Pinkdisease fruitcollapse and marblingdisease bacteria

Pink disease

Pink disease is of little importance in fresh fruit, but can be a very serious sporadic problem in processed fruit because of the lack of

Pineapple Inflorescence
Fig. 9.23. Pineapple inflorescence at mid- to late flower.

detection prior to canning. It was first reported in Hawaii by Lyon (1915) and is now known in Australia, the Philippines, South Africa and Taiwan (Rohrbach, 1983). Cultivars vary in their susceptibility (Rohrbach and Pfeiffer, 1975). At least three genera of bacteria have been reported to cause pink disease: Erwinia, Gluconobacter and Acetobacter (Rohrbach, 1976a; Kontaxis and Hayward, 1978). The Erwinia herbicola species has recently been redescribed as Pantoea citrea, based on an isolate from the Philippines (Cha et al., 1997). Species of the remaining genera are the acetic acid bacteria Gluconobacter oxydans and Acetobacter aceti (Plate 31; Cho et al., 1980).

Pink-disease bacteria are vectored to the pineapple flowers by insects and mites, probably attracted to the nectar. Honey-bees may play a role as important vectors of Gluconobacter and a lesser role for Acetobacter (Gossele and Swings, 1986). The nectar is thought to provide a nutrient source for the survival of the bacteria until they become latent in the nectary gland or stylar canal or locule. Gossele and Swings (1986) suggest that the bacteria may actually overwinter in honey-bee hives. Once the bacteria are inside the flower, they remain latent until the fruit matures, sugar concentrations increase and translucence occurs.

Fruit collapse

Bacterial fruit collapse, caused by E. chrysanthemi Burkh. et al., is only economically important in peninsular Malaysia, although the bacteria have been reported elsewhere on pineapple (Melo et al., 1974; Chinchilla et al., 1979; Rohrbach, 1983). The disease is thought to be indigenous to Malaysia (Lim and Lo wings, 1979b). The economic importance of fruit collapse in Malaysia is probably due to the use of the much more susceptible 'Singapore Spanish' cultivar (Lim, 1985).

Symptoms of fruit collapse usually appear on maturing fruit 2-3 weeks prior to normal ripening (Plate 32). Infected fruit are characterized by exudation of juice and release of gas, as evidenced by bubbles. Fruit shell colour becomes olive-green. Dissection of completely infected fruit shows only cavities within the skeletal fibres of the fruit (Lim, 1985).

Bacterial fruit collapse is caused by E. chrysanthemi Burkh. et al., and the initial inoculum comes from other infected fruit. Insects such as ants, beetles and flies, are vectors of the bacteria, transporting them to flowers from other collapsed fruit or from plants with bacterial heart rot. Ants, as well as other insects, are thought to be attracted to the nectar available there. Open flowers are the infection site where the bacteria enter the developing fruit. The bacteria remain latent in the ovary until 2-3 weeks before normal ripening when sugar levels begin to increase rapidly and polyphenoloxidase levels decline (Lim and Lowings, 1978).

Marbling disease

Marbling disease is a sporadically occurring pineapple disease of the hot, humid, lowland tropics. Symptoms include a brown granular appearance (Plate 33). Marbling disease is caused by the acetic acid bacteria Acetobacter peroxydans Visser 't Hooft and E. herbicola var. ananas (Serrano) Dye. As in pink disease, infection by marbling bacteria occurs through the open flower (Rohrbach and Pfeiffer, 1974). Infection has also been reported within 7 weeks of harvest through growth cracks in the fruit (Yow and Wu, 1972), although this was not confirmed in Hawaii (Rohrbach, 1989). Speculation exists that the bacteria are vectored to the flowers by insects, as in pink disease (Serrano, 1928). The fact that application of surfactants prior to and during flowering significantly increases disease in Hawaii without inoculation indicates that the bacteria are ubiquitously present on the plant and that the limiting factor is whether they get into the flower. Once in the flower, as with pink disease, they apparently remain latent until approximately 1 month before fruit maturity. Symptoms develop during the last month of fruit maturation (Rohrbach, 1989).

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