Southern leaf blight of maize (zea mays l.) in Kenya.

Abstract

The southern leaf blight of maize caused by Helminthosporium maydis is a serious disease that has recently been introduced into Kenya. The disease is causing concern in many maize growing areas of the world. Investigations were carried out on this disease and these included studies of disease incidence, severity, symptoms, characteristics of the pathogen, factors affecting colony growch 2nd conidial germination, screening 7 maize hybrids aud zco POSl te for resistance and host ranqe . Where the disease was observed to occur incidence was below 5~ dnd there was only slight to light infection possibly because of drought at the L~me the disease survey was carried out. Symptoms were observed to form only on leaves and consisted of greyish-ta or straightcoloured lesions showing u zonate pattern and measuring 0.5 x 0.5 to 3 x 10 ~m. Colonies of the fungus formed chlamydospores ':'.11 contaminated cultures along zones of anti-biotic activity between the fungus and bacteria. The conidla were curved, fusiform and pale to somewbat dark golden brown in colour. They measured 27 to 103 x 7 to 20]..[l1 and the nurrber of septa ranged from 3 to 10. Optimum conditions for conidial germination (xiv) were when relative humidity was between 90 and 100% o and when the temperature was 30 C. Conidial germination was bipolar. Germ tubes were produced within 6 hours of the inoculation of maize leaves and most appressoria formed between 6 and 18 hours after inoculation near stomata which seemed to aid penetration of the leaves by the pathogen. Fungal colony growth rate was highest when temperature was o 30 C at pH 6. Attempts to obtain the perfect stage of the fungal pathogen (Cochliobolus heterostrophus) were unsuccessful. Katumani Composite maize was highly susceptible to the disease in the field. Hybrids 511, 512, 612 and 622 were only moderately susceptible while hybrids 614C, 613C and 632 were somewhat resistant to attack by southern leaf blight. In the host range study only one host plant species, Rottboellia exaltata, showed infection which was only slight, out of 13 grass and 1 sedge species . • All three isolates of the fungus were similar in behaviour in all the aspects of the disease studied and w~re probably all r~ce 0 of the pathogen since they infected only leaves.