of a heat producing nature. This means that the manure in the compost must be thoroughly rotted.
DISEASES OF AZALEA
The literature on azalea diseases is very scant. This means either that the troubles of this plant are still to be investigated or that it is a remarkably healthy one.
Caused by Septoria Azaleae Vogl.
Symptoms. This disease is characterized by red-dish yellow spots on the leaves. It is not of great importance economically.
The Organism. The pycnidia are immersed, globose, depressed, black. he conidia are oblong, filiform, straight or curved,'' 1 to 3 or more septate, and constricted slightly at the septum. The Conidiophores are short and cylindric.
THE BEGONIA (Begonia sp.)
Cultural Considerations. Begonias have become a very popular plant commercially. The tuberous type is extensively grown under glass. Throughout the season, the plants require frequent applications of liquid cow manure. They require an abundance of light and air, but are very sensitive to draughts and to exposure to direct sunlight. The best temperature required is about 65 degrees F. In the summer, the house should be frequently syringed in order to keep it cool.
DISEASES OF THE BEGONIAS
Begonias, although considered hardy plants, are subject to a few important diseases.
Caused by Oidium sp.
Stewart records a powdery mildew attacking the stems but not the leaves of begonia. The trouble appears as a white powdery fungus growth characteristic of all similar mildews. Only the Oidium or conidial stage of the fungus is present. It is not likely that this disease will become troublesome in greenhouses where begonia is grown on a large scale.
Caused by Rhizoctonia solani Kuhn.
The symptoms of root rot on begonia are the same as those described for alternanthera. The fungus also causes a damping off disease on young begonia cuttings.
Root Knot (fig. 43), see Nematode, p. 28.
CALADIUM (Caladium sp.)
Cultural Considerations. Caladiums should never be allowed to become pot bound. They require a medium temperature, plenty of water, ventilation, and drainage. As the growing season is over and the plants lose their leaves, the pots should be laid on their sides and the water withheld sufficiently to prevent growth.
DISEASES OF THE CALADIUM
Caladiums, it seems, are very hardy. The fungi recorded on matured parts of this plant may be mentioned :
Cercospora caladii Cke., Macrophoma surinamensis (B. and C.) Berl. and Vogl., Monilia prunosa Cke. and Mass. ; Spherella caladii (Schw.) Sacc., Uromyces caladii (Schew.) Farl.
CALCEOLARIA (Calceolaria arachnoidea)
Cultural Considerations. Calceolarić are green-house annuals grown for decorative purposes. It requires a soil made of equal parts of leaf mold, sand, and sand loam. The plants require frequent repotting to prevent them from becoming potbound, although the flowers are usually better when pot-bound. The plants require a northern exposure during the summer, plenty of ventilation, and a cool house. A temperature of 70 degrees F. may seriously injure them. Partial shading should be provided, and no water should be permitted to accumulate on the foliage.
DISEASES OF THE CALCEOLARIA
The Calceolaria is apparently a very resistant plant. Halsted, however, records a leaf blight that affects it. The trouble appears as brownish patches on the leaves just about blossoming time. The patches are many sided and seem to be bound by the smaller veins of the leaf (fig. 44). The spots are water-soaked, and transparent when held against any light. The cause of this trouble seems to be a bacterial organism which, however, needs further investigation. The same is true for methods of control.
Cultural Considerations. The canna, although an outdoor plant, is also extensively grown in the green-house for propagation and for decoration. The varieties best adapted for forcing may be mentioned: Queen Charlotte, Madame Crazy, Explorateur. For flowering in the greenhouse it is best to start with dormant plants.
FUNGI RECORDED ON THE CANNA
Cannas seem to be unusually free from diseases. With the exception perhaps of the rust, Uredo canna, the others here mentioned are saprophytes or semi-saprophytes attacking old and weakened plants.
Anthostomella achira Speg., Macrosporium bulbotrichum Cke., Ophiobolus linosporoides Speg., Uredo canna Wint.