the Symposium on Wheats for More Tropical Environments - cimmyt

183Documented Foot RotDlseaBes in Warmer ClimatesBased on personal observations.discussions with colleagues andsearches of <strong>the</strong> literature. <strong>the</strong>re is littledoubt that <strong>the</strong> main wheat foot rotpathogen in tropical areas at present is&lerotium rolfsU sacc.: reports fromsou<strong>the</strong>rn India. as well as from BrazU.have been published (6.7). S. rolfsU hasbeen observed on wheat in Ecuador.Peru and Bolivia. and it has been notedin many countries of Sou<strong>the</strong>ast Asia(E.E. Saari. personal communication).Sclerotium rolfsH is an omniverous.soU-inhabiting organism withworldwide distribution. attacking morethan 500 species in over 100 plantfamUtes (2). It survives as sclerotia in<strong>the</strong> soU for long periods of time and.although <strong>the</strong> organism has been <strong>the</strong>object of a great deal of research. fewcontrol measures exist o<strong>the</strong>r thanra<strong>the</strong>r costly chemical treatments (2).Due to its polyphagous nature andlongevity in <strong>the</strong> soU. this organism wUlbe dttncult to control without chemicalmeans.Rhtzoctonia solani Kuhn is ano<strong>the</strong>rWidespread soU pathogen that has beenobserved on wheat in <strong>the</strong> semitropicalareas of BrazU (7). As wheat is testedand grown in warmer areas. and aspathologists begin making moreintensive observations. R. solani willsurely become more obvious. Betterprogress has been made in breeding forresistance to R. solani than to S. rolfsUbut. overall. resistance breeding has notbeen very successful. One reason maybe that R. solani is made up ofgenetically distinct groups and. thus.R. solani is not a single species butconsists of diverse populations that maybe recognized through anastomosisgrouping (1). As more pathologists andbreeders take this into consideration.progress may be made in breeding fordisease resistance.Helminthosportum sativum P.K. and B.causes a severe foot rot problem inmany areas of BrazU (7). Up to now.major efforts in more tropical areashave dealt with <strong>the</strong> follar diseasecaused by H. sativum. but closerexamination of roots and crowns mayindicate <strong>the</strong> presence of <strong>the</strong> foot rotphase of this organism as well.The above examples point to some of<strong>the</strong> foot rot organisms that pathologistsand breeders will have to deal with indifferent areas. The foot rots are causedby some of <strong>the</strong> most dttncult pathogensto control in temperate climates. Due tosuboptimal growth conditions for <strong>the</strong>host. coupled with optimal conditionsfor <strong>the</strong> pathogens in tropical areas. <strong>the</strong>achievement of economic controlmeasures may be dttncult to obtain.Diseases of PossibleImportance in <strong>the</strong> FutureBased on <strong>the</strong> nature of <strong>the</strong> foot rotorganism. pathogens that attackrotation crops may become problems inwheat. Two examples may be sufficientto Ulustrate this point. Recently.R. oryzae-sativa.e sawada hasincreased in severity and incidence onrice in California: <strong>the</strong>re is an apparentcorrelation between this and <strong>the</strong>increased use of semidwarf cultivars (4).Three factors might indicate <strong>the</strong>possible importance of an R. oryzaesativae-typeorganism in tropical areas.First. wheat-rice rotations will beimportant: second. <strong>the</strong> organism Isadapted to warm climates and. third. itattacks many hosts. Thus it not onlymight attack wheat. but could becomeincreasingly important on rice.A second example is related to a wheatsoybeanrotation in Indiana whereGaeumannomyces graminis (Sacc.)Arx and Ollv.• normally infecting wheatand grasses. was isolated from