Stem end rot (SER) is a significant postharvest disease of mangoes, particularly in Queensland (Muirhead and Grattidge 1986; Johnson and Coates 1993). SER is caused by Botryosphaeria dothidea (Bd, Johnson and Coates 1993). Bd can infect mango fruit from conidia produced from mummified fruit within the canopy, or by ascospore production from fruiting bodies in leaf litter (Johnson et al. 1989a). Bd also endophytically colonises the stem and inflorescence of mango, and subsequently the fruit (Johnson et al. 1992). The current control method is a hot dip of carbendazim; however this is only partially effective (Cooke et al. 1999). Other more effective, and preferably non chemical methods of control of SER are necessary.
The development of Bd induced SER and Bd colonisation of mango plants subjected to water deficit stress was studied with a view to developing novel management strategies. It was hypothesised that by modifying the growing environment of the mango, it may be possible to alter the infection patterns of Bd within a mango tree and therefore reduce subsequent infection of the fruit (Johnson et al. 1992). Bd infects several other tree crops, including peach (Pusey 1989) and apple (Brown and Britton 1986), causing both canker and fruit rot. Predisposition to disease due to droughting or winter injury has been linked to the development of such Bd diseases. Water deficit stress in particular has been shown to significantly increase the incidence and severity of Bd stem cankers of white birch and peach and Bd blight of pistachio leaves (Crist and Schoeneweiss 1975; Pusey 1989; Ma et al. 2001).
Water was withheld from mature mango trees from flowering until fruit harvest to investigate the effects of water deficit stress on colonisation of the trees by Bd. Two sites were chosen; one with krasnozem soils and the other with a sandy soil. On the krasnozem soil, consistent differences in stomatal conductance were not detected between irrigated and non irrigated trees even after 17 months without irrigation. Leaf water potentials, measured with pressure bomb apparatus, were also similar. At the second site, trees subjected to water deficit stress showed significantly lower stomatal conductance measurements and had lower soil water content than irrigated trees. This water deficit stress resulted in decreased fruit number, size and yield, and markedly increased dry matter content of harvested fruit. Fruit ripening rates were not affected, although the respiratory peak was higher in the Suit from non irrigated trees.
It was anticipated that water deficit stress would shift the host-pathogen balance between the mango tree and endophytic Bd in favour of Bd. Endophytic colonisation by Bd was measured from a sample of inflorescences taken on several occasions between flowering and harvest. Segments of this tissue were plated out after triple sterilisation and fungal colonies identified. However, detection of Bd in stem pieces from water deficit stressed trees was not consistently higher than those detected in stem pieces from non stressed trees.
The effects of water deficit stress on SER of harvested mango fruit were complex and indicated that mango SER caused by endophytic infection may be increased by water deficit stress. This result, however, was not consistently detected in fruit harvested from mature trees. Evidence of potential endophytic colonisation by Bd was measured by assays of rigorously surface sterilised pieces of the fruit pedicel. Bd was detected in the pedicel of only 25% of fruit that subsequently developed SER. Other inoculum sources of Bd appear to be responsible for many SER infections of mango fruit. Management strategies for SER within mango orchards will need to encompass both types of infection processes.
The effect of water deficit stress on Bd colonisation of mango seedlings was also examined. Endophytically infected seedlings were established by inoculation harvested fruit with Bd at the stem. As SER lesions developed the seeds were removed and planted. Seedlings thus infected with Bd were subjected to water stress at different stages of development to determine whether colonisation of the mango seedling by Bd could be manipulated by water deficit stress.
Inoculation of mango fruit with Bd significantly reduced germination of the seed and subsequent growth of the endophytically infected seedlings. Water deficit stress added to the stress already introduced through fungal inoculation often resulted in seedling death. This was particularly evident in seedlings inoculated with a more aggressive Bd isolate. Endophytic colonisation was again measured by assaying triple sterilised stem segments for Bd colonisation. As water deficit stress increased, Bd was recovered further from the hypocotyl, or inoculation point, indicating increased colonisation.
The infection pathway of Bd resulting in SER of harvested fruit was unknown prior to these studies. Continuity of the xylem system during fruit ripening was studied by means of dye, introduced into the fruit through partial pressure infiltration. The dye penetrated into the vascular system of the stem and fruit, but not the seed. Dye was limited to the xylem, and was not observed in the lactifers. No dye infiltrated into the seed, which indicates that the tissues of the seed are not attached to the vascular system of the fruit. Dye infiltration in immature fruit was more extensive than that of fruit which had commenced ripening. As harvested fruit ripened, infiltration decreased even further.
The vascular tissue, as highlighted by the partial pressure infiltration of dye, was extensively used as an infection pathway by Bd inoculated onto the cut pedicel of harvested mango fruit. This was shown by assays of tissue from 19 points within the fruit for the presence of Bd over the period from inoculation after harvest to the development of external SER lesions. Colonisation of the vascular tissue of the fruit preceded visible lesion development, and Bd was detected earlier in the vascular tissue than the surrounding fruit flesh. However lateral transmission of Bd was also evident as the infection developed, with adjacent tissues not connected via vascular tissue colonised at similar times.
Histopathological studies of Bd in artificially and naturally inoculated mango tissue showed fungal hyphae residing primarily in phloem cells, but also in the xylem. Hyphae similar in appearance were also observed in non inoculated mango tissues. Inability to identify the fungi further than correlation with previous plate assays of adjacent tissue prevented any further conclusions being drawn. It was shown, however, that fungi reside latently in naturally infected mango. Fungi other than Bd were regularly detected after triple sterilisation of stem pieces and it is likely that these also show latent endophytic phases to their lifecycles.
Through these studies, significant progress in understanding Bd as an endophytic pathogen of mango has been made. Bd was recovered after triple sterilisation of mango tissues, again indicating endophytism as proposed by Johnson et al. (1992). Water deficit stress applied to mature trees did not clearly indicate an increase in endophytic colonisation by Bd, although subsequent SER of harvested fruit was higher in one season. Correlation between pedicel and finit infections showed that endophytic colonisation was implicated in only 25% of SER infections. Water deficit stress was shown to increase the colonisation of mango seedlings endophytically infected with Bd. In the future, management strategies for SER in mango fruit will need to encompass endophytic and epiphytic infection pathways.