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Do\1'I!Y Mildew ojMaize in Asia 281emerging. An inoculum threshold value beyond which a gene for resistance becomes less effective isanother explanation already indicated in inheritance studies (Kaneko and Aday, 1980). Thisthreshold theorem is obviously an indication of the apparent instability of the MR genes that mightlead to a breakdown of resistance.The emergence of tolerance of Peronosc!erospora species to metalaxyl has been anticipated forquite sometime. The chemical has started to show signs of declining efficacy judging fromobservations of plants from treated seed lots infected by P. philippinesnsis (A. Patonona, personalcommunication) and by P. nza}'dis (S.c. Dalmacio. personal communication). Although the infectionmight have occurred as a result of improper treatment procedures, as when some seeds are not coatedwith the slurry. this phenomenon is consistent with the predictable shift in the pathogen population asan evolutionary response for survival. In theory, the time that has elapsed since. metalaxyl wasinitially used would be enough for the pathogen to mutate into a form capable of overcoming theeffect of the chemical. This. however, may not happen yet in places where present plantingsinvolving metalaxyl-treated seeds constitute only a small portion of the total area planted to maize.The pre:;ent pathogen population can be expected to change when the pressure brought about byexpanded usage of metalaxyl becomes increasingly significant. This change can be due to slowadaptation of the pathogen to the chemical resulting in a metaJaxyl-to[erant strain. The adaptation ofthis strain may have evolved infection mechanisms of current DMR cultivars.Suggested Research ThrustsThe genetic structure of downy mildew pathogen population should be elucidated. The inherentdifficulty of determining physiologic speciation can possibly be circumvented by modern moleculartechniques such as RFLP and PCR as has been done with other genera of plant pathogens. Corollaryto this should be a marker-assisted selection for resistance. This is now being undertaken with otherdiseases and has actually been applied to downy mildew in sorghum (Gowda et al.. 1994).Downy mildew pathogen speciation appears to be unclear in some of the downy mildews. Forinstance, the existence of a S. thailandensis earlier described by M. Payak (c. D~ Leon, personalcommunication) appears to be supported by the findings of Bonde et aI, 1984) based on isozymeanalysis on the downy mildew isolate in Thailand. In the Philippines, P. phihppinensis has long beenpresumed the dominant if not the only species attacking maize. Despite the occurrences ofdevastating droughts and other natural calamities that have wiped out known alternate hosts of P.philippinensis, local epidemics have persisted. Since this species has not been observed to produceoospores. the suspicion that other species with different survival mechanisms has gained credence.These possibilities whose significance is immense can now be explored through the application ofmolecular biology.The phenomenon of slow mildewing (Baria and Raymundo, 1989; Raymundo et a!., 1997)should be explored. When combine with mature-plant resistance (Sun et aI., (966) the result could bea drastic reduction in downy mildew incidence.The population biology of downy mildew-maize interaction should be pursued. After a DMRcultivar has been released, its durability and stability should be determined. This requires monitoringof its performance in time and space (Ebron and Raymundo, 1987a). Such approach will indicate if abreakdown of resistance in imminent.Literature CitedAday, B. A. 1974. The Philippine program in breeding for resistance to downy mildew of maize.Proc. Symp. On Downy Mildew of Maize. Trop. Agr. Res. Ser. No.8. pp.207-219.Asnani, V. L. and B. Bhusan. 1970. Inheritance study on the brown stripe downy mildew of maize.Indian Phytopathol. 23: 220-230.