A number of reports in the print and social media about what was called ‘a new banana disease’ were received by the National Agricultural Research Laboratories of NARO at Kawanda. These reports from a number of districts in western Uganda described a strange disease that makes the bananas rusty brown. Information of a similar disease had earlier been received from extension officers of districts in 2017 that further indicated that the disease later disappeared with the onset of the heavy rains.
The officers were informed that this was a typical thrip damage problem. Thrips are insects which cause damage by feeding on the tender green skin of young banana fruits. As the fruits mature, the injury marks become visible and give the fruits the dark appearance. Sometimes opportunistic fungi enter the injuries and worsen the appearance of the fruits. In all cases, the pulp of banana fruits remains clean. This problem has been in Uganda for a very long time but is always affects very few farms.
On February 24, 2020, a team from NARO’s banana research programme at Kawanda visited the affected Isingiro district to verify the recent reports and assess the extent of damage to bananas. The scientists met district officials including OWC and all extension workers from the 21 different sub-counties for a briefing about the suspected disease.
Later, the scientists visited farms in the worst-hit areas of Rugaga and Kikagate sub-counties.
During their interaction, the farmers reported that the intensity of the disease increased when the rains became heavy between September 2019 and January 2020. The LC3 chairperson for Rugaga sub-county, Mathias Tushabomwe and the extension officer for Rugaga, Besiga Felix confirmed the information from farmers.
Although in 2017, the symptoms disappeared at the onset of rains, the recent outbreak has persisted, and farmers reported an increase in the number of bunches and farms affected during the rainy season.
Farmers in Isingiro reported that the symptoms are on the peel of the fruit and the pulp is not affected, they can peel and cook it for food. However, it is not as soft as it should be.
According to the extension workers and what the NARO scientists observed on the farms across sub-counties in Isingiro district, the incidence (percentage plants affected in a farm) was unusually high. On some farms, it was as high as 90%.
Cause of the problem
The reported problem is caused by banana rust thrips. They are small yellow insects with narrow fringed wings. The insects’ attack by feeding on the green surface on the banana fingers. The bananas appear brown, purplish to black. The discolouration, however, is on the surface of the fruit peel. In extreme cases of severely affected fruits, the whole bunch appears blemished.
They cause more severe damage to younger fruits, often the symptoms do not manifest until after 2 months as the fruits begin to mature. When the peel gets damaged as a result of feeding by the thrips, the affected fruits first appear as grey and dusty, later turning rusty brown.
As affected fruits continue to grow, sometimes the peel cracks causing scarring. The scars provide an opening for opportunistic fungi including pathogenic [disease causing] ones especially anthracnose that take advantage and creep into the affected areas. In severe cases the fungus appears ashy/sooty, the fingers start to shrivel and rot starting from the cracked points due to the fungal infection.
Female thrips lay eggs in the banana plant tissue which hatch and pupate at the base of the banana plant, maturing into adult thrips in a matter of two weeks. It is the adult thrips which feed on fruits. Banana is the primary host of banana rust thrips. However, alternate hosts are reported in immature fruits of citrus, tomatoes and green beans.
It is not clear why the problem is becoming much bigger than usual and why it did not reduce during the rain. Investigations to establish the reasons and to devise appropriate control measures have been planned.
Obtain thrips-free planting material and, if possible, hot water treat prior to planting out. Destroy all volunteer plants and old neglected plantations that harbour the pest and that could act as a source of thrips to spread to other plantings.
Sound (unbroken) bunch covers (which cover the full length of the bunch) do provide some protection if applied very early. These cannot be relied upon to fully protect fruit, particularly during severe infestations. Regular checking of fruit under the bunch covers is essential to ensure that damage is not occurring. Ensure treatments are applied immediately after detection to prevent further damage.
General predators such as lacewings and ladybird beetles exert some control over rust thrips on the plant, and ants may be effective in removing some of the pupae in the soil.
Chemical control should be directed at both the soil-dwelling pupal stage and the adults and larvae on the fruit and plant. Failure to control the pest at both sites will result in continuous reinfestation, especially during the hot, humid periods of the year.
Soil treatments: The treatments aimed at the common banana weevil borer in September/October will provide temporary control of rust thrips.
Fruit/plant treatments: All bunches, the pseudostem and the suckers should be sprayed with an approved pesticide. This control method can disrupt beneficial insects so exercise caution.
Bunch treatments: Pesticide injection for scab moth provides early bunch protection against rust thrips. Extended protection up to harvest should then be provided by one insecticide application to the bunch.
By Wilberforce Tushemereirwe, the Institute Director of the National Agricultural Research Laboratories- NARL-Kawanda, National Agricultural Research Organisation