STUDIES ON THE EPIDEMIOLOGY OF THE TAR SPOT DISEASE COMPLEX OF MAIZE IN MEXICO

During the period 1986-1988 field studies were conducted on the epidem iology of the tar spot disease complex (TDC) of maize (Zea mays) cause d by Phyllachora maydis, Monographella maydis and Coniothyrium phyllac horae. Under field conditions we found that P. maydis symptoms always appeared first, followed by symptoms of either M. maydis or C. phyllac horae. M. maydis causes leaf necrosis and has the most devastating eff ect. The primary symptoms covered about 12% of the leaf area below the ear leaf, whereas the total necrotic leaf area amounted to 30-60%, he re considered as a secondary effect. Maximum TDC severity occurred dur ing the winter season of 1988, which was characterized by a temperatur e range of 17-22 degrees C, a mean RH > 75%, and > 7 h of leaf wetness per night. The highest numbers of windborne ascospores of P. maydis w ere trapped at an RH > 85% and at temperatures of 17 to 23 degrees C i n the winter of 1987 and 1988, although large numbers were also caught at temperatures of > 23 degrees C and RH < 70%. Spore release was str ongly influenced by light conditions and followed a similar diurnal cu rve throughout three seasons, reaching a maximum at 17.00-21.00 hours. The spread of P. maydis within the field was very homogeneous, The in cubation period of P. maydis was 12 to 15 days, and most of the ascosp ores were released within 3 weeks after formation of the ascostromata. M. maydis inoculum in plant debris was reduced by 90% within 3 to 4 m onths.