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Pests and diseases of quarantine importance
Anthurium Blight
Production of Anthurium andreanum, a member of the Araceae family, is an important economic activity in many countries with tropical and sub-tropical climates. The spathe, which is the most attractive part of the plant, is sold as a flower because of its colour, shape and long-vase-life. A number of different varieties exhibit different colours, from dark red to white, green, orange and pink. The industry is worth about US$ 20 m annually and Mauritius is the third world producer. The most serious disease problem to strike the industry is bacterial blight caused by the pathogen Xanthomonas axonopodis pv. Dieffenbachiae.
The first visible symptoms are yellowed (chlorotic), water-soaked lesions along the leaf margins that grow rapidly to form dead (necrotic) V-shaped lesions characteristic of this disease.
The bacteria infect Anthurium plants by entering pores (hydathodes) along the leaf margins . Bacteria may also enter if leaf tissues become torn through pruning, or if leaf tissues are punctured by insects. When flowers are harvested, bacteria can enter via wounds.
Hawaii was amongst those countries, which had most severely suffered from this disease. As the organism can be propagated though latently infected plants, tissue cultured plantlets do not constitute a safeguard.
Hence, many countries, including Mauritius, had to impose a ban on the import of Anthurium plants. Although the disease is known to be present in Reunion island, Mauritius has not so far been affected.
Sugarcane Mosaic Virus (SMV)
Sugarcane Mosaic Virus (SMV) is a plant pathogenic virus of the family Potyviridae. The virus was first noticed in Puerto Rico in 1916 and spread rapidly throughout the southern United States in the early 1920s. SMV is of great concern because of the high economic impact it has on sugarcane and maize
Symptoms of Sugarcane Mosaic Virus include intense mottling throughout the laminar region of the plant, characterized by discoloration of the plants leaves, and growth stunting. In maize, the infection occurs first in the youngest leaves with symptoms such as irregular, light or dark green mosaic coloring developing along the veins. The virus can result in severe yield loss of the infected host and the disease eventually leads to necrosis.
In Mauritius, Sugarcane Mosaic Virus has not been reported so far.
Smut
Smut of sugarcane is caused by the fungus Ustilago scitmainea (Sydow, 1924). The first report of the disease incidence came from Natal, South Africa in 1877 and it was speculated to be confined in the eastern hemisphere, until it was reported in Argentina. The disease spread is worldwide covering most of the sugarcane producing areas like Mauritius, Rhodesia, Indonesia, the islands of Java, Sulawesi and Sumbawa. The most recognizable diagnostic feature of sugarcane infected with smut is the emergence of a long, elongated whip. The whip morphology differs from short to long, twisted, multiple whips. The disease is of high economic importance in the African region. It affects the number of millable stalks, Cane diameter, cane yield and sugar recovery.White Grub
The white grub Heteronychus licas is an occasional insect pest of sugarcane. It feeds on the underground parts of the plant most damaging to sugar cane. It feeds on all its underground parts. The root system and even cane setts can be destroyed. Both adults and larvae of Heteronychus licas can cause damage to sugarcane. Adults feed at the base of young shoots causing dead hearts
Clavibacter michiganensis
The bacterium Clavibacter michiganensis subsp. sepedonicus (synonym Corynebacterium sepedonicum) is the causal agent for bacterial wilt in growing crops and for ring rot on tubers.
This bacterium is typical of areas with a cool climate: Scandinavia, North Europe, North Asia, North America and temperate zones in South America. However, it has been discovered more recently in countries with a warmer climate such as Greece and Spain. The natural host of the bacterium is the potato, although it can also be transmitted to eggplant or tomato by artificial contamination.
Ring rot is a quarantine disease of the potato that is listed in the European Plant Health Directive and which may be very destructive to the potato crop (wilting, yield losses up to 50%, tuber rot) when cutting seed tubers at the time of planting. As the bacterium is highly contagious and persistent in plant and soil debris, this disease is submitted to strict regulation to avoid its dissemination by plant transport.
Ralstonia solanacearum
Ralstonia solanacearum is the causal agent of bacterial wilt (also known as brown rot on potato), one of the most destructive plant diseases on many crops in tropical and subtropical areas, which leads to huge losses in food production.
At the early stages of disease, the first visible symptoms of brown rot are usually on the foliage of plants. These symptoms consist of wilting of the youngest leaves at the ends of the branches during the hottest part of the day. At this stage, only one or half a leaflet may wilt, and plants may appear to recover at night, when the temperatures are cooler. As the disease develops under favorable conditions, all leaves in a hill may wilt quickly and desiccate although dried leaves remain green, leading to general wilting and yellowing of foliage and eventually plant death. Another common symptom that can be associated with brown rot in the field is stunting of plants. These symptoms may appear at any stage of plant growth.
In young potato stems, infected vascular bundles may become visible as long, narrow, dark brown streaks. In young, succulent plants of highly susceptible varieties, collapse of the stem may also be observed. In well-established infections, cross-sections of stems and stolons may reveal brown discoloration of infected tissues
Potato Cyst Nematode
Potato root nematodes or potato cyst nematodes are 1-mm long roundworms belonging to the genus Globodera, which comprises around 12 species. They live on the roots of plants of the family Solanaceae, such as potatoes and tomatoes.
Potato cyst nematodes have the ability to cause a large scale devastation in crops due to the massive amounts of nematode embryos in each cyst. Many continents across the world such as Australia, North America, Asia, Europe, and Africa have had many epidemics of potato cyst nematodes that continue to persist year after year. Potato cyst nematodes are important economically due to the fact that they reduce the crop yield exceedingly. A species of potato cyst nematodes called pale cyst nematodes (Globodera pallida) are able to cause 80% yield loss in a potato field if left untreated.
Fusarium wilt (Panama disease)
Panama disease (also known as fusarium wilt) is caused by the soil-borne fungus Fusarium oxysporum f. sp. cubense. Panama disease is considered to be the most destructive disease of banana in modern times.
The Fusarium fungus invades young roots or root bases, often through wounds. Some infections progress into the rhizome (rootlike stem), followed by rapid invasion of the rootstock and leaf bases. Spread occurs through vascular bundles, which become discoloured brown or dark red, and finally purplish or black. The outer edges of older leaves turn yellow. Within a month or two, all but the youngest leaves turn yellow, wilt, collapse, and hang downward, covering the trunk (pseudostem) with dead brown leaves. All aboveground parts are eventually killed, although fresh shoots may form at the base. These later wilt and the entire plant dies, usually within several years. The Fusarium fungus then continues to thrive in surrounding soil, preventing the success of future plantings