Improving management of potato taste defect in coffee and elucidating its mechanisms of occurrence

"Potato taste defect (PTD) is a raw potato-like smell and taste found in green and roasted coffee beans and also in brewed cups of coffee. This defect diminishes the flavor experience and the perception of quality of finished coffee, reducing its value or causing it to be rejected by consumers or international buyers. Occurrence of PTD in coffee has been associated with feeding by Antestia bug, Antestiopsis thunbergii (Hemiptera: Pentatomidae). The outcome of laboratory bioassays indicated that pyrethroid (Alpha-cypermethrin) and pyrethrins (Pyrethrum EWC) provided 100% mortality and significantly greater mortality than the neonicotinoid (Imidacloprid) twelve hours after sprays. Under field conditions, pruning plus application of Fastac or Pyrethrum EWC or Pyrethrum 5EW provided significantly greater mortality compared to other treatments. Additionally, pruning combined with insecticide application, especially Fastac, significantly reduced PTD incidence up to 1%. An expansive survey of PTD occurrence in Rwanda demonstrated that the defect is distributed throughout the coffee producing regions of the country with the highest incidence in Central Plateau, Granitic Ridges and Eastern Plateau. High incidence of PTD was shown to be strongly associated with high density and damage of Antestia bug. In contrast, coffee berry borer (CBB), Hypothenemus hampei (Coleoptera: Scolytidae) damage was not significantly related to the occurrence of PTD. Although CBB bores holes into coffee berries that could allow the bacterium to enter, survival of the pathogen may be impeded by the presence of the beetle. In light of these findings, there is need to improve Antestia bug control, especially in the areas where the highest PTD incidence was observed. Antestia bug is distributed in all coffee growing regions of Rwanda with the highest density in the Northern region and the lowest in the Eastern region. In a two - year study (2016 and 2017), the economic damage due to Antestia bug ranged from 0 to 92% in 2016 and from 0 to 81% in 2017, underscoring an urgent need for pest control. Temperature and relative humidity were positively related to Antestia bug density and their increase due to climate change may lead to future pest outbreaks. However, wind speed was not significantly associated with Antestia bug density possibly because of the bug's physical structure and behavior, as it is a relatively a large insect that prefers to hide in dense canopies. In a study conducted to elucidate the mechanisms of PTD occurrence and to assess the relationship between cupping method using the Specialty Coffee Association of America (SCAA) cupping protocol and chemical analysis using Gas Chromatography Mass Spectrometry (GCMS) to detect the presence of PTD, findings indicated the highest incidence of PTD was found in coffee beans damaged by Antestia bug and the lowest was found in undamaged control beans. It was demonstrated for the first time that mechanical damage alone in coffee berries can cause PTD and that Antestia bug is not the absolute cause of PTD, but wounds to the berries including feeding damage by this bug allow entry of the bacterium responsible for PTD. The lowest amount of 2-isopropyl -3-methoxypyrazine (IPMP), the main compound associated with PTD, was found in the control beans while the highest levels was in the coffee beans that had been poked with a needle to simulate bug damage. Additionally, there was a positive relationship between the cupping method to detect PTD and IPMP detection using GCMS. Further research is justified to better understand the ecology of Pantoea coffeiphilia and to design methods and techniques for early detection and prevention of this bacterium in the effort to supply PTD free coffee."--Pages ii-iii.