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 epidemiology of the tar spot disease complex (TDC) of maize ( Zea mays ) caused by Phyllachora maydis, Monographella maydis and Coniothyrium phyllachorae. Under field conditions we found that P. maydis symptoms always appeared first, followed by symptoms of either M. maydis or C. phyllachorae. M. maydis causes leaf necrosis and has the most devastating effect. The primary symptoms covered about 12% of the leaf area below the ear leaf, whereas the total necrotic leaf area amounted to 30–60%, here considered as a secondary effect. Maximum TDC severity occurred during the winter season of 1988, which was characterized by a temperature range of 17–22°C, a mean RH >75%, and > 7h of leaf wetness per night. The highest numbers of windborne ascospores of P. maydis were trapped at an RH > 85% and at temperatures of 17 to 23°C in the winter of 1987 and 1988, although large numbers were also caught at temperatures of >23°C and RH <70%. Spore release was strongly influenced by light conditions and followed a similar diurnal curve throughout three seasons, reaching a maximum at 17.00–21.00 hours. The spread off. maydis within the field was very homogeneous. The incubation period of P. maydis was 12 to 15 days, and most of the ascospores were released within 3 weeks after formation of the ascostromata. M. maydis inoculum in plant debris was reduced by 90% within 3 to 4 months.