Morphological characterization of Cercospora cof-fiecola on Robusta coffee berries in Uganda

Abstract

Coffee contributes enormously to Uganda’s economy; however, farmers cur-rently lose over 50% yield of the crop to coffee red blister disease (CRBD), a fungal infection caused by Cercospora coffeicola. Currently, the pathogen pos-es the greatest challenge to Robusta coffee production though its ecology is not fully studied. This study aimed at characterizing C. coffeicola on berries of Robusta coffee in Uganda. A total of 47 Robusta coffee farms were randomly sampled from seven (7) districts which were purposively selected from three (3) Robusta coffee growing regions. Fifty (50) coffee berries were sampled along a transect at each farm and assessed for RBD symptomatology variabil-ity. The Coffee Red Blister Disease (CRBD) was found in all the sampled re-gions and districts of Uganda with a prevalence rate of 38%. The highest CRBD prevalence (47%) was observed in central Uganda and was significant-ly different from that of Southwestern region (16%) (P <0.0001). On average, the lesion incidence on diseased Robusta coffee berries was 10.6%. The high-est lesion incidence was recorded in central region (13.1%) followed by mid-eastern region (12.0%) and least in the southwestern region (6.1%). CRBD le-sion area coverage on the coffee berry averaged at 27.3% but varied signifi-cantly across the regions (P=0.0069) and districts (P<0.0001). Berries collected from mid-eastern and central regions recorded a statistically higher lesion area coverage (30%) than southwestern region (21.8%). The average number of CRBD lesions per coffee berry ranged from 4.4 to 8.4. Luuka district with 10 lesions per berry registered the highest number of lesions and lowest Isingiro district (0.5). The pathogen, C. coffeicola, was isolated from infected berries and the isolates assessed for morphological variability. Based on the isolates’ pigmentation, radial growth, colony elevation, sporulation ability, and shape, the assessment resulted into six symptom morphotypes (SM) that were ran-domly assigned the capital letters of the alphabets ―A‖ to ―F‖. Isolated C. cof-feicola fungi grew linearly on PDA at a rate of 2.9 to 3.6 mm/day to attain a diameter of 27 to 70 mm in 5-14 days. Morphotype IV (colony radius = 44mm/day) had the highest growth rate, while morphotype V had the slowest growth rate (colony radius = 35mm. Morphologically, the morphotype colonies were peripherally flat with slightly raised front view and umbonate to radially folded centers. Pathogenicity tests that were conducted on one susceptible Ro-busta coffee variety (KR6) and two tolerant varieties (KR3 and KR10) using three distinct morphotypes, L1C (Morphotype II), M1C (Morphtype III) N1C (Morphotype IV) revealed that all the isolates were pathogenic to the three se-lected coffee varieties. L1C caused the most cercosporiosis to KR 6 (DI = 41%) while M1C caused the least disease (DI=1.3%) on KR3. N1C did not infect KR10. KR6 had the highest disease index of 41.2%, while the resistant variety, KR10 had the lowest (6.8%). In conclusion, there is variability in the C. coffeicola pathogen complex across the Robusta coffee growing regions, as exhibited through symptomatology of the disease on berries. This study pro-vides baseline information on CRBD symptomatology and pathogenicity to inform further research on the disease. This information can guide target CRBD management strategies as well as development of new CRBD resistant varieties.