EPIDEMIOLOGY AND INTEGRATED MANAGEMENT OF SUGARCANE SMUT (Ustilago scitaminea Syd.) IN ETHIOPIA

Abstract

Sugarcane is one of the most important commercially grown crop in Ethiopia. However, production of the crop is adversely affected by several biotic constraints among which sugarcane smut (Ustilago scitaminea) is the most important. Thus, developing integrated management methods is essential for sustainable sugarcane production and productivity. Disease survey was carried out to determine sugarcane smut intensity, distribution and its association with biophysical factors. Moreover, laboratory, lath-house and field experiments were conducted during 2017 to 2020 to determine the progress and disease development; and to develop various smut management technologies as components of integrated management of sugarcane smut in Ethiopia. Sugarcane smut survey was carried out on 311 fields from eight Sugar Estates. Besides, study on in vitro evaluation of native Trichoderma bioagents against sugarcane smut pathogen, U. scitaminea was conducted using dual culture technique at Wonji Laboratory. Likewise, the suitability of four sugar factory by-products for the growth of the five effective Trichoderma isolates was done under in vitro condition at Wonji. In vitro compatibility of five Trichoderma isolates with fungicides Opera and Apron Star in suppressing U. scitaminea was also carried out using poisoned dual culture technique. Furthermore, two experiments on in vivo evaluation of seedcane treatment with fungicides and Trichoderma isolates against smut with each consisting 9 treatments in three replications were done at Wonji. Lastly, the experiment on the efficacy of integrated smut management consisting of 36 treatments in three replications was conducted at Wonji and Metehara. Survey of sugarcane smut demonstrated that smut prevailed in 62.7% of the fields. Mean smut incidence of the fields varied from 0.99 to 26.33%, while mean severity ranged from 2.06 to 58.27%. Besides, high smut intensity (incidence >5% and severity >25%) was highly associated with Metehara, Kesem, Kuraz, Tendaho fields, and also with Demerera, Mexican, Natal, and NCo 334 sugarcane varieties, altitude ≤ 1000 m.a.s.l., ratoon crop, heavy and intermediate textured soils and tillering growth stage. However, low smut intensity (incidence ≤5% and severity ≤25%) had high probability of association with Cuba and Barbados varieties, furrow irrigated fields, an altitude of 1001-1500 m.a.s.l., weed-free and fertilized- xxi fields. Twenty one Trichoderma isolates were recovered from 120 soil samples. Among these, twelve of them achieved pathogen suppression of >90% at 7 days after inoculation (DAI). However, five of the isolates (WI4-27, MKI2-k17, MI3-m33, WI12-25 and MKI3) were selected as potential candidates based on distinct suppression features, growth rate and antagonistic performance. These isolates were identified as T. harzianum, T. asperellum, T. viride, T. viride and T. harzianum, respectively. The experiment on suitability of sugar factory by-products for growth and mass production of the Trichoderma isolates demonstrated Bagasse as a suitable substrate. Moreover, study on the compatibility of the five native Trichoderma isolates with fungicides, against U. scitaminea discovered that isolates MKI4-27, MKI2-k17, WI12-25 and MKI3 at a rate of 5 mL Opera and 8 g Apron Star L-1 of PDA as the most effective combination against U. scitaminea. Furthermore, the experiment on seedcane treatment with the five native Trichoderma isolates demonstrated that three of them [WI4-27 (T. harzianum), MKI2-k17 (T. asperellum) and MKI3 (T. harzianum)] completely protected sugarcane from smut with 0% disease intensity. Similarly, the experiment on the efficacy of seedcane treatment with three rates Opera and Apron Star against smut indicated that 5 mL Opera and 8 g Apron Star L-1 water completely subdued sugarcane smut from the crop. Finally, study on efficacy of integrated disease management options against smut showed that mean smut incidence, severity and AUDPC values that ranged from 0 to 21.50%, 0 to 67.22%, 0 to 5476.52%-day over the two locations. Mex 54-245 treated with apron + native T. harzianum attained the highest sugarcane yield of 270.78 tons ha -1at Metehara with untreated NCo 334 delivered the lowest cane yield of 99.23 tons ha -1 at Metehara. However, NCo-334 treated with native T. harzianum attained the highest sugar yield of 31.86 tons ha -1at Wonji with untreated NCo 334 delivered the lowest sugar yield of 10.31 t ha-1 at Metehara. The result from survey of sugarcane smut indicated that the disease was widely distributed with higher intensity and recommended the need to develop sustainable integrated management strategies. The results of the in vitro and in vivo experiments recommended seedcane treatment with WI4-2 (T. harzianum), 8 g of Apron Star and 5 mL of Opera L-1 of water for integration with different management strategies. Finally, seedcane treatment with Opera and Opera + native T. harzianum could be used as alternative seedcane treatment.