Specific diseases of ornamental pot crops and protection methods

In addition to diseases that affect many types of pot crops at once, there are those that produce typical symptoms only on a single host. Some specific diseases are listed below; for a complete picture, common diseases already described above are also noted for each plant species. APHELANDRA Leaf spot (Corynespora cassiicola)

Round black or brown spots 10–20 mm in diameter appear on the leaves. Apparently, this pathogen primarily colonizes wound surfaces. Disease development can be prevented by avoiding mechanical damage to plants: spraying with benomyl is carried out if necessary.

Azalea

During the propagation of azaleas, root rots very often develop, with the rooting medium serving as the main source of infection. In addition, root rots caused by Phytophthora species are widespread; B. cinerea settles on damaged tissues, infecting both leaves and stems. Control measures are carried out according to the type of pathogen. |

This disease, which rarely affects azaleas when forced for sale at the end of December, is common on plants grown in greenhouses for planting in open soil. The fungus causes typical leaf changes: thickening and reddening. Sometimes the lesion covers stems and flowers. The reddened tissues are covered with a white bloom of pathogen sporulation. Spores (basidiospores) are released into the air and widely dispersed by air currents. The range of host plants of E. vaccinii is limited to members of Ericaceae. Wild species of this family hardly play an important role in the spread of infection; the main source of the disease is diseased azalea mother plants.

Control. At the first appearance of disease symptoms, the affected parts of the plants must be removed, and then spraying should be carried out. There is little data on the comparative effectiveness of fungicides, but dithiocarbamates (zineb) and copper-containing preparations appear to suppress the pathogen to some extent. Among systemic fungicides, benodanil should be effective, as it suppresses pathogens related to E. vaccinii.

Begonia

Among begonia species, the most frequently grown in pot culture are Begonia rex, Rieger begonia, and large, also flowering tuberous begonias. All these types are affected by the same pathogens: Pythium and Phytophthora species, which cause rot of roots and tubers; Thielaviopsis basicola, the causative agent of black root rot; and Botrytis cinerea, the causative agent of gray mold. On rex and Rieger types, powdery mildew is particularly harmful. Fig. 17.3. Azalea leaf gall (causative agent Exobasidium vaccinii). Affected leaves thicken, deform, and become covered with white bloom.

In addition, bacterial leaf spot and verticillium wilt cause significant damage to begonias. Bacterial leaf spot (Xanthomonas campestris pv. begoniae)

Bacterial leaf spot of begonia: symptoms and spread

This disease is most dangerous for Rieger begonias and tuberous begonias. The first symptoms of infection are small brown spots on aging leaves. As the disease develops, the spots enlarge and gradually cover large areas of the leaf blades.

Severely affected leaves become yellowish-green from the very beginning of the disease, and marginal necrosis and a typical V-shaped sector of diseased tissues often appear. Chlorotic zones are sometimes visible around the necrotic areas. The pathogen is rapidly dispersed by water droplets; a warm, humid environment is favorable for its development. The infection also spreads during work with diseased plants (contamination of knives and hands).

Control:

• Only healthy mother plants should be used for propagation.
• To reduce the spread of infection, water splashing must be avoided.
• Apply pesticides by spraying only when necessary.
• Replace overhead irrigation with another method.

There are no cultivars resistant to bacterial spot, and effective chemical control measures have not been developed.

Verticillium wilt of begonia and preventive methods

This disease causes enormous damage to the Rieger begonia crop. Upon its appearance, plant growth is stunted, and areas of slightly discolored tissue appear on the leaf blades. Symptoms on the leaves gradually intensify, and the plant dies.

The source of infection is infected cuttings and soil. If cuttings are taken from infected mother plants, it is impossible to combat an outbreak of epiphytotic.

Control:

• Verticillium wilt is not a problem if healthy mother plants are available.
• Use of infection-free compost for propagation.
• If the disease appears, it is advisable to cull all affected plants.