By: Mayra Cathme, Mayra Llamatumbi and José B. Ochoa.
Maize is an important food crop in the highlands valleys of Ecuador. Among other reasons farmers maintain maize diversity to reduce biotic stress losses (hypothesis). Northern leaf blight (NLB) ( Exserohilum turcicum(Pass.) Leonard & Suggs is an important disease. The avirulence/resistance gene for gene relationship has been postulated for NLB (Welz, H. G and Geiger, H. H. 2000).Therefore, plant/pathogen co evolution might have been taking place in Ecuador. A study to explain plant/pathogen co evolution and to characterize resistance on the NLB pathosystem was carried out with local cultivars from Cotacachi and Saraguro at seedling (green house) and at adult plant (field) stages.
The disease severity was not enough to differentiate between virulence and avirulence. Long incubation period, chlorotic lesion and size of the lesion helped discriminating virulence. (Hooker, A. and Kim, S. 1980; Welz, H. G and Geiger, H. H. 2000). Integrating DS, incubation period, chlorosis and lesion size led to four Reaction Types (RT) being discriminated: Inm = No symptoms (immunity); R = long incubation period, chlorosis and/or small lesion size (resistant); I = Intermediate incubation period and small to medium lesion size (Intermediate) and S = short incubation period and large lesion size (Susceptible). Isolates were virulent on Ht1, Ht2 and Ht3 (Race 123). All isolates were avirulent to HtN. However, two phenotypic resistance reactions were identified to HtN gene: a) long incubation period, the typical resistance reaction of HtN b) a mile chlorosis called yellow pinhead (Muiru, 2007).
Resistance to NLB was present in most cultivars from Cotacachi and Saraguro. Inm and R appears associated with major genes identified so far (Hooker, A. and Kim, S. 1980).Long incubation period and the small lesion size have also been associated with Partial Resistance (Hooker, A. and Kim, S. 1980; Welz, H. G and Geiger, H. H. 2000). The intermediate reaction type (I) could be considered lower levels of susceptibility. High variation in RT was observed among cultivars, but mostly within cultivars. Most cultivars were a mixed reacting population. Cultivars from Cotacachi varied from very resistant to very susceptible. Some Cultivars contained different population frequencies of all reaction types. Some cultivars showed differential plant/pathogen interactions.
Cultivars from Saraguro reacted more similarly than Cotacachi cultivars except, no uniformly reacting cultivar as Morocho amarillo was observed. A higher frequency of intermediate and susceptible populations was observed in Saraguro. Resistance/susceptible interactions in cultivars Morochillo and Puka sara suggests that sources of resistance of major nature are derived. A similar interaction was observed in the Saraguro variety morocho dulce suggesting variation in resistance too. The resistance of Mmorocho amarillo and others appears new. Field severity of specific cultivars might vary year by year, depending on the frequency of the infection types involved. Under these conditions is difficult to discern major and minor gene resistance however, both resistant types appear to be operating in the field.
