EPIDEMIOLOGY OF GRAY LEAF SPOT (CERCOSPORA ZEAE- MAYDIS) AND GENETICS OF RESISTANCE IN MAIZE TO THE DISEASE IN ETHIOPIA

Abstract

Maize (Zea mays L.) is an important component of farming systems and staple food crop in sub- Saharan Africa. In Ethiopia it is a staple food crop and one of the main sources of calories in the major maize producing regions. It is cultivated on about 2.135 million hectares of land. The national average yield of maize under subsistence production is 3667.5kg/ha. This is too much below the world’s average yield which is over 6520 kg/ha. This low yield is attributed to several factors among which foliar diseases and insect pests are the major once. Gray leaf spot (GLS) caused by the pathogen Cercospora zeae-maydis is one of the necrotrophic and polycyclic foliar diseases of maize that poses a serious problem to maize production and productivity in Ethiopia. Field surveys were conducted in Western and South western parts of the Oromia National Regional State, Ethiopia during 2015 and 2016 main cropping seasons to determine the distribution, prevalence, importance and the association of gray leaf spot (Cercospora Zeae maydis) epidemics with environmental factors and cultural practices. In both years a total of 480 maize fields were surveyed in four zones and 90% of the fields were infested with gray leaf spot. Gray leaf spot incidence and severity varied among zones, between years, altitude range, soil types and field management practices. The mean disease incidence in 2 years ranged from 35% in Horro Guduru to 95% in Jimma zone and severity ranged from 25% in west Shewa to 75% at Jimma and East Wollega zones where it was significantly higher. The incidence in 2016 cropping season was higher by 55% than in 2015. Logistic regression was used to analyze the association of gray leaf spot incidence and severity with environmental factors and cultural practices as independent variables. Analysis of result (p=0001) showed Zones, varieties, cropping year, soil type, maize crop residues and weed management practices were significantly associated with gray leaf spot incidence and severity in a multiple variable model. Higher incidence of gray leaf spot was significantly associated with east Wollega and Jimma zones, crop residue, rain fall and sowing dates. Lower incidences had a high probability of association with Horro Guduru and west Shewa in (2015) cropping year and good weed management practices. In a reduced multiple variable xiv model, sowing date, variety, area, year, weed management practices and tillage frequency were significantly associated with higher gray leaf spot incidence and severity. Increased incidence was observed in maize intercropped with haricot bean than brassica spps. The survey revealed high occurrence and distribution of gray leaf spot in the study area. Thus, effective and feasible management options need to be developed in combating this foliar disease of maize. In experiment two fourteen advanced maize inbred lines and locally adapted hybrid maize (BH-540) as a check were used to investigate their reaction to GLS disease. Field experiments were conducted at Bako National Maize Research Centre in 2015 and 2016 main planting seasons arranged in a randomized complete block design (RCBD) with three replications. Artificial inoculation with Cercospora zeae-maydis was conducted by applying dry, ground, infected maize leaves into the whorls of younger maize plants. Data on agronomic and disease parameters (latent period, disease severity, disease incidence and lesion type) were recorded from the middle two rows. From the combined analysis of variance, maize genotypes showed significant differences with reaction to GLS indicating the existence of genetic variability among the selected genotypes. Highly significant differences were also observed among entries for all agronomic parameters in both seasons. Gray leaf spot incidence and severity varied among genotypes and between years. The mean GLS incidence and severity were higher in 2016 than 2015. GLS disease incidence in two years ranged from 35% on Sc22 to 95% on CML-387 and severity ranged from 15% on A-7016 to 75% on CKL05003. Significant differences in epidemic variability were also observed among genotypes and seasons. From the analysis of disease progress curves Logistic model (R2=94.55) better described the disease progress curves than the Gompertz model (R2=91.50). Parents; P6 and P8 had the most desirable quality for the most of agronomic traits whereas P2, P7 and P9 were the best parents for grain yield. Among all inbred lines, P6, P7 and P14 were identified as the most desirable sources of genes for GLS disease resistance. But P6, P7, P8 and P14 were identified as the best genotypes in yield, yield related traits and GLS disease parameters. Thus, these parents were recommended to be used in breeding programs with a purpose of developing high yielder and GLS disease resistant open pollinated varieties. In conclusion this study identified potential and promising high yielding and GLS resistant open pollinated genotypes (CKL05017-B-B, CML-395, CML-387, A-7016, Gutto and Sc22). Therefore, it is recommended that these OPVs can be used by resource poor farmers for direct production where this disease is the most prevalent and/or for further breeding programs in generating novel hybrids for future use. Similarly in experiment three ten elite maize inbred lines were selected based on all over per se performance and gray leaf spot disease reaction. Crosses were made in a 10 x 10 half-diallel mating design to produce 45 F1 single cross hybrids. The experiment was conducted at Bako national maize research center in xv 2015 and evaluation of the crosses were made at Bako and Jimma research centers in 2016 by using alpha lattice design with three replications including three commercial checks. Artificial inoculation with Cercospora zeae-maydis was conducted by applying dry, ground, infected maize leaves into the whorls of younger maize plants. All the necessary yield, agronomic and GLS disease data were recorded. In all the studied traits highly significant genotypic differences were observed indicating the existence of genetic variability among the crosses. Analysis of variance for the combining ability indicated GCA and SCA mean squares were significant at (P<0.001) for all traits except for anthesis-silking interval, ear per plant, ear diameter, lesion length and width. The ratios of GCA/ SCA variances for agronomic parameters and all disease parameters were greater than unity except for that of first disease appearance implying the predominance of additive gene actions. P3 and P4 had the most desirable quality for most agronomic traits whereas P2, P7 and P9 were the best combiners for grain yield with the general combining ability (GCA) effects of 0.80, 0.75 and 1.56 respectively. Among all inbred lines, P1, P4, P7, P8 and P9 were identified as desirable sources of resistant genes for GLS disease resistance with positive days of first disease appearance and negative disease incidence, severity and AUDPC values for GCA effects. Some of the cross combinations showed significant SCA effects, suggesting that these crosses have performed significantly higher or lower than what was predicted based on their parental performances implying the importance the non additive gene action. From the analysis of epidemiological data and disease progress curves the Logistic model (R2=96.5) better described the disease progress curves than the Gompertz model (R2=92.5) indicating the presence of delayance in epidemics and the inflection point of the GLS disease. P1, P7 and P8 were identified as a good general combiners for yield, yield related traits and GLS disease parameters. Thus, these parents were recommended to be used in breeding programs with a purpose of developing high yielder and GLS disease resistant single cross hybrids. In conclusion this study identified potential high yielding and GLS resistant single cross hybrids (CML-395/ CML383, CML395/Sc22, CML- 395/CML-197 and CML-383/CML-197). Therefore, it is recommended that these hybrids can be used for direct production where this disease is the most prevalent and/or for further breeding programs in generating novel hybrids for future use