Anthracnose | Bacterial blight | Bacterial leaf spot | Bacterial wilt | Nematode decline | Leaf cutting ants | Snails and slugs
Anthracnose is caused by the fungus Colletotrichum gloeosporioides. It is of minor significance to the anthurium industry in the Caribbean. However, if left uncontrolled, this disease could cause significant cut-flower loss. Symptoms start as tiny brownish spots on the flower spadix. During high humidity these spots enlarge, appear water-soaked and turn necrotic. Sometimes the entire spadix will turn black as lesions coalesce. The shape of most lesions is, however, angular due to the shape of the spadix tissue. As the disease becomes more severe, masses of orange coloured spores form on necrotic areas. Leaves and spathes are rarely if ever infected.
Mancozeb (Manzate 200) and benomyl (Benlate) at the recommended dosages can easily control the disease at an economic level. Some other fungicides that can be used include chemicals such as Tilt, Bayleton, Baycor, and Prochloraz, but they are a bit expensive. It is worthwhile to note that several anthurium varieties grown by Caribbean farmers are indeed resistant to Anthracnose disease.
Bacterial blight disease caused by Xanthomonas campestris pv. dieffenbachiae (= Xanthomonas axonopodis pv. dieffenbachiae) affects a broad range of ornamental and edible aroids including anthurium, Colocasia (taro), Aglaonema, Syngonium, Xanthosoma, Dieffenbachia, Epipremnum, Dracaena, Alocasia, Spathiphyllum, Rhaphidophora, Caladium, and Philodendron. Bacterial blight disease of anthurium was first reported in Brazil in 1960 and Hawaii in 1971. In the humid tropics, high rainfall coupled with year-round high temperatures increases the severity and spread of bacterial blight disease making it a very important impediment to the cultivation of anthurium in the Caribbean.
The disease became a major problem in Hawaii in the 1980s and caused major decline in the production of anthurium there in the 1980s. In 1982, bacterial blight disease was accidentally introduced from Venezuela to Guadeloupe, causing severe decline of anthurium grown under traditional and greenhouse methods. This decline caused significant losses to commercial anthurium growers and was identified as an important limiting factor for the development of the crop in Martinique, Guadeloupe and indeed the rest of the Caribbean.
The disease was first reported in Jamaica in 1985 and along with the burrowing nematode, Radopholus similis, wiped out the anthurium industry in Jamaica. It was first identified in Trinidad in 1986 and has been responsible for the decline of the anthurium industry from 16 large anthurium farms in the early 1990s to only five large farms at present.
Bacterial blight disease has now been reported in most anthurium producing countries including the mainland United States (Florida, New Jersey and California), the Netherlands and all other anthurium producing Caribbean islands as well as South America, Tahiti and Taiwan.
Foliar symptoms usually start at, or close to the leaf margins and occasionally in the centre of leaves. The first sign of infection is the appearance of irregular water-soaked spots on areas on the underside of leaves and spathes and faint chlorosis when viewed from the upper side. As infection proceeds, the spots become brown or black at the centre and bright yellow at the edges. Bacterial blight damage spreads quickly and will eventually cover large sections of the leaf or spathe. Infections often become systemic as the bacteria enter the vascular system of the plant. A characteristic symptom of advanced systemic infection is discoloration of the vascular system. In most cases, systemic infection leads to plant death.
Foliar bacterial blight disease can severely decrease farm productivity, and hence the profitability of anthurium cultivation. Infected leaves are removed, sometimes to a bare stem to reduce further disease spread. The reduced photosynthetic capacity as a result of leaf removal causes plant stunting. In cases where most of the leaves have been removed, flowers are not produced at all, plants take longer to mature or are unmarketable, being either deformed or small.
Systemic infection can occur independently of foliar infections. In such cases, the pathogen enters the plant via wounds or through the root system. The disease advances without apparent symptoms until the infected plant wilts and collapses. Symptoms of systemic infection start with yellowing of older leaves and petioles. Systemically infected leaves and flowers break off easily and may show dark-brown streaks at their base, which gradually enlarge. Cut-petioles show yellow-brown vascular bundles. Systemic infection may also show water-soaked leaf spots when the bacteria enter the leaf parenchyma from the infected vascular bundles. Many farmers and researchers have observed that some cultivars resistant to foliar bacterial blight disease succumb rapidly to systemic infection and vice versa, indicating a two-phase differential basis for the disease.
Bacterial blight is a contagious disease that is difficult to control. The pathogen spreads by excessive water splashing (rain or irrigation), movement of infected soil, and contact with infected plant material, tools (from leaf pruning, harvesting) and persons walking through infected plants. The bacterial blight pathogen could survive for many months in tissue-cultured plantlets without showing any symptoms. Furthermore, the pathogen can survive in leaf lamina, petiole, and root residues for as long as four months when tissues are left on the ground or buried 15 cm deep; the pathogen retains its pathogenicity during that time.
Preventative measures to manage bacterial blight disease at economically feasible levels include:
(a) use of clean planting material
(b) installing footbaths with disinfectant at entrances to greenhouses
(c) adjusting plant density on beds to facilitate good aeration
(d) working from clean to diseased fields to minimise pathogen movement
(e) discarding and burning of infected plants
(f) disinfecting tools and clothing regularly, changing tools on different plots and avoiding exchange of material between greenhouses
(g) ensuring good drainage of the growing substrate
(h) using drip irrigation or micro-sprinklers
(i) avoiding the presence of visitors not attached to the farm
- removing all debris and old media (or fumigate old media with methyl bromide) and letting beds fallow for two months (or) fumigate with metam-sodium and tarped in prior to replanting
producing disease-free planting material from bud- or tissue-cultured plants away from production fields and
ensuring that the plants’ nutritional needs are met to improve their vigour.
Where the disease has been already established, sanitation practices should be performed.
(a) In addition to adhering to all the preventative measures
(b) keeping plants as dry as possible
(c) removing diseased leaves early and burning them
(d) avoiding extreme climatic conditions particularly high temperatures
(h) adapting a fertilization regime to keep the production of glutamine in plants at a low level, as glutamine is a major food source for the bacteria. For instance, ammonium nitrate can be left out and desirable levels of potassium maintained.
Chemical control of bacterial blight is ineffective and to date no chemicals have been described that can control or reduce the disease. Some chemicals used in the past have been shown to actually increase the severity of the disease.
The most effective control measure for bacterial blight disease is the use of resistant cultivars. The use of such cultivars will reduce the cost and risk associated with anthurium cultivation. A rapid screening method for determining resistance to the disease at both the foliar and systemic phases have been developed at the University of the West Indies, which for the first time provides a practical means of identifying resistance to bacterial blight. With this two-stage screening method several anthurium cultivars with resistance/ high tolerance to the disease combined with good horticultural attributes have been developed. The method is now being routinely used on a commercial farm in Trinidad with great success.