the Symposium on Wheats for More Tropical Environments - cimmyt

182Reflections on Foot Rots of Wheat inWarmer, Nontraditional Wheat-Growing ClimatesH.J. Dubio, Wheat Program, CIMMYT, Quito, EcuadorAbstractInJormation is presented on Sclerotium rolfsii. Rhizoctonia solani, andHelminthosporium sativumJoot rots in warmer areas. It is noted that <strong>the</strong>Jacultative nature oj mostJoot rot pathogens mayJacilitate <strong>the</strong>ir movingJromrotation crops to wheat in tropical areas. Effective control may require <strong>the</strong>integration oj breedingJor resistance or tolerance. appropriate agronomicpractices and chemical or biological control.The introduction of wheat as a possiblecommercial crop in warmer or tropicalclimates has been increasing in recentyears (8). The difficulty of achievingeconomic yields should not beunderestimated; <strong>the</strong>re are a myriad ofproblems inherent in transposing atemperate climate crop to tropicalareas. One of <strong>the</strong> principal problemswill be <strong>the</strong> unique diseases encounteredin <strong>the</strong>se nontraditional areas and howto control <strong>the</strong>m.Foot rot is one of <strong>the</strong> diseases of wheatthat may have to be confronted andcontrolled in subtropical or tropicalareas. Wheat scientists must be awareof <strong>the</strong> type of pathogens <strong>the</strong>y aredealing with. and that <strong>the</strong> ecological orhost-pathogen relationships in <strong>the</strong>hotter. humid areas may be quitedifferent from those preVailing intemperate climates. An exhaustivereview of <strong>the</strong> literature of foot rots ofwheat in <strong>the</strong> tropics is not presented;indeed, very little publishedinformation exists. What little is citedshall serve only as examples.Fungal pathogens constitute <strong>the</strong> focusof this paper, with some mention alsomade of nematode diseases. since <strong>the</strong>ymay be of some importance. However.it should be noted that bacteria, virusesand related organisms could becomeproblems also. The fungal organismscausing foot rots are. without exception,facultative parasites that spend most of<strong>the</strong>ir life cycles as saprophytes in <strong>the</strong>soil or on plant debris. Plant parasiticnematodes generally are obligateparasites that feed and reproduce onlyon <strong>the</strong>ir liVing hosts. Never<strong>the</strong>less.although <strong>the</strong>y are obligate parasites. inmany cases <strong>the</strong>y do not appear to havedeveloped high levels of pathogenicspecificity, and so often <strong>the</strong> samespecies can attack many diverse typesof plant hosts (10).As noted. fungal facultative parasitesprimarily exist as saprophytes andbecome parasitic when <strong>the</strong> appropriateconditions are met. These parasiteshave a minimum of pathogenicspecialization and probably haveevolved to a level of speciescompatibility. ra<strong>the</strong>r than cultivarspecificity or compatibility (5). Since<strong>the</strong>y are not as fastidious as <strong>the</strong> fungalobligate parasites, <strong>the</strong>y may, if given<strong>the</strong> opportunity. become parasitic on anontraditional crop when it isintroduced into an area. It can beexpected that cropping patterns andhusbandry practices may have animportant impact on foot rots of wheat.Diseases of rice and soybeans. forexample. might be important on wheat.and vice versa. This would also holdtrue for <strong>the</strong> facultative. foliarpathogens. This must be kept in mindwhen considering present foot rotproblems. diseases of potentialimportance in <strong>the</strong> future and suggestedcontrol strategies.