Plant diseases

Phytophthora blight is caused by the soilborne fungus. On carnations, symptom expression depends on infection site. Infections occurring on the basal portion of the stem cause a typical Wilting and collapse of the stem or plant. The fungus may also invade the stem at any point on the plant, causing stem collapse and eventual death of the branch. If the infection is on the upper leaves and stem, blighting occurs. The most common symptom of this disease, however, is a Wilting, collapse, and bleaching of the tips of healthy shoots, which resembles bromide toxicity symptoms. This is a common disease of carnations grown in areas where soil temperatures rise above 25°C in the daytime.

Rhizoctonia stem rot is caused by the soilborne fungus and the symptoms on rooted cuttings is a moist greyish-black rot at the soil line, which causes the top of the plant to wilt and die. On older plants the rot may extend a couple of inches above the soil line, inducing bark decay, which is easily rubbed off. Occasionally brown mycelial strands may be seen on the surface of diseased tissue. The fungus causing this disease in Kula, Maui, has been found to belong to a genetically distinct stock, known as anastomosis grouping AG2-2 (15).

Septoria leaf spot is caused by the fungus and the symptoms on leaves and stems appear as light brown spots with purple margins. Small black specks are present at the centre of the spots. These are the spore producing structures of the fungus. Individual lesions may enlarge and coalesce with adjacent lesions to cause death of the leaf.

This fungal disease appears as tan to light brown soft rots on the petals of flower buds and opened flowers. Affected petals soon become covered with the greyish growth of the fungus filaments, which later are covered by a powdery mass of greyish spores. The disease usually occurs when the environment is cool and damp at night, and warm with high relative humidity in the daytime.

Initial leaf symptoms of Alternaria blight of carnation, caused by the fungus are tiny purple dots. When moist weather prevails, the spots enlarge, developing into large lesions with a purple margin and a yellow-green border surrounding a grey-brown centre covered with black spores. Several lesions may expand and coalesce to form large, irregular necrotic areas that eventually kill the entire leaf. The branches are most frequently infected at the nodes and branch base. These infection centres enlarge to form cankers, which eventually girdle the stem, causing the branch to wilt and the girdled portion turn to yellow and die.

Rust, caused by the fungus and is the most common foliar disease of carnation in Hawaii. The first symptoms on leaves, stems, or flower buds are small, slightly raised blisters that eventually rupture, forming pustules filled with powdery reddish-brown spores. A yellow margin surrounds the pustules and, when infections are severe, entire leaves turn yellow and die. Stems may be girdled when several pustules develop around the shoot, resulting in decreased flower production and quality. Plants may be attacked at any stage of development. Disease is favoured by cool nights alternating with warm humid days. This induces dew at night and the formation of a film of water on the leaf surface. Because rust spores require free water for nine to 12 hours on the plant surfaces to germinate and infect, this disease is most severe in open air culture and in plastic film greenhouses, where dew formation is common.

The fungus affects the collar portions of their plants. In the nursery, the infection results in damping-off and is characterized by soil moisture. In the field, the infected plants show wilting. French marigold and dwarf varieties are less susceptible whereas the African Kinds are highly susceptible to the disease. It can be controlled by soil treatment with Captan, Mancozeb, Metalaxyl, and Fosetyl-Al to control this disease.

The symptoms are in the form of black lesions developed on the main stem. Rotting in the collar regions causes the death of the plant. It can be controlled by soil sterilization and controlled release of watering.

The symptoms of this disease include brown necrotic spots develop on leaves, which have enlarged in the next stage of infection. The entire Foliage becomes damaged and results in poor vegetative growth. It can be controlled by spraying fungicides for controlling this disease.

The symptoms of this disease are in the form of whitish powdery growth on the aerial parts of the plant. It can be controlled by spraying Sulfex of 3grams/liter of water to control this effect.