DISTRIBUTION, PATHOGENIC VARIATION AND INTEGRATED MANAGEMENT OF FABA BEAN GALL DISEASE (Olpidium viciae Kusano) IN NORTHWESTERN ETHIOPIA

Abstract

Faba bean (Vicia faba L.) is an important food legume crop used for various dishes due to its high protein and essential nutrient content. In recent years, faba bean production and productivity are constrained by faba bean gall disease (Olpidium viciae Kusano) in the highlands of Ethiopia. Thus, this research was conducted to (1) assess the spatial distribution and intensity of faba bean gall disease and its association with agro-ecological factors; (2) determine Olpidium viciae pathogenic variation using faba bean varieties and other legume crops; and (3) evaluate an integrated effect of fungicide, host resistance and intercropping on gall disease epidemic and yield and yield components of faba bean in northwestern Ethiopia. The field survey and experiments were done during the 2019 and 2020 main cropping seasons. A total of 304 faba bean fields were assessed from ten districts in three zones to measure disease and biophysical data during the survey periods. A pathogenic variability assay of 30 isolates was conducted twice on six faba bean varieties in the growth chamber, and seven pathogenic isolates were tested on other legumes in the screen house in a factorial experiment and the treatments were arranged in a completely randomized design with three replications. Each faba bean variety and legume was inoculated with zoospore suspension (106 spores ml ‒1 ) of each isolate and the disease severity was recorded. Integrated management was conducted using Bayleton fungicide for foliar spray and/or seed treatment, three faba bean (Bulga 70, Degaga and Tumsa) varieties, and two intercropping (wheat and mustard as a component of crops) systems as treatments. The treatments were arranged in split-split plot design with three replications under field conditions at Farta, South Gondar, Ethiopia. Disease intensity and crop data were collected from field trail. The highest gall disease prevalence (94.07%), incidence (43.41%), and severity (25.36%) were estimated in South Gondar during the field survey. The gall disease was more severe in Tach Gayint (32.58%), followed by Lay Gayint and East Estie districts, each estimated 25.34% severity. The gall symptom was also observed on field pea, lentil, and vetch legume crops during xiv the field assessment. The associations of faba bean gall disease intensity with agro-ecological factors were analyzed using the ordinal logistic regression model. Variables such as geographic area (zone), altitude, and sowing date were highly and significantly (P<0.0001) associated with high disease incidence (>50%) and severity (>25%) in the model. Lower disease intensity had a strong relationship with intercropping, July to 15 August sowing, good weed management practice, and altitude less than 2800 m.a.s.l. and that could be considered as management options to reduce the gall disease intensity. There were highly significant (P<0.01) variations among faba bean varieties, other legumes, isolates, and interactions in components of disease development for both growth chamber and screen house experiments. Seven pathogenic isolates were identified as causing more disease on faba bean varieties than the remaining 23 isolates considered and tested on other legumes under screen house conditions. The interaction of the isolates with other legumes varied in components of gall disease development. The inoculation of grass pea by SEL and WQZ isolates caused higher (20.99%) severity and AUDPC (156.64%- days), respectively, next to faba bean than other legumes tested. The isolates varied in their infection ability on faba bean varieties and other legumes, and the pathogen had a wide range of hosts. Highly significant (P<0.01) differences were detected among integrated management treatments in disease parameters and yield and yield components of faba bean. The integration of varieties, intercropping and Bayleton foliar sprays and/or seed treatment significantly lowered disease incidence, severity, AUDPC, and disease progression rate and increased the yield of faba bean compared with other treatments. The gall disease intensity was completely managed in fungicide sprayed plots at 90 days after planting. Intercropping of Degaga and Tumsa with wheat and mustard significantly reduced the gall disease epidemic compared to Bulga 70-wheat and mustard intercropping in seed-treated plots. A combination of Bayleton (three sprays at 15-day intervals) with Degaga gave the highest (5.27 t ha–1 ) grain yield, followed by integration of foliar sprays + seed treatment with Degaga (4.97 t ha–1 ) and both had high net benefits. The yield losses due to faba ben gall disease were estimated at 60.00, 67.61 and 68.61% on Bulga 70- wheat, Bulga 70-mustard intercropping and sole cropped Bulga 70 in untreated plots, respectively. Hence, it can be concluded that the integration of seed treatment using Bayleton (1.5 g kg–1 seeds) with moderately resistant varieties reduced the yield losses in both cropping systems by decreasing the gall disease intensity, and thus, it could be used as a cost-effective and environmentally safe management option in the study areas and other related agro-ecologies in xv the country and elsewhere. Further study on the epidemic of gall disease on faba bean, alternate hosts, and weeds should be conducted to determine the source of inoculum, survival mechanisms of the pathogen, and the effects of alternate hosts on faba bean gall disease epidemiology and management. Future research is also needed on designing a breeding method for faba bean gall resistant variety development programs in the country