Study on Corn Rust Caused by Puccinia polysora Undrew

Thammasak Sommartya, Prachoom Jutawantana, Sutathip Sangkul, and Jeeranan Yhamsoongnern

Department of Plant Pathology, Kasetsart University Bangkok 10900 Thailand

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Verification of the effects of spore suspension concentration and temperature on infection of seedling maize cultivars showed that the lowest concentration of infection level was 100,000 spore/ml.  No significant differences of the infectivity were demonstrated, although the concentration was increased to 300,000 spore/ml.  The uredospore of P.polysora could develop a phatogenicity and pustule formation at 25^oC and 30^oC, the seedling maize cultivars were, however, responded to the temperature at 25^oC and the pustule of P.polysora were produced apparently within 9-10days after inoculation.  The uredospores of susceptible maize were more strongly developed than those of resistant maize due to 14 days period of infection reaction.  The inbred line, Ki 11 had high susceptible rate, as classified in class “4”, where it was found that severe infection pustule density showed five or more pustule per square centimeter.  The resistant inbred line, Ki 21 was classified in class “1” while the composite and hybrid varieties, SW 1(S)C₉ and KTX 2602 were classified in class “3” as well as the resistant hybrid varieties, KTX 2801 and KTX 2802 were classified in class “2”.  The maize cultivars were maintained in optimum temperature 25^oC, 95% R.H. throughout the testing period.

            After the temperature was increased up to 30^oC, the infection intensity of corn rust of seedling maize cultivars showed decreasingly of pustule numbers since the higher temperature could prevent of greatly reduce infection and survival of corn rust uredospore.  The inbred line, Ki 11 was always susceptible to southern corn rust than that of Ki 20.  The Ki 3 and Ki 21 were, however, resistant to the rust and no pustules formation were found apparently on leave surface.  The hybrid varieties KTX 2602 performed more susceptible to corn rust than those of KSX 2301, KTX 2801 and KTX 2802 due to no pustule formation on those cultivars under high temperature.  Investigations on the morphology of uredospore on infected seedling maize by scanning electronmicroscope appeared that their characteristics were similar to those of uredospores in nature field.  Owing to the direct penetration in cell surface of uredospore germ tube and producing of rust mycelium under the pustule.

            Surveying the virulence of corn rust at Suwan Farm showed that most of maize plants could be severely infected by southern corn rust in late season rather than those in early rainy season, but they still had low infection in dry season.  Their symptoms appeared at lower leaves under wind direction and the-planted corn had more severely infected.

            Besides, significant correlation was found between the seedling maize cultivars disease evaluation and the late-planted maize measuring in the field.