The cause, distribution and economic importance of fruit speckle of banana in north Queensland (BA05001)

This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

New research had found the causes of banana fruit speckle, an unsightly skin blemish in Cavendish and Ladyfinger bananas responsible for significant fruit losses. Fruit speckle typically appeared as a small reddish-brown to black spot (0.5-1 mm in diameter) with a water-soaked margin. All surfaces of the fruit could be affected but symptoms were often more pronounced at the neck and the flower end of fingers. These symptoms were not to be confused with the normal brown spots which develop on ripe bananas.

A national mail-out survey to banana growers revealed that more than 50 per cent of growers believed that fruit speckle increased the amount of rejected fruit at the shed and twice the number of southern growers than northern growers reported greater than 20 per cent of fruit being discarded as a result of fruit speckle.

In a laboratory tests, Department of Primary Industries and Fisheries’ scientists found that of 11 species of fungi recovered from speckle affected fruit only three of these, Colletotrichum musae, Fusarium oxysporum and Fusarium semitectum produced typical speckle-like spots. Significant sources of the spores of these fungi were found to be the banana flowers, fruit bracts and dead leaves.

Research also showed that a 10 per cent sap solution applied to fruit prior to infection with these fungi significantly increased the severity of speckling. Sap had much more of an effect on the infection caused by Fusarium than the Colletotrichum.

In another experiment, banana fruit was shown to be less susceptible as it matured. At bract-lift, Colletotrichum caused significantly more speckle lesions than the Fusarium suggesting it was more aggressive at this stage of fruit maturity.

During the 2007 wet season, the examination of banana bunches at early bract-lift and bract lift for insects showed flower thrips were more common than any other insect. However the damage to fruit they caused (dark to black pimples of less than 1 mm in diameter without a watersoaked halo) was not typical of fruit speckle. Researchers however thought that thrips and other insects might cause some superficial damage to the skin whilst feeding, which would allow more fungal infection and fruit speckle. A laboratory experiment subsequently proved that flower thrips had little effect on the incidence of Colletotrichum-related fruit speckle but caused a 10-fold increase in the incidence of Fusarium-related fruit speckle.

Further test results showed that all registered fungicides used with oil and sprayed during the hot dry months of November-December in 2005, 2006 and 2007 developed severe spotting on fruit. However, the researchers concluded that while all fungicide sprays with or without oil have the potential to cause fruit ‘burn’, none of this chemical-induced spotting was typical of fruit speckle.

Based on the results, the researchers concluded that control of fruit speckle of banana during the warmer, wetter months of the year could be assisted by:

• Adequate crop hygiene (deleafing, desuckering) particularly prior to and during the ‘wet season’
• Under-tree spraying with mancozeb to help reduce the fungal spore load in the crop
• Treatment of bunches with mancozeb as Tatodust® (applied to emerging bunch before bracts were fully open and again at bunch covering)
• Effective control of insect bunch pests
• Avoiding physical damage to the developing bunch that may cause sap release onto fruit.