Diseases Of The Alternanthera
( Originally Published 1920 )
The Alternanthera is comparatively free from disease and it is generally considered a hardy plant.
Caused by Phyllosticta sp.
Symptoms. Alternanthera blight was first reported by Halsted* as being very serious in green-houses of the eastern states. It is especially severe in the "cutting" benches. The trouble is characterized by a premature defoliation. The affected leaves coil up and drop off. In an early stage, the leaves become spotted merely, and it is only when the spots become numerous that the foliage drops off. The trouble is usually overlooked, because of the variegated foliage of the host. The cause of the blight is a Phyllosticta fungus, probably the same as P. amaranthi, which attacks the pigweed, Amaranthus retroflexus.
Control. It is probable that spraying with a standard fungicide will control the trouble.
Caused by Rhizoctonia solani Kuhn.
This form of injury is commonly met with in propagating benches of Alternanthera. The young cuttings often rot off before setting roots. On well established plants, the Rhizoctonia fungus is found as strands on the sides of the branches which touch the ground. In this case there is apparently no injury. It seems that the reddish varieties of Alternanthera is covered with more Rhizoctonia strands than are the green or the variegated varieties. For a description of the causal organism and methods of control, see p. 20.
ANTIRRHINUM (Antirrhinum Mafia).
Cultural Considerations. Antirrhinums have be-come important plants, forced primarily as cut flowers. The plants require a light sandy loam compost. In filling the benches, we must avoid fresh and undecomposed manure. It is necessary also to avoid the excessive use of nitrogen. Where this is overlooked, the flowers will have a tendency to "sport" and possess too much yellow color, which is objectionable to the trade. Some growers prefer to add to the soil a liberal application of rock phosphate and finely ground limestone. Antirrhinums thrive very poorly in wet soils. The plants should not be syringed in winter and especial care should be taken in watering on cloudy days. The plants are not injured by a night temperature of 45 degrees F., al-though 48 to 55 degrees suits them best. The day temperature should never run above 70 degrees F.
DISEASES OF THE ANTIRRHINUM
The antirrhinum, although considered a hardy plant, is subject to several diseases, most of which are of economic importance.
Caused by Puccinia antirrhini Diet. and Halw.
Symptoms. The Uredo stage is the one most commonly found. It is manifested as small roundish, reddish brown pustules, usually grouped circularly on the under side of the leaf or on the stem (fig. 40, a and b). The affected tissue becomes yellow. The fungus was first described by Dietel who found the Teleuto and the Uredo stage on specimens collected in California. The fungus is very commonly found to attack snapdragons out of doors. It is also a serious trouble to growers of greenhouse plants. However, the Teleuto stage is not frequently met with. The exact life history of this fungus is as yet imperfectly known.
Control. In the greenhouse, the disease is only prevalent on snapdragon propagated by cuttings taken from outdoor plants. In this case, the dis-ease is brought in directly with infected cuttings. The only remedy known is to use healthy cuttings. The safest is to use plants started from seeds sown indoors.
Caused by Colletotrichum antirrhini Stew.
Symptoms. Anthracnose is a common disease on greenhouse and garden snapdragons. In the green-house, it is more troublesome in the fall and spring. The disease attacks the stems (fig. 40, d) in all stages of development. It appears as a large spot on the stems or lateral shoots, resulting in their death (fig. 41, a and b). The spots are at first dirty white with a narrow border. Soon, however, the center turns black and under conditions of moisture, becomes covered with the acervuli of the causal organism. On the leaves (fig. 40, d) the spots are circular, slightly sunken, at first yellowish green with indefinite outline, and later becoming dirty white or greenish, definitely outlined and limited by a narrow brown border. The spots, when numerous, spread and blend together. The affected foliage shrivels, clings to the stems and dies.
The Organism. The stroma is well developed; the conidia are straight to curved, with both ends rounded. The conidiophores are short, the setæ abundant, dark brown, simple, and mostly straight (fig. 40, e and f).
Control. The disease is often introduced in the greenhouse with infected cuttings. Cuttings should therefore be secured from healthy plants. This disease attacks only the snapdragon. It should therefore be an easy matter to prevent its introduction indoors. If the disease makes its appearance, spraying with Bordeaux mixture should be resorted to. All diseased material should be destroyed by fire.
Caused by Phoma poolensis Taub.
Symptoms. The disease seems to be confined to the tender and growing shoots. It seldom affects the older and more woody stems. Affected parts wilt, and become discolored without showing any definite spotting. Later, however, numerous pycnidia appear on the dead parts.
The Organism.—Stewart has proved by artificial inoculation that the causal organism is a parasite. The writer's investigation of this organism has confirmed the work of Stewart. In 1916 and 1917, a careful study of this disease was made, as it occurred in several greenhouse establishments in San Antonio, Texas. It was proved definitely that the organism is parasitic and that it was also apparently an undescribed species to which the name Phoma poolensis Taubenhaus was given. The pycnidia are minute, numerous, black, with distinct mouths (ostioles). The spores ooze out in a colorless gelatinous rope-like mass. They are small, elliptical, hyaline, and one celled.
Control. The methods of control for this disease should be the same as those used, for anthracnose.
Caused by Septoria antirrhini Desm.
This disease is greatly dreaded by English gardeners. It was first described by Chittenden* who claims that it is very prevalent in Great Britain. Fortunately, it is not yet known to occur in the United States. The disease is characterized by a general blighting and dying of the leaves and branches.
Caused by Verticillium sp.
This disease, although new, is prevalent all over the United States. Little is known of the disease and of the causal organism. The Verticillium, how-ever, may be introduced with infected soil or manure, or with diseased cuttings. To prevent the disease from getting a foothold in the greenhouse, it is necessary to secure cuttings from healthy plants. Infected soils should be steam sterilized or treated with formaldehyde (see pp. 32-43). Spraying in this case will be useless since the causal organism works in the interior of the roots and stems.
RooT KNOT (fig. 40, b), see NEMATODE, p. 28.
ASPIDISTRA (Aspidistra Lurida)
Cultural Considerations. This plant is very easy to grow. It is valued mostly as a foliage plant. It grows well in dark halls and in dwelling houses. The plant requires an abundance of water. It is propagated by division of rhizomes in late winter.
DISEASES OF ASPIDISTRA
The Aspidistra is a very hardy plant. It is subject to the attacks of but few diseases.
Caused by Colletotrichum omnivorum Hals.
This disease causes irregular ragged dry spots on the leaves. The spores are sickle shaped, hyaline, one celled. The setæ are black, elongate, and pointed. The disease may be kept in check by spraying with a standard fungicide. All infected material should be destroyed by fire.
Caused by Ascochyta aspidistre Mas.
This disease is characterized by roundish, whitish spots on the leaves. The trouble is as yet of no economic importance. Little is known of the causal organisms. Of the other fungi recorded on aspidistra may be mentioned Pyrenochaeta bergevini Roll.