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Exotic Fusarium diseases
Fusariosis is one of the key diseases of pineapple but has not been recorded in Vietnam. It is caused by Fusarium subglutinans (Wollenw. & Reink.) Nelson, Toussoun & Marasas. This disease has caused major losses in Brazil. Growers in Vietnam are now expanding the production of the Smooth Cayenne cultivar of pineapple for export markets rather than the traditional pineapple cultivars. It is possible that susceptible varieties of Smooth Cayenne may increase the incidence and severity of Fusariosis disease.
Sunken light brown lesions form on one or more fruitlets on the fruit surface. Light pink to grey mycelium usually develops on the lesions and gum can exude from the diseased fruitlets. The lesions may be superficial or extend into the fruit core. The fruit is the most commonly affected part of the plant but all plant parts can be infected and develop display symptoms. The plant may be stunted and leaves chlorotic, and in some cases the stem apex may be bent or dead.
This disease is found in South America. It has not been found in Vietnam.
Pineapple Ananas comosus L.
Plants are most affected when infection occurs during the early stages of flowering. Wounding, particularly from insects encourages disease development. Once the flower or fruit is infected, secondary infections can occur on the other plant parts, such as the base of developing fruit.
The pathogen is spread primarily in infected propagation material. Local spread within crops can occur in the wind and on insects.
Since this pathogen does not produce chlamydospores, it can only survive in the soil for a few months. Therefore there should be a break (fallow) or rotation crop following a diseased crop.Infected fields should not be replanted immediately with pineapples until inoculum levels have decreased.
Click here for the Fusarium diseases already present in Vietnam.
Malformation of mango is caused by F. subglutinans (Wollenw. & Reinking) Nelson, Toussoun & Marasas and refers to the abnormal dense, bunchy and compacted appearance of the inflorescence of infected trees. The symptoms of the disease have been attributed to altered levels of the hormone cytokinin produced in the plant. The pathogen is spread into new areas in infected nursery stock, however dissemination within crops is not clearly understood. This disease has not yet been reported in Vietnam, but there have been no extensive surveys of fungi associated with mango blossoms. It is likely that it would cause significant problems if introduced into Vietnam.
The flowers of infected trees develop abnormally. The ratio of male to hermaphroditic flowers increases and the number of flowers and flower size also increases. The length of the branches supporting the flowers is shortened. The result of these abnormalities is a very dense or bunchy appearance of the inflorescences. The greater number of male flowers leads to poor fruit yield and the hermaphroditic flowers that are produced may be sterile or die. The blossoms may also produce vegetative structures.
Vegetative shoots can display a compacted, bunchy appearance, particularly in young plants in nurseries. The leaves are small, bent and narrow with short internodes.
The disease is found in many countries within Africa, Asia and America. It has not been reported in Vietnam.
Mango Mangifera indica L.
The pathogen is disseminated into new areas in infected nursery stock however the mechanism for spread within a crop though is not known. The disease is more common in arid areas. The seed is not infected with this pathogen, but it has been found to spread through nurseries in the soil, where seedlings were propagated in the shade of malformed mango trees. Wounding of the host tissue has been found to favour disease development.
There have been suggestions that the mango bud mite Eriophyes mangiferae causes the disease because it is often present in high numbers. However the mites occur in mango buds in Australia, but the disease has not been reported there. The mites are thought to have a role in the disease cycle, but it has not yet been defined. The mites could cause wounding of the bud which would predispose the tissues to infection by the pathogen. Furthermore, the mites may act as vectors of the fungus and so introduce it to the wound site.